Additive manufacturing using materials that form a weak gel

Abstract

A formulation usable in the additive manufacturing of three-dimensional objects is provided. This formulation comprises one or more monofunctional curable materials, one or more hydrophilic multifunctional curable materials, and one or more water-miscible non-curable materials, such that the total amount of curable materials is 20% or less by weight, and the weight ratio of the total weight of the monofunctional curable materials to that of the hydrophilic multifunctional curable materials ranges from 1:1 to 10:1. Once cured, the formulation exhibits the properties of a weak, fluid gel. Additive manufacturing processes using this formulation as a support material are also provided.

Description

Additive manufacturing using materials that form a weak gel Related requests This application claims the benefit of priority with respect to U.S. Provisional Patent Application No. 62 / 786,809, filed on December 31, 2018. Field and background of the invention The present invention, in some embodiments thereof, relates to additive manufacturing and, more particularly, but not exclusively, to curable formulations that, when hardened, provide materials exhibiting weak gel properties, and to the additive manufacturing of three-dimensional objects using such materials. Additive manufacturing (AM) is generally a process in which a three-dimensional (3D) object is manufactured using a computer model of the object. This process is used in various fields, such as design-related fields for visualization, demonstration, and mechanical prototyping, as well as for rapid manufacturing (RM).The basic operation of any AM system consists of slicing a three-dimensional computer model to produce fine-cut sections, translating the result into two-dimensional position data and feeding the data to the control equipment that manufactures a three-dimensional structure in the form of layers. One type of AM is three-dimensional inkjet printing. In this process, a build material is dispensed from a print head with a set of nozzles to deposit layers onto a support structure. Depending on the build material, the layers can then be cured or solidified using a suitable device. There are several three-dimensional inkjet printing techniques and they are disclosed in, for example, U.S. Patents Nos. 6,259,962, 6,569,373, 6,658,314, 6,850,334, 7,183,335, 7,209,797, 7,225,045, 7,300,619, 7,479,510, 7,500,846, 7,962,237. Several AM processes allow the additive formation of objects using more than one modeling material. For example, U.S. Patent No. 9,031,680, held by the present Assignee, discloses a system comprising a solid freeform manufacturing apparatus having a plurality of dispensing heads, a material supply apparatus configured to supply a plurality of materials to the manufacturing apparatus, and a control unit configured to control the manufacturing and supply apparatus. The system has several modes of operation. In one mode, all dispensing heads operate during a single build scan cycle of the manufacturing apparatus. In another mode, one or more of the dispensing heads are inoperative during a single build scan cycle or part thereof. Construction materials may include modeling materials and support materials, which form the object and temporary support structures that hold the object while it is being built, respectively. The modeling material, also referred to herein as "model material," (which may include one or more materials, included in one or more formulations) is deposited to produce the desired object or objects. The support material, also known in the art as "support material," (which may include one or more materials) is used, with or without modeling material, to support specific areas of the object during construction and to ensure proper vertical placement of subsequent layers. For example, in cases where objects include protruding elements or shapes, such as curved geometries, negative angles, hollows, and the like, the objects are typically constructed using adjacent support structures, which are used during printing. In all cases, the support material is deposited in the vicinity of the modeling material, allowing the formation of complex object geometries and filling the gaps in the objects. In all currently implemented technologies, the deposited support material and the modeling material harden, typically after exposure to a curing condition (e.g., curing energy), to form the required layer shape. After printing is complete, the support structures are removed to reveal the final shape of the fabricated 3D object. When using currently available commercial printheads, such as inkjet printheads, the support material must have a relatively low viscosity (approximately 10–20 mPa·s [10–20 cP]) at the operating temperature, i.e., the jetting temperature, so that it can be jetted. Furthermore, the support material must harden quickly to allow for the build-up of subsequent layers. Additionally, the hardened support material must have sufficient mechanical strength to hold the model material in place and minimal distortion to prevent geometric defects. Known methods for removing support materials include mechanical impact (applied by a tool or water injection) and chemical methods, such as dissolution in a solvent, with or without heating. Mechanical methods are labor-intensive and generally unsuitable for small and intricate parts. To dissolve support materials, the manufactured object is usually immersed in water or a solvent capable of dissolving the support materials. Solutions used to dissolve support material are also referred to herein and in the art as "cleaning solutions." In many cases, however, the support removal process may involve the use of hazardous materials, manual labor, and / or specialized equipment requiring skilled personnel, protective clothing, and costly waste disposal. Furthermore, the dissolution process is often limited by diffusion kinetics and can require very long periods of time, especially when support structures are large and bulky. Additionally, further processing may be necessary to remove any remaining "mixed layer" from the object surfaces.The expression "mixed layer" refers to a residual layer of hardened pattern materials and mixed support materials that forms on the contact surface between the two materials on the surfaces of the object being manufactured, as the pattern and support materials mix together on the contact surface between them. Furthermore, methods requiring high temperatures during support removal can be problematic, as some model materials, such as waxes and certain flexible materials, are temperature-sensitive. Both mechanical and dissolution methods for removing support materials are particularly problematic for use in an office environment, where ease of use, cleanliness, and environmental safety are primary considerations. Water-soluble materials for 3D construction are described, for example, in U.S. Patent No. 6,228,923, where a water-soluble thermoplastic polymer, poly(2-ethyl-2-oxazoline), is indicated as a support material in a 3D construction process involving the high-pressure, high-temperature extrusion of ribbons of selected materials onto a plate. A water-containing support material comprising a fusible crystalline hydrate is described in U.S. Patent No. 7,255,825. Suitable formulations for forming a hardened support material in the construction of a 3D object are described, for example, in U.S. Patents Nos. 7,479,510, 7,183,335, and 6,569,373, all in the name of the present Assignee. Generally, the compositions disclosed in these patents comprise at least one UV-curable (reactive) component, for example, an acrylic component, at least one non-UV-curable component, for example, a polyol or glycol component, and a photoinitiator. After irradiation, these compositions provide a semi-solid or gelatinous material capable of dissolving or swelling upon exposure to water, an alkaline or acidic solution, or an aqueous detergent solution. In addition to its ability to swell, another characteristic of this type of support material can be its ability to decompose upon exposure to water, an alkaline or acidic solution, or an aqueous detergent solution, since the support material is composed of hydrophilic components. During the swelling process, internal forces cause fractures and breakage of the hardened support. Furthermore, the support material may contain a substance that releases bubbles upon exposure to water, for example, sodium bicarbonate, which transforms into CO2 upon contact with an acidic solution. The bubbles aid in the process of removing the support from the model. Several additive manufacturing processes allow for the additive formation of objects using more than one modeling material. For example, U.S. Patent Application No.Publication No. 2010 / 0191360, of the present Assignee, discloses a system comprising a freeform solid manufacturing apparatus having a plurality of dispensing heads, a material supply apparatus configured to supply a plurality of materials to the manufacturing apparatus, and a control unit configured to control the manufacturing and supply apparatus. The system has several modes of operation. In one mode, all dispensing heads operate during a single scan cycle of the manufacturing apparatus. In another mode, one or more of the dispensing heads are not operational during a single scan cycle or part thereof. In a 3D inkjet printing process, such as PolyJet™ (Stratasys Ltd., Israel), the build material is selectively jetted from one or more print heads and deposited onto a build tray in successive layers according to a predefined configuration defined by a software file. U.S. Patent No. 9,227,365, herein assigned, discloses methods and systems for the freeform fabrication of covered solid objects, constructed from a plurality of layers and a layered core constituting core regions and a layered cover constituting enveloping regions. Additive manufacturing processes have been used to create rubber-like materials. For example, rubber-like materials are used in PolyJet™ systems, as described herein. These materials are formulated to have a relatively low viscosity, allowing them to be dispensed, for example, by inkjet printing, and to develop a glass transition temperature (Tg) below room temperature, for example, -10 °C or lower. This is achieved by formulating a product with a relatively low degree of crosslinking and using monomers and oligomers with an inherently flexible molecular structure (for example, acrylic elastomers). An example family of rubber-like materials useful in PolyJet™ systems (marketed under the family trade name "Tango™") offers a variety of elastomeric characteristics of the resulting hardened material, including Shore A hardness, elongation at break, tear strength, and tensile strength. Another family of rubber-like materials useful in PolyJet™ systems (marketed under the family trade name "Agilus™") is described in PCT International Application No. IL2017 / 050604 (published as WO2017 / 208238), of the present Assignee, and uses a curable elastomeric formulation comprising an elastomeric curable material and silica particles. International Patent Application Publications PCT Nos. WO 2019 / 021291, WO 2019 / 021292 and WO 2019 / 021295, all of which are from the present Assignee, describe formulations that are useful in the additive manufacturing of three-dimensional objects and that provide, after exposure to a curing condition, a liquid or liquid-like material. Document JP 2017 / 078123 A describes a photoenergy-curable resin composition comprising a (meth)acrylate (A) represented by the formula (1) CH=CR-CO-(O-CH)m-OR in an amount of 10.0 to 60.0% by mass, a cationic monomer (B) from 1.0 to 20.0% by mass, a polyfunctional unsaturated compound (C) from 0.5 to 10.0% by mass, and a non-reactive diluent (D) from 30.0 to 80.0% by mass. A support material for supporting a form is prepared using the above resin composition. In the support element for supporting a form, in particular, the (meth)acrylate (A) is preferably a monomer represented by the formula (2) CH=CH-CO-(O-CH)n-OR. WO 2017 / 029657 A1 describes support material formulations characterized by providing a cured support material with an improved dissolution rate while maintaining sufficient mechanical strength. The formulations comprise a non-curable, water-miscible polymer, a first curable, water-miscible material, and a second water-miscible material selected for its ability to interfere with the intermolecular interactions between the polymer chains formed after exposing the first water-miscible material to curing energy. US patent 2017 / 252971 A1 describes an actinically curable inkjet printing ink composition for 3D printing comprising an acrylate monomer A capable of forming a homopolymer having a glass transition temperature of 25°C to 120°C; an acrylate monomer B capable of forming a homopolymer having a glass transition temperature of -60°C or higher and lower than 25°C; a bifunctional acrylate oligomer C having a weight average molecular weight of 2,000 to 20,000; and an acylphosphine oxide compound, wherein the mass content of bifunctional or higher functionality acrylate compounds is 15% by mass or less. Summary of the invention According to the present invention, a formulation useful in the additive manufacturing of a three-dimensional object is provided, which is also referred to herein as formulation FG, comprising: at least one monofunctional curable material; at least one hydrophilic multifunctional curable material; and at least one non-curable, water-miscible material, where a weight ratio of a total weight of at least one monofunctional curable material and a total weight of at least one hydrophilic multifunctional curable material ranges from 1:1 to 10:1. A total amount of at least one monofunctional curable material varies from 1 to 10, or from 3 to 10, or from 5 to 10, % by weight, of the total weight of the formulation. A total amount of at least one hydrophilic multifunctional curable material varies from 1 to 5% by weight of the total weight of the formulation. The total amount of at least one non-water-miscible curable material is at least 85% by weight of the total weight of the formulation. Each of the curable materials is a UV-curable material. According to any of the embodiments described herein, at least one hydrophilic multifunctional curable material provides, when cured per se, a material that is insoluble in water. According to any of the embodiments described herein, at least one of the one or more monofunctional curable materials comprises a hydrophilic monofunctional curable material. According to any of the embodiments described herein, at least one of the one or more monofunctional curable materials comprises a hydroxyalkyl moiety and / or an alkylene glycol moiety. According to any of the embodiments described herein, at least one of the one or more hydrophilic multifunctional curable materials comprises one or more alkylene glycol moieties. According to any of the embodiments described herein, at least one non-curable material is a water-soluble material. According to any of the embodiments described herein, the at least one non-curable material comprises a polymeric material. According to any of the embodiments described herein, the at least one non-curable material further comprises a non-polymeric material. According to any of the embodiments described herein, a weight ratio of a total weight of at least one non-curable polymeric material and a total weight of at least one non-curable non-polymeric material ranges from 2:1 to 1:2. According to an example that does not include all the features of the claimed invention, the formulation comprises: at least one monofunctional curable material, in an amount of 3 to 10% by weight; at least one multifunctional curable material, in an amount of 3 to 5% by weight; at least one polymeric non-curable material, in an amount of 30 to 60% by weight; and at least one non-polymeric non-curable material, in an amount of 30 to 60% by weight, wherein a total amount of the non-curable materials is at least 80% by weight. According to any of the embodiments described herein, the formulation is water-free. According to any of the embodiments described herein, the formulation exhibits a viscosity of 8 to 40 mPa·s (8 to 40 centipoises) at 75 °C, measured using a Brookfield viscometer. According to any of the realizations described in this document, additive manufacturing is 3D inkjet printing. According to any of the embodiments described herein, the formulation, when hardened, exhibits a Young's modulus of 10 to 100, or 10 to 80 kPa. According to any of the embodiments described herein, the formulation presents, when hardened, a gel material that can flow after the application of positive pressure. According to any of the embodiments described in this document, the pressure varies from 10 to 150 kPa (0.1 to 1.5 bar), or from 20 to 120 kPa (0.2 to 1.2 bar), or from 20 to 100 kPa (0.2 to 1 bar). According to any of the realizations described in this document, each of the curable materials is an acrylic material. According to any of the embodiments described herein, the formulation also comprises a photoinitiator. According to any of the embodiments described herein, the amount of the photoinitiator is not greater than 2% by weight of the total weight of the formulation. According to one aspect of the present invention, a method is provided for the additive manufacturing of a three-dimensional object, the method comprising sequentially forming a plurality of layers in a configured pattern corresponding to the shape of the object, thereby forming the object, where the formation of at least some of the layers comprises: Dispense at least two formulations of construction material, the at least two formulations of construction material comprising a modeling material formulation M which, upon exposure to a curing condition, forms a hardened modeling material M, and an FG formulation in accordance with any of the respective realizations described herein, which, upon exposure to the curing condition, forms a hardened support material FG. According to any of the embodiments described herein, the dispensing is such that the hardened modeling material M forms at least one hollow structure, and the FG material is enclosed, at least partially, in the hollow structure. According to any of the embodiments described herein, the hollow structure is selected from a tubular structure, a branched tubular structure, and a plurality of intertwined tubular structures. According to any of the embodiments described herein, the diameter of at least one of the tubular structures is less than 1 cm. According to any of the embodiments described in this document, the FG material is completely enclosed in the hollow structure. According to one aspect of the present invention, a method is provided for the additive manufacturing of a three-dimensional object having a cavity, the method comprising sequentially forming a plurality of layers in a configured pattern corresponding to a combined shape of the object and a sacrificial object, such that the sacrificial object is enclosed by a sacrificial cover, and the sacrificial cover is enclosed in the cavity; and remove the sacrificial object and the sacrificial cover from the cavity; wherein the sacrificial object comprises a modeling material M, and the sacrificial cover is made of a support material FG formed from the formulation FG as described herein in any of the respective embodiments. According to any of the embodiments described herein, the sacrificial object further comprises the FG material, the FG material being reinforced by the modeling material M. According to any of the embodiments described herein, the modeling material M occupies from approximately 60% to approximately 80% of the volume of the sacrificial object. Unless otherwise defined, all technical and / or scientific terms and expressions used herein have the same meaning as those commonly understood by a person skilled in the art to which the invention pertains. Although similar or equivalent methods and materials to those described herein may be used in the practice or testing of embodiments of the invention, example methods and / or materials are described below. In case of conflict, the patent specification, including the definitions, shall prevail. Furthermore, the materials, methods, and examples are merely illustrative and are not necessarily intended to be limiting. Implementing the method of embodiments of the invention may involve performing or completing selected tasks manually, automatically, or a combination thereof. Furthermore, depending on the actual instrumentation and equipment used in the embodiments of the method of the invention, several selected tasks could be implemented using hardware, software programs, or firmware, or a combination thereof, using an operating system. For example, the hardware for performing selected tasks according to the embodiments of the invention could be implemented as a chip or a circuit. As software, the selected tasks according to the embodiments of the invention could be implemented as a plurality of software instructions executed by a computer using any suitable operating system. In an exemplary embodiment of the invention, one or more tasks according to exemplary embodiments of the method as described herein are performed by a data processor, such as a computing platform for executing a plurality of instructions. Optionally, the data processor includes volatile memory for storing instructions and / or data and / or non-volatile storage, for example, a magnetic hard disk and / or removable media, for storing instructions and / or data. Optionally, a network connection is also provided.A display and / or user input device, such as a keyboard or mouse, are also optionally provided. Brief description of the different views of one or more drawings This document describes some embodiments of the invention, by way of example only, with reference to the accompanying drawings. It is emphasized that the features shown are illustrative and for the purpose of analyzing embodiments of the invention. In this respect, the description, taken in conjunction with the drawings, makes clear to those skilled in the art how embodiments of the invention can be implemented. In the drawings: Figures 1A-1D are schematic illustrations of an additive manufacturing system; Figures 2A-2C are schematic illustrations of printheads; Figures 3A and 3B are schematic illustrations that demonstrate coordinate transformations; Figure 4 is a flowchart of a suitable method for the AM of a three-dimensional object according to various example embodiments of the present invention; Figures 5A-C present photographs of a printed object with a DM core made from an example FG formulation and a grid of an Agilus™ M modeling formulation and a coating made of Agilus™ as M material (approximately 1 mm) (Figure 5A), and of the same object after immersion in a 2% NaOH solution (Figure 5B) and after complete dissolution of the hardened FG material (Figure 5C). Figures 6A and 6B present schematic illustrations of digital material modes that can be realized in additive manufacturing methods employing example formulations according to some of the present embodiments; Figures 7A-C present schematic representations of a "bone-type test specimen" model manufactured in experiments conducted in accordance with some embodiments of the present invention. Figure 8 presents comparative graphs showing the tear strength of bone-like specimen models prepared using an Agilus™ formulation as the modeling formulation M and example FG formulations, SUP706 or a liquid formulation L, as the support material formulation. Figures 9, 10 and 11 present series of photographs showing the removal of an FG material according to some of the present realizations from models that have hollow structures. Figures 12A and 12B are schematic illustrations of a tubular structure. Description of specific embodiments of the invention The present invention, in some embodiments thereof, relates to additive manufacturing and, more particularly, but not exclusively, to curable formulations that, when hardened, provide materials exhibiting weak gel properties, and to the additive manufacturing of three-dimensional objects using such materials. Before explaining at least one embodiment of the invention in detail, it should be understood that the invention is not necessarily limited in its application to the construction details and arrangement of components and / or methods set forth in the following description and / or illustrated in the drawings and / or examples. The invention is capable of other embodiments or can be practiced or carried out in various ways. The present inventors have observed that, when applying currently known additive manufacturing processes to produce three-dimensional objects, difficulties arise in forming and cleaning parts with intricate geometries, such as geometries containing hollow structures like cavities, enclosed volumes, thin and / or interlocking and / or branching hollow tubular structures (e.g., pipes, tunnels), and sponge-like structures with narrow openings or complete enclosures. Non-limiting examples of such intricate geometries include objects with body organ structures, such as blood vessels, internal parts of bones, and hearts. More specifically, the present inventors have found that when manufacturing such objects using, for example, 3D inkjet printing, the intricate parts of the object, such as cavities, enclosed volumes, tubes, pipes and / or intertwined and branching tunnel systems, are typically filled with a support material, typically a material exhibiting gel-like properties, so that the removal of the support material after the object has been manufactured using conventional mechanical and / or chemical techniques is difficult, inefficient and time-consuming and, moreover, causes damage to the intricate parts and / or the entire object. Mechanical techniques for removing support material, such as water injection and compressed air, are ineffective for enclosed or partially enclosed hollow structures because physical accessibility to the support material is limited, and the pressure required for complete removal can damage the object. Chemical techniques for removing support material, which involve dissolving the support material by contacting it with a cleaning solution, are also ineffective for enclosed or partially enclosed hollow structures because they require diffusion of the cleaning solution into the object. As discussed earlier in this document, the support material formulations currently in use typically include curable and non-curable materials that, when hardened, form gel or gel-like materials. In this document and in the art, the term "gel" describes a material, often referred to as a semisolid material, comprising a three-dimensional solid network, typically made up of chemically or physically bonded fibrous structures, and a liquid phase enclosed within this network. Gels are typically characterized by a solid consistency (e.g., they are not fluids) and exhibit relatively low tensile strength, a relatively low shear modulus (e.g., less than 100 kPa), and a shear loss modulus to shear storage modulus (tan delta, G" / G') ratio of less than 1.Gels can be characterized as fluids when subjected to a positive pressure of at least 50 kPa (0.5 bar), preferably at least 100 kPa (1 bar) or higher, or, alternatively, as non-fluids when subjected to a pressure lower than 100 kPa (1 bar), or lower than 50 kPa (0.5 bar), or 30 kPa (0.3 bar) or lower. Gel-like materials according to the present embodiments are typically soft materials, which may be gels or solids, exhibiting mechanical and rheological properties of a gel, as described herein. Support material formulations currently in use typically comprise a mixture of curable and non-curable materials and, when hardened, form hardened support materials that are also referred to herein as gel-like support material or gel support material (e.g., material S). Most hardened support materials currently in use are typically water miscible, water dispersible, or water soluble, as defined herein. International Patent Application Publications PCT Nos. WO 2019 / 021291, WO 2019 / 021292 and WO 2019 / 021295, all of which are from the present Assignee, describe formulations that are useful in the additive manufacturing of three-dimensional objects and that provide, after exposure to a curing condition, a liquid or liquid-like material. The present inventors have conceived of using curable formulations that form a weak gel material upon exposure to a curing condition. In this document and in the art, the term "gel" describes a material, often referred to as a semisolid material, comprising a three-dimensional solid network, typically made up of chemically or physically bonded fibrous structures, and a liquid phase enclosed within this network. Gels are typically characterized by a solid consistency (e.g., they are not fluids) and exhibit relatively low tensile strength, a relatively low shear modulus (e.g., less than 100 kPa), and a shear loss modulus to shear storage modulus (tan delta, G" / G') ratio of less than 1.Gels are typically characterized as fluids when subjected to a positive pressure of at least 50 kPa (0.5 bar), preferably 100 kPa (1 bar) or higher, and as non-fluids when subjected to a pressure lower than 100 kPa (1 bar) or lower than 50 kPa (0.5 bar) or 30 kPa (0.3 bar) or lower. Gel-like materials are typically soft materials, which can be gels or solids, that exhibit the mechanical and rheological properties of a gel. A "weak gel" is defined as a material that has a gel-like consistency at room temperature and a shear modulus in the lower to middle range for gels, i.e., less than 80 kPa, or less than 50 kPa or lower. The present inventors have devised formulations that provide such weak gels, which also exhibit fluidity when subjected to relatively low pressure, for example, in a range of 10 kPa to 150 kPa (0.1 bar to 1.5 bar), or 20 kPa to 150 kPa (0.2 bar to 1.5 bar), or 10 kPa to 120 kPa (0.1 bar to 1.2 bar), or 20 kPa to 120 kPa (0.2 bar to 1.2 bar), or 20 kPa to 100 kPa (0.2 bar to 1 bar), or 50 kPa to 150 kPa (0.5 bar to 1.5 bar), or 50 kPa to 100 kPa (0.5 bar to 1 bar), or 10 kPa to 80 kPa (0.1 bar to 0.8 bar), or from 20 kPa to 80 kPa (0.2 bar to 0.8 bar), or from 50 kPa to 80 kPa (0.5 bar to 0.8 bar), or from 10 kPa to 50 kPa (0.1 bar to 0.5 bar), or from 20 kPa to 50 kPa (0.2 bar to 0.5 bar), including any intermediate values ​​and sub-intervals between them. Such formulations are also referred to herein as FG formulations or fluid gel formulations.The material formed when these formulations harden is referred to herein as FG material or fluid gel material or hardened fluid gel material. Throughout this document, the terms "printed object" or "manufactured object" describe a product of the additive manufacturing process. This term refers to the product obtained by a method as described herein, before the removal of the support material (e.g., FG material, optionally in conjunction with support material S). Therefore, a printed object consists of hardened (e.g., cured) modeling material and hardened (e.g., cured) support material. The expression "printed object", as used herein, refers to a complete printed object or a part thereof. The terms "model object," "final object," "object," and "model," as used herein, describe a final product of the manufacturing process. This term refers to the product obtained by a method as described herein, after the removal of one or more support materials. Therefore, the model essentially consists of a hardened (solidified, cured) modeling material, unless otherwise specified. The terms and expressions "model", "model object", "final object" and "object", as used throughout this document, refer to a complete object or a part thereof. By "hollow structure" in an object, it is meant that the object comprises one or more parts having cavities (non-solid portions) therein. The cavity may be completely or partially enclosed by solid material. When partially enclosed, the cavity may have a narrow opening (e.g., less than 10 mm in diameter) to the outermost surface of the object. The cavities may be of any shape, e.g., tubular, spherical, cylindrical, cuboid, pyramidal, and more complex shapes, such as, but not limited to, a shape that is not merely connected, including, e.g., interlocking and / or branching shapes (e.g., interlocking and / or branching tunnels and / or pipes). In some embodiments, the cavity has at least one dimension on the millimeter scale, i.e., from 0.1 mm to 10 mm, or from 0.1 mm to 8 mm, or from 0.1 mm to 5 mm, or from 1 mm to 5 mm. Example hollow structures include, but are not limited to, thin tubular structures such as tubes, pipes, and tunnels, which may be branched and / or intertwined, and which have, at least in some part thereof, a diameter on the millimeter scale, as defined herein. In some embodiments, the cavity is completely enclosed; that is, it is an internal hollow structure within the object that is not exposed to the surrounding environment. Such an internal hollow structure can have any shape / geometry, as described herein, and its dimensions can range from a millimeter scale, as defined herein, to a few centimeters (e.g., 1-20, 1-10, or 1-5 cm) and up to 30, 40, 50 cm or even more. Such example objects include, but are not limited to, sealed bottles, glasses, and any other sealed containers. The embodiments of the present invention relate to a novel formulation providing a fluid gel (FG) material, as defined herein, and to methods employing these formulations in the additive manufacturing of a three-dimensional object, as described herein. The method and system manufacture three-dimensional objects based on computer object data in the form of layers by forming a plurality of layers in a configured pattern corresponding to the shape of the objects.Computer object data can be in any known format, including, without limitation, a Standard Tessellation Language (STL) format or a Stereolithography Contour Format (SLC), Virtual Reality Modeling Language (VRML), Additive Manufacturing File Format (AMF), Drawing Exchange Format (DXF), Polygonal File Format (PLY), or any other format suitable for computer-aided design (CAD). Each layer is formed by an additive manufacturing device that scans and models a two-dimensional surface. During the scan, the device visits multiple target locations on the two-dimensional layer or surface and decides, for each target location or group of locations, whether the location or group of locations should be occupied by the building material formulation and, if so, what type of building material formulation should be supplied to them. The decision is made based on a computer image of the surface. In preferred embodiments of the present invention, AM comprises three-dimensional printing, more preferably three-dimensional inkjet printing. In these embodiments, a building material formulation is dispensed from a dispensing head having one or more nozzle assemblies to deposit the building material formulation in layers onto a support structure. The AM apparatus thus dispenses the building material formulation into target locations to be occupied and leaves other target locations empty. The apparatus typically includes a plurality of dispensing heads, each of which may be configured to dispense a different building material formulation from the same nozzle assembly or from a different nozzle assembly. Therefore, different target locations may be occupied by different building material formulations. Throughout this document, some embodiments of the present invention are described in the context of additive manufacturing, specifically 3D inkjet printing. However, other additive manufacturing processes, such as, but not limited to, SLA and DLP, are also considered, as described in more detail later in this document. An uncured building material may comprise one or more modeling material formulations, and may be dispensed so that different parts of the object, after curing, are made from different cured modeling formulations or different combinations thereof and are therefore made from different cured modeling materials or different mixtures of cured modeling materials. The formulations that form the building material (modeling material formulations and optionally support material formulations) comprise one or more curable materials, which, when exposed to a curing condition (e.g., curing energy), form a hardened material (e.g., cured, solidified). Throughout this document, a "curable material" is a compound (typically a monomeric or oligomeric compound, although optionally a polymeric material) that, when exposed to a curing condition (e.g., curing energy), as described herein, solidifies or hardens to form a cured material. Curable materials are typically polymerizable materials that undergo polymerization and / or crosslinking when exposed to a suitable curing source, typically an energy source. A curable material, according to the present embodiments, can harden or solidify (cure) while exposed to a curing condition which can be a curing energy and / or another curing condition, such as contact with a chemical reagent or exposure to the environment. The terms "curable" and "solidifiable", as used herein, are interchangeable. According to some embodiments of the present invention, a curable material, as described herein, hardens upon undergoing polymerization and is also referred to herein as a polymerizable material. Polymerization can be, for example, free-radical polymerization, cationic polymerization, or anionic polymerization, and each can be induced when exposed to curing energy such as, for example, radiation, heat, etc., as described herein, or to a curing condition other than curing energy. The curable material is a UV-curable material that polymerizes and / or undergoes crosslinking upon exposure to UV or UV-vis radiation, as described herein. In some embodiments, a curable material as described herein is a photopolymerizable material that polymerizes via photo-induced free-radical polymerization. Alternatively, the curable material is a photopolymerizable material that polymerizes via photo-induced cationic polymerization. In any of the embodiments described herein, a curable material may be a monomer, an oligomer, or a short-chain polymer, each being polymerizable and / or crosslinkable as described herein. In any of the embodiments described herein, when a curable material is exposed to a curing condition (e.g., radiation), it hardens (solidifies, cures) by any one, or a combination, of chain elongation and crosslinking. In any of the embodiments described herein, a curable material is a monomer or a mixture of monomers that can form a polymeric material upon exposure to a curing condition (e.g., curing energy) under which the polymerization reaction takes place. These curable materials are also referred to herein as monomeric curable materials. In any of the embodiments described herein, a curable material is an oligomer or a mixture of oligomers that can form a polymeric material upon polymerization when exposed to a curing condition (e.g., curing energy) under which the polymerization reaction takes place. Such curable materials are also referred to herein as oligomeric curable materials. In any of the embodiments described herein, a curable material, whether monomeric or oligomeric, may be a monofunctional curable material or a multifunctional curable material. In this document, a monofunctional curable material comprises a functional group that can undergo polymerization when exposed to a curing condition, such as curing energy (e.g., radiation). A multifunctional curable material comprises two or more, e.g., 2, 3, 4, or more, functional groups that can undergo polymerization when exposed to curing energy. Multifunctional curable materials may be, e.g., difunctional, trifunctional, or tetrafunctional curable materials, comprising 2, 3, or 4 groups that can undergo polymerization, respectively. The two or more functional groups in a multifunctional curable material are typically linked together by a linker, as defined herein. When the linker is an oligomeric or polymeric linker, the multifunctional group is an oligomeric or polymeric multifunctional curable material.Multifunctional curable materials can undergo polymerization when subjected to curing energy and / or act as crosslinkers. The final three-dimensional object is produced from a modeling material formulation, a combination of modeling material formulations, or a modification thereof (e.g., after curing). All these operations are well known to experts in the technique of manufacturing solid freeforms. In some exemplary embodiments of the invention, an object is manufactured by dispensing two or more different building material formulations, each formulation from a different dispensing head of the AM. The building material formulations are deposited optionally and preferably in layers during the same pass of the print heads. The formulations and the combination of formulations within the layer are selected according to the desired properties of the object. Throughout this document, the term "uncured build material" or "build material formulation" collectively describes the materials dispensed during the manufacturing process to sequentially form the layers, as described herein. This term encompasses uncured materials (also referred to herein as one or more build material formulations) dispensed to form the printed object, specifically one or more uncured modeling material formulations, and uncured materials dispensed to form the support, specifically uncured support material formulations. The types of construction material formulations can be classified into two main categories: modeling material formulations and support material formulations. Support material formulations can serve as a matrix or support structure for supporting the object or parts of the object during the manufacturing process and / or for other purposes, for example, providing hollow or porous objects. Support structures may additionally include elements of modeling material formulations, for example, for increased support strength. A construction material formulation that yields a liquid or liquid-like material upon exposure to a curing condition can also be classified, according to some embodiments of the present invention, as a support material formulation. Throughout this document, the terms "cured modeling material" and "hardened modeling material," or simply "modeling material," used interchangeably, describe the portion of the building material that forms a model object, as defined herein, after the dispensed building material has been exposed to curing and the support material has been removed. The cured or hardened modeling material may be a single hardened material or a mixture of two or more hardened materials, depending on the modeling material formulations used in the method, as described herein. A building material formulation that yields a liquid or liquid-like material upon exposure to a curing condition may also be classified, according to some embodiments of the present invention, as a modeling material formulation. Throughout this document, the term "modeling material formulation," also referred to interchangeably as "modeling formulation," describes a portion of the uncured construction material dispensed to form the model object, as described herein. Modeling formulation is an uncured modeling formulation that, upon exposure to a curing condition, forms the final object or a portion thereof. An uncured building material may comprise one or more modeling formulations and may be dispensed in such a way that different parts of the model object are formed by curing different modeling formulations and are therefore made of different cured modeling materials or different mixtures of cured modeling materials. Throughout this document, the expression "hardened support material" is also referred to interchangeably as "cured support material" or simply "support material" and describes the part of the construction material intended to support the final object manufactured during the manufacturing process, and which is removed once the process is completed and a hardened modeling material is obtained. Throughout this document, the term "support material formulation," also referred to interchangeably as "support formulation" or simply "formulation," describes a portion of the uncured construction material dispensed to form the support material as described herein. The support material formulation is an uncured formulation. When a support material formulation is a curable formulation, it forms, upon exposure to a curing condition, a hardened support material. The support materials, which may be liquid or liquid-like materials, or hardened materials, typically gel or gel-like materials, are also referred to herein as sacrificial materials, which can be removed after the layers have been dispensed and exposed to a curing energy, thereby exposing the shape of the final object. The support materials currently in use typically comprise a mixture of curable and non-curable materials and are also referred to herein as gel-like support material or gel support material. The support materials currently in use are typically water miscible, water dispersible, or water soluble. Throughout this document, the term "miscible in water" describes a material that is at least partially soluble or dispersible in water, meaning that at least 50% of its molecules pass into the water upon mixing at room temperature. This term encompasses both "water-soluble" and "water-dispersible." Throughout this document, the expression "water-soluble" describes a material that, when mixed with water in equal volumes or weights, at room temperature, forms a homogeneous solution. Throughout this document, the expression "water dispersible" describes a material that forms a homogeneous dispersion when mixed with water in equal volumes or weights, at room temperature. Throughout this document, the term "dissolution rate" describes the rate at which a substance dissolves in a liquid medium. In the context of the present embodiments, the dissolution rate can be determined by the time required to dissolve a given quantity of support material. The measured time is referred to herein as the "dissolution time." Throughout this document, whenever the expression "weight percentages" is indicated in the context of realizations of a formulation (for example, a carrier material formulation), it shall be understood as weight percentages of the total weight of the respective formulation. The expression "percentages by weight" is also referred to in this document as "% by weight" or "% p". The fluid gel formulation: According to one aspect of some embodiments of the present invention, a formulation is provided that is useful as a support material formulation in the additive manufacturing of a three-dimensional object. According to some embodiments of the invention, the formulation, after exposure to a curing condition, provides a material exhibiting the properties of a weak gel, as defined herein. According to some embodiments of the invention, the formulation, after exposure to a curing condition, provides a material exhibiting the properties of a fluid gel or a flowable gel, as defined herein. The formulation is also referred to herein as a "fluid gel formulation," "FG formulation," "FLG formulation," or "FG formulation." According to embodiments of the present invention, the formulation comprises: at least one monofunctional curable material; at least one hydrophilic multifunctional curable material; and at least one water-miscible non-curable material. The total amount of the at least one monofunctional curable material ranges from 1 to 10, or from 3 to 10, or from 5 to 10, wt.% of the total weight of the formulation. The total amount of the at least one hydrophilic multifunctional curable material ranges from 1 to 5, wt.% of the total weight of the formulation. The weight ratio of the total weight of the at least one monofunctional curable material to the total weight of the at least one hydrophilic multifunctional curable material ranges from 1:1 to 10:1. The total amount of the at least one water-miscible non-curable material is at least 85% by weight of the total weight of the formulation. Each of the curable materials is a UV-curable material. According to any of the embodiments described herein, the formulation is water-free. In any of the embodiments described herein, the support material formulation is devoid of a silicon polyether. "Devoid of" means that a quantity of the indicated material (e.g., water) is not greater than 2%, or not greater than 1%, or not greater than 0.5%, or not greater than 0.1%, or not greater than 0.5%, or not greater than 0.1%, or not greater than 0.05%, or not greater than 0.01%, by weight, and may even be less than or zero. According to any of the embodiments described herein, a weight ratio of a total weight of one or more monofunctional curable materials and a total weight of one or more multifunctional curable materials may be, for example, 1:1, or 1:2, or 1:3, or 1:4, or 1:5, or 1:6, or 1:7, or 1:9, or 1:10. According to any of the embodiments described herein, a total quantity of one or more monofunctional curable materials may include any intermediate value and sub-intervals in the range of 1 to 10% by weight, of the total weight of the formulation. According to any of the embodiments described herein, a total amount of one or more multifunctional curable materials varies from 1 to 5% by weight of the total weight of the formulation. According to any of the embodiments described herein, one or more, or each, of the one or more monofunctional curable materials is a hydrophilic material, as defined herein. According to any of the embodiments described herein, one or more, or each, of the one or more monofunctional curable materials is a water-miscible or water-soluble material, as defined herein. Throughout this document, the term "hydrophilic" describes a physical property of a compound or a portion of a compound (e.g., a chemical group in a compound) that accounts for the transient formation of one or more bonds with water molecules, typically through hydrogen bonds. A hydrophilic compound or a portion of a compound (for example, a chemical group in a compound) is one that typically has a polarized charge and is capable of forming hydrogen bonds. Hydrophilic compounds or groups typically include one or more electron-donating heteroatoms that form strong hydrogen bonds with water molecules. Such heteroatoms include, but are not limited to, oxygen and nitrogen. Preferably, the ratio of the number of carbon atoms to the number of heteroatoms in hydrophilic compounds or groups is 10:1 or less and may be, for example, 8:1, more preferably 7:1, 6:1, 5:1, or 4:1 or less. It should be noted that the hydrophilicity of compounds and groups can also result from the ratio of hydrophobic to hydrophilic moieties in the compound or chemical group and is not solely dependent on the ratio indicated above. Hydrophilic compounds dissolve more readily in water than in oil or other hydrophobic solvents. Hydrophilic compounds can be identified, for example, as having a LogP value less than 0.5, when the LogP is determined in the octanol and water phases at temperatures below 50 °C, 40 °C, 35 °C, or 30 °C (e.g., 25 °C). Alternatively, it can be determined that hydrophilic compounds, by means of, for example, Hansen parameters, have a relative energy distance (RED) greater than 1, when calculating to determine the interaction with water as a solvent, at a temperature below 50, or below 40 °C, or below 35 °C or below 30 °C, for example, at 25 °C. A hydrophilic compound can have one or more hydrophilic groups that confer hydrophilicity to the compound. Such groups are typically polar groups, comprising one or more electron-donating heteroatoms, such as oxygen and nitrogen. The hydrophilic group can be, for example, one or more substituents of a monofunctional monomeric curable material or two or more substituents or switching groups of a monofunctional oligomeric curable material. The hydrophilic group can be, for example, one or more substituents of a multifunctional monomeric curable material or one or more substituents or switching groups of a bonding residue of a multifunctional monomeric curable material. The hydrophilic group can be, for example, two or more substituents or switching groups of an oligomeric bonding residue in an oligomeric multifunctional curable material. Example hydrophilic groups include, but are not limited to, an electron-donating heteroatom, a carboxylate, a thiocarboxylate, oxo(=O), a linear amide, hydroxy, a (C1-4) alkoxy, a (C1-4) alcohol, a heteroalicyclic (e.g., having a carbon-to-heteroatom ratio as defined herein), a cyclic carboxylate such as a lactone, a cyclic amide such as a lactam, a carbamate, a thiocarbamate, a cyanurate, an isocyanurate, a thiocyanurate, urea, thiourea, an alkylene glycol (e.g., ethylene glycol or propylene glycol), and a hydrophilic polymeric or oligomeric moiety, as these terms are defined hereinafter, and any combination thereof (for example, a hydrophilic group comprising two or more of the indicated hydrophilic groups). In some embodiments, the hydrophilic group is, or comprises, an electron-donating heteroatom, a carboxylate, a heteroalicyclic, an alkylene glycol and / or a hydrophilic oligomeric moiety. A hydrophilic polymeric or oligomeric residue, as used herein, comprises a polymer chain comprising hydrophilic groups as defined herein. The hydrophilic groups may be heteroatoms within the main chain of the polymeric residue, as, for example, in poly(alkylene glycols), or hydrophilic pendant groups. A polymeric or oligomeric residue, according to some embodiments of the present invention, preferably has from 10 to 40 repeating main chain units, more preferably from 10 to 20 repeating main chain units. A hydrophilic monofunctional curable material according to some embodiments of the present invention may be a vinyl-containing compound represented by Formula I: Formula 1 wherein at least one of R1 and R2 is and / or comprises a hydrophilic group, as defined herein. The (=CH2) group of Formula I represents a polymerizable group, and is typically a UV-curable group, so that the material is a UV-curable material. For example, R1 is a hydrophilic group as defined herein and R2 is a non-hydrophilic group, for example, hydrogen, C(1-4) alkyl, C(1-4) alkoxy, or any other substituent, provided that the compound is hydrophilic as defined herein. In some embodiments, R1 is a carboxylate, -C(=O)-OR' group, and R2 is hydrogen, and the compound is a monofunctional acrylate monomer. In some of these embodiments, R2 is methyl, and the compound is a monofunctional methacrylate monomer. In other embodiments, R2 is a hydrophilic substituent, specifically a substituent that is, or comprises, a hydrophilic group as described herein. In any of these embodiments, the carboxylate group, -C(=O)-OR', comprises R', which is a hydrophilic group. Example R' groups include, but are not limited to, heteroalicyclic groups (having a 5:1 or lower ratio of carbon atoms to electron-donating heteroatoms, such as morpholine, tetrahydrofuran, oxalidine, and the like), hydroxyl, C(1-4) alkoxy, thiol, alkylene glycol, or a polymeric or oligomeric moiety, as described herein. An example monomeric monofunctional acrylate is acryloyl morpholine (ACMO). In some embodiments, R1 is an amide, and in some embodiments, it is a cyclic amide, such as a lactam, and the compound is a vinyl lactam. In some embodiments, R1 is a cyclic carboxylate, such as a lactone, and the compound is a vinyl lactone. When one or both of R1 and R2 comprise a polymeric or oligomeric moiety, for example, a hydrophilic oligomeric moiety, as defined herein, the monofunctional curable compound of Formula I is an example oligomeric monofunctional curable material. Furthermore, it is an example monomeric monofunctional curable material. Examples of oligomeric monofunctional curable materials include, but are not limited to, a mono-(meth)acrylated urethane oligomer derivative of polyethylene glycol, a mono-(meth)acrylated polyol oligomer, a mono-(meth)acrylated oligomer having hydrophilic substituents, and a mono-(meth)acrylated polyethylene glycol (e.g., methoxypolyethylene glycol). (Meth)acrylated means that the oligomer or polymer comprises an acrylate or methacrylate functional group. In some embodiments, R1 is a carboxylate and R' is a poly(alkylene glycol), as defined herein. An example of such a hydrophilic monofunctional curable material is hexa(ethylene glycol) acrylate, (6-PEA). In some embodiments, R1 is a hydrophilic heteroalicyclic group, as defined herein. ACMO is an example of such a hydrophilic monofunctional curable material. In some embodiments, if there are two or more curable monofunctional materials, all the curable monofunctional materials are hydrophilic and / or water-soluble or water-miscible, and in some embodiments, only one of these materials is a hydrophilic and / or water-soluble or water-miscible material. In some embodiments, one or more of the monofunctional curable materials, when hardened per se, provide a material that is water-soluble. ACMO is an example of such a material. In some embodiments, each of the monofunctional curable materials provides, when it hardens per se, a material that is soluble in water. In some embodiments, one or more of the monofunctional curable materials, when cured per se, provide a material that is insoluble in water. Examples of such materials include PEA6 and HEAA. In some embodiments, each of the monofunctional curable materials provides, when per hardened, a material that is insoluble in water. In some embodiments, one or more of the monofunctional curable materials provide, when hardened per se, a material that is water-soluble, and one or more of the monofunctional curable materials provide, when hardened per se, a material that is water-insoluble. In any of the embodiments described herein, at least one of the one or more monofunctional curable materials is a curable material comprising a hydroxyalkyl (e.g., HEAA) and / or an alkylene glycol moiety (e.g., a poly(alkylene glycol) acrylate, such as, e.g., PEA6). According to any of the embodiments described herein, each of the one or more multifunctional curable materials is a hydrophilic material. A multifunctional hydrophilic curable material according to some embodiments of the present invention can be represented by Formula II: where: each of R3, R4 and R5 is independently hydrogen, C (1-4) alkyl, or a hydrophilic group, as defined herein; each of L1, L2 and L3 is independently a junction remnant or absent; each of P1 and P2 is independently a hydrophilic group as defined herein or is absent; each of X1, X2 and X3 is independently either a C (1-4) alkyl or a hydrophilic group, as defined herein, or is absent; and each of n, myk is 0, 1, 2, 3 or 4, provided that n+m+k is at least 2, and provided that at least one of R3, R4, R5, X1, X2, X3, P1 and P2 is a hydrophilic group, as defined herein. The multifunctional curable materials of Formula II, in which one, two, or all of X1, X2, and X3, when present, are oxo, are multifunctional acrylates, which may further be substituted with a hydrophilic group, as described earlier herein. When one or more of R3, R4, and R5, when present, are methyl, the curable materials are multifunctional methacrylates. Multifunctional curable materials, in which one, two or all of X1, X2 and X3, when present, are oxo, may include a combination of functional remnants of acrylate and methacrylate. In some embodiments, the multifunctional curable acrylate or methacrylate material is monomeric, such that neither P1 nor P2 is a polymeric or oligomeric moiety. In some of these embodiments, one or both of P1 and P2 are a hydrophilic group, as described herein, for example, an alkylene glycol, or any other hydrophilic bonding group, or a short chain (for example, of 1-6 carbon atoms), a substituted or unsubstituted hydrocarbon moiety, as defined herein. In some embodiments, one or both of P1 and P2 is a polymeric or oligomeric residue, as defined herein, and the curable compound is a multifunctional oligomeric curable material, for example, a multifunctional oligomeric acrylate or methacrylate, as described herein, in the case of X1, X2, and / or X3. If both P1 and P2 are present, L2 may be, for example, a bonding residue, such as a hydrocarbon comprising alkyl, cycloalkyl, aryl, and any combination thereof. Such example curable materials include ethoxylated or methoxylated polyethylene glycol diacrylate and ethoxylated bisphenol A diacrylate. Other non-limiting examples include polyethylene glycol diacrylate, polyethylene glycol dimethacrylate, polyethylene glycol-polyethylene glycol urethane diacrylate, and a partially acrylated polyol oligomer. In some embodiments, one or more of P1 and P2 are, or comprise, a poly(alkylene glycol) residue, as defined herein. In some embodiments of a Formula II multifunctional curable acrylate or methacrylate material, one or more of R3, R4, and R5 are hydrophilic groups as described, for example, for R1 and R2 in Formula I herein. In these embodiments, P1 and / or P2 may be present or absent and may be, or comprise, a hydrophilic group or not, provided that the material is hydrophilic as defined herein. Alternatively, one, two, or all of X1, X2, and X3, when present, may be -O-, such that at least one functional residue in the multifunctional curable material is vinyl ether. In some embodiments, nym are each 1, k is 0, X1 is 0, X2 is absent, and the compound is a vinyl ether, which may or may not be substituted. In some of these embodiments, L1, L2, L3, P1, and P2 are absent, and the compound is a monomeric vinyl ether. Examples of monomeric vinyl ethers include ethyl vinyl ether, propyl vinyl ether, isobutyl vinyl ether, cyclohexyl vinyl ether, 2-ethylhexyl vinyl ether, butyl vinyl ether, ethylene glycol monovinyl ether, triethylene glycol divinyl ether, butanediol divinyl ether, hexanediol divinyl ether, cyclohexanedimethanol monovinyl ether, and the like. In some embodiments, P1 and P2 are absent, and one of L1 and L2 is an alkylene chain substituted with one or more hydrophilic groups. Such an example curable compound is 1,4-cyclohexanedimethanol divinyl ether. In some embodiments, one or more of P1 and P2 is a hydrophilic polymeric or oligomeric moiety, as defined herein. In some embodiments, one or more of P1 and P2 is, or comprises, a poly(alkylene glycol) moiety, as defined herein. In some embodiments, the polymeric moiety is substituted with one or more vinyl ether substituents. In some of any of the embodiments related to Formula II, one or more of the polymerizable group substituents, R3, R4 and R5, may be a hydrophilic group as described for R1 and R2 in Formula I hereof. In any of the embodiments related to Formula II, when P1 and P2 are a polymeric or oligomeric residue, this residue may comprise hydrophilic heteroatoms as defined herein, within the main chain, or the main chain may be substituted with hydrophilic groups, as described herein. In any of the embodiments described herein, more than one, or each of the multifunctional curable materials, is a hydrophilic material as defined herein. In any of the embodiments described herein, one or more, or each of the multifunctional curable materials is a water-soluble or water-miscible material as defined herein. In any of the embodiments described herein, one or more, or each of the multifunctional curable materials provides, when hardened, a material that is insoluble in water. In any of the embodiments described herein, one or more, or each, of the one or more multifunctional curable materials comprises one or more alkylene glycol moieties. In some of these embodiments, a multifunctional curable material comprises a poly(alkylene glycol) moiety. In any of the embodiments described herein, one or more, or each, of the one or more multifunctional curable materials is a difunctional material. In any of the embodiments described herein, the one or more multifunctional curable materials comprise or are a poly(alkylene glycol) diacrylate. The poly(alkylene glycol) diacrylate may be from 20 to 1000 alkylene glycol units, including any intermediate values ​​and sub-ranges therein. In one example embodiment, the one or more multifunctional curable materials comprise or are a poly(ethylene glycol) diacrylate, from 20 to 1000, or from 100 to 1000, or from 200 to 1000, or from 100 to 800, or from 200 to 800, or from 400 to 800 ethylene glycol units, including any intermediate values ​​and sub-ranges therein. The formulation, as described herein, further comprises non-curable materials. According to the invention, the total amount of the non-curable materials is at least 85% by weight of the total weight of the formulation. The term "non-curable" encompasses materials that are not polymerizable under any conditions or materials that are not curable under conditions in which monofunctional and multifunctional curable materials, as described herein, are curable, or under any conditions used in the manufacture of an object according to the present embodiments. Such materials are typically devoid of a polymerizable group or a UV-curable group. In some embodiments, the material is non-reactive toward the curable material, as written herein; that is, it does not react with the curable material and is unable to interfere with the curing of the curable materials under the manufacturing conditions, including the curing conditions. In some of any of the embodiments described herein, the non-curable material is water-soluble or water-dispersible, as defined herein. In any of the embodiments described herein, one or more, or each, of the one or more non-curable materials is a water-miscible or water-soluble material. In any of the embodiments described herein, one or more of the non-curable materials is a polymeric material, for example, a water-miscible or water-soluble polymeric material. In some embodiments, the non-curable material is a polymeric material comprising a plurality of hydrophilic groups, as defined herein, within the polymer backbone or as pendant groups. Such example polymeric materials are polyols. Representative examples include, but are not limited to, polyol 3165, polypropylene glycol, polyethylene glycol, polyglycerol, polyglyme, ethoxylated forms of these polymers, paraffin oil and the like, and any combination thereof. In example embodiments, the non-curable polymeric material comprises poly(ethylene glycol) and / or polyol 3165. The polymeric materials can be of any molecular weight. In some embodiments, one or more polymeric materials and a quantity thereof are selected so that the formulation exhibits a viscosity suitable for the additive manufacturing method, as described herein. In any of the embodiments described herein, a polymeric material has a molecular weight of at least 500 or at least 600 grams / mol. In any of the embodiments described herein, a polymeric material has a PM not exceeding 3000, or not exceeding 2500, or not exceeding 2000 or not exceeding 1500, grams / mol. In any of the embodiments described herein, a polymeric material has a molecular weight of approximately 500 to approximately 2500 grams / mol, including any intermediate values ​​and sub-intervals between them. In any of the embodiments described herein, the one or more non-curable materials comprise a non-polymeric material, for example, a water-soluble or water-miscible non-polymeric material.Such non-curable non-polymeric material examples include, but are not limited to, propanediol (e.g., 1,2-propandoyl, also referred to herein and in the art as propylene glycol), propanetriol, glycerol, butyl diglyme (butyl diglycol acetate, diethylene glycol butyl ether acetate, 2-(2-butoxyethoxy)ethyl acetate), diethylene glycol monobutyl ether (BDG), (EDGAc), diethylene glycol monoethyl ether acetate (EDGAc; DGMEA), di(ethylene glycol) ethyl ether (DEGEE), tri(propylene glycol) methyl ether, dipropylene glycol monomethyl ether (DPGME), di(propylene glycol) methyl ether acetate (DPGMEA), propylene carbonate (cyclic carbonate of 1,2-propanediol, 4-methyl-1, 3-dioxolan-2-one), diethylene glycol methyl ether (DGME), diethylene glycol methyl ether (TGMME), 1-methoxy-2-propanol (PGME / PM; propylene glycol monomethyl ether) and propylene glycol monomethyl ether acetate (PGMEA). In any of the embodiments described herein, the formulation comprises a non-curable, water-miscible material comprising a mixture of two or more of the non-curable, water-miscible polymeric and non-polymeric materials described herein. Such an example mixture may comprise two or more of a poly(ethylene glycol), a propanediol, glycerol, and a polyol, such as polyol 3165. In some of these embodiments, the weight ratio of the total weight of one or more non-curable polymeric materials to the total weight of one or more non-curable non-polymeric materials ranges from 2:1 to 1:2, including any intermediate values ​​and sub-intervals between them. A non-limiting example formulation (which does not include all the features of the claimed invention) comprises: One or more monofunctional curable materials, as described herein in any of the respective embodiments, in an amount of 3 to 10% by weight; One or more hydrophilic multifunctional (e.g., difunctional) curable materials, as described herein in any of the respective embodiments, in an amount of 3 to 5 wt%; One or more non-curable polymeric materials, as described herein in any of the respective embodiments, in an amount of 30 to 60 wt%; and One or more non-curable non-polymeric materials, as described herein in any of the respective embodiments, in an amount of 30 to 60% by weight, and the total amount of non-curable materials is at least 80% by weight of the total weight of the formulation. In any of the embodiments described herein, the formulation is useful in additive manufacturing, such as 3D inkjet printing. In any of the embodiments described herein, the formulation exhibits properties (e.g., viscosity, surface tension, jettiness) that are suitable for additive manufacturing, such as 3D inkjet printing, as described herein. In some embodiments, the formulation exhibits a viscosity of 8 to 40, or 8 to 30, or 8 to 25 centipoises at the injection temperature (e.g., 75 °C). In any of the embodiments described in this document, and as discussed earlier in this document, the formulation provides, when hardened, a material that exhibits weak gel properties. In any of the embodiments described herein, and as discussed earlier herein, the formulation, when hardened, provides a material exhibiting an elastic modulus (e.g., Young's modulus) of 10 to 100, or 10 to 80, or 20 to 80, or 10 to 50, or 20 to 50, or 10 to 70, or 20 to 70, or 30 to 80, or 30 to 70, or 30 to 100, or 20 to 90, or 10 to 90, or 30 to 90, or 40 to 90, or 40 to 100, or 40 to 80, or 40 to 70, or 20 to 60, or 10 to 60, or 30 to 60, or 40 to 60, or 50 to 100, or 50 to 90, or 50 to 80, or 50 to 70, or 50 to 60, kPa, including any intermediate values ​​and sub-intervals between them. According to any of the embodiments described herein, the formulation provides, when hardened, a gel material that can flow upon application of positive pressure, as described herein. According to some of these realizations, the positive pressure is such that it is applied by pressing the formulation by a human being. According to the claimed invention, each of the monofunctional and multifunctional curable materials is a UV curable material. In any of the embodiments described herein, one or more, and preferably each, of the monofunctional and multifunctional curable materials is an acrylic material, as defined herein. In any of the embodiments described in this document, the formulation also comprises one or more photoinitiators. In some of these embodiments, the total amount of the photoinitiator is not greater than 2% by weight of the total weight of the formulation, and may be, for example, in a range of 0.1 to 2%, or 0.1 to 1.5%, or 0.5 to 1.5% by weight, including any intermediate values ​​and sub-ranges between them. The photoinitiator can be a free radical photoinitiator, a cationic photoinitiator, or any combination thereof. A free-radical photoinitiator can be any compound that produces a free radical upon exposure to radiation, such as ultraviolet or visible radiation, and thus initiates a polymerization reaction. Non-limiting examples of suitable photoinitiators include phenyl ketones, such as alkyl / cycloalkylphenyl ketones; benzophenones (aromatic ketones) such as benzophenone, methylbenzophenone, Michler ketone, and xanthones; photoinitiators of the acylphosphine oxide type, such as 2,4,6-trimethylbenzoyldiphenylphosphine oxide (TMPO), 2,4,6-trimethylbenzoylethoxyphenylphosphine oxide (TEPO), and bisacylphosphine oxides (BAPO); benzoins and benzoin alkyl ethers, such as benzoin, benzoin methyl ether, and benzoin isopropyl ether, and the like. Examples of photoinitiators are alpha-amino-ketone and 1-hydroxycyclohexylphenylketone (i.e., marketed as Irgacure® 184).A free-radical photoinitiator can be used alone or in conjunction with a co-initiator. Co-initiators are used with initiators that require a second molecule to produce a radical that is active in UV systems. Benzophenone is an example of a photoinitiator that requires a second molecule, such as an amine, to produce a curable radical. After absorbing radiation, benzophenone reacts with a ternary amine via hydrogen abstraction to generate an alpha-amino radical that initiates acrylate polymerization. A non-limiting example of a class of co-initiators is alkanolamines, such as triethylamine, methyldiethanolamine, and triethanolamine. Suitable cationic photoinitiators include, for example, compounds that form aprotic acids or Brønsted acids upon exposure to sufficient ultraviolet and / or visible light to initiate polymerization. The photoinitiator used may be a single compound, a mixture of two or more active compounds, or a combination of two or more different compounds—that is, co-initiators. Non-limiting examples of suitable cationic photoinitiators include aryldiazonium salts, diarylodonium salts, triarylsulfonium salts, triarylselenonium salts, and the like. An example of a cationic photoinitiator is a mixture of triarylsulfonium hexafluoroantimonate salts. In any of the embodiments described herein, the formulation may further comprise one or more additional agents that are advantageously used in the manufacturing process. Such agents include, for example, surfactants, inhibitors, and stabilizers. In some embodiments, a support material formulation as described herein comprises a surfactant. A surfactant can be used to reduce the surface tension of the formulation to the value required for injection molding or other printing processes, which is typically 0.03 N / m (30 dynes / cm). Such a surfactant is, for example, a silicone surface additive, such as, but not limited to, commercially available surfactants marketed as the BYK family. In some embodiments, a support material formulation, as described herein, further comprises an inhibitor that inhibits the prepolymerization of the curable material during the manufacturing process and before it is subjected to curing conditions. An example of a stabilizer (inhibitor) is the aluminum salt Tris(N-nitroso-N-phenylhydroxylamine) (NPAL) (e.g., as marketed as FirstCure®NPAL). Suitable stabilizers include, for example, thermal stabilizers, which stabilize the formulation at high temperatures. Model manufacturing: According to one aspect of some embodiments of the present invention, a method is provided for manufacturing a three-dimensional model object, using a formulation as described herein as a support material formulation. The method is also referred to herein as a manufacturing process or model-making process. In some embodiments, the method comprises dispensing an uncured construction material to sequentially form a plurality of layers in a configured pattern corresponding to the shape of the object. In some embodiments, the (uncured) construction material comprises one or more modeling material formulations and one or more support material formulations, and one or more of the support material formulations is a formulation as described herein in any of the respective embodiments. The modeling material formulation can be any modeling material formulation useful in additive manufacturing, such as inkjet 3D printing, and is preferably curable under the same conditions as the support material formulation. The manufacturing method is the additive manufacturing of a three-dimensional model object. According to some embodiments of this aspect, the formation of each layer is effected by dispensing at least one uncured building material, and exposing the dispensed building material to curing energy or curing conditions, thereby forming a cured building material, which is composed of a cured modeling material and a cured support material. According to any of the embodiments described in this document, additive manufacturing is preferably carried out by three-dimensional inkjet printing. The method of the present realizations manufactures three-dimensional objects in the form of layers, forming a plurality of layers in a configured pattern corresponding to the shape of the objects. Each layer is formed by an additive manufacturing device that scans and models a two-dimensional surface. During the scan, the device visits multiple target locations on the two-dimensional layer or surface and decides, for each target location or group of target locations, whether that point or group should be occupied by building material, and what type of building material (e.g., a modeling material formulation or a support material formulation) should be supplied at that point. The decision is made based on a computer image of the surface. When AM is performed by three-dimensional printing, an uncured build material, as defined herein, is dispensed from a dispensing head that has an array of nozzles to deposit the build material in layers onto a support structure. Therefore, the AM apparatus dispenses the build material into target locations to be occupied and leaves other target locations empty. The apparatus typically includes a plurality of dispensing heads, each of which can be configured to dispense a different build material. Therefore, different target locations can be occupied by different build materials (e.g., a modeling formulation and / or a support formulation, as defined herein). A representative, non-limiting example of a system 110 suitable for the AM of an object 112 is illustrated in Figure 1A. The system 110 comprises an additive manufacturing apparatus 114 having a dispensing unit 16 comprising a plurality of dispensing heads. Preferably, each head comprises one or more nozzle assemblies 122, as illustrated in Figures 2A-C described below, through which a liquid building material 124 is dispensed. The 114 appliance can operate at a temperature not exceeding 35 °C. Preferably, but not necessarily, apparatus 114 is a three-dimensional inkjet printing apparatus, in which case the dispensing heads are print heads and the build material is dispensed by inkjet technology from a print head having one or more sets of nozzles to deposit the build material formulation in layers onto a support structure. This is not necessarily the case, since for some applications, it may not be necessary for the additive manufacturing apparatus to employ three-dimensional printing techniques. Representative examples of additive manufacturing apparatuses considered include, but are not limited to, fused deposition modeling apparatuses and molten material deposition apparatuses. The term "printhead", as used herein, represents a dispensing head useful in 3D printing, such as inkjet 3D printing. The expression "dispensing head" encompasses the expression "printing head" in the context of embodiments relating to 3D inkjet printing. Each dispensing head is optionally and preferably fed via one or more build material formulation reservoirs, which may optionally include a temperature control unit (e.g., a temperature sensor and / or a heating device) and a material formulation level sensor. To dispense a build material formulation, a voltage signal is applied to the dispensing heads to selectively deposit droplets of a selected formulation, or a selected combination of two or more formulations, through the dispensing head nozzles, for example, as in piezoelectric inkjet printing technology. Another example includes thermal inkjet printheads.In this type of printhead, heating elements are in thermal contact with the build material formulation. These elements heat the formulation and form gas bubbles within it after being activated by a voltage signal. The gas bubbles generate pressure within the build material formulation, causing droplets of the formulation to be ejected through the nozzles. Piezoelectric and thermal printheads are well-known to experts in solid-free form manufacturing. For any type of inkjet printhead, the dispensing speed depends on the number of nozzles, the nozzle type, and the applied voltage signal rate (frequency). Preferably, but not necessarily, the total number of dispensing nozzles or nozzle assemblies is selected such that half of the dispensing nozzles are designated for dispensing the support material and the other half are designated for dispensing the modeling material; that is, the number of nozzles that inject modeling materials is the same as the number of nozzles that inject support material. In the representative example in Figure 1A, four dispensing heads 16a, 16b, 16c, and 16d are illustrated. Each of the heads 16a, 16b, 16c, and 16d has one nozzle assembly. In this example, heads 16a and 16b can be designated for one or more modeling materials, and heads 16c and 16d can be designated for the support material.Therefore, print head 16a can dispense a first modeling material, print head 16b can dispense a second modeling material, and print heads 16c and 16d can both dispense a support material. In an alternative embodiment, print heads 16c and 16d, for example, can be combined into a single print head having two sets of nozzles for deposition of a support material. In a further alternative embodiment, any one or more of the print heads can have more than one set of nozzles for depositing more than one material formulation, for example, two sets of nozzles for depositing two different modeling material formulations, or one modeling material formulation and one support material formulation, with each formulation being dispensed through a different set or number of nozzles. However, it should be understood that the scope of the methods of the present invention is not intended to be limited and that the number of modeling material printheads (modeling heads) and the number of support material printheads (support heads) may differ. Generally, the number of nozzle assemblies dispensing modeling material formulation, the number of nozzle assemblies dispensing support material formulation, and the number of nozzles in each respective array are selected to provide a predetermined ratio, a, between the maximum dispensing rate of the support material formulation and the maximum dispensing rate of the modeling material formulation. The value of the redetermined ratio, a, is preferably selected to ensure that, in each layer formed, the height of the modeling material is equal to the height of the support material.Typical values ​​for a range from approximately 0.6 to approximately 1.5. For example, for a = 1, the overall dispensing rate of the support material is, in general, the same as the overall dispensing rate of the modeling material when all nozzle assemblies are in operation. For example, apparatus 114 may comprise M modeling heads, each with m sets of p nozzles, and S support heads, each with s sets of q nozzles, such that M x m x p = S x s x q. Each of the M x m modeling sets and S x s support sets may be manufactured as a separate physical unit, which may be mounted and dismounted from the group of sets. In the present embodiment, each of such sets optionally and preferably comprises a temperature control unit and a proprietary material formulation level sensor and receives an individually controlled voltage for its operation. In some embodiments, the temperature control unit of at least some of the assemblies is configured not to exceed 45°C, or 40°C, or 35°C. The apparatus 114 may further comprise a hardening device 324, which may include any device configured to emit light, heat, or the like that can cause the deposited material to harden. For example, the hardening device 324 may comprise one or more radiation sources, which may be, for example, an ultraviolet, visible, or infrared lamp, or other sources of electromagnetic radiation, or an electron beam source, depending on the modeling material used. The hardening device 324 may serve to cure or solidify the modeling material. As used herein, the terms "dispensing head" or "deposition head" encompass printheads, which are dispensing heads useful in 3D printing, such as inkjet 3D printing. The one or more dispensing heads and the radiation source are preferably mounted on a frame or block 128 that is preferably designed to move reciprocally on a tray 360, which serves as a work surface. The radiation sources can be mounted on the block so that they follow the path of the dispensing heads to cure or solidify, at least partially, the materials freshly dispensed by those heads. The tray 360 is positioned horizontally. In accordance with common conventions, a Cartesian coordinate system XYZ is selected such that the XY plane is parallel to the tray 360. The tray 360 is preferably configured to move vertically (along the Z direction), typically downwards. The apparatus 114 may further comprise one or more leveling devices 132, for example, a roller 326.The leveling device 326 is used to straighten, level, and / or establish the thickness of the newly formed layer before the formation of the subsequent layer on top of it. The leveling device 326 preferably includes a waste collection device 136 for collecting excess material generated during leveling. The waste collection device 136 may include any mechanism that delivers the material to a waste tank or waste cartridge. During use, the dispensing heads of unit 16 move in a scanning direction, referred to herein as direction X, and selectively dispense build material in a predetermined configuration as they pass over the 360 ​​tray. The build material typically comprises one or more types of support material and one or more types of modeling material. The passage of the dispensing heads of unit 16 is followed by curing of the one or more modeling materials by radiation source 126. On the reverse pass of the heads, returning to their starting point for the newly deposited layer, additional dispensing of build material may occur, according to a predetermined configuration.During the forward and / or backward movements of the dispensing heads, the layer formed in this way can be straightened by the leveling device 326, which preferably follows the path of the dispensing heads during their forward and / or backward movement. Once the dispensing heads return to their starting point along the X direction, they can move to another position along an indexing direction, referred to herein as the Y direction, and continue building the same layer by reciprocating along the X direction. Alternatively, the dispensing heads can move in the Y direction between forward and backward movements or after more than one forward-backward movement. The series of scans performed by the dispensing heads to complete a single layer is referred to herein as a single scan cycle. Once the layer is complete, the 360 ​​tray is lowered in the Z direction to a predetermined Z level, according to the desired thickness of the layer to be printed subsequently. The procedure is repeated to form the three-dimensional object 112 in layers. In another embodiment, the 360 ​​tray can be moved in the Z direction between the forward and reverse steps of the dispensing head of unit 16, within the layer. This Z movement is performed to cause the leveling device to make contact with the surface in one direction and prevent contact in the other direction. System 110 optionally and preferably comprises a construction material supply system 330 comprising construction material containers or cartridges and supplies a plurality of construction materials to the manufacturing apparatus 114. A control unit 152 controls the manufacturing apparatus (e.g., printing) 114 and, optionally and preferably, also the supply system 330. The control unit 152 typically includes an electronic circuit configured to perform the control operations. The control unit 152 preferably communicates with a data processor 154 that transmits digital data pertaining to manufacturing instructions based on computer object data, for example, a CAD configuration represented on a computer-readable medium in the form of a Standard Tessellation Language (STL) format or similar. Typically, the control unit 152 controls the voltage applied to each dispensing head or nozzle assembly and the temperature of the build material in the respective print head or nozzle assembly, as described herein. The control unit 152 can be operated in such a way that the temperature of the construction material (uncured) does not exceed 40 °C or 35 °C. Once the manufacturing data is loaded into the control unit 152, it can operate without user intervention. In some embodiments, the control unit 152 receives additional input from the operator, for example, using the data processor 154 or a user interface 116 that communicates with the unit 152. The user interface 116 can be of any type known in the art, such as, but not limited to, a keyboard, a touchscreen, and the like. For example, the control unit 152 can receive, as additional input, one or more types and / or attributes of construction materials, such as, but not limited to, color, characteristic distortion and / or transition temperature, viscosity, electrical properties, and magnetic properties. Other attributes and groups of attributes are also contemplated. Another representative and non-limiting example of a system 10 suitable for the AM of an object according to some embodiments of the present invention is illustrated in Figures 1B-D. Figures 1B-D illustrate a top view (Figure 1B), a side view (Figure 1C), and an isometric view (Figure 1D) of the system 10. In the present embodiments, system 10 comprises a tray 12 and a plurality of inkjet printheads 16, each having one or more nozzle assemblies with one or more respective pluralities of separate nozzles. The tray 12 may be disc-shaped or annular. Non-round shapes are also acceptable, provided they can rotate about a vertical axis. The printheads 16 may be any of the printheads described above with respect to system 110. The tray 12 and the printheads 16 are optionally and preferably mounted to allow relative rotation between the tray 12 and the printheads 16. This can be achieved by (i) configuring the tray 12 to rotate about a vertical axis 14 with respect to the printheads 16, (ii) configuring the printheads 16 to rotate about the vertical axis 14 with respect to the tray 12, or (iii) configuring both the tray 12 and the printheads 16 to rotate about the vertical axis 14 but at different rotation speeds (e.g., rotation in opposite directions). Although the following embodiments are described with particular emphasis on configuration (i) where the tray is a rotating tray configured to rotate about the vertical axis 14 with respect to the printheads 16, it should be understood that this application also covers configurations (ii) and (iii).Any of the embodiments described herein can be adjusted to be applicable to any of the configurations (ii) and (iii), and a person skilled in the art, provided with the details described herein, would know how to make such an adjustment. In the following description, a direction parallel to tray 12 and pointing outwards from axis 14 is called radial direction r, a direction parallel to tray 12 and perpendicular to radial direction r is called azimuthal direction, and a direction perpendicular to tray 12 is called vertical direction z. The radial direction r in system 10 represents the indexing direction and in system 110, and the azimuthal direction represents the scanning direction x in system 110. Therefore, the radial direction is interchangeable and is referred to herein as the indexing direction, and the azimuthal direction is interchangeable and is referred to herein as the scanning direction. The expression "radial position," as used herein, refers to a position on or above tray 12 at a specified distance from axis 14. When the expression is used in relation to a printhead, it refers to a position of the printhead that is at a specified distance from axis 14. When the expression is used in relation to a point on tray 12, it corresponds to any point that belongs to a locus of points that is a circle whose radius is the specified distance from axis 14 and whose center is on axis 14. The term "azimuth position," as used herein, refers to a position on or above tray 12 at a specific azimuth angle with respect to a predetermined reference point. Therefore, radial position refers to any point that lies on a locus of points that is a straight line forming the specified azimuth angle with respect to the reference point. The expression "vertical position", as used herein, refers to a position on a plane that intersects the vertical axis 14 at a specific point. Tray 12 serves as a support structure for 3D printing. The build area in which one or more objects are printed is usually, but not necessarily, smaller than the total area of ​​Tray 12. The build area can be annular. The build area is shown in Figure 26. Tray 12 can rotate continuously in the same direction throughout the object formation process, and in some examples, the tray reverses its direction of rotation at least once (e.g., oscillating) during the object formation process. Tray 12 is optional and preferably removable. Removing Tray 12 can be for system maintenance (Figure 10) or, if desired, to replace the tray before printing a new object.The system 10 may be equipped with one or more different replacement trays (e.g., a replacement tray kit), where two or more trays are designated for different types of objects (e.g., different weights), different modes of operation (e.g., different rotation speeds), etc. Replacement of tray 12 may be manual or automatic, as desired. When automatic replacement is employed, the system 10 comprises a tray replacement device 36 configured to remove tray 12 from its position under the print heads 16 and replace it with a replacement tray (not shown). The representative illustration of the tray replacement device 36 in Figure 1B depicts it as a unit 38 with a movable arm 40 configured to pull out tray 12, but other types of tray replacement devices are also contemplated. Example embodiments of printhead 16 are illustrated in Figures 2A-C. These embodiments can be used for any of the AM systems described above, including, without limitation, system 110 and system 10. Figures 2A-B illustrate a printhead 16 with one (Figure 2A) and two (Figure 2B) nozzle assemblies 22. The nozzles in the assembly are preferably aligned linearly, along a straight line. In embodiments where a particular printhead has two or more linear nozzle assemblies, the nozzle assemblies may optionally and preferably be parallel to each other. When a printhead has two or more nozzle assemblies (e.g., Figure 2B), all assemblies in the printhead may be fed with the same build material formulation, or at least two assemblies in the same printhead may be fed with different build material formulations. When a system similar to the 110 system is used, all 16 printheads are optionally and preferably oriented along the indexing direction, their positions along the scanning direction being offset from each other. When a system similar to system 10 is used, all printheads 16 are optionally and preferably radially oriented (parallel to the radial direction), with their azimuth positions offset from each other. Therefore, in these embodiments, the nozzle assemblies of different printheads are not parallel to each other but rather angled relative to each other, the angle of which is approximately equal to the azimuth offset between the respective heads. For example, one head may be radially oriented and positioned at azimuth 1, and another head may be radially oriented and positioned at azimuth 2. In this example, the azimuth offset between the two heads is 1-2, and the angle between the linear nozzle assemblies of the two heads is also 1-2. In some embodiments, two or more printheads can be mounted on a printhead block, in which case the printheads in the block are usually parallel to each other. A block including several inkjet printheads 16a, 16b, 16c is illustrated in Figure 2C. In some embodiments, the system 10 comprises a support structure 30 positioned beneath the printheads 16 such that the tray 12 is between the support structure 30 and the printheads 16. The support structure 30 can serve to prevent or reduce vibrations of the tray 12 that may occur while the inkjet printheads 16 are in operation. In configurations where the printheads 16 rotate about axis 14, the support structure 30 preferably also rotates so that the support structure 30 is always directly beneath the printheads 16 (with the tray 12 between the printheads 16 and the tray 12). The tray 12 and / or the printheads 16 are optionally and preferably configured to move along the vertical z-direction, parallel to the vertical axis 14, to vary the vertical distance between the tray 12 and the printheads 16.In configurations where the vertical distance is varied by moving tray 12 along the vertical direction, the support structure 30 preferably also moves vertically along with tray 12. In configurations where the vertical distance is varied by means of the heads 16 along the vertical direction, while keeping the vertical position of tray 12 fixed, the support structure 30 is also kept in a fixed vertical position. The vertical movement can be set by means of a vertical drive 28. Once a layer is completed, the vertical distance between the tray 12 and the printheads 16 can be increased (for example, the tray 12 lowers relative to the printheads 16) by means of a predetermined vertical step, according to the desired thickness of the layer to be printed subsequently. The procedure is repeated to form a three-dimensional object in layers. The operation of the inkjet printheads 16 and, optionally and preferably, also of one or more system components 10, for example, the movement of the tray 12, are controlled by a controller 20. The controller may have an electronic circuit and a circuit-readable non-volatile memory medium, wherein the memory medium stores program instructions which, when read by the circuit, cause the circuit to perform control operations as detailed below. The controller 20 can also communicate with a main computer 24 that transmits digital data related to manufacturing instructions based on computer object data, for example, in the form of a Standard Tessellation Language (STL) format or a Stereolithography Contour Format (SLC), Virtual Reality Modeling Language (VRML), Additive Manufacturing File Format (AMF), Drawing Exchange Format (DXF), Polygonal File Format (PLY), or any other format suitable for computer-aided design (CAD). The object data formats are typically structured according to a Cartesian coordinate system. In these cases, the computer 24 preferably executes a procedure to transform the coordinates of each segment of the computer object data from a Cartesian coordinate system to a polar coordinate system.The optional and preferably computer 24 transmits the manufacturing instructions in terms of the transformed coordinate system. Alternatively, the computer 24 can transmit the manufacturing instructions in terms of the original coordinate system provided by the computer object data, in which case the coordinate transformation is performed by the controller circuit 20. Coordinate transformation enables three-dimensional printing on a rotating tray. In non-rotating systems with a stationary tray, the print heads typically move reciprocally across the stationary tray along straight lines. In such systems, the print resolution is the same at any point on the tray, provided the print head dispensing speeds are uniform. In System 10, unlike non-rotating systems, not all nozzles at the print head points cover the same distance across Tray 12 simultaneously. Coordinate transformation is optionally, and preferably, performed in a way that ensures equal amounts of excess material are dispensed at different radial positions.Representative examples of coordinate transformations are provided in Figures 3A-B, which show three segments of an object (each segment corresponds to the manufacturing instructions of a different layer of the objects), where Figure 3A illustrates a segment in a Cartesian coordinate system and Figure 3B illustrates the same segment after an application of a coordinate transformation procedure to the respective segment. Typically, controller 20 controls the voltage applied to the respective component of system 10, based on manufacturing instructions and based on stored program instructions, as described below. Generally, controller 20 controls printheads 16 to dispense, during the rotation of tray 12, drops of layered building material, such as to print a three-dimensional object on tray 12. The system 10 optionally and preferably comprises one or more radiation sources 18, which may be, for example, an ultraviolet, visible, or infrared lamp, or other electromagnetic radiation sources, or an electron beam source, depending on the modeling material used. The radiation source may include any type of radiation-emitting device, including, without limitation, light-emitting diodes (LEDs), a digital light processing (DLP) system, a resistive lamp, and similar devices. The radiation source 18 serves to cure or solidify the modeling material. The operation of the radiation source 18 may be controlled by the controller 20, which may activate and deactivate the radiation source 18 and, optionally, may also control the amount of radiation generated by the radiation source 18. System 10 may further comprise one or more leveling devices 32, which can be manufactured as a roller or a blade. The leveling device 32 serves to straighten the newly formed layer before the formation of the subsequent layer on top of it. In some embodiments, the leveling device 32 is in the form of a conical roller positioned so that its axis of symmetry 34 is inclined with respect to the surface of the tray 12 and its surface is parallel to the surface of the tray. The present embodiment is illustrated in the side view of system 10 (Figure 2C). The conical roller can be shaped like a cone or a truncated cone. The opening angle of the tapered roller is preferably selected so that there is a constant ratio between the radius of the cone at any location along its axis 34 and the distance between that location and axis 14. The present embodiment allows the roller 32 to efficiently level the layers, since as the roller rotates, any point p on the roller surface has a linear velocity that is proportional (e.g., the same) to the linear velocity of the tray at a point vertically below point p. In some embodiments, the roller has the shape of a truncated cone having a height h, a radius R1 at its nearest distance to axis 14, and a radius R2 at its farthest distance from axis 14, wherein the parameters h, R1, and R2 satisfy the relation R1 / R2 = (Rh) / hy, where R is the distance of the roller farthest from axis 14 (e.g., R may be the radius of tray 12). The operation of the leveling device 32 is optional and preferably controlled by the controller 20, which can activate and deactivate the leveling device 32 and optionally can also control its position along a vertical direction (parallel to axis 14) and / or a radial direction (parallel to tray 12) and pointing towards or away from axis 14. The printheads 16 can be configured to move reciprocally with respect to the tray along the radial direction r. These embodiments are useful when the lengths of the nozzle assemblies 22 of the printheads 16 are shorter than the width along the radial direction of the work area 26 on the tray 12. The movement of the printheads 16 along the radial direction is optionally and preferably controlled by the controller 20. Some embodiments involve manufacturing an object by dispensing different materials from different sets of nozzles (belonging to the same print head or to different print heads). These embodiments provide, among other things, the ability to select materials from a specified number of materials and define the desired combinations of the selected materials and their properties.According to the present embodiments, the spatial locations of the deposition of each material with the layer are defined either to effect the occupation of different three-dimensional spatial locations by different materials or to effect the occupation of substantially the same three-dimensional location or adjacent three-dimensional locations by two or more different materials to allow the spatial combination after deposition of the materials within the layer, thereby forming a composite material in the respective location or locations. Any combination or mixture of modeling materials after deposition is considered. For example, once a particular material is dispensed, it may retain its original properties. However, when dispensed simultaneously with another modeling material or other dispensed materials in the same or nearby locations, a composite material is formed that has different properties than the dispensed materials. Additional details concerning the principles and operation of an AM system suitable for the present embodiments are found in U.S. Patent No. 9,031,680 and International Publication No. WO2016 / 009426. Therefore, the present embodiments allow the deposition of a wide range of material combinations and the fabrication of an object that can consist of multiple material combinations in different parts of the object, according to the properties that are desired to characterize each part of the object. Figure 4 is a flowchart of a method for the additive manufacturing of at least one tubular structure exhibiting properties of a blood vessel, according to some embodiments of the present invention. The method begins at 200 and, optionally and preferably, proceeds to 201, where 3D printing data is obtained in any of the computer object data formats mentioned above. The method may be advanced to step 202, whereby drops of one or more uncured building material formulations are dispensed to form a layer. The building material formulation may be a modeling material formulation such as, but not limited to, an M formulation, which yields hardened material M when exposed to a curing condition as described herein, and / or a backing material formulation such as, but not limited to, an FG formulation, which yields hardened material FG when exposed to a curing condition as described herein, and / or an S formulation, which yields hardened material S when exposed to a curing condition as described herein, and / or an L formulation, which yields hardened material L when exposed to a curing condition as described herein. The modeling material formulation is preferably dispensed with a pattern configured to match the object's shape and according to the computer object data. Other construction material formulations are preferably dispensed according to the computer object data, but not necessarily according to the object's shape, since these construction material formulations are typically sacrificial. Optionally, before dispensing, the uncured building material, or a portion thereof (e.g., one or more formulations of the building material), is heated. These embodiments are particularly useful for uncured building material formulations that have a relatively high viscosity at the operating temperature of the working chamber of a 3D inkjet printing system. The heating of the one or more formulations is preferably carried out to a temperature that allows the respective formulation to be injected through a nozzle of a print head of a 3D inkjet printing system. In some embodiments of the present invention, the heating is carried out to a temperature at which the respective formulation exhibits a viscosity not exceeding X centipoises, where X is approximately 30 mPa·s (30 centipoises), preferably approximately 25 mPa·s.s (25 centipoises) and more preferably about 20 mPa.s (20 centipoises), or 18 mPa.s (18 centipoises), or 16 mPa.s (16 centipoises), or 14 mPa.s (14 centipoises), or 12 mPa.s (12 centipoises), or 10 mPa.s (10 centipoise), or even less. The heating process can be performed before loading the respective formulation into the printhead of the AM system (e.g., 3D inkjet printing), or while the formulation is in the printhead, or while the composition is passing through the printhead nozzle. In some embodiments, heating is performed before loading the respective formulation into the dispensing head (e.g., inkjet printing) to prevent clogging of the dispensing head (e.g., inkjet printing) by the formulation if its viscosity is too high. In some embodiments, heating is performed by heating the dispensing heads (e.g., inkjet printing) at least while one or more modeling material formulations are being passed through the nozzle of the dispensing head (e.g., inkjet printing). In some embodiments, during the dispensing of at least one of the FG formulation and the L formulation, the operation of the cooling system described below is temporarily interrupted to maintain a still air environment. As used herein, "still air environment" refers to an environment in which there is no airflow, or in which the air flows at a speed of less than 3 m / s. In 203, the freshly dispensed layer is straightened, for example, using a leveling device 32 or 132, which is optionally and preferably rotary. When the freshly dispensed layer contains formulation FG and / or formulation L, the rotation speed of the leveling device is preferably modified, typically decreasing, compared to its speed when straightening other layers. The rotation speed of the leveling device can be controlled by a controller (for example, controller 20 or controller 340). The method optionally and preferably progresses to step 204, where the deposited layer is exposed to a curing condition (e.g., curing energy is applied), for example, by means of a hardening device, e.g., a radiation source as described herein. Preferably, curing is applied to each individual layer after the deposition of the layer and before the deposition of the previous layer. Optionally, the deposited (dispensed) layers are exposed to a curing condition other than curing energy, such as, but not limited to, contact with a chemical reagent or exposure to the environment. Operations 202-204, and in some embodiments also 201, are preferably performed sequentially a plurality of times, such that a plurality of layers are dispensed and solidified sequentially. This is illustrated in Figure 4 by return arrows pointing from operation 204 to operations 201 and 202. The layers are dispensed to form a stack of model layers made of a modeling material formulation and a sacrificial structure, wherein the model layer stack and the sacrificial structure can be separated from each other in a manner that maintains the shape and size of the model layer stack without deformation. In various exemplary embodiments of the invention, operations 202-204 are performed such that the layers form an elongated core and a shell encapsulating the core, wherein the core is optional and preferably the sacrificial structure, as further detailed later herein.In some embodiments of the present invention, these operations are also performed to form an intermediate cover between the core and the cover, as further detailed later herein. Each of the core, the cover, and the intermediate cover (when formed) is optionally and preferably formed by dispensing a different construction material formulation or a different combination of construction material formulations. The core and the intermediate cover (when formed) are optionally and preferably formed by dispensing a construction material that can be removed after the object is completed and is therefore sacrificial, as described herein. In some embodiments of the present invention, the method dispenses with a digital material formulation for at least one of the layers. The term "digital material formulations," as used herein and in the art, describes a combination of two or more material formulations interlocked such that the printed areas of one material formulation occupy a voxel, a few voxels, or a block of voxels, at least partially surrounded by a voxel, a few voxels, or a block of voxels of another material formulation. Such digital material formulations may exhibit novel properties that are affected by the selection of material formulation types and / or the relationship and relative spatial distribution of two or more material formulations. In the example digital material formulations, the modeling or support material formulation of each voxel or block of voxels, obtained after curing, is independent of the modeling or support material formulation of a neighboring voxel or block of voxels, obtained after curing, such that each voxel or block of voxels can result in a different modeling or support material formulation, and the new properties of the complete object result from a spatial combination, at the voxel level, of several different model material formulations. In various example embodiments of the invention, operations 202-204 are performed to form, for at least a portion of the layers, voxel elements containing different building material formulations in interlocking locations. As used herein, a "voxel" of a layer refers to a three-dimensional physical elementary volume within the layer that corresponds to a single pixel in a bitmap describing the layer. The size of a voxel is approximately the size of a region formed by a building material once the building material is dispensed at a location corresponding to the respective pixel, leveled, and solidified. Throughout this document, whenever the expression "at the voxel level" is used in the context of different materials and / or properties, it is intended to include differences between blocks of voxels, as well as differences between voxels or groups of a few voxels. In preferred embodiments, the properties of the whole part result from a spatial combination, at the voxel block level, of several different model materials. In some embodiments, at least one, or at least some (for example, at least 10, at least 20, at least 30, at least 40, at least 50, at least 60, at least 80 or more), of the layers are formed by dispensing droplets of two or more building material formulations at interlocking locations, each building material formulation from a different set of nozzles. These building material formulations may include: (i) two or more modeling material formulations, as described herein in any of the respective embodiments; (ii) at least one modeling material formulation and at least one support material formulation, as described herein in any of the respective embodiments; or (iii) two or more support material formulations, as described herein in any of the respective embodiments. In some embodiments, the method proceeds to step 205, where the hardened material FG and / or the hardened material L, or a digital material containing one or more of these hardened materials, is removed from the printed object, thereby revealing the final object. Removal 205 can be performed in more than one way. In some embodiments of the present invention, the withdrawal 205 is effected by applying pressure to one or more cavities filled with these hardened materials. The pressure is optionally and preferably sufficient to cause the hardened material FG or the hardened material L to flow out of the cavity without causing pressure-induced damage to the cover or covers enclosing the cavity. Optionally, and preferably if the liquid or liquid-like material exhibits thermal thinning behavior, the object is heated, for example, to a temperature of approximately 40°C to approximately 95°C before the withdrawal of the liquid or liquid-like material. When removal 205 is carried out by applying pressure, the pressure can be, for example, air pressure or liquid pressure, for example, in the form of a jet of an aqueous solution (for example, water). The pressure is preferably not greater than 100 kPa (1 bar), or not greater than 50 (0.5), or not greater than 30 kPa (0.3 bar), and can be, for example, 10 kPa (0.1 bar), 20 kPa (0.2 bar) or 30 kPa (0.3 bar). Alternatively, and optionally in addition to the above, and particularly in cases where the material to be removed is not sufficiently fluid under ambient conditions, removal 205 is preceded by the application of a condition that renders the material fluid. Such conditions include, for example, the application of shear forces (e.g., when the material to be removed is a shear-thinning material) and / or the application of heat (e.g., when the material to be removed is a thermally thinning material). Optionally and preferably, the hardened support structures (e.g., made of hardened material S) are also removed in 205. When the hardened support structure forms the intermediate cover, its removal is optionally and preferably carried out by circulating a solution capable of removing the hardened support structure through the cavity occupied by the intermediate cover. For example, the hardened material S may be water-soluble or miscible, in which case it may be removed by contacting it with an aqueous solution in which it is soluble or dispersible (e.g., a cleaning solution; an aqueous solution comprising an alkaline substance, in an amount of approximately 1% to approximately 3% by weight of the solution). In some embodiments of the present invention, the hardened support structure forms a pull-out core surrounded by an intermediate cover made of a fluid material (e.g., hardened material FG or hardened material L), which is in turn surrounded by a non-fluid cover. In these embodiments, removal 205 can be effected by pulling out the pull-out core through an open end of the cover. The method terminates in 206. Figures 12A and 12B are schematic illustrations of a tubular structure 300. The tubular structure 300 is preferably manufactured by AM (e.g., by operating one of the AM systems 10 and 110) from construction material formulations, e.g., by performing selected operations of method 200. The tubular structure 300 may have the shape and, optionally and preferably, also the mechanical properties of a blood vessel. The tubular structure 300 may comprise an elongated core 302 and a solid shell 308 encapsulating the core 302. In some embodiments, the smallest dimension of the 308 cover (e.g., its outside diameter) is approximately 0.1 mm to approximately 5 cm, or approximately 1 mm to approximately 3 cm. In some embodiments, the wall thickness of the 308 cover is approximately 0.1 mm to approximately 5 mm, or approximately 0.1 mm to approximately 3 mm. Other dimensions are also contemplated. Core 302 is optional and preferably sacrificial. In some optional and preferred embodiments, the tubular structure 300 also comprises an intermediate cover 304 between the core 302 and the cover 308. In some optional and preferred embodiments, the tubular structure 300 also comprises more than one intermediate cover between the core 302 and the cover 308. Embodiments in which the tubular structure 300 comprises a single intermediate cover are illustrated in Figure 12A, and embodiments in which the tubular structure 300 comprises more than one intermediate cover are illustrated in Figure 12B (this example illustration shows two such intermediate covers 304 and 306, but any number of intermediate covers may be included). The one or more intermediate covers are also optional and preferably sacrificial. Each of the 302 core, 308 cover, and 304 and 306 intermediate covers is optionally and preferably made of a different material or a different combination of materials. The cover 308 may comprise hardened material M, as defined herein. The intermediate cover 304 may comprise material FG. Cover 304 may comprise only material FG and may be devoid of other materials. Cover 306 may, in some embodiments, comprise hardened material S. The 302 core can be made in more than one way. In one example, the 302 core is made of a digital material comprising FG material and M material, interwoven together. These embodiments are particularly useful when it is desired to manufacture tubular structures that have a non-uniform diameter along their length. In some embodiments, the 302 core is formed from M material alone, without being interwoven with any additional material. In these embodiments, the 302 core is pull-out, and these embodiments are particularly useful when it is desired to manufacture tubular structures that have a generally uniform diameter (e.g., with a tolerance of less than 10%). These latter embodiments are also useful when the shape of the 300 tubular structure is intricate with small-radius bends. It should be understood that other combinations of materials can be used for the structure 300. For example, the core 302 can be made of hardened material FG, and the intermediate cover 304 can be made of hardened material S or a digital material comprising hardened material S, as described herein. The inventors found that such an intermediate cover significantly reduces the probability of inward collapse. A configuration is also contemplated in which the core 302 is made of hardened material FG, as defined herein, and the intermediate cover 304 is made of material L, as defined herein. The advantage of these embodiments is that the non-solid intermediate cover 204 reduces friction and thus facilitates the removal of the core 302 from the tubular structure 300. The inventors found that the quality of manufactured three-dimensional objects can be affected by the orientation of parts of the object with respect to the direction of relative movement between the tray and the dispensing head. Therefore, the inventors devised a procedure in which the orientation of the object part and the formulation or combination of formulations used to manufacture that part are selected together. In particular, it was found that a judicious selection of the formulation or formulations based on the orientation of the object part can facilitate easier removal of the support material after the object has been manufactured. When the object part is aligned, generally, along the indexing direction (yor), it is preferred to manufacture the core using a digital material formulation comprising an FG formulation and a modeling material formulation M, wherein the M formulation serves to reinforce the FG formulation. When the object part is aligned, generally, along the scan direction (xo), it is preferable to fabricate the core using an FG formulation or using a digital material formulation comprising an FG formulation and a modeling material formulation M, wherein the formulation M serves to reinforce the FG formulation. Representative examples of M formulations include, without limitation, any of the modeling materials marketed by Stratasys, Israel, under the Agilus™ or Vero™ family trademarks. When the core is fabricated using a digital material formulation comprising an FG formulation and a modeling material formulation M, the preferred voxel-level ratio between the number of voxels p of the FG formulation and the number of voxels q of the M formulation is from a p:q ratio of approximately 60:40 to a p:q ratio of approximately 80:20, e.g., p:q of approximately 70:30. The inventors also discovered that when a part of the object is fabricated using a digital material formulation, the elementary units within the digital material formulation can affect the mechanical properties of the fabricated part. For example, suppose a digital material formulation includes a first formulation (e.g., an M formulation) and a second formulation (e.g., an FG formulation), where the formulations are dispensed such that the first formulation forms a plurality of cubic structures distributed within the second formulation, such that one or more, for example, each, of the cubic structures of the first formulation is surrounded by the second formulation. The inventors found that the mechanical properties of the part can be adjusted by a judicious selection of the orientation of the cubic structures with respect to the scanning direction.In particular, it was found that internal sacrificial parts are easier to remove when the cubic structures of the first formulation are arranged at an acute angle (e.g., from approximately 20° to approximately 70°, or from approximately 30° to approximately 60°, e.g., approximately 45°) with respect to the scanning direction. In a preferred embodiment, an object is formed comprising a core (e.g., core 302), a cover (e.g., cover 308), and one or more intermediate covers (e.g., intermediate covers 304 and 396), wherein the core of the object is fabricated using a digital material formulation comprising an FG formulation and a modeling material formulation M, wherein the M formulation is dispensed to form cubic structures oriented at an angle of approximately 45° with respect to the scan direction (xo), and wherein the intermediate cover is formed from a support formulation S. The present embodiment allows for easy removal of the core and is particularly useful when the objects mimic a blood vessel. The preferred size of the cubic structures is approximately 0.5 mm to approximately 1.5 mm along the major diagonal.The preferred thickness of the intermediate coating is approximately 0.01 mm to approximately 0.5 mm. The preferred voxel-level ratio between the number of p-voxels of the FG formulation in the core and the number of q-voxels of the M formulation in the core is a p:q ratio of approximately 60:40 to a p:q ratio of approximately 80:20, for example, a p:q ratio of approximately 70:30. A three-dimensional model object prepared by the method as described herein is provided, in any of its embodiments and any combination thereof. A 3D model object can be provided as described in this document. While the foregoing description of the system and method places particular emphasis on embodiments in which the layers are formed by selective dispensing and curing of the building material formulation (e.g., one or more modeling material formulations as described herein and, optionally, a support material formulation), it should be understood that a more detailed reference to such a technique should not be construed as limiting the scope of the invention in any way. For example, other practitioners in the field form the layers using vat-based techniques, such as, but not limited to, stereolithography and DLP (in this regard, see, for example, U.S. Patent Nos. 4,575,330 and 9,211,678 above). Therefore, embodiments are also considered in which the system comprises a vat-based system or apparatus, for example, a stereolithography or DLP system or apparatus. As previously stated, a vat-based technique comprises a vat containing the material that is exposed to a curing condition, typically irradiated with curing radiation, with a pattern configured corresponding to the shape of a section of the object, to form solid objects by successively building up thin layers of a curable material on top of one another. When additive manufacturing uses stereolithography, a programmed moving point beam of curing radiation is directed onto a surface or layer of a curable fluid medium (one or more modeling material formulations and, optionally, also a support material formulation) to form a solid layer of the object on the surface.The formulations are UV-curable, and the curing radiation is UV radiation. Once a solid layer of the object forms, the layer is displaced, in a programmed manner, away from the fluid surface by a distance corresponding to the thickness of one layer. Then, the next cross-section forms and adheres to the immediately preceding layer, defining the object. This process continues until the complete object is formed. When additive manufacturing uses DLP, a digital light processor projects curing radiation that creates a digital image of a cross-section of the object, preferably from below, to form a solid layer of the object on the surface of a dispensed layer of a curable formulation. The curable formulation is UV-curable, and the curing radiation is UV radiation. Once a solid layer of the object is formed, the layer is moved, in a programmed manner, away from the image plane of the digital light processor by a distance corresponding to the thickness of one layer. Then, the next cross-section is formed and bonded to the immediately preceding layer, defining the object. This process continues until the complete object is formed.It is anticipated that during the term of a patent derived from this application, numerous relevant curable and non-curable materials will be developed, and it is intended that the scope of the materials described and claimed herein will include, a priori, all such new technologies. As used herein, the term "approximately" refers to ±10% or ±5%. The terms and expressions "comprises", "comprising", "includes", "including", "having" and their variations mean "including, but not limited to". The expression "consisting of" means "including and limited to". The expression "consisting essentially of" means that the composition, method, or structure may include additional ingredients, steps, and / or parts, but only if such additional ingredients, steps, and / or parts do not materially alter the basic and novel features of the claimed composition, method, or structure. As used herein, the singular forms "a," "one," and "the" include plural references unless the context clearly indicates otherwise. For example, the expression "a compound" or "at least one compound" may include a plurality of compounds, including mixtures thereof. Throughout this application, various embodiments of the present invention may be presented in interval format. It should be understood that the interval format description is solely for convenience and brevity and should not be interpreted as an inflexible limitation on the scope of the invention. Accordingly, the description of an interval should be considered to have specifically disclosed all possible subintervals, as well as individual numerical values ​​within that interval. For example, the description of an interval such as 1 to 6 should be considered to have specifically disclosed subintervals such as 1 to 3, 1 to 4, 1 to 5, 2 to 4, 2 to 6, 3 to 6, etc., as well as individual numbers within that interval, for example, 1, 2, 3, 4, 5, and 6. This applies regardless of the range of the interval. Whenever a numerical range is stated herein, it is intended to include any number cited (fractional or integer) within the stated range. The expressions "ranging / varies between" a first stated number and a second stated number and "ranging / varies from" a first stated number "to" a second stated number are used interchangeably herein and are intended to include the first and second stated numbers, and all fractional and integer numbers between them. As used herein, the term "method" refers to ways, means, techniques, and procedures for performing a given task, including, but not limited to, those ways, means, techniques, and procedures both known and readily developed from ways, means, techniques, and procedures known to specialists in chemical, pharmacological, biological, biochemical, and medical techniques. In this document, the term "acrylic material" encompasses acrylate, methacrylate, acrylamide, and methacrylamide compounds. Throughout this document, the term "(met)acrylic" encompasses acrylic and methacrylic compounds. In this document, a liquid material L describes a liquid or liquid-like material. Throughout this document and in the art, the term "liquid" describes a fluid that does not change its volume in response to stress. Liquid materials are characterized by fluidity, i.e., the ability to flow as molecules move past one another; viscosity, i.e., shear stress; a very low or zero shear modulus (G); and a ratio of shear loss modulus to shear storage modulus (G'' / G', or tan delta) greater than 1, typically greater than 10. In this document, a "liquid-like material" describes a material that exhibits liquid-like properties, exhibiting, for example, a low shear modulus, for example, less than 100 kPa or less than 50 kPa or less than 10 kPa; and / or a ratio of shear loss modulus to shear storage modulus (tan delta) greater than 1, as described herein, optionally greater than 5 or greater than 10, and thus its fluidity, viscosity, and flowability resemble those of a liquid. A liquid-like material can exhibit the liquid properties described above after the application of shear forces. A liquid-like material can be either a shear-thinning material or a thixotropic material; that is, a material that exhibits shear-thinning or thixotropic behavior, respectively. A liquid-like material can exhibit the liquid properties mentioned above after the application of thermal energy. A liquid-like material can be a thermally thinning material, meaning a material that exhibits thermal thinning behavior. A liquid-like material may have the consistency and / or rheological properties of a gel or a paste. Liquid and liquid-like materials may exhibit one or more of the following characteristics: a viscosity not exceeding 10,000 mPa·s (10,000 centipoise); and / or a ratio of shear loss modulus to shear storage modulus (tan delta) greater than 1; and / or shear thinning and / or thixotropic behavior; and / or thermal thinning behavior; and / or a shear storage modulus less than 20 kPa; and / or the ability to flow when subjected to a positive pressure less than 100 kPa (1 bar) or less than 50 kPa (0.5 bar). The shear storage modulus, G', is also referred to interchangeably in this document as the "shear storage modulus" and reflects the elastic behavior of a material. Liquid materials are typically inelastic and therefore exhibit a low shear storage modulus. The shear loss modulus, G'', is also referred to interchangeably in this document as the "shear loss modulus" and reflects the viscous behavior of a material. The shear storage modulus and shear loss modulus can optionally be determined using a shear rheometer, for example, a strain-controlled rotational rheometer, at a specified temperature and frequency (for example, using procedures well known in the art). The ratio between the shear loss modulus and the shear storage modulus, G'' / G', also known as "tan delta," reflects the viscoelastic behavior of a material. Liquid materials are typically more viscous and inelastic, and therefore, for liquid or liquid-like materials, this ratio is greater than 1. Gels are typically elastic, and therefore, for gel-like or gel-like materials, this ratio is less than 1. Throughout this document, the term "shear thinning" describes a property of a fluid compound or material that is reflected in a decrease in its viscosity (increase in its fluidity) after the application of shear forces (under shear strain). In some of the embodiments herein, a shear-thinning material is one that exhibits a significant reduction, for example, of at least 100%, in its shear modulus when the shear strain is increased from approximately 1% to more than 50%. Throughout this document, the term "thixotropic" describes a property of a compound or fluid material that is reflected in a time-dependent shear thinning; that is, its viscosity decreases in correlation with the time during which shear forces are applied and returns to its original value when the application of shear forces ceases. In some of the present embodiments, a thixotropic material is one that exhibits a significant reduction, for example, of at least 100%, in its shear modulus under a 50% strain. Throughout this document, the term "thermal thinning" describes a property of a fluid compound or material that results in a decrease in its viscosity (increase in its fluidity) following the application of thermal energy (increase in temperature). In some of the embodiments presented herein, the thermally thinning materials exhibit a decrease in viscosity or shear modulus of at least 20%, or at least 50%, or even 100%, after heating to a temperature of 40 to 95 °C, including any intermediate values ​​and sub-ranges between them. Throughout this document, the term "linking group" or "bonding moiety" describes a group that connects two or more moieties or groups in a compound. A linking group typically comes from a bifunctional or trifunctional compound and can be considered a biradical or triradical moiety, which is connected to two or three other moieties through two or three atoms of itself, respectively. Example bonding residues include a hydrocarbon residue or chain, optionally interrupted by one or more heteroatoms, as defined herein, and / or any of the chemical groups listed below, when defined as bonding groups. When a chemical group is referred to in this document as a "terminal group", it should be interpreted as a substituent, which is connected to another group through an atom of the same. Throughout this document, the term "hydrocarbon" collectively describes a chemical group composed primarily of carbon and hydrogen atoms. A hydrocarbon may be composed of alkyl, alkene, alkyne, aryl, and / or cycloalkyl groups, each of which may be substituted or unsubstituted and may be interrupted by one or more heteroatoms. The number of carbon atoms may range from 2 to 20, and is preferably lower, for example, from 1 to 10, or from 1 to 6, or from 1 to 4. A hydrocarbon may be a linker group or a terminal group. Bisphenol A is an example of a hydrocarbon composed of 2 aryl groups and one alkyl group. As used herein, the term "amine" describes both an -NR'R'' group and an -NR'- group, wherein R' and R'' are each independently hydrogen, alkyl, cycloalkyl, aryl, as these terms are defined hereinafter. Therefore, the amine group can be a primary amine, where both R' and R'' are hydrogen, a secondary amine, where R' is hydrogen and R'' is alkyl, cycloalkyl or aryl, or a tertiary amine, where each of R' and R'' is independently alkyl, cycloalkyl or aryl. Alternatively, R' and R'' can each independently be hydroxyalkyl, trihaloalkyl, cycloalkyl, alkenyl, alkynyl, aryl, heteroaryl, heteroalicyclic, amine, halide, sulfonate, sulfoxide, phosphonate, hydroxy, alkoxy, aryloxy, thiohydroxy, thioalkoxy, thioaryloxy, cyano, nitro, azo, sulfonamide, carbonyl, C-carboxylate, O-carboxylate, N-thiocarbamate, O-thiocarbamate, urea, thiourea, N-carbamate, O-carbamate, C-amide, N-amide, guanyl, guanidine, and hydrazine. The term "amine" is used herein to describe an -NR'R" group where the amine is a terminal group, as defined herein, and to describe an -NR' group where the amine is a linker group or is part of a linker group. The term "alkyl" describes a saturated aliphatic hydrocarbon, including both straight-chain and branched-chain groups. Preferably, the alkyl group has from 1 to 30 or from 1 to 20 carbon atoms. Whenever a numerical range is used, e.g., "1-20" as stated herein, it means that the group, in this case the alkyl group, may contain 1 carbon atom, 2 carbon atoms, 3 carbon atoms, etc., up to and including 20 carbon atoms. The alkyl group may be substituted or unsubstituted.The substituted alkyl can have one or more substituents, so each substituent group can be, independently, for example, hydroxyalkyl, trihaloalkyl, cycloalkyl, alkenyl, alkynyl, aryl, heteroaryl, heteroalicyclic, amine, halide, sulfonate, sulfoxide, phosphonate, hydroxy, alkoxy, aryloxy, thiohydroxy, thioalkoxy, thioaryloxy, cyano, nitro, azo, sulfonamide, C-carboxylate, O-carboxylate, N-thiocarbamate, O-thiocarbamate, urea, thiourea, N-carbamate, O-carbamate, C-amide, N-amide, guanyl, guanidine, and hydrazine. The alkyl group can be a terminal group, as defined earlier in this document, where it is attached to only one adjacent atom, or a linking group, as defined earlier in this document, which connects two or more segments through at least two carbons in their chain. When the alkyl is a linking group, it is also referred to herein as an "alkylene" or "alkylene chain." In this document, a C (1-4) alkyl, substituted with a hydrophilic group, as defined herein, is included under the expression "hydrophilic group" in this document. Alkene and alkyne, as used herein, are alkyl, as defined herein, containing one or more double or triple bonds, respectively. The term "cycloalkyl" describes a monocyclic, fused ring or group of rings composed entirely of carbon (i.e., rings sharing an adjacent pair of carbon atoms) where one or more of the rings does not have a fully conjugated pi electron system. Examples include, but are not limited to, cyclohexane, adamantine, norbornyl, isobornyl, and the like. The cycloalkyl group may be substituted or unsubstituted. The substituted cycloalkyl can have one or more substituents, so each substituent group can be, independently, for example, hydroxyalkyl, trihaloalkyl, cycloalkyl, alkenyl, alkynyl, aryl, heteroaryl, heteroalicyclic, amine, halide, sulfonate, sulfoxide, phosphonate, hydroxy, alkoxy, aryloxy, thiohydroxy, thioalkoxy, thioaryloxy, cyano, nitro, azo, sulfonamide, C-carboxylate, O-carboxylate, N-thiocarbamate, O-thiocarbamate, urea, thiourea, N-carbamate, O-carbamate, C-amide, N-amide, guanyl, guanidine, and hydrazine.The cycloalkyl group can be a terminal group, as defined above in this document, where it is attached to a single adjacent atom, or a linking group, as defined above in this document, connecting two or more fractions at two or more positions thereof. Cycloalkyls of 1-6 carbon atoms, substituted with two or more hydrophilic groups, as defined herein, are included under the expression "hydrophilic group" in this document. The term "heteroalicyclic" describes a group of monocyclic or fused rings that have one or more atoms, such as nitrogen, oxygen, and sulfur, in the ring or rings. The rings may also have one or more double bonds. However, the rings do not have a fully conjugated pi electron system. Representative examples include piperidine, piperazine, tetrahydrofuran, tetrahydropyran, morpholino, oxalidine, and similar compounds. A heteroalicyclic compound can be substituted or unsubstituted. A substituted heteroalicyclic compound can have one or more substituents, so each substituent group can be, independently, for example, hydroxyalkyl, trihaloalkyl, cycloalkyl, alkenyl, alkynyl, aryl, heteroaryl, heteroalicyclic, amine, halide, sulfonate, sulfoxide, phosphonate, hydroxy, alkoxy, aryloxy, thiohydroxy, thioalkoxy, thioaryloxy, cyano, nitro, azo, sulfonamide, C-carboxylate, O-carboxylate, N-thiocarbamate, O-thiocarbamate, urea, thiourea, O-carbamate, N-carbamate, C-amide, N-amide, guanyl, guanidine, and hydrazine. The heteroalicyclic group can be a terminal group, as defined above in this document, where it is attached to a single adjacent atom, or a linking group, as defined above in this document, which connects two or more fractions at two or more positions thereof. A heteroalicyclic group that includes one or more electron-donating atoms, such as nitrogen and oxygen, and in which the numerical ratio of carbon atoms to heteroatoms is 5:1 or less, is included under the expression "hydrophilic group" in this document. The term "aryl" describes monocyclic or polycyclic groups with a fully fused carbon ring (i.e., rings sharing adjacent pairs of carbon atoms) that have a fully conjugated pi electron system. The aryl group may be substituted or unsubstituted. The substituted aryl group can have one or more substituents, so each substituent group can be, independently, for example, hydroxyalkyl, trihaloalkyl, cycloalkyl, alkenyl, alkynyl, aryl, heteroaryl, heteroalicyclic, amine, halide, sulfonate, sulfoxide, phosphonate, hydroxy, alkoxy, aryloxy, thiohydroxy, thioalkoxy, thioaryloxy, cyano, nitro, azo, sulfonamide, C-carboxylate, O-carboxylate, N-thiocarbamate, O-thiocarbamate, urea, thiourea, N-carbamate, O-carbamate, C-amide, N-amide, guanyl, guanidine, and hydrazine.The aryl group can be a terminal group, as defined earlier in this document, where it is attached to a single adjacent atom, or a linking group, as defined earlier in this document, connecting two or more fractions at two or more positions thereof. The term "heteroaryl" describes a monocyclic or fused ring group (i.e., rings sharing a pair of adjacent atoms) that has one or more atoms, such as nitrogen, oxygen, and sulfur, in its ring or rings, and that also has a fully conjugated pi electron system. Examples, without limitation, of heteroaryl groups include pyrrole, furan, thiophene, imidazole, oxazole, thiazole, pyrazole, pyridine, pyrimidine, quinoline, isoquinoline, and purine. The heteroaryl group may be substituted or unsubstituted.The substituted heteroaryl can have one or more substituents, so each substituent group can be, independently, for example, hydroxyalkyl, trihaloalkyl, cycloalkyl, alkenyl, alkynyl, aryl, heteroaryl, heteroalicyclic, amine, halide, sulfonate, sulfoxide, phosphonate, hydroxy, alkoxy, aryloxy, thiohydroxy, thioalkoxy, thioaryloxy, cyano, nitro, azo, sulfonamide, C-carboxylate, O-carboxylate, N-thiocarbamate, O-thiocarbamate, urea, thiourea, O-carbamate, N-carbamate, C-amide, N-amide, guanyl, guanidine, and hydrazine. The heteroaryl group can be a terminal group, as defined earlier in this document, where it is attached to only one adjacent atom, or a linker group, as defined earlier in this document, connecting two or more moieties at two or more positions. Representative examples include pyridine, pyrrole, oxazole, indole, purine, and the like. The terms "halide" and "halo" describe fluorine, chlorine, bromine, or iodine. The term "haloalkyl" describes an alkyl group as defined above, further substituted with one or more halides. The term "sulfate" describes a terminal group -OS (=O) 2-OR', as this term has been defined earlier herein, or a bonding group -OS (=O) 2-O-, as these expressions have been defined earlier herein, where R' is as defined earlier herein. The term "thiosulfate" describes a terminal group -OS (=S) (=O) -OR' or a bonding group -OS (=S) (=O) -O-, as these expressions have been defined above in this document, where R' is as defined above in this document. The term "sulfite" describes a terminal group -OS (=O) -OR' or a bonding group -OS (=O) -O-, as these expressions have been defined above in this document, where R' is as defined above in this document. The term "thiosulfite" describes a terminal group -OS (=S) -OR' or a bonding group -OS (=S) -O-, as these expressions have been defined above in this document, where R' is as defined above in this document. The term "sulfinate" describes a terminal group -S (=O) -OR' or a linking group -S (=O) -O-, as these expressions have been defined above in this document, where R' is as defined above in this document. The term "sulfoxide" or "sulfinyl" describes a terminal group -S(=O)R' or a bonding group -S(=O)-, as these expressions have been defined above in this document, where R' is as defined above in this document. The term "sulfonate" describes a terminal group -S(=O)₂-R' or a linker group -S(=O)₂-, as these terms have been defined earlier herein, where R is as defined herein. The term "S-sulfonamide" describes a terminal group -S(=O)₂-NR'R'' or a linker group -S(=O)₂-NR'-, as these terms have been defined earlier herein, with R and R' as defined herein. The term "N-sulfonamide" describes a terminal group R'S (=O) 2-NR''- or a linking group -S (=O) 2-NR'-, as these expressions have been defined above in this document, where R' and R'' are as defined herein. The term "disulfide" refers to a terminal group -S-SR' or a bonding group -SS-, as these expressions have been defined above in this document, wherein R is as defined herein. The term "phosphonate" describes a terminal group -P (=O) (OR') (OR'') or a linking group -P (=O) (OR') (O) , as these expressions have been defined above in this document, with R and R' as defined herein. The term "thiophosphonate" describes a terminal group -P (=S) (OR') (OR'') or a binding group -P (=S) (OR') (O) -, as these expressions have been defined above in this document, with R and R' as defined herein. The term "phosphinyl" describes a terminal group -PR'R'' or a binding group -PR'-, as these expressions have been defined above in this document, with R' and R'' as defined above in this document. The expression "phosphine oxide" describes a terminal group -P (=O) (R') (R") or a bonding group -P (=O) (R') , as these expressions have been defined above in this document, with R and R' as defined herein. The expression "phosphine sulfide" describes a terminal group -P(=S)(R')(R") or a bonding group -P(=S)(R'), as these expressions have been defined above in this document, with R and R' as defined herein. The term "phosphite" describes a terminal group -O-PR' (=O) (OR'') or a binding group -O-PH (=O) (O) , as these expressions have been defined above in this document, with R and R' as defined herein. The term "carbonyl" or "carbonate", as used herein, describes a terminal group -C(=O)-R' or a bonding group -C(=O)-, as these expressions have been defined above herein, with R' as defined herein. The term "thiocarbonyl", as used herein, describes a terminal group -C(=S)-R' or a bonding group -C(=S)-, as these expressions have been defined above herein, with R' as defined herein. The term "oxo", as used herein, describes a group (=O) , wherein an oxygen atom is bonded by a double bond to the atom (e.g., carbon atom) in the position indicated. The term "thiooxo", as used herein, describes a group (=S) in which a sulfur atom is double-bonded to the atom (e.g., carbon atom) in the position shown. The term "oxime" describes a terminal group =N-OH or a binding group =NO-, as these expressions have been defined earlier in this document. The term "hydroxyl" describes an -OH group. The term "alkoxy" describes both an -O-alkyl group and an -O-cycloalkyl group, as defined herein. The term "aryloxy" describes both an -O-aryl group and an -O-heteroaryl group, as defined herein. The term "thiohydroxy" describes an -SH group. The term "thioalkoxy" describes both an -S-alkyl group and an -S-cycloalkyl group, as defined herein. The term "thioaryloxy" describes both an -S-aryl group and an -S-heteroaryl group, as defined herein. "Hydroxyalkyl" is also referred to herein as "alcohol" and describes an alkyl, as defined herein, substituted with a hydroxy group. The term "cyano" describes a -CN group. The term "isocyanate" describes a -N=C=O group. The term "isothiocyanate" describes a -N=C=S group. The term "nitro" describes a -NO2 group. The expression "acyl halide" describes a group - (C=O) R, where R is halide, as defined above in this document. The term "azo" or "diazo" describes a terminal group -N=NR' or a bonding group -N=N-, as these expressions have been defined earlier in this document, with R' as defined earlier in this document. The term "peroxo" describes a terminal group -O-OR' or a linker group -OO-, as these expressions have been defined earlier herein, with R' as defined earlier herein. The term "carboxylate," as used herein, encompasses C-carboxylate and O-carboxylate. The term "C-carboxylate" describes a terminal group -C(=O)-OR' or a bonding group -C(=O)-O-, as these expressions have been defined above in this document, wherein R is as defined herein. The term "O-carboxylate" describes a terminal group -OC(=O)R' or a bonding group -OC(=O)-, as these expressions have been defined above in this document, wherein R is as defined herein. A carboxylate can be linear or cyclic. When cyclic, R' and the carbon atom are bonded together to form a ring, in the C-carboxylate, and this group is also called a lactone. Alternatively, R' and O are bonded together to form a ring in the O-carboxylate. Cyclic carboxylates can function as a bonding group, for example, when an atom in the ring is bonded to another group. The term "thiocarboxylate", as used herein, encompasses C-thiocarboxylate and O-thiocarboxylate. The term "C-thiocarboxylate" describes a terminal group -C(=S)-OR' or a binding group -C(=S)-O-, as these expressions have been defined above in this document, wherein R is as defined herein. The term "O-thiocarboxylate" describes a terminal group -OC(=S)R' or a binding group -OC(=S)-, as these expressions have been defined above in this document, wherein R is as defined herein. A thiocarboxylate can be linear or cyclic. When cyclic, R' and the carbon atom are bonded together to form a ring, in the C-thiocarboxylate, and this group is also called a thiolactone. Alternatively, R' and O are bonded together to form a ring in the O-thiocarboxylate. Cyclic thiocarboxylates can function as a bonding group, for example, when an atom in the formed ring is bonded to another group. The term "carbamate", as used herein, encompasses N-carbamate and O-carbamate. The term "N-carbamate" describes a terminal group R''OC (=O) -NR'- or a linking group -OC (=O) -NR'-, as these expressions have been defined above in this document, with R and R' as defined herein. The term "O-carbamate" describes a terminal group -OC(=O)-NR'R'' or a binding group -OC(=O)-NR'-, as these expressions have been defined above in this document, with R and R' as defined herein. A carbamate can be linear or cyclic. When cyclic, R' and the carbon atom are bonded together to form a ring, as in O-carbamate. Alternatively, R' and O are bonded together to form a ring, as in N-carbamate. Cyclic carbamates can function as a bonding group, for example, when an atom in the ring is bonded to another group. The term "carbamate", as used herein, encompasses N-carbamate and O-carbamate. The term "thiocarbamate", as used herein, encompasses N-thiocarbamate and O-thiocarbamate. The term "O-thiocarbamate" describes a terminal group -OC(=S)-NR'R'' or a binding group -OC(=S)-NR'-, as these expressions have been defined above in this document, with R and R' as defined herein. The term "N-thiocarbamate" describes a terminal group R''OC (=S) NR'- or a binding group -OC (=S) NR'-, as these expressions have been defined above in this document, with R and R' as defined herein. Thiocarbamates can be linear or cyclic, as described herein for carbamates. The term "dithiocarbamate," as used herein, encompasses S-dithiocarbamate and N-dithiocarbamate. The term "S-dithiocarbamate" describes a terminal group -SC(=S)-NR'R'' or a linker group -SC(=S)NR'-, as these expressions have been defined above herein, with R and R' as defined herein. The term "N-dithiocarbamate" describes a terminal group R''SC (=S) NR'- or a binding group -SC (=S) NR'-, as these expressions have been defined above in this document, with R and R' as defined herein. The term "urea", which is also referred to herein as "ureide", describes a terminal group -NR'C (=O) -NR''R''' or a linking group -NR'C (=O) -NR''-, as these expressions have been defined above herein, where R' and R'' are as defined herein and R''' is as defined herein for R' and R''. The term "thiourea", also referred to herein as "thioureide", describes a terminal group -NR'-C(=S)-NR''R''' or a linker group -NR'-C(=S)-NR''-, with R', R'' and R''' as defined herein. The term "amide", as used herein, encompasses C-amide and N-amide. The term "C-amide" describes a terminal group -C(=O)-NR'R'' or a linking group -C(=O)-NR'-, as these expressions have been defined above in this document, where R' and R'' are as defined herein. The term "N-amide" describes a terminal group R'C (=O) -NR''- or a linking group R'C (=O) -N-, as these expressions have been defined above in this document, where R' and R'' are as defined herein. An amide can be linear or cyclic. When it is cyclic, R' and the carbon atom are bonded together to form a ring, in C-amide, and this group is also known as a lactam. Cyclic amides can function as a bonding group, for example, when an atom in the ring is bonded to another group. The term "guanyl" describes a terminal group R'R''NC (=N) - or a linking group -R'NC (=N) -, as these expressions have been defined above in this document, where R' and R'' are as defined herein. The term "guanidine" describes a terminal group -R'NC (=N) -NR''R''' or a binding group -R'NC (=N) -NR''-, as these expressions have been defined above in this document, where R', R'' and R''' are as defined herein. The term "hydrazine" describes a terminal group -NR'-NR''R''' or a binding group -NR'-NR''-, as these expressions have been defined earlier herein, with R', R'', and R''' as defined herein. As used herein, the term "hydrazide" describes a terminal group -C(=O)-NR'-NR''R''' or a binding group -C(=O)-NR'-NR''-, as these expressions have been defined earlier herein, where R', R'', and R''' are as defined herein. As used herein, the term "thiohydrazide" describes a terminal group -C(=S)-NR'-NR''R''' or a binding group -C(=S)-NR'-NR''-, as these expressions have been defined above herein, where R', R'' and R''' are as defined herein. As used herein, the term "alkylene glycol" describes a terminal group -O-[(CR'R'') zO]y-R''' or a linker group -O-[(CR'R'') zO]y-, with R', R'', and R''' as defined herein, and z being an integer from 1 to 10, preferably from 2 to 6, more preferably 2 or 3, and y being an integer of 1 or more. Preferably, R' and R'' are both hydrogen. When z is 2 and y is 1, this group is ethylene glycol. When z is 3 and y is 1, this group is propylene glycol. When y is 2-4, the alkylene glycol is referred to herein as oligo(alkylene glycol). When y is greater than 4, the alkylene glycol is referred to herein as poly(alkylene glycol). In some embodiments of the present invention, a poly(alkylene glycol) group or moiety may have from 10 to 200 repeating alkylene glycol units, such that z is from 10 to 200, preferably 10-100, more preferably 10-50. The term "silanol" describes a -Si(OH)R'R'' group, or a -Si(OH)2R' group, or a -Si(OH)3 group, with R' and R'' as described herein. The term "silyl" describes a group -SiR'R"R"', with R', R'' and R''' as described herein. As used herein, the term "urethane" or the expression "urethane moiety" or "urethane group" describes a terminal group Rx-OC(=O)-NR'R'' or a linking group -Rx-OC(=O)-NR'-, where R' and R'' are as defined herein, and Rx is an alkyl, cycloalkyl, aryl, alkylene glycol, or any combination thereof. Preferably, R' and R'' are both hydrogen. The term "polyurethane" or "oligourethane" describes a residue comprising at least one urethane group as described herein in the repeating main chain units thereof, or at least one urethane linkage, -OC(=O)-NR'-, in the repeating main chain units thereof. It is noted that certain features, for the sake of clarity, are described in the context of separate embodiments, but may also be provided in combination in a single embodiment. Conversely, various features, for the sake of brevity, are described in the context of a single embodiment, but may also be provided separately or in any suitable subcombination or as appropriate in any other embodiment described. Certain features described in the context of various embodiments are not to be considered essential features of those embodiments, unless the embodiment cannot function without those elements. Examples The following examples, which together with the descriptions above illustrate some embodiments of the invention in a non-limiting manner, are referenced below. EXAMPLE 1 Fluid gel formulations Table 1 below presents the chemical compositions of example formulations that, when exposed to UV irradiation, provide a fluid gel formulation, as defined herein. Table 1 continuation All tested formulations exhibit a viscosity at 75 °C of 10-30 kPa (10-30 centipoises) (e.g., 10-15 kPa (10-15 centipoises)) and a surface tension at 25 °C of 20-50 (e.g., 30-40) mN / m2. The formulations were used successfully in 3D inkjet printing (they are suitable for inkjet printing), without curvature of the printed object. The stability of the formulations during storage was evaluated by measuring viscosity and surface tension after storage at 65 °C for 14 days. No change in viscosity or surface tension was observed. The stability of the formulation as measured in these tests over 14 days is indicative of stability when stored at room temperature for 8 months. Printed models using these formulations exhibited Young's modulus values, as determined, for example, according to ASTM E111, less than 0.1 MPa, typically approximately 0.05 MPa ± 20%. It should be noted that formulations comprising monofunctional curable materials in an amount of 10-15% by weight, but lacking a multifunctional curable material, provided materials that did not meet the suitability requirements for injection of some inkjet AM systems and produced hardened materials that were, for example, too sticky (data not shown). A formulation such as the one presented in Table 1 and described in this document is also called FLG. The solubility of the formulations was tested by immersing a 40 x 20 x 10 mm cube printed with the formulations, consisting of a hardened material, in a 2% aqueous solution of NaOH. Solubility was measured for objects made from a single formulation e (S), or from a mixture of two formulations, whether two support formulations or one support formulation and one modeling formulation, such as Agilus™ or Vero™, such that a core made of formulation FG was coated with a 1 mm layer of the other formulation (DM). The coated objects were tested after cutting a 40 x 10 mm side wall to expose the core (as shown, for example, in Figure 5C). The commercially available formulation SUP706 was used as a reference single support formulation (S). Tables 2A and 2B below present the observed dissolution times for each of the tested objects with stirring (Table 2A) and without stirring (Table 2B). Table 2A continuation Table 2B Additional objects, 40 x 20 x 10 mm cubes, were printed in DM mode, in which a DM core was made from an FG formulation containing a lattice made from a model formulation (e.g., Agilus™), and a coating approximately 1 mm thick was made from the model formulation (Agilus™). Figures 5A-C present photographs showing such an imprinted object (Figure 5A), as well as the object during (Figure 5B) and after (Figure 5C) dissolution of the core in a 2% NaOH solution. The side wall of the object was cut before carrying out the dissolution. EXAMPLE 2 3D printed objects using inkjet printing When the object is aligned, generally, along the x-direction, good results were obtained by printing a core made of an FG formulation or a DM core made of an FG formulation reinforced with a lattice made of a modeling material formulation M, as described herein (e.g., Agilus, Vero). When the object is aligned, generally, along the y-direction, good results were obtained by printing a DM core made of an FG formulation reinforced with a lattice made of a modeling material formulation M, as described herein. The DM structure consisted of approximately 30% M material lattice and approximately 70% FG material.A preferred DM (easy to clean) structure for printing within the internal space of blood vessel imitations is made of FG material with a predefined grid made of Agilus cubes, 0.5-1.5 mm in diameter, oriented at an angle of approximately 45° to the tray, and a thin (0.5 mm) outline of SUP706. The optimal injection temperature of the FG material is between 65-75 °C. The inventors found that when a modeling material formulation was incorporated during printing within an inclusion material, the mechanical properties of the modeling material formulation were modified by the inclusion material, even after the inclusion material was removed. To further investigate this phenomenon, the ability of four types of inclusion materials to modify the mechanical properties of Agilus™ was tested.The inclusion materials tested were a first example FG formulation according to the present embodiments, comprising hydrophilic curable materials that, when hardened per se, provide a water-insoluble material (referred to as FLG334A), a second example FG formulation according to the present embodiments, comprising hydrophilic curable materials that, when hardened per se, provide a water-soluble material (referred to as FLG-PA4), a SUP706 support formulation; or a liquid formulation (providing a liquid or liquid-like material upon exposure to a curing condition). To this end, "bone-like specimen" models made of Agilus™ as the model material were printed on a J750 (Stratasys Ltd., Israel) in High Mix mode, with the bone arms supported by the SUP706 support material and the bone hinge incorporated within the embedding material under investigation. This is illustrated schematically in Figure 7A. The Agilus™ model material is shown in 710, the support material in 712, and the embedding material in 714. Figure 7B illustrates the fabricated "bone-like specimen" model after removal of the support material. The Agilus™ model material, as modified by the embedding material 714, is shown in 716. Figure 7C illustrates the fabricated "bone-like specimen" model after removal of the embedding material. The unmodified (710) and modified (716) Agilus™ model materials are illustrated in Figures 7B and 7C using different line patterns.The stress-strain curve of the modified Agilus was measured and the results, for the four types of inclusion materials tested, are shown in Figure 8. As shown therein, FLG334A provides the obtained model with better mechanical properties than the reference formulation SUP706, the liquid formulation and the PA4 formulation. Figures 6A and 6B illustrate two DM printing modes using an FG support material formulation according to the present embodiments. Both Figures 6A and 6B depict a structure 600 having a tubular core 602 and one or more covers 604, 606, and 608 surrounding the core 602. Preferably, the structure 600 comprises the core 600 and covers 604 and 608. Optionally, the structure 600 also comprises cover 606 between covers 604 and 608. Cover 608 is the outermost cover and typically comprises an M material, such as Agilus™. Cover 604 is adjacent to the core 602 and typically comprises only the FG material. The optional cover 606 typically comprises an S support material, such as SUP706. Figure 6A illustrates an embodiment, referred to herein as geometry 1, in which the core 602 is formed from material FG reinforced with material M (e.g., Agilus™).The FG material and the M material are typically dispensed in an interlocking manner to form a digital material, as described herein. Figure 6B illustrates one embodiment, referred to herein as geometry 2, in which the core 602 is formed from the M material. In the present embodiment, a pull-out core 602 is formed, as shown below (see Figures 10 and 11). The printing method illustrated in Figure 6A is particularly useful when it is desired to manufacture hollow tubular structures that have a non-uniform diameter along their length. The printing method illustrated in Figure 6B is particularly useful when it is desired to manufacture hollow tubular structures that generally have a uniform diameter, and can also be used for intricate tubular shapes with small-radius bends. Figure 9 presents photographs demonstrating the removal of the support material from an object made according to geometry 1, as shown in Figure 6A, using an FG formulation according to the present embodiments. As shown therein, by applying slight pressure, the hardened support material is easily removed, thus revealing a hollow tubular structure. Printing according to geometry 2, as shown in Figure 6B, can be used when printing models that have hollow structures with intricate geometries (e.g., twisted thin tunnels). Figure 10 presents photographs demonstrating the removal of the support material from an object made according to geometry 2 using an FG formulation in accordance with the present embodiments. As shown therein, by applying slight pressure, the hardened support material is easily removed, thus revealing a hollow tubular structure. Figure 11 presents photographs demonstrating the removal of the support material from an object made according to Figure 6B, in accordance with the present embodiments.

Claims

1. A formulation useful in the additive manufacturing of a three-dimensional object, the formulation comprising: at least one monofunctional curable material; at least one hydrophilic multifunctional curable material; and at least one water-miscible non-curable material, wherein: a total amount of said at least one monofunctional curable material ranges from 1 to 10, or from 3 to 10, or from 5 to 10, wt.% of the total weight of the formulation; a total amount of said at least one hydrophilic multifunctional curable material ranges from 1 to 5, wt.% of the total weight of the formulation; a weight ratio of a total weight of said at least one monofunctional curable material and a total weight of said at least one hydrophilic multifunctional curable material ranges from 1:1 to 10:1; and a total amount of at least one non-water-miscible curable material is at least 85% by weight of the total weight of the formulation, and wherein each of said curable materials is a UV-curable material. 2.The formulation of claim 1, wherein said at least one hydrophilic multifunctional curable material provides, when cured per se, a material that is insoluble in water.

3. The formulation of claim 1 or 2, wherein at least one of said one or more monofunctional curable materials comprises a hydrophilic monofunctional curable material.

4. The formulation of any one of claims 1 to 3, wherein at least one of said one or more monofunctional curable materials comprises a hydroxyalkyl moiety and / or an alkylene glycol moiety.

5. The formulation of any one of claims 1 to 4, wherein said at least one water-miscible non-curable material comprises a water-miscible polymeric material and a water-miscible non-polymeric material. 6.The formulation of claim 5, wherein the weight ratio of the total weight of said at least one non-curable, water-miscible polymeric material to the total weight of said at least one non-curable, water-miscible polymeric material ranges from 2:1 to 1:

2.

7. The formulation of any one of claims 1 to 6, comprising: said at least one monofunctional curable material, in an amount of 3 to 10 wt%.; said at least one hydrophilic multifunctional curable material, in an amount of 3 to 5 wt%.; at least one non-curable, water-miscible polymeric material in an amount of 30 to 60 wt%.; and at least one non-curable, water-miscible, non-polymeric material in an amount of 30 to 60 wt%.

8. The formulation of any one of claims 1 to 7, wherein each of said curable materials is an acrylic material. 9.The formulation of any one of claims 1 to 8, wherein, when exposed to a curing condition, it presents a gel material that can flow upon application of a positive pressure of 10 kPa to 150 kPa (0.1 bar to 1.5 bar), or 20 kPa to 120 kPa (0.2 bar to 1.2 bar), or 20 kPa to 100 kPa (0.2 bar to 1 bar).

10. The formulation of any one of claims 1 to 9, further comprising a photoinitiator.

11. The formulation of claim 10, wherein the amount of said photoinitiator does not exceed 2% by weight of the total weight of the formulation. 12.A method for the additive manufacturing of a three-dimensional object, the method comprising sequentially forming a plurality of layers in a configured pattern corresponding to the shape of the object, thereby forming the object, wherein the formation of at least some of said layers comprises: dispensing at least two formulations of construction material, said at least two formulations of construction material comprising a modeling material formulation M which, after exposure to a curing condition, forms a hardened modeling material M, and a formulation according to any one of claims 1 to 11, which, after exposure to said curing condition, forms a hardened support material FG. 13.The method of claim 12, wherein said dispensing is such that said hardened modeling material M forms at least one hollow structure, and said material FG is enclosed, at least partially, in said hollow structure.

14. A method for the additive manufacture of a three-dimensional object having a cavity, the method comprising sequentially forming a plurality of layers in a configured pattern corresponding to a combined shape of the object and a sacrificial object, such that said sacrificial object is enclosed by a sacrificial cover, and said sacrificial cover is enclosed in said cavity; and removing said sacrificial object and said sacrificial cover from said cavity; wherein said sacrificial object comprises a modeling material M, and said sacrificial cover is made of a support material FG formed from the formulation according to any one of claims 1 to 11. 15.The method according to claim 14, wherein said sacrificial object also comprises said FG material, said FG material being reinforced by said modeling material M.

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