Apparatus for transferring particles

Abstract

Applying particles to a substrate which is at least partially coated with an adhesive 1000 uses a driven cylinder 2000 or a belt applicator and a particle dispenser for applying a uniform multilayer c

Description

CROSS-REFERENCE TO RELATED APPLICATIONS This application claims Paris Convention priority from Great Britain patent applications GB 2313841.5 filed on September 11, 2023, and GB 2400528.2 filed on January 15, 2024, and is related to international patent application No. PCT / IB2024 / 058781 filed on September DD, 2024. The entire disclosures of the latter applications are hereby incorporated by reference, as if fully set forth herein. FIELD The present disclosure relates to an apparatus for transferring particles to the surface of a substrate, as may be used in printing and coating. BACKGROUND The transfer of particles to a target surface, often called “deposition”, is critical in a variety of industries. In some, the particles are intended to continuously cover the surface of an object, in which case the transferred particles may be said to form a coat or a coating. Such coatings are typically functional, such as protective coatings (e.g., anti-corrosion, anti-wear, thermal barriers, fluid permeation barriers, radiation barriers etc.) that can be formed on the surface of components to enhance their durability in demanding industries (e.g., transport, aerospace, drilling etc.) and / or reduce their exposure to deleterious factors. The transferred particles can form active coatings releasing to the surroundings of the surface coated thereby an agent having a desirable effect. Biomedical coatings on medical implants may promote their biocompatibility, and in turn enhance their performance, particles can be deposited on surfaces for drug delivery or any other purpose (such as catalysts in fuel cells). Particles may also be transferred to a surface in a more selective manner, so as to form specific patterns. That can be the case in the manufacturing of electronic components, microchips, integrated circuits, semiconductors, optoelectronic devices and photovoltaic cells, to name a few industries. Whether transferred as coatings or patterns, the deposited particles can alternatively, or additionally, be decorative fulfilling aesthetic purposes. Printing is one of the common technologies in which particles are widely transferred to a substrate to form a two-dimensional (2D) image or a three-dimensional (3D) object, if prepared by additive manufacturing. While many printing technologies rely on particles (e.g., pigments) being dispensed in liquid or paste form (e.g., inkjet inks, lithographic inks, offset printing inks, screen printing inks, etc.), this requires the time- and energy-consuming elimination of the liquid carrier and the use of special printing substrates to prevent permeation of the liquid to an extent deforming the substrate, and consequently the pattern deposited thereon. Therefore, alternative printing techniques have been developed to reduce or totally eliminate the liquid from the ink substance being deposited, the particles being applied in substantially dry form. In 2D printing, a common process using dry particles of ink, known as toner, is called xerography, also known as laser printing or photocopying. In 3D printing, powder-based deposition involves transferring layers of powdered material onto a surface and then fusing them together using lasers or glue-acting binding agents. In all of these industries, precise control over the size, composition, and uniformity of the particles being transferred is essential for the performance and quality of the final product and drawbacks remain. Considering, for illustration, the printing industry, the rise of 3D printing imposes new challenges. By way of example, while in 2D printing the human eye can perceive an image as being complete even with partial coverage of 50-60%, thanks to a phenomenon called "visual completion" by which the brain is capable of filling in gaps in visual information, such coverage would be insufficient for 3D printing, the voids between the applied particles weakening the end-structure. SUMMARY With a view to mitigating at least some of the foregoing disadvantages, and in particular increasing the transfer of particles to a surface, there is provided in a first aspect of the present disclosure, a first particle application apparatus as hereinafter set forth in more detail and claimed in Claim 1 of the appended claims and claims depending therefrom. According to the present disclosure, there is provided in a second aspect, a second particle application apparatus as hereinafter set forth in more detail and claimed in Claim 15 of the appended claims and claims depending therefrom. The particle application apparatus comprises: a) an applicator having the form of a cylinder or a belt, b) a drive mechanism for urging a surface of the applicator into rolling contact with the surface of the substrate, and c) a particle dispenser (2000) for applying to at least a section of the applicator surface that is to make rolling contact with the substrate a uniform multilayer coating of the particles to be applied to the substrate. Generally, the cohesion between the particles being transferred by the present apparatus or method is such that they are transferred from one (transfer) surface to a (receiving) surface as overlying layers of particles, typically at least 2-3 particles deep. The particles being transferred typically have a higher cohesion with the receiving surface than with a preceding transfer surface. In some embodiments, the ultimate receiving surface which is on the substrate is coated with an adhesive at least in selected regions so as to further increase the cohesion of the particles to the substrate, in this case to the adhesive on the substrate. Advantageously, the adhesive in the adhesive coated areas of the substrate do not transfer “backward” from the substrate to the preceding transfer surface, also named the applicator surface. In other words, and considering only the last transfer of particles from the surface of the applicator to adhesive coated regions of a substrate, the present apparatus and method can be characterized in that the cohesion between the overlying layers of particles coating the surface of the applicator is less than the cohesion of the particles to the adhesive on the substrate, the latter cohesion being in turn less than the cohesion between the adhesive and the substrate, whereby particles are transferred from the applicator to the adhesive coated areas of the substrate without transfer of adhesive from the substrate to the applicator. In some embodiments, the particle dispenser comprises at least one particle supply and at least one transfer cylinder or roller for transferring particles from the, or a respective, particle supply, onto the surface of the applicator, prior to the surface coming into the contact with the substrate. The, or each, particle supply comprises a cartridge insertable into a respective supply chamber. When two or more transfer rollers are disposed (in parallel and / or in series) between the particle supply and the applicator, a transfer roller receiving particles from the, or a respective, particle supply, shall be termed a primary transfer roller, and a subsequent roller transferring the particles from the primary roller onto (or towards) the applicator shall be termed a secondary transfer roller. In some cases, to facilitate transfer of particles from one surface to another, the hardness of the surface of an upstream transfer cylinder is higher than that of the surface of subsequent downstream transfer cylinder, the hardness of the surface of the applicator being the lowest. Each supply chamber and / or cartridge includes one or more members for mixing the particles, at least one of the members being movable and extending so as to contact the surface of associated (e.g., primary) transfer cylinders, whereby the mixed particles are evenly smeared to form a homogeneous coating of particles on the surface of the transfer cylinder. In some embodiments, the particle dispenser comprises a housing common to all the particle supplies and transfer cylinders, an opening in the housing allowing one cylinder disposed in a chamber of the housing to partially protrude therethrough to enable direct or indirect contact with the substrate. Direct contact may take place (upon controlled urging of the dispenser), when the cylinder disposed adjacent to the opening and having its surface partially rotating outside of the housing is the applicator cylinder. Indirect contact may take place when the same ultimate cylinder of the dispenser is instead a final transfer cylinder, coming into controlled contact with an applicator entirely disposed outside the housing. One or more levelling blades can be disposed adjacent to the opening in the housing, the or each levelling blade controllably contacting the surface of the final transfer cylinder, the applicator or particles thereon, as the case may be. Levelling blades may further be configured to prevent egress of particles from the housing. In operation, the applicator surface is configured to rotate or circulate at a linear speed substantially identical to the linear speed of the substrate upon rolling contact therebetween, so as to avoid slippage between the applicator and the substrate. In contrast, the linear speed of at least one of the transfer rollers is at least 10% higher or lower than the speed of the applicator, so as to enable slippage between surfaces upstream of the applicator and rubbing of the particles prior to rolling contact between the applicator and the substrate. When the applicator is disposed externally to the housing of the dispenser, the particle application apparatus further comprises a second drive mechanism for urging the particle dispenser into rolling contact with a surface of the applicator, wherein, in operation, the second drive mechanism is configured to disengage the dispenser from the applicator whilst the drive mechanism of the apparatus is urging a surface of the applicator into rolling contact with the surface of the substrate. Features of different embodiments of the foregoing aspects and additional benefits of the present disclosure are set out in the following detailed description taken in conjunction with the figures, non-limiting examples and appended dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described further, by way of example, with reference to the accompanying drawings, where like reference numerals or characters (or last digits thereof) indicate corresponding or like components. The description, together with the figures, makes apparent to a person having ordinary skill in the art how some embodiments of the disclosure may be practiced. The figures are for the purpose of illustrative discussion and no attempt is made to show structural details of an embodiment in more detail than is necessary for a fundamental understanding of the disclosure. For the sake of clarity and convenience of presentation, some objects depicted in the figures are not necessarily shown to scale. In the Figures: Fig, 1 is a schematic representation of a first 3D printing system in which particles can be applied by an apparatus of the invention; Fig, 2 is a schematic representation of a second 3D printing system in which particles can be applied by an apparatus of the invention; Figs. 3 to 7 show schematically the sequence of steps and implementing modules including the present applicator of particles employed in forming a 3D article; Fig, 8 shows illustrations of the post-processing of an article after all its different layers including the particles dispensed by the present apparatus have been deposited; Fig, 9 illustrates in cross-sectional side view a device adapted to deposit particles on a transferring surface from which particles can be detached by thin layers of tacky adhesive according to one embodiment of the invention; Fig, 10 provides in perspective view an applicator of particles similar in principles to the device shown in Fig. 9; Fig, 11A displays a top view of a single coat of layers of particles as applied to an adhesive image by a conventional dispenser, whereas Fig, 11B displays a comparative top view of a coat of particles alternatively applied using an applicator according to the present teachings, such as illustrated in Figs. 9 and 10; Fig, 12 illustrates how devices, including a dispenser of particles according to the invention, can be arranged to form a 3D printing system able to form a stack of layers including regions of compacted first particles when the substrate and its transport mechanism are nonpl anar; Fig, 13 provides an exemplary alternative arrangement of devices, when the 3D printing system is intended for planar substrates and transport mechanisms; and Fig, 14 displays pictures captured by microscopy of particles applied according to embodiments of the present teachings (panels A-C) and of cross-sections through layers formed from said particles following their compaction (panels D-E) and their subsequent sintering (panel F). DETAILED DESCRIPTION The present apparatus and method for transferring particles to a substrate was developed and tested in a demanding 3D printing process, however this is not its sole use. Understandingly, it may also serve less challenging 2D or 3D printing processes or any other manufacturing process in which application of particles can be beneficial, in particular when elevated coverage is sought. Overview of Examplary 3D Printing Methods and Systems The 3D printing method of the present disclosure comprises forming a plurality of stacked layers, in which at least each intermediate layer is formed of contiguous regions of two or more different materials (e.g., particles having different compositions or properties and / or different adhesive compositions) that cover the whole of the area bounded only by the outer perimeter of each intermediate layer. In other words, the individual layers do not have any holes in them, though the perimeters of adjacent layers do not need to be the same. Consequently, during formation of each layer (or slice of a 3D object), there is a continuous underlying layer onto which material can be applied. Fig. 1 schematically shows an apparatus for carrying out a method of a first embodiment of 3D printing process in which the present invention can be implemented. A substrate 10 is mounted on a transport mechanism shaped as a drum 12 that transports it sequentially through different processing stations. In a first station 14 a thin adhesive layer is selectively applied to regions onto which first particles are to be adhered, said regions (which need not be contiguous for any single type of material) being also termed first regions or foreground regions. The selective application may for example be carried out by jetting of an adhesive composition to the first regions. While the adhesive is tacky, the substrate passes through a second station 16 where first particles are made to adhere only to regions that have been coated with an adhesive. This process is then essentially repeated for second particles by passing the substrate through a third station 18 where a second adhesive coating is selectively applied to second regions (also called background regions or remaining regions) of the substrate 10 and then through a fourth station 20 where second particles are adhered only to regions coated with the second adhesive. If desired, the process can be repeated for third particles. Once all the different particles forming the various regions of a layer have been applied to the substrate 10, it is passed through a station 22 where the particles are compacted, and optionally levelled, to create a flat continuous surface onto which a new layer of particles can be applied. Such a compacting station may additionally be found after previous types of particles have been adhered to selectively applied regions of thin adhesive layer preceding the completion of a layer. In the setup illustrated in Fig. 1, such a situation would be depicted by the presence of two compacting stations, one downstream of station 16 and upstream of station 18, and the other as currently illustrated by station 22 downstream of station 20 and upstream of station 14. If the compaction achieved downstream of the last compacting station passed through during any given cycle is such that the flat continuous surface has an average roughness Ra of 3 pm or less (as could have been achieved by milling), separate levelling may not be necessary. Yet, in some embodiments, an even smoother surface (e.g., having an average roughness Ra of 2 pm or less, 1 pm or less, or 0.5 pm or less), may be required of the continuous intermediate layer, in which case a separate levelling station (not shown in present figure) may be further included downstream of station 22 and upstream of station 14 where a new cycle can be initiated. Depending on the desired degree of smoothness, the levelling station can include a milling device (for Ra of about 3.5 pm or less), a grinding device (for Ra of about 1.5 pm or less) or an abrading device (also known as a polishing device) (for Ra of about 0.5 pm or less). The topographic properties of a surface compacted and / or levelled by a compacting module or a levelling module as herein described can be monitored by any suitable method. For illustration, optical sensors suitable for distance measurements, light detection and ranging (LiDAR), 3D mapping, and laser scanning of the surface being assessed, or cameras capturing such information, may be used in connection with computerized analysis systems translating the sensors output into a sought measurement. Control systems may further be used to modify an operational parameter of the 3D printing system in response to the measured property (e.g., surface topography measured by laser triangulation). After completion of multiple cycles according to similar principles, there is formed a stack as shown at (A) in Fig. 8. Within this stack, the first particles, shown around the centreline, may be made of a metal or a ceramic and the second particles, surrounding the first particles, may be formed of a plastics material. To complete the production of an article made of the first particles, the stack may be consolidated as shown at (B) in Fig. 8, such as by sintering, to fuse the different layers of the first particles to one another and finally, as shown at (C) in Fig. 8, the second particles may be removed to leave behind only the desired article. It will be seen from this exemplary figure that the first particles are those serving to form the desired article and will therefore also be termed herein model particles, while the second particles serve only to allow the model particles to be deposited wherever necessary and are herein also termed support particles. The support particles can be made of material different from the model particles in order to permit their removal by a physical or chemical process. The manner of the removal will depend on the choice of support particles. For example, the support particles may dissolve in a solvent in which the model particles are not soluble, to permit chemical removal of the support particles, or they may have a lower melting point, allowing their removal by application of heat to which the model particles would be resistant. Alternatively, or additionally, the regions made of different materials may include different adhesives, the particles themselves optionally being made of a same material. For illustration, a first adhesive intended to adhere the first particles can be rendered tacky at a first temperature and a second adhesive intended to adhere the second particles can be rendered tacky at a second temperature. This alone may permit the joint application (at a same station and / or concomitantly) of the first adhesive and second adhesive, each in its respective intended region, the first adhesive being activatable at a first temperature (e.g., 80°C) sufficiently lower than the second temperature (e.g., 120°C) so that the second adhesive remains “inactive” when the first particles are applied to the first adhesive at its first temperature. The temperature may then be raised to reach the second temperature, thereby activating the second adhesive in regions uncoated by the first particles, the regions now rendered tacky being then able to adhere second particles. If the regions made of different materials differ from one another inter alia or only by the choice of the adhesives, then the adhesives can additionally be selected to facilitate or prevent removal of one type of particles’ coating over the other, the adhesives having different solubility / resistance towards a removing solvent. For instance, the first adhesive can be water-insoluble and the second adhesive water soluble, water being used to separate particles bound in the different regions upon completion of superposition of all compacted layers. It should be noted that removal of the support material is not essential as it may form part of the end-product. For example, in the manufacture of a circuit board, the support particles may form the insulating substrate while the model particles may form the desired electrical conductors. Fig- 2 differs from Fig. 1 in that instead of adhesives being applied to selective regions according to the particles to be coated thereby, an adhesive coating is applied to the entire substrate 10 and then selectively activated, such as by exposure to radiation or a suitable chemical agent. Thus, in the apparatus schematically depicted in Fig. 2 there is only one adhesive application station 30 where an adhesive is applied but is not tacky (or tacky enough) by the time it reaches a first activator station 32 where selected regions of the adhesive (e.g., first or foreground regions) are activated to remain tacky until desired particles are applied thereto. After first particle are applied to the activated regions of the adhesive by a station 34, the remaining regions of the adhesive (e.g., second or background regions) are activated in an adhesive activating station 36 to remain tacky till desired particles are applied thereto and are coated with second particles in station 38. As with the previous embodiment, the final station 40 serves to compact (and optionally level) all the particles constituting a layer to provide a continuous surface for the next deposition cycle and, similarly, such a compacting station may additionally be found downstream of station 34 and upstream of stations 36 being an adhesive activator or 38 being a dispenser of second particles. In both embodiments, the compaction (optionally achieving levelling) can be carried out by passing the coated substrate between rollers urged against one another with a desired force but movable relative to one another to allow for the height of the substrate to increase from cycle to cycle. The embodiments of Figs. 1 and 2, which build an article onto a part-cylindrical substrate, are well suited to manufacture of flexible circuit boards, as the substrate can be bent after removal from the drum. To construct an article that is not necessarily flexible and that needs to have a flat base requires a different transport mechanism to move the substrate cyclically between different processing stations. It is possible for example, to replace the drum by an annular horizontal support surface that rotates about a vertical axis. As a further alternative, the substrate may be placed on a flat plate that is suspended from two continuous chains and each station may comprise features to retain the plate in a predetermined position during processing, to ensure correct alignment of the different regions. Though it has been assumed above that both the model and the support materials are made of particles, it would be possible for the support regions to be applied as a liquid that is then solidified (e.g., dried or cured if a curable liquid, such as a radiation curable resin that can be solidified by ultraviolet (UV), visible or IR light, electron beam (EB), or thermal energy). Liquid may be selectively applied to the support regions, such as by jetting, or one may coat with a liquid the entire substrate (including regions already coated with particles) and, following curing at a curing station, the solidified (e.g., cured) support materials may be removed from regions coated with model particles. In this case, a separate levelling device adapted to remove surplus support material will be required in addition to the compacting station adapted to compress the regions coated with model particles. The levelling device of the separate levelling station may, for example, employ an abrasive having a hardness greater than that of the cured support material but lower than that of the compacted model particles. While for the sake of efficient stacking of layers it may be desired to remove as little as possible from the regions of compacted model particles, this is not essential. It may, on the contrary, be desirable to expose the model particles compacted in an underlying layer to model particles to be freshly applied in a subsequent layer. This can be the case when model particles of adjacent layers are to be applied in overlapping regions, the exposure of the model particles of a previous layer facilitating later binding to model particles of a following layer in a consolidation process. When such exposure is required (e.g., for the additive manufacturing of a PCB), the levelling of the intermediate layer may be such as to achieve an average roughness Ra of 500 nm or less, 400 nm or less, 300 nm or less, or 200 nm or less. Figs. 3 to 7 illustrate schematically different apparatus for implementing different embodiments of a 3D printing method in which the present apparatus can be used to apply the particles, wherein an arrow indicates the direction (X) of the process for completing the formation of a single layer of the stack. While illustrated with a transport mechanism adapted for a flat planar backing surface for stacked layers having a flat base, this should not be construed as limiting. Fig-3 shows the steps performed by the embodiment of Fig. 2 and uses the same reference numerals. A thin adhesive applicator 30 is followed by a selective adhesive activator 32. After passing through the model particle dispenser 34 while the selectively activated adhesive is still tacky, the substrate passes beneath a background adhesive activator 36 and a support particle dispenser 38 while the activated background adhesive is still tacky, before reaching the compacting station 40. While the second adhesive activator 36 can selectively activate (e.g., by selective irradiation or jetting of an activating chemical) the second regions of the substrate to be thereafter coated by second particles, this is not essential. For instance, if the thin layer of adhesive applied by applicator 30 over the entire surface of the substrate due to form a subsequent layer is heat activatable, then activation of the adhesive layer in the background regions not coated by model particles can be achieved by applying heat to the entire substrate, the model particles in the coated first regions blocking access to subsequently applied support particles. Fig. 4 differs from Fig. 3 only in that it comprises two compaction stations, the model particles being compacted in a station 42 before activation of the background adhesive (at 36), deposition of support particles (at 38) on the exposed regions of the activated adhesive, and compaction of the support particles to the substrate in compacting station 40, where the model particles may be further compacted. In Fig. 5, which is based on the embodiment of Fig. 1 and uses the same reference numerals, the adhesive for the model particles is applied by a selective applicator 14A that is followed by an optional activator 14B that is not selective which can be used to increase, maintain or restore tackiness of the selective adhesive image. Likewise, the adhesive for the support particles is a selective applicator 18A that is followed by an optional activator 18B that is not selective. Otherwise, the process is the same as in Fig. 3. For illustration, selective applicators 14A and 18A may apply a thin layer of same or different heat activatable adhesives, which can optionally be non-selectively activated by heat at stations 14B and 18B, to the extent necessary to increase or renew their tackiness for adhering to the model or support particles respectively downstream dispensed. While in the present figure, the selective applicators of adhesives have been illustrated as being each downstream followed by a respective dispenser of particles 16 or 20, this arrangement should not be construed as limiting. For instance, if the adhesive compositions are distinct, e.g., the first adhesive being activatable at a first temperature and the second adhesive being activatable at a second temperature greater than the first (and being inactive at the first temperature), then applicators 14A and 18A may be adjacent one another so as to form at once a layer with different regions of distinct adhesives. The first particles may then be applied by particle dispenser 16 at the first temperature (optionally with the assistance of heater 14B), this temperature rendering tacky only the regions of the first adhesive, and the second particles may be thereafter applied by particle dispenser 20 at the second temperature (optionally with the assistance of heater 18B), this higher temperature rendering tacky the regions of the second adhesive. As the regions of the first adhesive are by then already coated by the first particles, the second particles will only adhere to the second activated adhesive. In this case, the different adhesive compositions, even if jointly applied before entering a first dispenser of particles, may be viewed as serving the purpose of “differential” adhesion to a backing surface or underlying layer during the formation of the superposed layers. Alternatively, or additionally, the different adhesives may serve the purpose of “differential” removal of the different materials, if desired, following completion of superposition of the layers. For instance, the first adhesive can be insoluble in a “removing solvent” whereas the second adhesive would be soluble in the same solvent. Again, adhesive applicators 14A and 18A can be adjacent one another forming at once the different regions of distinct adhesives. Furthermore, in this case, the particles serving as model or support can be the same, since their removal may rely on the distinct adhesives binding them. Then, in such embodiment, a single particle dispenser 16 may suffice (optionally with the assistance of a device activating the layer comprising the distinct adhesives, e.g., heater 14B). As readily appreciated from these exemplary alternatives, an intermediate layer is said to be made of different materials in different regions of the layer, a) if the “building” material, whether particles or solidifiable liquids, are of different chemical and / or physical composition; b) if the “connecting” material, namely the adhesive compositions, similarly have different chemical and / or physical properties; or c) both. Fig. 6 illustrates an embodiment in which the support material is applied in liquid form. The first four processes in this embodiment can be the same as in Fig. 4 and have been allocated the same reference numerals. Following application and compaction of the model particles, in the station designated 60 a liquid resin is applied over the entire surface of the substrate. A squeegee 62 removes most of the liquid overlying the compacted model particles while leaving a film of the liquid resin over the remainder of the substrate. Following curing of the liquid resin in the station designated 64 the substrate passes through a levelling station 66 in which any cured resin overlying the compacted model particles is removed by milling, grinding, and / or abrasion, the material so eliminated being sucked away. If desired, an excess of model particles may be applied during the first four processes and the levelling may remove some of the model particles as well as the overlying cured resin. While levelling is described in this figure as being performed on a cured resin, such a step may alternatively be performed when the materials forming an intermediate layer are all in the form of particles. Taking Fig. 6 for illustration, in this case station 60 would dispense support particles, squeegee 62 would ensure the support particles fill the regions of the substrate not previously coated by the model particles, station 64 would in turn ensure all particles are compacted before entering the levelling station 66. While Figs. 1 to 6 have schematically illustrated different methods and some alternative apparatuses for their implementation showing that all layers of the stack can be formed on a common transport mechanism (e.g., adapted to convey a flat planar or a curved backing surface and growing substrate), this should not be construed as limiting. For instance, some layers can be formed on a separate surface before being transferred to the transport mechanism onto an initially vacant surface or on layers previously formed thereon or transferred thereto from additional separate surfaces. Fig- 7 illustrates such an embodiment and differs from Fig. 6 in that there are two thin adhesive applicators. A first thin adhesive applicator 30a is configured to coat non-selectively the entire surface of the substrate transported by the flat transport mechanism 52 illustrated horizontally in the figure. A second thin adhesive applicator 30b is configured to non-selectively coat a separate surface, shown as drum 54, the downstream selective adhesive activator 32 and model particle dispenser 34 being now adjacent to and operative on drum 54 so as to form thereon a layer including regions selectively coated with model particles. These regions of model particles are transferred in mirror image from drum 54 to the thin adhesive applied on the upper surface conveyed by the flat transport mechanism 52 at the nip 56 therebetween. This relocation of the layer at the transfer nip 56 can take place if the thin adhesive applied by applicator 30a to form a flat adhesive is still tacky, and the model particles have a greater adhesion to the flat adhesive than to the adhesive to which they were initially adhered to on the surface of the drum. If this is not the case, an additional adhesive activator (not shown) can be included downstream of the thin adhesive applicator 30a and upstream of the nip 56 to ensure that the thin layer of adhesive be tacky enough to detach the regions of model particles from the drum to effect their transfer. While a compacting station 42 is shown downstream of the transfer nip 56, this may not be necessary if the pressure applied to effect transfer at the nip can additionally achieve a desired compaction of the model particles. The following processes can be as illustrated in Fig. 6, the support material being applied as a liquid resin in the station designated 60, most of the liquid overlying the compacted model particles being removed by squeegee 62 while leaving a film of the liquid resin over the remainder of the substrate which is solidified in the station designated 64. The substrate then passes through a levelling station 66. Alternatively (and not shown in Fig. 7), the support material can be applied in the form of particles, in which case the processes following compaction of the transferred model particles can be as illustrated for instance in Fig. 4. While Figs. 1 to 7 have for simplicity schematically illustrated different 3D printing methods and some exemplary printing systems for their implementation showing layers formed of two different types of material, this should not be construed as limiting. As can be appreciated, some steps of the method may be repeated for additional (e.g., a third, a fourth, and so on) material, the apparatus accordingly including additional corresponding devices adapted to the performance of such steps for the materials desired in addition to the first and second one. A set of devices adapted to the performance of any of the steps described herein, such as enabling the formation of regions of a specified material on the backing surface, can be referred to as a sub-system. Hence, if a third material is to be applied similarly to a second material, the apparatus may further comprise a sub-system identical to the one selected for the application of the second material. However, they need not be identical, as some devices of such two sub-systems can be redundant and omitted in one of the two sub-systems. Alternatively, the additional material can be applied by any other equipment, means or devices, suitable for said material and / or its intended purpose in the layer or stack thereof, the method being accordingly adapted to further comprise appropriate steps. Regardless of the number of materials that may be used to generate any intermediate layer, the form in which they are applied (e.g., as particles or a jettable liquid) and of the way by which each such material is applied or transferred to form such a layer, the present method is believed to achieve such additive manufacturing in a relatively fast manner. The efficiency of such a process, its productivity over time, can be provided in volume of materials than can be formed by unit of time, for instance in cubic centimeter per hour (cm3 / h) or liter per hour (1 / h) under the operating conditions (e.g., speed of the transport mechanism) selected for the 3D printing system of relevance. In some cases, the present printing methods and / or apparatuses enable a 3D build speed of at least 5 cm3 / h, at least 10 cm3 / h, at least 25 cm3 / h, at least 50 cm3 / h, at least 100 cm3 / h, at least 200 cm3 / h, at least 300 cm3 / h, or at least 400 cm3 / h for manufacturing of superposed layers involving a liquid, such as can be required for the preparation of PCBs or similar electronic circuits. When the present printing methods and / or apparatuses rely on solid materials substantially devoid of a liquid carrier, the 3D build speed can be further increased and be of at least 0.5 1 / h, at least 1 1 / h, at least 2 1 / h, at least 4 1 / h, at least 6 1 / h, at least 8 1 / h, or at least 10 1 / h. Once the layers including regions of at least particles of first material being compacted have been superposed one on the other to form a 3D-object, or a part thereof, the stack may optionally be detached from the backing surface upon which the layers were prepared. The materials providing at least for the first particles (hence the first regions of each intermediate layer) can be functionalized (e.g., by consolidation, as in debinding processes, sintering or any like treatment adapted to render the object functional). A dispenser of particles as schematically illustrated by devices or modules 16, 20, 34, and 38 in the exemplary processes described above will now be considered in more detail in the following, together with specific devices with which the particles dispensed thereby may interact. Adhesive Application Regardless of the intended use of the particles and the corresponding substrate, it is possible when employing a particle dispenser according to the invention to use a relatively low amount of adhesive on the surface of the substrate. The layer required to retain the stack of particles typically have a thickness corresponding to about a fifth to a tenth of the average size of the particles. Considering the examplary 3D printing method previously described, such a relatively low initial amount of adhesive may not only reduces costs, but also the adverse effects associated with the presence of excessive binder on the quality of the end-product. While the elimination of adhesive is expected to be more difficult from relatively narrow voids, as compared to relatively larger ones, the compaction step nevertheless facilitates the debinding process. First, the small size of the voids by itself ensures that only a minute amount of adhesive material would require elimination between particles that would only at most deform to disc shape. If the model particles are made of a crushable conglomerate of particles, the crushing of the powder “open” larger passage for the elimination of the adhesive. If necessary, the debinding (elimination of the adhesive) can be done layer by layer, or after the stacking of a limited number of layers, rather than at the end of the process when passages out are more limited and occluded in part by repeated compactions. Adhesive can be selectively applied to desired regions of a layer by any device suitable for ink-jetting. Such devices are known to persons skilled in the printing industry relying on ink-jetting and need not be further detailed herein. Since in this case, the adhesive is applied relatively liquid (with a viscosity adapted to the jetting method and vice versa), it may be desired to dry the adhesive partially, by removing part of the liquid carrier. The drying may take place after the selective application of the adhesive liquid, before and / or after the application of particles, the drying being such that the adhesive remains sufficiently tacky to attach the particles, and sufficiently deprived of liquid for the compaction of the particles not to inadvertently displace them by slippage. Alternatively, an adhesive can be applied to entirely coat the surface of an underlying layer within the perimeter of the layer to be subsequently formed, regions of the adhesive being later selectively rendered tacky by suitable activation. Any apparatus capable of applying thin film of liquid can be used, such as the adhesive applicator schematically depicted as 1000 in Fig. 12. Such devices and similar ones are known need not be detailed herein. The adhesive applicator can be preceded by a module adapted, selectively or otherwise, to treat, chemically or physically, regions of the substrate upon which the adhesive is to be deposited as an adhesive image. If present, this module can be termed an adhesive priming module. If the priming treatment involves a chemical substance to be applied selectively to desired regions of a layer, the adhesive priming module can include any device suitable for inkjetting. If the adhesive priming substance is to be applied on the entire surface of a preceding layer, any suitable apparatus capable of applying a chemical substance can be used such as a module similar in principle to the exemplified adhesive applicator 1000, can be used. If the priming treatment is physical, a device capable of discharging corona plasma can be disposed upstream of any suitable adhesive applicator. The pretreatment of the surface can alternatively seek to decrease the presence of the adhesive in undesired areas surrounding the intended image (which can be referred to as the negative image). In such a case, the module preceding the adhesive applicator can be referred to as an adhesive masking module. Particle Applicator Here, an active adhesive image (selectively formed or activated) is brought into contact with (e.g., model) particles dispensed by a dispenser capable of reliably providing an even layer of particles, without necessarily forming a monolayer, but forming a coating of densely packed and uniformly distributed particles. The dispenser may in principle be similar to a dispenser of particles as used in xerography, into which one may optionally introduce a rubbing effect to enhance the uniform attachment of the particles transferred to the glue. The rubbing effect can be achieved by controlling slipping between a transfer roller and the applicator, or between any two cylinders disposed upstream of the applicator, the applicator having itself substantially no slippage with the substrate. As moving parts are rarely ideally entirely devoid of any slippage, as used herein a slippage between any two surfaces is said to be essentially null when the relative speed between the two surfaces is 2% or less of the fastest, or 1% or less, or 0.5% or less. While in theory, only the particles coming in direct contact with the adhesive layer would transfer, prospectively forming a “single particle deep” layer of particles on the activated glue, in practice additional particles stick to the first layer due to cohesivity (e.g., as resulting from the presence of attractive van der Waals interactions between the particles, the cohesion typically increasing with decreasing particle size). The particles would however not transfer to “inactivated” regions of the adhesive or to regions lacking an adhesive. If parasitic transfer should nevertheless occur, the particles having inadvertently transferred to unintended areas of the inactivated adhesive layer / uncoated tray could be removed by gentle suction or blowing away, optionally assisted with mechanical means of dislodgment of loosely attached particles (e.g., with a rotating brush or sponge). A mechanism or station adapted to such a purpose in a printing system according to the present teachings can be referred to as a cleaning device, module or station (e.g., depicted as 730 in Fig. 13) to be later detailed. An exemplary device configured for the application of particles to tacky adhesive images is schematically partially illustrated in the side view of Fig. 9, an alternative perspective view emphasizing different details being shown in Fig. 10. The dispenser of particles 2000 is formed of a chamber 200 and a housing 260. Particles may be supplied to the chamber by way of cartridges 210 loaded into adapted cavities of the housing, the cartridges or the recesses into which they are lodged including mechanisms 212 (e.g., paddles) and 214 (e.g., flexible sweeps) for mixing the particles and for smearing the mixed particles onto the surfaces to be contacted thereby. In some cases, the smearing mechanisms 214 are sufficient to also achieve mixing of the particles, so that additional mechanisms 212 dedicated to mixing can be omitted. For instance, smearing of the particles can be performed by e.g., four sweeps regularly distanced around a pivot and made of very thin metal shims, consisting for instance of stainless steel having a thickness of about 0.065 mm. In operation of the applicator, the mixing and / or smearing mechanisms (which are typically secured to an axis disposed at the center of the cavity or cartridge) rotate constantly at a controllable desired speed. The particles may alternatively be supplied from a reservoir of particles in communication with the supply cavity(ies) via supply pipe(s), the particles being delivered by way of gravitation or any suitable mechanical device (e.g., a single ortwin screw(s) extruder). As the particles are supplied from cartridges or (e.g., tube-shaped) cavities in the housing, the openings facing a subsequent component of the dispenser are designed so as to permit sufficient exposure and contact with the particles, while preventing surplus presence in a downstream chamber. A component downstream of the particle supply and disposed at least in part in a chamber of the applicator can be referred to as a transfer member or transfer cylinder. To ensure sufficient exposure of the surface of a transfer cylinder to the particles being supplied, the length of the opening is typically corresponding to the length of the cylinder or outer surface to which the particles are to be applied for subsequent transfer. To facilitate contact, the width of the opening may permit rotation of at least a part of the transfer cylinder surface within the cavity or the cartridge. The opening may also be dimensioned in accordance with the dimensions of the radially extending smearing mechanisms 214 due to spread the mixed particles on the outer surface of the transfer cylinder during its partial rotation through the supply cavity / cartridge. Considering rotating flexible sweeps for illustration, while schematically represented as straight lines in Fig. 9, in practice such strips bend in contact with the walls of the particle supply cavities or cartridges, the openings’ dimensions being such that the strips retain sufficient curvature upon contacting with a transfer surface circulating through the opening to effect smearing of the particles. Typically, the openings in the particle supply cavities or cartridges have an area corresponding to at least 15% of the circumferential area of the surface of the downstream transfer cylinder, the area of opening generally being between 17.5% and 22.5% of the area of the downstream cylinder. While the dimensions of the opening typically remain constant in operation of the dispenser, in an alternative architecture the effective dimensions of the opening may be controlled (e.g., using a sliding lid altering the width of the opening), the communication between the supply of particles and a subsequent transfer cylinder being even closed when desired. The particles (e.g., from each cartridge) are smeared upon a respective sleeve 222 which can be made of a sponge-like material (e.g., consisting of a closed cell foam) mounted on a rigid axial support 220, the combination being referred to herein as a roller, or cylinder 220. Alternatively, the outer surfaces of the cylinders 220 may be made of a silicone elastomer. The particles are then transferred from the primary transfer cylinders 220 to a common secondary transfer cylinder 230 which can have an outer surface made of a release material, such as can be provided by a silicone sleeve 232 mounted on the cylinder 230. In contrast with the parts of the applicator of particles 2000 previously described, which were all entirely disposed within the boundaries of housing 260, the transfer cylinder 230, which is able to releasably transfer particles out of the housing, is only partly disposed within the chamber 200. Blades 240a and 240b are disposed at the rim of the aperture in the housing through which cylinder 230 can cyclically rotate. These blades may not only ensure the formation of a uniform layer of particles on the outer surface of sleeve 232, but also prevent egress of particles from chamber 200. The blades can each be controllably disposed to form any desired gap with the outer surface of sleeve 232 on cylinder 230. They can be made of a same or different material (e.g., a metal or a plastic polymer), have a same or different flexibility (a contacting levelling blade being typically more flexible than a gap-forming sealing one), and / or have a same or different shape, such blades being known to the skilled persons. Typically, at least one of the blades (e.g., 240a) has a tip forming an acute angle, a slope of which being in contact with the outermost layer of particles on the coat formed on transfer cylinder 230 so as to level the coat of particles upon exit from the housing. A blade (e.g., 240b) only serving to reduce egress of free particles from the housing need not contact the coat, being disposed with a small spacing therefrom. Such a sealing blade may have a round tip. In the event that blades 240a and 240b fulfil slightly different functions as exemplified above, the one predominantly serving for levelling and the one predominantly serving for sealing would be positioned on the housing according to the direction of rotation of cylinder 230. In order to be sufficiently flexible so as to permit efficient levelling, the levelling blade can be made of a relatively thin metal shim, such as from stainless steel having a thickness between 0.1 and 0.5 mm. A blade intended to partially seal the housing of the applicator need not have a same degree of flexibility and can be made from materials relatively more rigid and / or with a relatively larger thickness reducing the bendability of the blade. For illustration, blades intended only for sealing can be made of polycarbonate having a thickness between 1 and 2 mm, e.g., being of 1.5 mm. In the perspective view of Fig. 10, a few cogwheels 270 secured to the shafts of respective (e.g., transfer) cylinders disposed in the housing 260 of the applicator 2000 are visible. They may engage additional toothed wheels hidden in the present figure by the gear 272 connected by a transmission belt 274 to a motor 276. Depending on the respective sizes of the cogwheels associated with each rotating cylinder and / or optionally interconnecting them, the speed and / or direction of the motion of the mixing and smearing mechanisms of the supply cavity(ies) and of each transfer cylinder, as transmitted by the motor, can be controlled. As better seen in Fig. 12 to be later detailed, transfer cylinder 230 can then transfer the particles to a similarly constructed applicator 250 also having on its outer surface a release material, such as can be provided by a silicone sleeve 252 mounted on the cylinder 250. The cylinder 230 and the applicator 250 need not be the same, neither in dimensions nor in the properties of their outer surfaces. For illustration, the hardness of the silicone sleeve 252 may be lower than the hardness of the silicone sleeve 232, to facilitate transfer of particles from the last transfer cylinder partially circulating within the dispenser housing to the applicator cylinder (entirely found externally to the dispenser housing). The applicator cylinder can then be urged, for instance using a pneumatic or hydraulic piston 254, into contact with a surface including tacky regions of adhesive to be coated by the particles. The outer surface 252 of the applicator cylinder 250 actually urged into rolling contact with the surface of the substrate for particles to transfer to the adhesive coated areas of the substrate can also be referred to as a continuous blanket. The continuous blanket of the applicator can, instead of a sleeve mountable on a cylinder, form a belt (preferably seam-less) circulating over two or more rollers including a driving one. Hence, the applicator can be a rotating cylinder or a circulating belt, a drive mechanism for urging a surface of the applicator into rolling contact with the surface of the substrate being accordingly selected. For illustration, while the drive mechanism may entirely displace cylinder 250 or one of the rollers supporting a belt, it may alternatively move an impression cylinder located within the perimeter of the circulating belt, on a rear side thereof. The continuous blanket (and any other outer surface made of an elastomer facilitating release) can be made of any suitable materials and prepared by any appropriate method. By way of example, a flexible blanket or sleeve can comprise in their release surface elastomers formulated as disclosed inter alia in WO 2018 / 100541 and cast as detailed in WO 2017 / 208155 to the same Applicant. By way of non-limiting example, the elastomeric sleeves can be prepared by centrifugal casting elastomers including or consisting of silicone-based polymers in a drum having dimensions (e.g., diameter, length) adapted to the sought sleeves or belt, said materials being suitable to release particles transiently transferring therefrom. Suitable silicone polymers can be selected from liquid silicone resins (LSR), room temperature vulcanization (RTV) silicones, vinyl methyl silicone (VMQ), phenyl silicone rubber (PMQ, PVMQ), fluorosilicone rubber (FMQ, FMVQ), and polydialkyl siloxanes (PDAS) capable, alone or in combination, of providing once cured a mechanically resilient silicone matrix. The silicone polymers can be addition-curable or condensation-curable, depending on their particular chemistry, and can be formulated to include any desirable and suitable additive (e.g., cross-linkers, catalysts, curing inhibitors, etc.}. The silicone polymers can also be mixed with elastomers of different chemical families (e.g., polyurethanes) to further tailor the properties of the sleeves as desired. Persons knowledgeable in the art of elastomeric blankets can readily adapt the exact composition of the fluid polymer to be centrifugally casted to the desired properties (e.g., hardness) of the sleeve, the mixing, defoaming, spinning and curing (e.g., temperature) conditions being also derivable from the properties sought of the cured product. For instance, the continuous blanket and the sleeves of the transfer rollers can be prepared from a two-parts LSR, the relative proportion of each part in the formulation being casted into a continuous, seam-less, sleeve setting inter alia the final hardness of the sleeve. Taking for illustration, two-compartments LSR commercialized by KCC Silicones as SL9508A for Part A and SL9508B for Part B, a weight-to-weight ratio of Part A to Part B of 10:1 yields a silicone SL9508AB having a Shore A hardness of 50 when cast and cured as detailed herein-below. Increasing the relative amount of Part A to Part B decreases the hardness of the resulting cured silicone polymer. In this particular example, Part A:Part B ratios of 20:1, 30:1, 35:1, 40:1 and 50:1, respectively result in silicone elastomers having a hardness of 40, 30, 24, 15 and 9 Shore A, any desirable other values being obtained by suitably modifying the ratios between the two parts. The hardness of the cured elastomer can be determined by routine analysis, such as described in ASTM D 2240. While the property monitored for the elastomer can be a physical property (e.g., its hardness as determined on a suitably prepared sample), the different proportions of components of two-parts LSR may also result in changes in chemical properties of the elastomer. In the present illustration, SL9508A contributes vinyl groups while SL9508B additionally contributes hydride groups, the final amounts of which can be modified according to the respective proportion of each part in the final elastomer. Following the aforesaid principles, a continuous blanket of an applicator cylinder or sleeves of upstream disposed transfer cylinders can be prepared, for example, by mixing SL9508A with SL9508B at any ratio of parts providing a desirable hardness, the mixed LSR being first cast-centrifuged for 10 to 15 minutes at 1200 to 1400 revolutions per minute (rpm) at room temperature so as to build an even tubular structure on the inner walls of a smooth casting drum (e.g., made of Aluminum 6061 and having an average roughness Ra between 0.2 pm and 1.6 pm). The tube-like liquid elastomer is then cured for 1 to 2 hours at 80°C whilst the drum is rotating at 500 to 1000 rpm (depending on the ratio elected for the two parts). The sleeve is then (actively or passively) cooled to room temperature under the same ongoing spinning conditions. The sleeve sufficiently cured and cold can be then gently detached from the surface of the drum inner walls and transferred for final curing to an oven set at 170°C for 30 minutes. The fully cured sleeve (e.g., 232, 252), typically having a cured thickness of about 2 to 3 mm, can then be mounted on a cylinder (e.g., 230, 250). When the continuous blanket is urged into contact with the substrate at least selectively coated with an active adhesive so as to transfer particles thereto (e.g., by moving downward in Fig. 12), the dispenser 2000 can be disengaged from the applicator cylinder 250. This can be achieved by any mechanism (schematically represented by double arrows 256) able to displace the housing 260 away (or towards) the continuous blanket, so that a gap is formed (or closed) between the outer surface of the last transfer cylinder (e.g., 230) and the blanket on the applicator. Once the applicator disengages from the substrate, the dispenser may reengage with the surface of the continuous blanket so as to replenish the coat of particles in regions partially depleted by the preceding transfer. To facilitate the reformation of a homogenous coat of particles on the continuous blanket, it can be advantageous to set the last transfer cylinder coming into contact with the outer surface of the applicator to be in relative slippage therewith, e.g, by having a different linear speed, to favorize a rubbing effect on the particles. As used herein, the term “rubbing” with respect to the particles is to be understood broadly to encompass any action on the particles that may further homogenize at least one property of the particle coating (whether on the transfer surface or the adhesive regions), be it the thickness of the layer, the orientation of the particles, their distribution on the surface, their size or any other like characteristic of the particles forming the coat. Typically, a rubbing effect can be achieved when two surfaces are in motion one relative to the other (e.g., the linear speed of the relatively faster surface being at least 10% higher than the linear speed of the relatively slower surface, or at least 15% higher, or at least 20% higher), the resulting friction causing the arrangement of the particles into a relatively even coat of stacked particles. In one prototype of particle dispenser as shown in Fig. 9, the transfer and applicator rollers have been set to have a maximal relative speed of 200%, the two surfaces in rolling contact being optionally counter-rotating. A rubbing effect was observed when the speed of the faster rotating surface was within a range of 110% to 200% of the slower rotating surface, but it may suffice that the speed of the faster surface be within the range of 110% to 150%, or 110 to 125% of the slower one. While in the particle applicator exemplified in these figures, two cavities are depicted to provide a supply of particles (e.g., optionally by way of cartridges insertable in the relevant cavities), this should not be construed as limiting. The applicator may alternatively include only one supply of particles or more than two. When two or more supply of particles are included, the particles supplied thereby can be the same (e.g., to accelerate or increase coverage) or different (e.g., in size and / or composition of matter). Considering the path followed by particles supplied by a single source, the present illustrations depict a series of two transfer cylinders (220 and 230) when the applicator is used in connection with an external applicator cylinder 250 (see Fig. 12); or with a single transfer cylinder (220) when the last cylinder (230) partly circulating out of the housing may serve as applicator cylinder (see Fig. 13). This should not be construed as limiting and any other number of transfer cylinders may be used along the path between one supply of particles and the last cylinder due to act as applicator cylinder, as long as permissible in the housing. Typically, all cylinders arranged to transfer particles between the supply and the final applicator are selected to have outer surfaces facilitating such transfer one from the other. For illustration, an upstream primary transfer cylinder (e.g., 220) can have a sponge-like outer surface and a subsequent downstream secondary transfer (or applicator) cylinder (e.g., 230 or 250) can have a releasy outer surface, or an upstream transfer cylinder can have a releasy outer surface with a predetermined hardness (e.g., within a range of 20-50 Shore A) and a subsequent downstream transfer (or applicator) cylinder may have a releasy outer surface with a relatively lower hardness (e.g., being 10 Shore A smaller than a preceding roller). Conveniently the outer surfaces of any of the cylinders involved in the dispensing of the particles can be provided in the form of a sleeve to be mounted on a cylindrical support, so they can be replaced with ease in case of need. Alternatively, an entire cylinder can be replaced by a new one when desired. When two or more supply of particles are included, the paths followed by their respective particles need not be the same. For illustration, they may have a different number of transfer cylinders, which may additionally differ in individual properties, direction and speed of rotation etc. By way of example, when two or more transfer cylinders are used in parallel to transfer particles to a common cylinder, one of the upstream transfer cylinders can be configured to have the same linear speed as the common cylinder (i.e., transferring the particles with essentially no slippage to increase efficiency of transfer), whereas a second upstream transfer cylinder can be configured to have a different linear speed and / or direction of rotation, the slippage resulting with respect to the common cylinder providing a rubbing effect (e.g., to increase homogeneity of the coat of particles to be further transferred). The relative velocity and direction of rotation between any two cylinders can, for illustration, be set according to the dimensions of the toothed wheels associated with their respective axis of rotation and the presence of intermediate cogwheels therebetween. The degree of slippage between any two cylinders effecting transfer of particles from one to the other may inter alia affect the properties (e.g., thickness) of the coat of particles ultimately transferring, the cohesivity between the particles also contributing to the prospective end-result. Particles smaller than 100 pm are believed to display sufficient cohesion to form a stack, the smaller the particles the higher the expected cohesion. Without wishing to be bound by any particular theory, it is believed that the adhesive image may also control the thickness of the coat of particles that would transfer and adhere thereto. Depending on the thickness of the adhesive image and the viscosity of the glue, particles being inserted therein during transfer may displace the adhesive away (e.g., upwardly) from its supporting surface, the adhesive material so lifted by an initial set of particles being then able to detach a subsequent set of particles and so on, until there is no more adhesive (or active one) capable of transferring particles from the continuous blanket to the substrate. To ensure that no adhesive may adversely transfer from the substrate to the blanket, the number of overlying layers of particles on the blanket typically exceeds the number of layers that may transfer to the adhesive image being considered. Typically, the number of layers of particles forming a coat on any transfer cylinder or applicator before application, or on the adhesive image after a single transfer can be estimated by measuring the average thickness of the coat and comparing to the average thickness of the constituent particles, such dimensions being determined by any suitable standard methods (e.g., microscope analysis). While the coats of particles may rarely be only one particle thick, they typically are vastly at least two particles thick, and predominantly at least three particles thick. Importantly, for the sake of 3D printing, the particles should preferably be packed relatively densely in the coat of particles. As the particles are to be compacted (reducing gaps therebetween), a coverage by 100% of particles of any surface being considered (as can be assessed by top-view image analysis) may not be necessary (even if achievable). In some embodiments, the proportion of the (e.g., blanket) surface that is covered by the layers of particles forming the transferable or transferred coat may need only be 95% or less, or 90% or less. That being said, the present dispenser may suitably achieve a coverage of 75% or more, 80% or more, or 85% or more. In some cases, the coverage of any surface of transfer (donating or receiving the particles) can be comprised between 75% and 100%, 80% and 95 % or 85% and 95%. Fig. 11 shows how an applicator of particles according to the present teachings provides an improved coverage of particles. The figure shows top view captured by optical microscopy (Olympus OLS 4000 LEXT, at a magnification of x20) of particles applied on an adhesive image, the picture on the left side (Fig. 11 A) showing the coverage achieved by a conventional dispenser of particles, such as a toner cartridge including a sponge roller and an elastomeric one but deprived of rubbing effect, whereas the picture on the right side (Fig. 11B) shows the coverage achieved by an applicator as discussed with reference to Figs. 9 and 10, the present applicator being operated to include a rubbing effect and the transfer of the particles to the adhesive image being achieved by rolling contact with cylinder 230 serving as applicator. A same layer of adhesive (including a polyamide resin, such as Uni-Rez™ 2224 commercialized by Kraton, supplemented with 10 wt.% rosin gum, such as Foral™ AX-E commercialized by Eastman) having a thickness of about 4 pm was applied on a transparent foil of PET as substrate and pre-heated to 115°C to remain tacky whilst particles are applied thereto. Polyamide particles having an average diameter (Dvso) of about 40-50 pm and a predominant distribution of size between 10 and 100 pm (commercially available from BASF as Ultrasint® PA11) were supplied to each device being compared. The PET foil was transported below the dispensers at a distance enabling contact with the outermost roll of each device and the adhesive image now coated with the particles applied thereto was brushed with a rotating brush 190 disposed in a cleaning device 2020, as illustrated in Figs. 14 and 15 to be later detailed. Top views of the coats of particles were then captured. With the conventional dispenser, a coverage of less than 75% was achieved when the PET foil conveying the adhesive image was transported at a speed of 100 mm / s relative to the dispenser, see Fig. 11 A, the percentage of coverage being estimated by suitable image analysis based on a larger field of view. Increasing the speed of the adhesive image detracted from the coverage that could be achieved. At the same substrate speed of 100 mm / s, the present dispenser including a rubbing effect provided a particles coverage of about 100%. The rubbing effect was achieved by setting one of the transfer rollers to be at a relative linear speed at least 10% higher than that of the applicator surface, the linear speed of the applicator corresponding to the linear speed of the substrate. Speed of the substrate was step-wisely increased up to ten-fold and the coverage remained at about 100% at all tested speeds, as can be seen in Fig. 11B which shows the results obtained at 1 m / s. A similarly high coverage was observed when the cylinder serving as applicator was mounted with elastomeric sleeves having hardnesses of 50 Shore A, 40 Shore A, 30 Shore A, 24 Shore A, and 15 Shore A (prepared from LSR as previously described). Thus, the inclusion of rubbing between the transfer rollers of the present applicator of particles not only enhanced the coverage of particles that may be achieved on the adhesive image, but also significantly increased the speed at which such improvement can be obtained. Understandingly, dispensers of particles conventionally used in 2D printing or copying industry do not need to achieve high coverage, human brains automatically compensating for lower ink coverage (even not exceeding 50%) so that we may perceive a full image regardless of true density of ink spots on the printing substrate and the relative amount of interstices. Similar experiments were performed with particles made of different materials and / or having a different particle size distribution, and the particle applicator of the invention (as discussed with reference to Figs. 9 and 10) was found similarly effective. For instance, spherical metallic particles made of copper coated with silver (at 10 vol.%) having an average diameter (Dvso) of about 10 pm and a predominant distribution of size between 4 and 22 pm (commercially available from Fukuda as Cu - HWQ - 10 pm) were supplied to the present applicator and tested for transfer to a similar adhesive layer applied on a PET support, the adhesive now having a thickness of about 2 pm. The extent of transfer (e.g., coverage of particles) was tested as previously detailed at various speeds from sleeves 232 having hardnesses of 50 Shore A, 40 Shore A, 30 Shore A, 24 Shore A, and 15 Shore A. A particles coverage of about 100% was observed regardless of the hardness of the sleeve and at all speeds tested from 100 mm / s up to 400 mm / s. The same particles of plastics and metals were tested in similar experiments, the linear speed of the applicator surface (and of the PET substrate) being set at 500 mm / s and the rubbing being achieved by a relative speed of an upstream transfer roller corresponding to 10%, 17% or 200% of said speed. The outer surface 232 of the applicator 230 had a hardness of 40 Shore A. The adhesives layers tested were about 2 pm thick for the copper particles coated with silver and about 4 pm thick for the polyamide particles. The adhesive layers applied on the substrate were pre-heated to 80°C so as to be tacky upon rolling contact. The adhesives so tested included a polyimide binder, Uni-Rez™ 2215 or Uni-Rez™ 2224, a copolyester, Dynacoll® 7360, or a rosin gum combined with pentaerythritol tetrastearate (9:1), all binders including 10 wt.% of carbon black pigments (Printex® 35 by Orion) to alternatively enable their activation by EM radiation. All adhesives tested with the aforesaid particles as dispensed by the present dispenser enabled a coverage of at least 90%. Printing Systems Exemplary arrangements including the present applicators of particles into printing systems are depicted in Figs. 12 and 13. While these systems are intended for 3D printing, using them for a unique printing cycle can alternavely achieve 2D printing. Fig. 12 displays for clarity only a part of the devices as may be arranged in a printing system 1400 to form stack of layers as previously illustrated in Fig. 2, the transport mechanism of the substrate (hence, its backing surface) being a drum. In this figure the transport mechanism drum 12 rotates counterclockwise along a circular path, the first station being a non-selective adhesive applicator 1000, which comprises a dipping cylinder 110 rotating in a heated bath containing a melt adhesive, which transfers therefrom to a first rotatable cylinder 100 and therefrom as a thin layer of adhesive to the outer surface of drum 12. A metering device 130 is schematically illustrated by a metering blade configured to control the thickness of the adhesive layer forming on the outer surface of cylinder 100. A pneumatic piston 124 which may engage (or disengage) the cylinder 100 transferring the this film adhesive to the substrate transporting drum 12 is also depicted. The thin layer of adhesive transferred to a substrate on drum 12 can then be selectively activated by device 500. Device 500 can be a laser emitting device such as detailed in the Applicant’s earlier patent applications WO 2016 / 189510, WO 2016 / 189511 and WO 2019 / 030694, capable of selectively heating and rendering tacky regions of a thermally activatable adhesive. The layer of adhesive including its selectively activated regions is then brought into contact with particles applied thereto by a particle applicator 250 being coated by dispenser 2000, following which the particles having adhered to the activated adhesive may be compacted by a compacting device 3000. The compacting device 3000 is schematically illustrated as comprising a compacting roller 310, a heater 312 and a pneumatic piston 300 for urging, when desired, the compacting roller into contct with the applied particles. For sake of clarity, this figure lacks stations 36, 38 and 40 as shown in Fig. 2. Double arrows positioned next to pneumatic pistons 124, 254 and 300, indicate that the outermost cylinder of the devices including these pistons may be brought into contact with, or away from, the transporting drum and the layers being built thereon, the same controlled engagement / disengagement of the dispenser 2000 being illustrated by double arrows 256. In Fig. 12, adhesive applicating cylinder 100 of device 1000 and compacting roller 310 of compacting device 3000 are shown engaged with drum 12, whereas particle applicator cylinder 250 of particle dispensing device 2000 is shown disengaged from the transport drum 12. In Fig. 13, in contrast to Fig. 12, the transport mechanism of a printing system 1500 is planar. In this figure, in which the direction of printing is from left to right, a platform 600 that can be controlled to move back and forth on guiding rails 610 (and forming therewith a transport mechanism 630) is illustrated with a substrate 620 thereon. The substrate is passed by the transport mechanism 630 through a first station 700 at which an adhesive can be selectively applied to desired regions of the substrate. The station 700 may comprise an inkjet head capable of moving in any direction parallel to the plan of the substrate (e.g., typically in a scanning direction perpendicular to the printing direction), but may alternatively comprise a static print bar constituted from a plurality of inkjet heads, the nozzles (or arrays of nozzles) of the inkjet heads spanning the width of the substrate in a direction traverse to the printing direction. Inkjet heads are known and need not be detailed herein. They can be purchased from several reputable manufacturers, amongst them Eastman, Epson, Fujifilm Dimatix, Konica Minolta, Kyocera, Ricoh, Toshiba, Seiko and Xaar, each having models adapted to any desired viscosity and jetting method (including the controllable varying of droplet volume). Depending on the inkjet head, the adhesive liquid to be jetted thereby and the particles to be adhered thereto, lines may be formed to have a width of 50 pm or more, the minimal spacing between adjacent lines being typically of at least 50 pm. Following the formation of the desired image of adhesive, the substrate is transported to a second station at which particles of a first material can be applied to the adhesive. The device capable of transferring the particles to the adhesive can be the particle applicator 2000 previously detailed. In the present illustration, cylinder 230 which acted as a last transfer cylinder preceding the applicator cylinder 250 in Fig. 12 can now directly serve to transfer the particles to the adhesive and de facto serve as applicator cylinder, its outer surface 232 serving as the continuous blanket. The mechanism 256 which previously served to disengage the dispenser from an external applicator cylinder 250 whilst it was urged into contact with the substrate, may now serve to urge the applicator cylinder 230 into contact with the substrate, so that layers of particles may transfer from blanket surface 232 to the adhesive image formed at the first station. The substrate partially coated with the adhesive and the particles thereon can proceed to a third station 710 at which excess liquid from the adhesive can be removed. Device 710 can be a drier, heating the substrate by convection or radiation (e.g., IR). While a single device 710 is illustrated in this figure downstream of applicator 2000, a second one may be positioned upstream of the applicator of particles if the adhesive selectively applied by the inkjet head(s) 700 is too liquid / insufficiently tacky to retain the particles upon passing beneath the applicator 2000. Following its drying, by one or more driers 710, the substrate and the particles adhered thereto arrive at the compacting nip formed by urging the compacting device 3000 against the platform 600. The layer including the compacted stack of particles can then be sintered by any suitable sintering device 720, the sintering being performed either for each cycle, for every few cycles, or upon completion of all cycles of additive manufacturing. In the latter case, the sintering device 720 need not be included in the printing system 1500 and can be an off-line device. If needed, following compaction and optional sintering, the substrate can be cleaned by a cleaning device 730, configured to remove particles having parasitically transferred to and / or sintered on unintended regions of the substrate. An additional, or alternative, cleaning device 730 (not shown in the figure) can be disposed upstream of the compacting device 3000 and downstream of the optional dryer 710. The cleaning device 730 can comprise a rotating brush and suctions means to remove the surplus material. In the apparatus exemplified in this figure, the second material is applied in liquid form by a suitable applicator 740 which may further include a squeegee 742 to ensure that the curable liquid uniformly coats the surface of the substrate. While in the present illustration, the squeegee 742 would wipe the liquid in direction opposite the printing direction as the substrate proceeds to a following station, alternatively it (or an additional squeegee) can be movable and configured to wipe the liquid in a direction transverse to the printing direction. The substrate then proceeds to a curing station 750, the exact curing device being selected in accordance with the liquid material to be cured (e.g., the device including a UV source when the liquid is UV-curable). Finally, the upper surface of the substrate can be milled, ground or abraded by a levelling device 760, following which the levelled surface can be inspected by an inspecting device 770 before being sent back to the first station for a new cycle and a new layer. While not apparent in the figure, an encoder is provided in the transport mechanism, allowing the substrate to be correctly positioned with respect to the inkjet head(s) for the adhesive of a subsequent cycle / layer to be properly registered with respect to the adhesive selectively applied in a previous cycle / layer. Alternatively, or additionally, proper registration between cycles (or between stations, if selective) can be achieved by optical sensors suitably disposed in the apparatus to monitor signals (e.g., fiducial marks) indicative of positioning of the substrate with respect to any device due to interact therewith. As new layers are formed, the distance between the outermost layer being formed and the stations and devices interacting therewith is modified. A desired gap (or lack thereof) can be maintained by moving the substrate 620 on the platform 600 up or down in a Z-direction, to a Z-level as desired, alternatively, or additionally, the devices (e.g., 2000, 3000, 730, 740-742, and 760) can be displaced up or down to achieve the desired distance. For instance, while the compacting device 3000 and the levelling device 760 are positioned to contact the uppermost surface of the substrate or layers formed thereon when the transport mechanism is moving in the printing direction, these same devices could be lifted to avoid contact in the reverse direction when the substrate is brought back to home position in order to start a new cycle. Particles Particle dispenser 2000 can apply any type of particles which can be made of a material identical to the one sought in the end-product, the material and the dimensions of the particles being selected according to the functional and / or decorative effect being sought. Alternatively, the particles can be made-up of the intended constituents of the final material (providing a master alloy, mixed or treated powders to reduce yield strength and increase plasticity), some being intentionally selected to be relatively softer, the relatively harder cores being then “protected” by the softer constituents and the resulting layers of particles being compressible at relatively lower pressures preventing detachment. Alternatively, the particles can be made-up of aggregates of micro and nanoparticles of the intended material, a pressure which can be relatively moderate crushing the aggregates into their sub-particulate constituents. Interestingly, such “composite particles” are not only advantageous during printing of the model and the compaction of the layers, but also for some future end-products. For instance, the “composite particles” could undergo sintering under conditions relatively milder than “conventional particles”. Alternatively, or additionally, the “composite particles” may provide a superior effect as compared to conventional ones, if similarly post treated. For illustration, composite particles could be sintered at a lower temperature to provide a same conductivity as regular particles, but could alternatively provide a superior conductivity if sintered under the same conditions. For simplicity, model or support powders that are predominantly constituted of elementary particles of materials that can be deformed during compaction according to the present teachings can be referred to as deformable powders or particles. Deformable particles can be made of a single material (e.g., a metal, a homo-polymer) or of a mix of materials (e.g., an alloy, a co-polymer). Model or support powders that predominantly include non-deformable elementary particles assembled (e.g., with a binder) in clusters that can be dismantled during compaction can be referred to as crushable powders or particles. They can be made of a single material with a narrow particle size distribution (PSD) (e.g., particles of ceramic having relatively homogeneous dimensions assembled with a binder) or of a mix of distinct PSDs and / or materials (e.g., microparticles of a first metal and nanoparticles of a second metal). The binder for such particles can be selected from a group consisting substances that may be removed with relative ease when desired while being sufficiently resilient when necessary for the present process. Suitable binders can be polymers, such a polyvinyl pyrrolidones (PVP), polyvinyl alcohols (PVA), polyvinyl butyrals (PVB), polyamides (PA), polyacryl amide (PAM), poly(N-isoprolylacrylamide)s (PNIPAM), poly(acrylic acid)s (PAA), polymethacrylates (PMMA), polypropylenes (PP), polyethylenes (PE), poly(ethylene oxide)s (PEO), polyethylene terephthalates (PET), polyethylene glycols (PEG), sulfonated polyesters and poly(2-ethyl-2-oxazoline) (PEOX). They can also be selected from sugars, oligosaccharides and polysaccharides, such as glucose, maltose, cellulose, hydroxyethyl cellulose, cellulose acetate, amylose, chitosan, carnauba wax, agar agar, acrawax, and starch. A binder can further be combinations of the foregoing materials. As can be appreciated by skilled persons, some of the aforesaid binders may also be used as adhesives, thermoplastic materials being particularly suitable foe heat activation. The powders’ grains, which can be made of one or more elementary particles, may have a longest dimension in a range of 1 pm to 100 pm. As the grains are often relatively globular, their longest dimension can typically be determined as their average diameters. In some embodiments, the average longest dimension or diameter of grains suitable for additive manufacturing by the present methods is 75 pm or less, 50 pm or less, 40 pm or less, 30 pm or less, 20 pm or less, or 10 pm or less. In some embodiments, the average longest dimension or diameter of the grains is 2.5 pm or more, 5 pm or more, 7.5 pm or more, or 9 pm or more. In some embodiments, the average longest dimension or diameter of the grains is between 2.5 pm and 75 pm, between 5 pm and 50 pm, between 7.5 pm and 40 pm, or between 9 pm and 30 pm. The elementary particles constituting the powders’ grain may have their longest dimension in a range of 5 nm to 10 pm, particles having a longest dimension not exceeding 1 pm, being typically smaller than 500 nm can be referred to as nanoparticles, larger particles being conversely termed microparticles. As the individual particles are often relatively globular, their longest dimension can typically be determined as their average diameters. In some embodiments, the average longest dimension or diameter of particles that may be used individually or as part of larger grains in the present methods is 5 pm or less, 4 pm or less, 3 pm or less, 2 pm or less, 1 pm or less, 500 nm or less, 250 nm or less, or 100 nm or less. In some embodiments, the average longest dimension or diameter of the particles is 10 nm or more, 50 nm or more, or 75 nm or more, if the particles are nanoparticles not exceeding 1 pm; or 1 pm or more, 2.5 pm or more, or 5 pm or more, if the particles are microparticles. In some embodiments, the particles can be nanoparticles having an average longest dimension or diameter between 5 nm and 1000 nm, between 10 nm and 500 nm, between 50 nm and 250 nm, or between 10 nm and 100 nm. The particles can also be microparticles having an average longest dimension or diameter between 1 pm and 10 pm, between 1 pm and 5 pm, between 1 pm and 2.5 pm, provided in any event that the size of the individual particle does not exceed the desired size of the grain. The dimensions and average size of particles (e.g., nanoparticles, microparticles or powder grains) can be assessed by any known technique, such as microscopy (e.g., SEM, TEM, FIB, laser confocal microscope) and analysis of images captured at an appropriate magnification. Dynamic Light scattering (DLS) or Light Scattering (LS) can be used for sampling larger populations, providing values statistically more significant. DLS is more suited for relatively smaller particles (e.g., of up to 6 pm) and LS for relatively larger particles (e.g., of up to 3.5 mm). In DLS or LS techniques, the particles are approximated to spheres of equivalent behavior, this method being therefore particularly suitable for globular particles, and the size can be provided in terms of hydrodynamic diameter. DLS or LS also allows assessing the size distribution of a population of particles, the results being typically provided in terms of the hydrodynamic diameter for a given percentage (e.g., 10%, 50% and 90%) of the cumulative particle size distribution, either in terms of numbers of particles or volumes. The size of the grains or particles can be assessed by this method on a sample suspended in a suitable liquid (e.g., water optionally supplemented with a dispersant), in which case their average diameter can be estimated by the median hydrodynamic diameter D50 (maximum hydrodynamic diameter below which 50% of the sample volume (Dv50) or the number (Dn50) of particles exists). D10 and D90 (by volume or by number) provide the range within which a predominant portion of the population of particles exists and may serve to estimate if a population of particles is, or not, of relatively uniform size. In some embodiments, the values as measured by DLS or LS refer to Dv values. In some embodiments, the size of the particles is relatively uniform (or their size distribution is relatively narrow and / or relatively symmetrical with respect to a median value of the population). Such relative size uniformity is believed to increase the reproducibility of the printing method. Size uniformity is however not essential as some variations may assist in achieving a better packing of the particles once compacted on the substrate, smaller ones being able to fill voids in between larger ones, hence resulting in a denser coverage of any particular layer by the materials applied in the respective regions. If the particles are to be functionalized (e.g., sintered for the operability of a 3D object formed therewith, their improved packing (as may result from the deformability of the grains or their crushability into densely packed particles of same or different PSDs and / or materials) may facilitate such functionalization, and in turn optionally improve a property of the resulting object (e.g., a conductivity of a PCB). A particle size distribution is said to be relatively narrow if at least one of the following conditions applies: A) If the particles are nanoparticles, the difference between the hydrodynamic diameter of 90% of the nanoparticles and the hydrodynamic diameter of 10% of the same is equal to or less than 250 nm, equal to or less than 200 nm, equal to or less than 150 nm, or equal to or less than 100 nm, which can be mathematically expressed by: (D90 - D10) <250 nm and so on; B) If the particles are microparticles or grains, the difference between the hydrodynamic diameter of 90% of the microparticles or grains and the hydrodynamic diameter of 10% of the same is equal to or less than 2.5 pm, equal to or less than 2.0 pm, equal to or less than 1.5 pm, or equal to or less than 1.0 pm, which can be mathematically expressed by: (D90 - D10) <2.50 pm and so on; C) The ratio between a) the difference between the hydrodynamic diameter of 90% of the nanoparticles, microparticles or grains and the hydrodynamic diameter of 10% of the same; and b) the hydrodynamic diameter of 50% of the same, is no more than 2.5, no more than 2.0, no more than 1.5, or even no more than 1.0, which can be mathematically expressed by: (D90 - D10) / D50 <2.5 and so on; and D) The poly dispersity index of the nanoparticles, microparticles or grains is equal to or less than 0.5, equal to or less than 0.4, equal to or less than 0.3, or equal to or less than 0.2, which can be mathematically expressed by: PDI = o2 / d2 <0.5 and so on, wherein a is the standard deviation of the particles distribution and d is the mean size of the particles, the PDI optionally being equal to 0.01 or more, 0.05 or more, or 0.1 or more. The PDI information is generally readily obtained from the instrument used to measure the hydrodynamic diameter of the particles being considered. The 3D printing processes wherein the particles were applied by a dispenser of the invention have been tested for model particles made of metals (aluminium, copper, silver- coated copper, ferrous metals, stainless steel, titanium alloys), plastics (polyamide, polyethylene terephthalate (PET), acrylonitrile butadiene styrene (ABS)) and ceramics (alumina, silicon carbide, tungsten carbide zirconia, titanium hydride and ferrum carbonate). Fig. 14 shows photos captured by scanning electron microscopy (SEM) of particles according to embodiments of the present teachings and of cross-sections through compacted layers formed therefrom, taken using a Focused Ion Beam (FIB) setting of the microscope. Fig. 14A is a photo of deformable particles made of the alloy AlSiioMg, the particles initially having a globular shape with an average diameter Dv50 of about 40 pm, while Fig. 14D is a cross-sectional photo of layers, each formed by compaction at a pressure of about 300 MPa, of the same particles using an apparatus as schematically illustrated in Fig. 12, the drum 12 rotatably transporting the substrate and the layers sequentially built on having a linear speed of 1 m / s. The grains previously globular now appear as flattened discs, which look like a brick wall in the section. Fig. 14B is a photo of crushable grains made of particles of silica having a relatively uniform PSD with a Dv50 of about 1 pm and a PDI of 0.2, the particles being bound into grains by polyvinylpyrrolidone (PVP) at 2 wt.%. The grains have on average a longest dimension of about 50 pm. Fig. 14E shows layers compacted from the same particles at a pressure of about 100 MPa under otherwise similar operating conditions of the apparatus, the crushability of the grains into their elementary particles blurring the borders between different layers. Fig. 14C is a photo of crushable grains made of two types of particles both in materials and PSDs. First the grains are constituted of micro-particles of copper coated with a thin envelop of silver, these particles having an average diameter of about 1 pm, and secondly of nanoparticles of silver having an average diameter of about 50 nm, the two types of particles being bound to form crushable grains of about 8 pm. Fig. 14F shows sintered layers following compaction of the same particles at a pressure of about 100 MPa under otherwise similar operating conditions of the apparatus, the crushability of the grains into elementary particles of different size providing for a mosaic of larger particles, with nanoparticles intercalated in the voids. The conductivity of a line prepared by this method, following a sintering at about 220°C for 5 minutes, was measured and found to amount to 50% of the ideal conductivity of pure copper as established under similar conditions. While the materials adapted for the present 3D printing method have been described mainly as particular solid matters or curable liquids, this should not be construed as limiting. For instance, some particles (typically nanoparticles of metals) may be replaced by salts (e.g.. copper nitrate hydroxide or copper sulfate, for copper) or by precursors (e.g., Metal-Organic-Decomposition (MOD)). MOD materials can precipitate as metal nanoparticles upon heating and may then serve as “binders” to larger particles, enhancing the mechanical strength of the region including the MOD and / or improving its function (e.g., electric conductivity following sintering). Thus, while for brevity it might be referred herein to materials by specific chemical names, salts or precursors of the same are encompassed, when relevant. Additional details on crushable particles as may be used in the present method or system for 3D printing, or in any other manufacturing process requiring such particles, are provided in the following clauses. Adhesive Compositions The initial requirements common to any composition suitable for the formation of a thin adhesive layer to be thermally activated (e.g., selectively for the foreground image or non-selectively for the background image) are: a) it can be melted to a suitable viscosity for application as a thin layer (e.g., having a dynamic viscosity of no more than 1 Pascal-second (Pa.S) at 150°C and a shear stress of 1000 sec'1); b) it should have an open-time sufficiently short (e.g., <1 sec, 0.5 sec or 0.2 sec) to readily solidify as a thin layer of even thickness on an application roller made of silicon (e.g., 102) and / or to prevent parasitic attachment of particles in inactivated regions of the adhesive; c) it is sufficiently cohesive to transfer as a uniform layer from the application roller to the support or compacted layers previously applied thereon; d) it should have an open-time sufficiently long (e.g., up to 0.5 sec, 0.25 sec or 0.1 sec) to remain tacky between the time the layer is activated and the time it is contacted by the particles to be attached thereon. Additional requirements may depend on the intended use of the 2D or 3D printed object, as residual amounts of adhesive may remain in the end-product, in which case the adhesive compositions may additionally be required to be compatible with such use. Fundamentally, a material or a chemical composition is compatible with another composition or with a method if it does not prevent its activity or performance, or if it does not reduce it to an extent that would significantly affect the intended purpose. Such compatibility may be from a chemical standpoint, for instance, materials having respective moieties or active groups that may desirably interact with one another, for illustration, adhesive materials being relatively acid on their own (e.g., rosin gums), by way of suitable additives (e.g., organic acids), or both, can beneficially reduce the amount of metal oxide that may surround metal particles, such decrease in the presence of an oxide relatively to the unoxidized metal typically increasing the conductivity once the particles are sintered. From a physical standpoint the compatibility of materials may relate, for instance, to the adhesive composition having a softening temperature and / or a debindability (ease of elimination) adapted to the printing process or operating conditions of the printing apparatus, or two materials intended to form contiguous regions having a similar coefficient of thermal expansion. From the standpoint of an intended use in a present process, an adhesive composition would be compatible if, for instance, being capable of absorbing the radiation of laser emitting elements intended for its activation or not impairing the consolidation of the layers or their functionalization, such as permitting proper sintering, conductivity, RF or dielectric properties, as may be desired. Some adhesive materials may by themselves be capable of fulfilling a particular set of desirable requirements, but in some embodiments (or for adhesives lacking a particular capacity) the adhesive compositions may be supplemented with suitable additives to enable or enhance a desired property or compatibility as above exemplified. For instance, an additive can facilitate the activation of activatable adhesives by the elected method of activation, can facilitate the consolidation (e.g., sintering) of the stacked layers by the method elected for this purpose or can adapt the property of a region made therewith to a sought value. For illustration, a radiation absorbing agent can be added to facilitate radiation triggered activation, a sintering additive can be added to facilitate sintering or a filler can be added to bring CTE of the stacked layers to a desired level. Adhesive compositions, in particular activatable ones, may comprise at least one of a natural or a synthetic polymer (e.g., a polyacrylate, a polyalkene (also known as polyolefin, such as a polyethylene, a polypropylene), a polyamide, a polycarbonate, a polyester, a polyketone, a thermoplastic polyurethane (TPU) or a gum of natural origin, such as gum Arabic, gum rosin etc., or any like thermoplastic material), if desired a plasticizer (e.g., fatty compounds, phtalates, resins, waxes, etc.} added to ensure the composition would have a required tackiness once activated, and any other optional additive which may facilitate activation. For instance, if the selectively activating device (e.g., as illustrated by referral 500 in Fig. 12) is a laser emitting device where the laser emit in the near-infrared (NIR) light range then the adhesive composition can be supplemented with (absorbance types) carbon black, dyes, pigments, cesium tungsten oxide, lanthanum hexaboride, nanoparticles of metals having an average size not exceeding 100 nm, such as nanoparticles of stainless steel 316L, carbon coated nickel copper, copper and nickel, or like additives, which may enhance its absorption in the NIR range at which the lasers operate. This was experimentally confirmed by the Applicant who tested carbon black at 20 wt.% and 30 wt.%, cesium tungsten oxide at 34 wt.%, lanthanum hexaboride at 45 wt.%, and nanoparticles of stainless steel 316L at 50 wt.%, the absorption enhancers being incorporated in various adhesive compositions, all being found activatable at energy levels of 0.6 Joule per square centimeter. Similar principles may be followed (and materials used) for the preparation of liquid adhesive compositions which can be selectively applied where desired and for the preparation of binders adapted to maintain elementary particles as crushable powders’ grains. However, as the selectively applicable liquid and the binder need not be responsive to a specific trigger, as are the activatable adhesive compositions, additional materials may be used for this particular purpose. For illustration, while thermoplastic materials are preferred for the activatable adhesive compositions and can be used for the jettable counterparts or for the binders, the latter may also include thermosetting polymers. In a test carried out by the Applicant, an adhesive suitable for the formation of a continuous thin layer was prepared from a commercially available flexographic printing ink, Uni-Rez™ 2224 (a polyamide resin) supplemented with 10 wt.% rosin gum (Foral™ AX-E) and 10-30 wt.% of radiation absorbers. Another suitable adhesive was found to be a hot melt adhesive Dynacoll® 7360 (which is a saturated copolyester). Additional suitable adhesives have been already described with respect to the feasibility of the particle dispenser to apply particles thereon and more materials are listed as binders to the particles. The thickness of the adhesive layer may be adapted to the size of the particles to be attached thereon, and for instance could be only 0.5-2 pm or 0.5-1 pm thick for particles of about 10 pm, and 2-4 pm to 2-3 pm thick for particles of about 40 pm, the general rule of thumb being that the thickness of an adhesive image is about a tenth to a fifth of the average size of the particles. It is preferred that the thickness of the adhesive layer does not to exceed 10 pm, being typically of no more than 7 pm. This allows very small amount of glue to be used which is beneficial for the debinding process and allow the preparation of more accurate models. In embodiments where the adhesive is selectively applied for the model and support particles, it is possible to use different adhesive compositions. For example, the adhesive used for the support particles may be water soluble while that used for the model particles is not soluble in water. The same approach may be applied to composite particles, with model particles relying on a first kind of (e.g., water-insoluble) binder and the composite support particles relying on a second type of (e.g., water-soluble) binder. Selectively applicable adhesive compositions should have a viscosity adapted to their method of application, for instance enabling their jetting, this being typically achieved by the inclusion of a liquid (e.g., a solvent) lowering the relative presence of adhesive solids. The nature of the solvent used for the preparation of a selectively applicable liquid adhesive composition may depend on the nature of the material dissolved therein which would provide the required tackiness upon at least partial evaporation of the solvent. If the adhesive material is a water-soluble thermoplastic polymer such as polyvinylpyrrolidone (PVP) or polyacrylic acid (PAA), the solvent can be water, short chain alcohols of no more than six carbon atoms (e.g., methanol, ethanol, isopropyl alcohol, etc.} which tend to rapidly evaporate, long chain alcohols (also known as fatty alcohols, which are often considered as plasticizers since their relatively lower rate of evaporation allows them to sufficiently remain with the polymer to lower the temperature at which it would become or remain tacky) and glycol derivatives (e.g., ethylene glycol, diethylene glycol (DEG), diethylene glycol butyl ether (DEGBE), dipropylene glycol methyl ether (DPM), etc.}. Materials that can serve to impart adhesive properties to a jettable liquid are known and include, in addition to the resins and polymers already mentioned as activatable adhesives or binders, phenolic resins, such as cresols, novolacs and bakelites, and PVP, to name a few.

Claims

1. A particle application apparatus for applying particles to a substrate (10) which is coated in at least selected areas with an adhesive, the apparatus comprising:a) an applicator having the form of a cylinder (250) or a belt,b) a drive mechanism (254) for urging a surface of the applicator into rolling contact with the surface of the substrate, andc) a particle dispenser (2000) for applying to at least a section of the applicator surface that is to make rolling contact with the substrate a uniform multilayer coating of the particles to be applied to the substrate,wherein the cohesion between the overlying layers of particles coating the surface of the applicator is less than the cohesion of the particles to the adhesive on the substrate, the latter cohesion being in turn less than the cohesion between the adhesive and the substrate, whereby particles are transferred from the applicator to the adhesive coated areas of the substrate without transfer of adhesive from the substrate to the applicator.

2. The particle application apparatus as claimed in claim 1, wherein the particle dispenser (2000) comprises at least one particle supply (210), and at least one transfer cylinder (230) for transferring particles from the, or a respective, particle supply, onto the surface of the applicator, prior to the surface coming into the contact with the substrate.

3. The particle application apparatus as claimed in claim 2, wherein the, or each, particle supply comprises a cartridge insertable into a respective supply chamber.

4. The particle application apparatus as claimed in claim 2 or claim 3, in which the particle dispenser comprises at least one primary transfer cylinder (220) receiving particles from the, or a respective, particle supply (210) and a secondary transfer cylinder (230) receiving particles from the primary transfer cylinder(s) and transferring the particles to the applicator (250).

5. The particle application apparatus as claimed in claim 4, wherein at least the surface (222) of the or each primary transfer cylinder (220) is made of a sponge-like material.

6. The particle application apparatus as claimed in claim 4 or claim 5, wherein the, or each, supply chamber and / or cartridge includes one or more members (212,214) for mixing the particles, at least one of the members (214) being movable and extending so as to contact the surface (222) of associated primary transfer cylinders (220), whereby the mixed particles are evenly smeared to form a homogeneous coating of particles on the surface of the primary transfer cylinder.

7. The particle application apparatus as claimed in any one of claim 1 to claim 6, wherein the particle dispenser comprises a housing (260) common to all the particle supplies and transfer cylinders, and wherein the secondary transfer cylinder comprises a cylinder disposed (230) in a respective chamber (200) in the housing so as to protrude partially through an opening in the housing, in order to contact the applicator (250), the latter being disposed outside the housing.

8. The particle application apparatus as claimed in claim 7, wherein at least one levelling blade (240) is disposed adjacent to the opening in the housing (260), the or each levelling blade controllably contacting the surface (232) of the secondary transfer cylinder (230) or particles thereon.

9. The particle application apparatus as claimed in claim 8, wherein the or each levelling blade (240) is configured to prevent egress of particles from the housing (260).

10. The particle application apparatus as claimed in any one of claim 7 to claim 9, wherein the hardness of the surface of the applicator (250) is lower than that of the surface (232) of the secondary transfer cylinder (230).

11. The particle application apparatus as claimed in claim 4 or claim 10, wherein the hardness of the surface of the applicator (250) is lower than that of the surface (232) of the secondary transfer cylinder (230) that is lower than that of the surface (222) of the primary transfer cylinder (220).

12. The particle application apparatus as claimed in any one of claim 1 to claim 11, wherein, in operation, the applicator surface (252) is configured to rotate or circulate at a linear speed substantially identical to the linear speed of the substrate (10) upon rolling contact therebetween, so as to avoid slippage between the applicator (250) and the substrate.

13. The particle application apparatus as claimed in any one of claim 2 to claim 12, wherein, in operation, the applicator surface (252) is rotated or circulated at an absolute linear speed at least 10% higher or lower than the linear speed of at least one of the transfer cylinders (220, 230), so as to enable slippage between surfaces upstream of the applicator (250) and24 10 25rubbing of the particles prior to rolling contact between the applicator (250) and the substrate (10).

14. The particle application apparatus as claimed in any one of claim 1 to claim 13, further comprising a drive mechanism (256) for urging the particle dispenser (2000) into rolling contact with a surface (252) of the applicator (250), wherein, in operation, the drive mechanism (256) is configured to disengage the dispenser from the applicator whilst the drive mechanism (254) is urging a surface of the applicator into rolling contact with the surface of the substrate.

15. The particle application apparatus as claimed in claim 1, wherein the applicator is in the form of a cylinder (230) configured to make rolling contact with the surface of the substrate.

16. The particle application apparatus as claimed in claim 15, wherein the applicator cylinder (230) is mounted within a housing (260) further containing at least one particle supply (210), and at least one transfer cylinder (220) for transferring particles from the, or a respective, particle supply, onto the surface (232) of the applicator.

17. The particle application apparatus as claimed in claim 16, wherein the applicator cylinder (230) is disposed in a respective chamber (200) in the housing (260) so as to protrude partially through an opening in the housing, in order to contact the substrate (10).

18. The particle application apparatus as claimed in claim 15 or claim 16, wherein the, or each, particle supply comprises a cartridge insertable into a respective supply chamber.

19. The particle application apparatus as claimed in any one of claim 16 to claim 18, wherein at least the surface (222) of the or each transfer cylinder (220) is made of a sponge-like material.

20. The particle application apparatus as claimed in claim 18 or claim 19, wherein the, or each, supply chamber and / or cartridge includes one or more members (212, 214) for mixing the particles, at least one of the members (214) being movable and extending so as to contact the surface (222) of associated transfer cylinders (220), whereby the mixed particles are evenly smeared to form a homogeneous coating of particles on the surface of the transfer cylinder.

21. The particle application apparatus as claimed in any one of claim 17 to claim 20, wherein at least one levelling blade (240) is disposed adjacent to the opening in the housing (260), the or each levelling blade controllably contacting the surface (232) of the applicator22. The particle application apparatus as claimed in claim 21, wherein the or each levelling blade (240) is configured to prevent egress of particles from the housing.

23. The particle application apparatus as claimed in any one of claim 19 to claim 22, wherein the hardness of the surface (232) of the applicator (230) is smaller than that of the surface (222) of the transfer cylinders (220).

24. The particle application apparatus as claimed in any one of claim 15 to claim 23, wherein, in operation, the applicator surface (232) is configured to rotate or circulate at a linear speed substantially identical to the linear speed of the substrate (10) upon rolling contact, so as to avoid slippage between the applicator (230) and the substrate.

25. The particle application apparatus as claimed in any one of claim 16 to claim 24, wherein, in operation, the applicator surface (232) is rotated or circulated at an absolute linear speed at least 10% higher or lower than the linear speed of at least one of the transfer cylinders (220), so as to enable slippage between surfaces upstream of the applicator (230) and rubbing of the particles prior to rolling contact between the applicator and the substrate (10).

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