Method an apparatus for 3D printing

Abstract

The method comprises 3D printing a plurality of layers, wherein each intermediate layer is formed by depositing 14 at least one coating of a first material by selectively applying, or activating, an a

Description

Various methods have hitherto been proposed for digital manufacture of 3D objects, which fall into two categories, namely subtractive and additive. In the former, material is removed from an initial solid blank to leave behind the desired finished article. This method is used for example to make dental crowns and fillings. The present invention is concerned with the additive method, in which the end-product is built up layer by layer, with each of all but the lowest layer being deposited onto, and bonded to, a layer beneath it. The additive method of 3D printing has been used for plastics materials, ceramics and metals, the method of deposition of each layer and the method of binding of each layer to the one beneath it being dependent upon the material from which the end-product is to be formed. To fonn a desired object by the additive method of manufacture, it is known to deposit stacked layers of two types of materials. A first type includes the material from which the object is to be formed and is termed herein the model material. The second type provides support for the formation of subsequent layers and is herein termed the support material. Upon completion of the printing process, or any other desired post-printing finishing process, the support material can be removed, and the remaining shape of model material can be referred to as the “green” printed part or the green model. If the model material is itself constituted of particles of the material intended to form the desired object (e.g., metal or ceramic) and a binder as used in Binder Jetting (BJT), Material Extrusion (MEX), Material Jetting (MJT) and Vat Photopolymerization (VPP) technologies, the binder would be mostly eliminated at the end of the 3D printing. This step is known as debinding, which may be either thermal, solvent based, or catalytic, depending on the binder. Following debinding, the remaining shape, which may still contain sufficient amount of binder to provide enough structural integrity, is referred to as the “brown” printed part or the brown model. Depending on the amounts of binder eliminated and shapes and / or density of the remaining particles, the brown model can be relatively porous. The brown model can be further treated, according to the type of model material being used, to provide to the treated end-product desired mechanical properties (e.g., can be cured, melted, or sintered to consolidate the powder material to the desired final density, etc.). Existing additive manufacture processes are relatively slow and energy-consuming, hence expensive, and mainly relevant for prototyping, or manufacturing of spare parts and such uses that would excuse the relatively high cost. To form items of relatively high resolution, particles of relatively small size need to be used, in which case it may take a high number of layers to form an object having dimensions in other orders of magnitude. Moreover, applying relatively small particles as dry powders can be problematic, as the particles may remain suspended in the air rather than attached to a substrate. One solution, termed nano-particle jetting, involves as its name suggests, the alternating jetting of droplets of ‘‘model ink" and “support ink” in respective regions of a layer being formed on the substrate. For a solid material to be amenable to ink jetting, its size should preferably be below 1 micrometer (pm). Thus, this process is expensive, both for the preparation of suitable inks and the need to dry' each layer, before a new one can be applied. Moreover, it could be extremely time consuming for the preparation of relatively large objects. To facilitate the attachment of powder particles to a substrate, without having to disperse them in a liquid, relatively large particles should be used (which can be at the expense of detailed resolution). In this method, abed of powder particles, the bed having athickness of SO-SOO pm, for individual particles typically larger than 20-30 pm and usually in the 70-100 pm range, is targeted where desired with a glue acting material. This process is known as binder jetting and may require a relatively large amount of binder to enrobe the particles to be jointly adhered. In this process, the bed is constituted of similar particles that have a dual role, a) serving as model particles, where glued by a jetted binder and b) serving as support particles everywhere else. This may impose conflicting demands on the particles and be wasteful, even if under some circumstances tire support particles could be recycled following their removal. Moreover, larger particles and / or larger amounts of binder (the removal of which being often challenging) yields aggregations of particles having relatively large voids therebetween. Too large voids may reduce tire bulk density of the particles in the model to an extent detrimental to its function. The present invention lends itself to the manufacture inter alia of printed circuit boards (PCB), which are currently manufactured by one of the following main approaches. In a first approach, a foil of conductive material (typically copper) is laminated onto a dielectric board and an image is formed by coating the laminated copper foil with a resist material that can be etched to leave a desired circuit layout. The conductivity that can be achieved by this method is very good, as one starts with a foil of conducting material. However, this method is very wasteful and limited to relatively simple structures. In a second approach, more complex structures can be formed by jetting a would-be conductive ink on a dielectric board directly according to the desired circuit layout. While this approach reduces the waste of material, it is still relatively slow and limited in the conductivity which can be achieved after sintering of the particles jetted in the ink. Typically, tire conductivity of a PCB prepared by such printing method does not exceed -30% IACS, this abbreviation standing for International Annealed Copper Standard and the number preceding "IACS" being the percentage of conductivity relative to copper, which is considered to be 100% conductive. SUMMARY According to the present disclosure, there is provided in a first aspect a method of 3D printing which comprises forming a plurality of superposed compacted layers as hereinafter set forth and claimed in Claim 1 of the appended claims. According to the present disclosure, there is provided in a second aspect a printing system for printing of a three-dimensional object by superposition of compacted layers as hereinafter set forth and claimed in Claim 25 of the appended claims. The printing system comprises: a) a transport mechanism comprising a movable carrier adapted to displace a backing surface along a direction of travel; b) an adhesive application module adapted to render tacky desired regions of the backing surface, or of a compacted layer previously formed thereon; c) a first dispensing module for applying model particles to adhere selectively to the regions rendered tacky; d) a second dispensing module for applying a support material to coat at least regions of the backing surface, or of a compacted layer previously formed thereon, not coated with the model particles; and e) at least one compacting module for compressing the model particles and / or the support material to form thereon a compacted layer or a part thereof; the transport mechanism serving to displace the movable carrier to pass cyclically through the adhesive application, dispensing and compacting modules so as to form superposed compacted layers on the movable carrier. In some embodiments, the second dispensing module is configured to apply a support material in the form of support particles, the second dispensing module further comprising an adhesive forming unit adapted to form a background adhesive in regions of the backing surface, or of a compacted layer previously formed thereon, uncoated by the model particles. The background adhesive forming unit can be selected in accordance with the adhesive application module and the means used thereby to form a tacky adhesive image. If the module applying the adhesive intended to adhere the model particles is configured to apply a layer of adhesive over the entire backing surface (or underlying layer) said module further comprising a device adapted to selectively render tacky desired regions of the adhesive layer (foreground adhesive regions), then the adhesive forming unit of the second dispensing module can be a heater capable of rendering tacky regions of the previously applied adhesive not already coated with model particles (the background adhesive regions). Alternatively, if the module applying the adhesive intended to adhere the model particles is configured to selectively apply a layer of adhesive only in the desired foreground adhesive regions, then the adhesive forming unit of the second dispensing module can also be capable of selectively applying a same or different adhesive to the background adhesive regions to be coated by the model particles. In some embodiments, at least one of the adhesive application module and the adhesive forming unit of the printing system comprises a plurality of individually controllable jetting nozzles forming a liquid writing unit adapted to jet an adhesive liquid according to the predetermined patterns of the desired region or in complementary regions surrounding the predetermined patterns, respectively, said adhesive application module and / or adhesive forming unit optionally furflier comprising a drying unit adapted to render the adhesive liquid tacky, the adhesive liquid of the adhesive application module and the adhesive forming unit optionally being different. In some embodiments, at least one of the adhesive application module and the adhesive forming unit of the printing system comprises a plurality of individually controllable jetting nozzles forming an activator writing unit adapted to jet an adhesive activating liquid upon a thin layer of adhesive according to the predetermined pattern of the region desired tacky or in complementary regions surrounding the predetermined pattern, respectively. In some embodiments, at least one of the adhesive application module and the adhesive forming unit comprises a plurality of individually controllable electromagnetic (EM) radiation emitting elements forming a writing unit adapted to heat a thin layer of thermoplastic adhesive according to the predetermined pattern of the region desired tacky or in complementary regions surrounding the predetermined pattern, respectively. While the writing unit can be directed to heat an adhesive layer directly applied to the backing surface (or to an underly), it may alternatively be directed to enable selective transfer of adhesive regions. In such embodiments, the thin layer of thermoplastic adhesive according to the predetermined pattern is first formed as a mirror image on a transfer member, the adhesive application module further comprising said transfer member and a transfer station at which the mirror image is transferred from the transfer member to the backing surface or a layer formed thereon, the thin layer of thermoplastic adhesive towards which the writing unit is directed being formed on the transfer member. In some embodiments, the individually controllable electromagnetic (EM) radiation emitting elements of the writing unit are laser beam emitting elements, the plurality of laser beam emitting elements optionally being constituted of one or more arrays of vertical-cavity surface-emitting lasers (VCSELs). In some embodiments, the dispensing modules, first or second when adapted to apply particles, each comprises one or more powder containers, each configured to contain same or different (e.g., model) particles, each powder container being connected via one or more passageways to a respective applicator adapted to apply the model (or the support) particles upon the tacky adhesive image formed in the desired regions of the backing surface, or of a compacted layer previously formed thereon. The applicator can be a contact-less applicator, such as a spray blowing the particles towards the tacky adhesive without contacting it, or can alternatively be a contacting applicator. In some embodiments, the applicator of a module adapted to dispense particles consists of one or more rollers, the roller adjacent to the movable carrier being configured to rotate in the direction of travel of the backing surface whilst slipping with respect to the backing surface. In some embodiments, the second dispensing module is configured to apply a support material in liquid form, the second dispensing module further comprising a solidifying unit adapted to solidify the applied liquid support material. If the liquid support material can be solidified by drying, the solidifying unit can be a dryer capable of removing the liquid vehicle of the support material. If the support material is a curable liquid which can be solidified by curing upon exposure to predetermined factors (e.g., UV light), then the solidifying unit can be a curing unit exposing the liquid to the factors adapted to harden it. The second dispensing module can be configured to apply a liquid support material in a non-selective manner over the entire surface of the layer, excess being wiped away from the regions already coated by the model particles, for instance with a squeegee. Alternatively, the second dispensing module can be configured to selectively apply the liquid support material in regions uncoated by the model particles, the module including for instance a digitally controlled jetting head. hi some embodiments, the printing system may comprise two compacting modules, each disposed downstream of a respective dispensing module and adapted to compress the materials dispensed thereby. Regardless of the number of compacting modules included in the printing system, each may comprises a) a rotatable compactor controllably urgeable towards the backing surface to be in rolling contact with the model particles and / or the support material of each to be compacted layer; b) a countersurface supporting the movable carrier and a backing surface thereon at least at the point of rolling contact with the compactor; and c) a heating element adapted to heat at least one of the backing surface and the layer(s) formed thereon. In some embodiments, the compacting module may further comprise a device providing vibration to die rotatable compactor. In some embodiments, the printing system further comprises a levelling module adapted to ensure that the layer formed thereby has an even continuous surface and / or has a desired thickness. The levelling module can be selected to remove at least part of any material protruding over a desired flat surface and / or exceeding the thickness sought for the layer. The levelling module can include a milling, grinding, and / or abrading device and means of removing the excess material taken away from a layer by such device. For illustration, tire levelling device can be disposed in a chamber further including suction means to eliminate the residues milled, grounded, and / or abraded away. The desired flatness or thickness of a layer can be achieved by relative motion between the layer and the levelling module in a direction transverse to the printing direction to an extent determining the flatness / thickness of the layer being levelled. For illustration, and considering a flat horizontal backing surface, the levelling module can be raised to a height above an intended outermost surface of the layer being formed corresponding to its desired thickness, or tire backing surface can be lowered to a similar extent, or each could be partially moved respectively up and down, the total relative displacement corresponding to the thickness of the new layer to be levelled. The printing system may further comprise any suitable sensors adapted to monitor and control the 3D printing process and the parts (e.g., modules) implementing steps thereof. Sensors can be temperature sensors, pressure sensors, motion sensors, distance sensors, level sensors, flow sensors, torque sensors, vibration sensors, presence sensors and the like, as adapted to the parameter to be monitored and controlled by the relevant feedforward or feedback mechanism. In some embodiments, the printing system further comprises an alignment sensor to align a positioning of the tacky adhesive image of each cycle with respect to edges of, or fiducial markers on, the backing surface. hi some embodiments, the transport mechanism further comprises a motor, a driving unit adapted to displace the movable carrier (e.g., horizontally, vertically, rotationally about a horizontal or a vertical axis, etc.), a control unit, a monitoring unit, a power supply and a process control software (e.g., to control speed, direction of displacement, distance between modules and backing surface or outermost surface of last superposed layer, temperature and any like parameter suitable for the operability of each module). Hie transport mechanism may further comprise a heater facilitating heat activation of heat activatable adhesives. In some embodiments, the printing system further comprises a pre-treating module adapted to treat the backing surface upstream of the adhesive application module. For illustration, the pre-treating module can be selected and configured to apply a sacrificial layer or otherwise treat the backing surface to facilitate subsequent operations, such as the separation of the superposed compacted layers upon completion of printing or furthering the adhesivity of the first layer to be formed on the backing surface. In some embodiments, the printing system further comprises a cleaning module adapted to remove excess model particles and / or support material from unintended regions, the cleaning module optionally comprising a brush, a soft roll, or a sponge, the cleaning module optionally being in a confined environment to permit gas flow or suction. hi some embodiments, the printing system further comprises a model enhancer module comprising an applicator configured to apply to voids between model particles a model enhancer liquid and a unit for treating the model enhancer liquid. In some embodiments, the printing system further comprises a detaching module for detaching the superposed compacted layers from the backing surface. In some embodiments, the printing system further comprises a post-printing module for treating the, optionally detached, superposed compacted layers so as to form the three-dimensional object, tine treatment being at least one of i) separating the compacted model particles from the support material, the post printing module comprising at least one of a mechanical disrupter, a chemical desintegrator, an air knife and a suction port; and ii) consolidating the model particles of the superposed compacted layers, or a part separated therefrom, the consolidation being by application of energy, the post-printing module accordingly comprising an energy source selected from the group consisting of: ultraviolet (UV) source, infrared (IR) source, electron beam source, and a heat source. In some embodiments, the printing system further comprises a controlled environment compartment which optionally provides a covered space around at least one of the adhesive applicator module, tire first dispensing module, the second dispensing module, the compacting module(s), and the optional foregoing modules, the at least one covered space being bounded by the movable carrier and optionally providing a gas controlled environment. Compositions that can be used inter alia in the present methods and / or printing systems are also disclosed to illustrate how the invention can be practiced. Features of different embodiments of the foregoing aspects are set out in the following detailed description and in the 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 tire Figures: Fig, 1 is a schematic representation of a first apparatus for 3D printing of an article; Fig. 2 is a schematic representation of a second apparatus for 3D printing of an article; Figs. 3 to 7 show schematically the sequence of steps employed in forming a 3D article by different embodiments of the invention; Fig, 8 shows illustrations of the post-processing of an article after all its different layers have been deposited; Fig, 9 shows in the form of a flowchart various sequences of steps that can be employed in forming a 3D article by different embodiments of the invention; Fig, 10 illustrates a device adapted to non-selectively apply a thin layer of adhesive; Fig, 11 illustrates a device adapted to deposit particles on a transferring surface from which particles can be detached by thin layers of tacky adhesive; Fig, 12 illustrates a device adapted to compact particles on a supporting surface; Fig, 13 illustrates how devices, such as illustrated in Figs. 10-12, can be arranged to form an apparatus able to form a stack of layers including regions of compacted first particles when the substrate and its transport mechanism are non-planar; Fig, 14 provides an exemplary alternative arrangement of devices, when the apparatus is intended for planar substrates and transport mechanisms; and Fig, 15 displays pictures captured by microscopy of particles 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 OF THE DRAWINGS The method of 3D printing of the present invention comprises fonning 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 fonnation of each layer, there is a continuous underly ing layer onto which material can be applied. Fig. 1 schematically shows an apparatus for carrying out a method of a first embodiment of the invention. A substrate 10 is mounted on 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. Tire 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 tire continuous intermediate layer, in which case a 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 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, I20°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 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 regions) are activated in a 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 or 38. 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 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 die method of the invention, 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 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 tire 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 m 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 embodiments of apparatus 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 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 m 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. Fig. 9 shows in the form of a flowchart various sequences of steps that can be employed in forming a 3D article, as exemplified in preceding figures illustrating different embodiments of the present teachings. In a first step Sil, a tacky image having a selective shape is formed as a thin layer of adhesive on a backing surface (e.g., a bare surface of a transport mechanism or the outermost surface of a growing substrate made of previously applied layers). Such a step of forming a tacky image in a first region of a layer can be carried out by three main approaches referred to as alternative steps Sila, SI lb and Sllc. In the approach designated by step Sila, the surface upon which the tacky image is to be formed is entirely coated with a thin layer of adhesive and regions of the layer corresponding to the desired image are selectively activated (e.g., by physical of chemical means adapted to the adhesive of the layer). This approach is illustrated by stations 30 and 32 in Figs. 3, 4 and 6. In the alternative approach designated by step Sllc, the tacky image is formed on an underlying surface by selectively depositing a thin layer of adhesive according to a desired shape. If, for illustration, the adhesive is ink-jetted to the backing surface as a liquid, then the formation of the tacky image may include at least partial removal of the liquid carrier, the partly dried image having a desirable tackiness by the time it is used to attach particles dispensed thereover. This approach is illustrated by station 14A in Fig. 5. In the alternative approach designated by step Sllb, the tacky image is formed either by selectively activating or selectively depositing a thin adhesive layer in regions of the backing surface to be coated with model particles. The tacky image is first formed on different surfaces (in manners similar to those already described for Sila and Sllc) and only then transferred to the backing surface, this situation being illustrated in Fig. 7. Regardless of the steps used to form the selective tacky image of SI 1, the adhesive image (also called the foreground adhesive) is then exposed to the particles to be adhered thereto. For brevity, particles made of a first material can also be referred to as “first particles”. While the first particles applied to the selective tacky image in step S12 have been previously described as being the model particles, this should not be construed as limiting. As readily appreciated, the first particles to be applied to suitable regions of a layer can alternatively be support particles. Particles made of a second material can also be referred to as “second particles”, and so on. In a following step designated S13, the first particles can be compacted by application of pressure (with or without heat) such as illustrated by station 42 in Figs. 4, 6 and 7. This step can be optional, as shown by the omission of such a compacting station from Fig. 3 and 5, wherein a unique compacting station 40 or 22 is present downstream of a station at which regions of background adhesive are coated with second particles. In a following step designated S14, a thin layer of adhesive is formed in the regions of the backing surface not previously coated by the first particles, (i.e., in regions other than the first region). Such a step of fonning a thin adhesive layer in previously uncoated background regions can be carried out by three main approaches referred to as alternative steps SI4a, SI4b and SI 4c. the selection of which may depend on the approach selected for step Sil as explained below. The step designated by S14a can follow the application of first particles (e.g., model particles) to a tacky image formed according to step Sila, in which a thin layer of adhesive entirely coating an underlying surface is selectively activated to yield the first region (in which the first particles can be optionally compacted). In such a case, as the background regions not coated by the particles are already coated by a thin layer of adhesive, it suffices for step SI 4a to activate the adhesive in the background regions. Such an activation of the thin adhesive layer can be selective (e.g., by physical of chemical means targeting only the areas not previously coated by particles) or can be non-selectively applied to the entire layer, based on the understanding that first regions already coated by first particles would not be “reactivated" to lose their adhered particles. This approach is illustrated by station 36 in Figs. 3 and 4. The step designated by S14b can follow the application of first particles to a tacky image selectively formed and / or transferred according to step Sllb or Sllc to yield the first (optionally compacted) region. In such a case, as the background regions not coated by the particles are also devoid of a thin layer of adhesive, step S14b requires such selective application of a thin layer of adhesive. Step S14b can be performed like step Sllc, but the adhesives need not be the same. In such a case, the adhesive selectively deposited or transferred to adhere the first particles to the foreground regions can be referred to as the first adhesive, while the adhesive selectively deposited or transferred to adhere the second particles to the background regions can be referred to as the second adhesive. This approach in which a background adhesive region is selectively formed following the application of first particles is illustrated by station 18A in Fig. 5. Regardless of the method by which the background adhesive region is selectively formed, e.g., according to step S14a or S14b, when the second material is in the form of particles, the second particles are applied to adhere to the background adhesive. Second particles can be applied (e.g., by a support particle dispenser 38) to overcoat the entire backing surface, being urged into the regions not previously coated, and removed from the coated ones, by a squeegee or like device. Such a step is designated S15A in the figure. In a following step designated S16A, the second particles can be compacted by application of pressure (with or without heat) such as illustrated by station 40 or 22 in Figs. 3-5. In an alternative approach the second material is in the form of a liquid. The step designated by S14c can follow the application of first particles to a tacky image formed according to any of steps Sila, Sllb and Sllc to yield the first region, though starting from step Sila can be wasteful in material, though easier in implementation. Therefore, step S14c is typically preceded by the compaction of the first particles in step S13. If the second material is applied as a liquid overcoating the entire surface (including the first regions where the first particles are already adhered and compacted), as can be formed by using a squeegee to level the liquid to a thickness commensurate with the heights of the particles in the first regions, then the following stages first include a step designated S15B in which the liquid second material is solidified. For instance, if the liquid applied in S14c is a curable resin, then it can be cured by the suitable curing agent in S15B or if the applied liquid of second material is a molten wax, then it can be solidified by cooling. In a step designated S16B excess of solid second material is removed so as provide an even continuous surface to the completed layer. In this step, part of the first particles coating the first regions can be removed, the tips of the particles being milled away or abraded, depending on the method used for levelling. This approach is illustrated by the stations downstream of 42 in Figs. 6 and 7. Each of the above sequence of steps leading to the formation of one layer in which at least the particles of the first material are compacted can be repeated as necessary to obtain a desired stack of superposed layers (as mentioned in steps SI 7A or S17B, depending on the preceding steps). 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. Ure 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). This functionalization, denoted in the figure by optional step S18 can further optionally include removing parts of the layers (e.g., second particles in second regions) not intended to remain in the final object (or part thereof) and or cleaning of the remaining parts. Features of the various exemplary processes described above will now be considered individually in more detail, some enabling devices and suitable compositions being also described. Adhesive application It is possible to use a relatively low amount of adhesive, the layer remaining between printing layers following compacting being as thin as about 1 pm. This 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 relatively more difficult from relatively narrow voids, as compared to relatively larger ones, the compaction step nevertheless facilitates tine 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), it may be desired to partially dry the adhesive to remove part of the liquid carrier. The drying may take place after tire 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. An exemplar}- device configured for this purpose is schematically illustrated in Fig. 10. Fig. 10 shows an adhesive applicator 1000, such as depicted by station 30 in Fig. 2, the applicator comprising a rotatable cylinder 100 upon which a sleeve 102 made of a release material, such as a silicone blanket, is mounted and secured by any suitable system, such as shown herein by numeral 104. Heating elements can be located in and / or about the cylinder so as to maintain a desirable temperature on its outer surface 106 (e g., on the surface of the silicone sleeve). Such a heating device is depicted as a central heater 108 in the present figure. In operation, cylinder 100 is in rolling contact with dipping cylinder 110 which rotates in chamber 112 within a bath of molten adhesive 114. The adhesive is maintained in relatively liquid molten state by means of a heater 116 positioned in the walls of the melting chamber 112, the chamber also including a thermocouple or any like instrument 118 adapted to measure temperature, so as to monitor the temperature of bath 114 and control the heat applied thereto by heater 116 to maintain the adhesive in suitably molten form. Rollers 120 can be disposed in contact with dipping cylinder 110 to ensure that the adhesive coat formed on the cylinder following its rotation within the bath of molten adhesive 114 is uniform over the surface of the cylinder 110, rollers 120 being also capable of removing excess coating if any, surplus still liquid adhesive dripping back to the bath. A roller 122 having a similar function may be positioned downstream of the nip where the dipping cylinder 110 comes into contact with cylinder 100 to transfer to its surface 106 a thin layer of adhesive. Downstream of the nip is a metering device 130 consisting of a metering blade 132 which can be heated by a heater 134 located m a body 136 of the metering device, the body further including a mechanism 138 for pivoting and / or displacing the metering blade with respect to the outer surface 106 of cylinder 100 so as to ensure that a uniform distance is maintained between the blade 132 and the outer surface 106, hence an adhesive coat having a predetermined thickness is formed on cylinder 100 downstream of the metering device 130. Temperature sensors 140, such as infra-red (IR) sensors, may be positioned around cylinder 100 to measure the temperature of its outer surface 106, before its coating with a molten adhesive (e.g., upstream of cylinder 110), after its coating with a metered layer of adhesive (e.g., downstream of metering device 130) or both to monitor and control temperature as needed. Hie thin layer of adhesive of known thickness can then be transferred from cylinder 100 to the surface to be coated therewith at a nip 150. This can be achieved by urging cylinder 100 towards the relevant surface by way of a pneumatic piston 124 capable of pressing chamber 112, its cylinder 110 and in turn cylinder 100 to form intimate contact with the intended surface. In the present figure, the applicator of the adhesive is illustrated in contact with a partial surface of a curved drum 12, as depicted in Fig. 2, but it may alternatively be in contact with a planar backing surface. 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 final rubbing effect to enhance the uniform attachment of the particles transferred to the glue. The rubbing effect can be achieved by controlling slipping between the applicator and the substrate. 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. 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, optionally assisted with mechanical means of dislodgment of loosely attached particles (e.g., with a rotating brush or sponge). An exemplary device configured for the application of particles to tacky adhesive images is schematically partially illustrated in Fig. 11. The dispenser of particles 2000 is formed of a chamber 200 and a housing 250. 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. The particles from each cartridge are smeared upon a respective cylinder 220 made of a sponge-like material mounted on a rigid axial support 222. The particles are then transferred from the cylinders 220 to a common cylinder 230 having 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 250, the cylinder 230, which is able to releasably transfer particles out of the housing, is only partly disposed within the chamber 200. Levelling blades 240 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. As better seen in Fig. 13, cylinder 230, which may be considered a first applicator cylinder, can then transfer the particles to a second similarly built 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 first and second cylindrical applicators 230 and 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 first applicator to the second applicator cylinder. The second applicator cylinder can then be urged, for instance using a pneumatic piston 254, into contact with a surface including tacky regions of adhesive to be coated by the particles. Compaction By using a relatively significant force urging a hard roller to compact the powders on the substrate, one can achieve very good compacting, even deformation of the powder particles when made of deformable materials, resulting in a relatively high density of the green printed model. Such greater density can have significant technological advantages, for instance if the model particles are made of a metal to be sintered to develop a desired function. As the layers add up, the pressure applied at the nip to compact the particles should be gradually decreased, otherwise the stack of compacted layers, or part of it, may detach from the substrate or from previously fonned layers, a process termed “delamination”. There are three main modes of compacting, each being able to control the pressure and prevent delamination in a different manner. All share a compacting roller, in rolling contact with the layers to be compacted to the substrate. Hie compaction and its extent (which may for example exceed the ultimate tensile strength of the particles being compacted) can be controlled by several parameters, which include inter alia the size of the compacting roller, its hardness (or other surface properties), the force applied to the compacting roller, the temperature applied at compaction (which also depends on the materials to be compacted and the backing surface), the coexistence of additional compacting enhancing powers (e.g., vibration of the compacting roller or ultrasonication of the particles to increase their packing) etc. In a first approach, pressure can be controlled according to the force applied to the substrate and the layers of particles building-up thereon. The force can be applied, for instance, by a hydraulic or a pneumatic press urging the compacting roller towards the substrate, the force being decreased as the layers pile up. In a second approach, the pressure perceived by the outermost layer being compacted can be decreased by increasing the size of the gap between the rollers, to accommodate the growing thickness of the layers, the thickness of each depending on the particles being used and typically not exceeding 50 pm. In these two approaches, the compacting roller can be rotated by mere friction with the printed object uppermost surface, having a substantially null relative linear velocity as compared to the substrate. In a third approach, the compacting roller can be independently driven by a motor setting a relative velocity with the substrate, which can be adjusted to reduce shear forces perceived by the object. Controlling (and preventing) slipping between layers may also reduce the risks of delamination. In all cases, heat can be applied at the nip of compaction to reduce yield strength and increase the plasticity of the material being layered, increasing temperature therefore allows decreasing the force applied by the heated pressure roller. An exemplary device configured for the compaction of particles to an underlying thin adhesive layer is schematically illustrated in Fig. 12. The compacting device 3000 includes a pneumatic piston 300 capable of urging a compacting roller 310 into contact with regions or layers of particles on a substrate conveyed and supported by a suitable transport mechanism the compacting roller and the surface compacted therewith forming a compacting nip 330. In the present figure, the compacting roller 310 is illustrated in contact with a partial surface of a curved drum 12, as depicted in Fig. 2, but it may alternatively be in contact with a planar backing surface. If the backing surface is planar and rotatable about a vertical axis, then the compacting roller may be frustoconical, so that there should be no relative slip between the surface of the compacting roller and the backing surface in any part of the nip between them. The compacting roller 310 may include heating elements in and / or around it, such as illustrated by internal heater 312. The temperature of the outer surface of the compacting roller can be monitored by any suitable device, such as by an IR sensor 314 which in turn may send data to a controller of the heater so as to achieve a desired temperature. There may also be provided a blade 320 for removing from the surface of the compacting roller any residual particles that would have unduly transferred thereto during compaction. It is believed that the present compaction step and the device allowing its implementation can significantly reduce the presence of voids between the compacted particles, voids constituting only 20% or less, 15% or less, 10% or less of the volume of the compacted layer. This can translate in even lower presence of voids if the particles are later consolidated or functionalized, e.g., by sintering, a region of compacted particles containing less than 10% of voids being expected for illustration to contain less than 5% of voids once sintered. Having provided schematic layouts for devices suitable to implement steps of the present 3D printing method, their exemplary arrangements are depicted in Figs. 13 and 14. Fig. 13 displays for clarity only a part of the devices as may be arranged 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 is rotating counterclockwise, the first station being the adhesive applicator 1000, the thin layer of adhesive applied thereby to the outer surface of drum 12 being 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 2000, following which the particles having adhered to the activated adhesive may be compacted by a compacting device 3000. 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. In Fig. 13, 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. 14, in contrast to Fig. 13, the transport mechanism 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) is illustrated with a substrate 620 thereon. The substrate is passed by the transport mechanism 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, and Seiko, each having models adapted to any desired viscosity and jetting method. Following the formation of the desired image of adhesive, the substrate is transported to a second station 2000 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. 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. 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. Following its drying, by one or more driers 710, the substrate arrives at the compacting nip formed by urging the compacting device 3000 against the platform 600. The layer including the compacted 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. If needed, 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. 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. The substrate then proceeds to a curing station 750, the exact curing device being selected in accordance with the liquid material to be cured. 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 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. While in Fig. 14 the various devices and stations of the apparatus are depicted in a linear arrangement considering their projection on the X-Y-plan, this should not be construed as limiting, an oval or circular arrangement being also envisioned. Regardless of the type of transport mechanisms (e.g., planar or not) used for apparatuses according to tire present teachings, they may displace the substrate at a linear speed of at least 0.5 m / sec, at least 1 m / sec, at least 2 m / sec, or at least 3 m / sec. Advantageously, the afore-described exemplary apparatuses enable relatively high throughput additive manufacturing. In common with their conventional additive manufacturing counterparts, the present apparatuses may include digital data feeding equipment and controls which enable slicing a computer model of a desired 3D object into thin cross sections, translating the result into two-dimensional position data and feeding the data to control equipment which fabricates the corresponding 3D structure in a registered layered manner. Hardware, software, firmware, or combinations thereof using an operating system, as well computer object data formats adapted for such Computer-Aided Design (CAD) are known and need not be detailed herein. For clarity, such computer assisted manufacturing (CAM) systems and other conventional equipment (such as supply systems for the different building materials) are omitted in the figures. Consumables Of the three main approaches introduced above regarding the operation of a compacting device 3000, the first one “force control” is the relatively easier one to implement from an engineering point of view, but the relatively more limited with respect to prevention of delamination. Surprisingly, it has been found that modifying the constitution of the particles may also prevent “delamination” from the substrate, by modifying the forces that would be required for compaction. Specifically, instead of using particles entirely made of the desired material at a hardness that could render the layers built therefrom ‘’oversensitive” to the pressure, the particles can be made-up of the intended constituents of tire 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. An effect similar to the use of “composite” particles, can be obtained by applying nanoparticles into the voids remaining following the compaction of the model particles. This can be achieved by ink jetting followed by sintering of the nanoparticles. While the feasibility of this method has been experimentally established, from a commercial standpoint jetting a liquid material requires the subsequent elimination of the liquid carrier, such drying accordingly slowing down the process. It is believed that instead of jetting ink that typically contains no more than 10% nanoparticles, one could jet a slightly more viscous slurry, containing for example 20% of nanoparticles, the nanoparticles migrating from the slurry to the interparticulate voids by capillarity, and the ink of the nanoparticles being applied by roll or blade coating. 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 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, tire 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, tire 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 the 3D object formed therewith, their improved packing (as may result from the defonnability 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 tine 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 polydispersity 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. Materials suitable for additive manufacturing, including powders, adhesives and curable liquids, are known but shall be exemplified in a non-limiting manner in the following, as many more similar or equivalent materials can be used in the practice or testing of embodiments of the invention. Composition of the particles The materials to be selected for the formation of the model particles, should take into consideration not only the intended function, but also prospective post-printing processes. For instance, if the method is used to form a pattern due to remain on a substrate (e.g., a board for printed circuitry), then the materials should be “formulated” according to the temperature each substrate could withstand for the sintering of the particles of conductive material. For example, glass-filled epoxy boards, such as FR4, are relatively heat sensitive, so that sintering of metallic particles stacked thereon should not exceed 220°C. In such case, it is preferable to use model particles constituted of both micro and nano-particles of metal. While their overall conglomerated size could be similar to regular powders, the fact that their individual constituents would be relatively smaller enables their sintering at relatively lower temperature. Sintering, however, need not only be thermal and could alternatively (or additionally) be achieved by laser / sonication / induction / microwave sintering. One could combine a layer-by-layer sintering to achieve preliminary bonding, then finalize with sintering of the stack of particles, the final sintering step being optionally less demanding, if intermediate sintering was performed every layer or every few layers. Delamination can occur if internal stresses exceed the cohesion forces in the printed layers and between them. To avoid delamination, one can use the following materials: agglomerated powders where particle is composed of nano / micro particles with binder, under certain pressures the powder will crush into the previous printed layer, this composite material that include major powder, binder and voids can absorb extra energy and prevent delamination. Another type of “composite” particles that may reduce the risks of delamination can consist of at least two different types of materials (e.g.. soft and hard), where the soft materials can absorb the energy and deform where the hard powder can function as gap control for the compacting roller. Finally, in some applications, the thickness of the adhesive layer can be increased to absorb the compacting roller energy to prevent delamination. This process has been tested for model particles made of metals (aluminium, 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. 15 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. 15A 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. 15D 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. 13, the drum 12 rotatably transporting the substrate and the layers sequentially built on having a linear speed of 1 m / sec. The grains previously globular now appear as flattened discs, which look like a brick wall in the section. Fig. 15B 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. 15E 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. 15C 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. 15F shows sintered layers following compaction of the same particles at a pressure of about 100 MPa under otherwise similar operating conditions of the apparatus, tire 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. Composition of the curable liquids When the support material can remain in the end-product, it may be provided in the form a liquid which can undergo hardening, m addition to the form of grains or particles. Materials may relatively solidify by building more complex structures starting from simpler ones (which can be liquid at the time they are applied to the substrate). A classical example is provided by pre-polymers which may form larger (e.g., thermosetting) polymers by ongoing cross-linking, a polymerization process also known as curing. Curing of curable materials can be induced by chemical additives (also called curing agents) or by physical means (light, heat, radiation). Chemically curable materials can be epoxy resins, their typical curing agents including polyamines, polyamides, anhydrides, imidazoles and phenols; polyester resins, their typical curing agents including styrene monomers, MEKP (Methyl Ethyl Ketone Peroxide), and cobalt naphthenate: polyurethane resins, their typical curing agents including isocyanates and polyols; acrylic resins, their typical curing agents including free radical initiators such as peroxides or azo compounds; silicone resins, their typical curing agents including silanes and organometallic compounds; phenolic resins, their ty pical curing agents including formaldehyde-based resins or other phenolic compounds; vinyl ester resins, their typical curing agents including peroxides and amines; cyanate ester resins their typical curing agents including anhydrides; and diallyl phthalate (DAP) resins, their typical curing agents including peroxides. Other specialty curing agents can be used with the afore-said chemically curable resins. Resins that can be cured by using peroxides, azo compounds, or like initiators are often polymerizing through a free radical mechanism, and may therefore alternatively be referred to as free radical curable resins. Physically curable materials can be resins that harden through different mechanisms when exposed to specific types of energy, the resins being for instance heat curable, UV-curable, photo-curable, and e-beam curable. UV-curable resins include aciylate and methacrylate-based oligomers and photoinitiators that are acrylate or methacrylate derivatives; photo-curable resins include cationic epoxy resins and vinyl ether resins, their polymerization upon exposure to light being initiated by photoinitiators, such as diaryliodonium salts or sulfonium salts; e-beam curable resins can be epoxy resins and acry lic or methacry lic resins. The curable liquid can be adapted to the model particles, and compatible therewith, and can additionally be selected to preferably rapidly solidify when sufficiently exposed in the present methods to the relevant curing agent or curing energy. The curable liquids may further contain any desirable additive, such as reinforcement fillers (such as glass fibers or silica particles). 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 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, or from a physical standpoint, for instance 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. In a test developed to select suitable adhesive compositions, a substrate (e.g., a PCB board, a transparent glass slide, etc.) is coated with a thin layer of the material being tested and exposed to laser emission adapted to prospectively render it tacky, and the irradiated sample is then brought into contact with a layer of particles to be detached from a blanket support having a releasy outer surface made of polydimethyl siloxane (PDMS). The substrate is then separated from the blanket and the ability of the tested activatable adhesive composition to detach and retain the particles is determined. A similar test can be performed with any other desirable method of activation or deposition of the adhesive candidate. 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. 13) 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, or like additives, which may enhance its absorption in the NIR range at which the lasers operate. 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) by Kraton supplemented with 10 wt.% rosin 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) by Evomk. For use in ink jetting embodiments, it was found that a commercially available jettable printing ink, silver ink Metalon® JS-A191A by NovaCentrix could be used. 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 0.5-1 pm thick for particles of about 10 pm, and 2-3 pm thick for particles of about 40 pm. It is preferred not to exceed 10 mp, being typically with no more than 7 pm. Tins 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 ty pically achieved by tire 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), 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, di ethylene glycol butyl ether, etc.). Materials that can serve to impart adhesive properties to a jettable liquid are known and include, m 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. The support particles can also be selected in accordance with the model particles. They should provide for easy removal, if so desired, upon completion of manufacturing and be, for instance, soluble in a desired solvent (preferably in water) or eliminated by burning at a temperature not affecting the model particles (e.g., salts, silica SiOz, ferrous carbonate FeCOs, calcium carbonate CaCOs). To the extent that the removal is not complete, residues of the support particles “contaminating” the model particles, such “contaminants” should be compatible with the intended function of the 3D shaped model. For instance, if the product is a PCB, residues of support particles should not affect conductivity of the model areas and their radio frequency (RF) properties, such as surface conductivity, surface roughness etc. For the manufacture of printed circuit boards, the model material could be electrically conductive (optionally following a treatment step, such as sintering) and the support material could be electrically insulating (e.g., epoxies, including glass or ceramic filled epoxies, ceramics, polyimide, polytetrafluoroethylene (PTFE) and combinations thereof, so as to separate and isolate the shapes of conductors formed on the PCB substrate. The support materials, which may remain for a PCB end-product, apart from being insulating should possess other properties required for desired PCB functioning such as dielectric constant, dielectric loss, temperature stability of the properties etc. In that case, residues of model particles and / or of adhesives in the support areas should not damage these properties. Depending on the materials used for the model particles, the process may serve for the manufacturing of different 3D objects, and vice versa. Taking for illustration printed electronics, the material resulting from the compaction of the support material (e.g., particles) may remain in the end-product (even if sintered). Micro particles of copper pressed between 100-300 MPa at the printing stage and followed during post processing with application of a pressure between 20-50 MPa at temperature between 250-350°C has been found to achieve 25-35 % of copper bulk conductivity. By using silver coated copper powder with same conditions, a conductivity of between 30-40% of bulk conductivity can be achieved. In order for post-processing of printed circuit boards to be able to use lower pressures and temperatures while achieving the required high conductivity, it has been found desirable to use as particles aggregates of nanoparticles. Even aggregates of microparticles (e.g., of copper) with nanoparticles (e.g., of silver) have been found advantageous to achieve a relatively higher conductivity of more than 35% of bulk (upon sintering of the model particles). For products where the substrate and the support are temporary' for the duration of the manufacturing, it should selected to be compatible with later separation of the manufactured object, hi such case, the support would be selected to have both suitable mechanical properties (to sustain the pressure ± heat) and release capability, or be coated with a sacrificial layer enabling later separation of the object. When the particles are composed of aggregates a binder might be needed. When the binder of some of the intended particles or the adhesive of each layer is to remain in the endproduct, their composition should be accordingly selected and adapted. For illustration, if the model particles are made of a plastics material that would be hardened at a temperature insufficient to eliminate the adhesive in-between, then the adhesive might also provide desirable mechanical properties to the resulting object. A benefit offered by the proposed method of 3D manufacture resides in the relatively low energy consumption as compared to more conventional 3D printing methods. For example, the known binder jetting method, which relies on the adhesive being mixed with the material to be selectively deposited, typically requires high amount of energy to eliminate the liquid carrier of the jetted material. In technologies involving lasers, such as Selective Laser Melting (SLM), Selective Laser Sintering (SLS), or Laser Metal Deposition (LMD), the energy required to melt or sinter a powder made of metal or polymer is significantly higher than the energy needed by a laser to heat a thin layer of adhesive. It is appreciated that certain features of the disclosure, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the disclosure, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable subcombination or as suitable in any other described embodiment of the disclosure. Certain features described in the context of various embodiments are not to be considered essential features of those embodiments, unless the embodiment is inoperative without those elements. Although the present disclosure has been described with respect to various specific embodiments presented thereof for the sake of illustration only, such specifically disclosed embodiments should not be considered limiting. Many other alternatives, modifications and variations of such embodiments will occur to those skilled in the art based upon Applicant’s disclosure herein. Accordingly, it is intended to embrace all such alternatives, modifications and variations and to be bound only by the spirit and scope of the disclosure and any change which come within their meaning and range of equivalency. In the description and claims of the present disclosure, each of the verbs “comprise”, “include” and “have”, and conjugates thereof, are used to indicate that the object or objects of the verb are not necessarily a complete listing of features, members, steps, components, elements or parts of the subject or subjects of the verb. Nevertheless, it is contemplated that the compositions of the present teachings also consist essentially of, or consist of, the recited components, that the methods of the present teachings also consist essentially of, or consist of, the recited process steps, and that the apparatus of the present teachings also consist essentially of, or consist of, the recited devices. Positional or motional terms such as “upper”, “lower”, “right”, “left”, “bottom”, “below”, “lowered”, “low”, “top”, “above”, “elevated”, “high”, “vertical”, “horizontal”, “front”, “back”, “backward”, “forward”, “upstream” and “downstream”, as well as grammatical variations thereof, may be used herein for exemplary purposes only, to illustrate the relative positioning, placement or displacement of certain components, to indicate a first and a second component in present illustrations or to do both. Such terms do not necessarily indicate that, for example, a “bottom” component is below a “top” component, as such directions, components or both may be flipped, rotated, moved in space, placed in a diagonal orientation or position, placed horizontally or vertically, or similarly modified. As used herein, the singular form “a”, “an” and “the” include plural references and mean “at least one” or “one or more” unless the context clearly dictates otherwise. As used herein, the term at least one of A and B is intended to mean either A or B, and may mean, in some embodiments, A and B. Unless otherwise stated, the use of the expression “and / or” between the last two members of a list of options for selection indicates that a selection of one or more of the listed options is appropriate and may be made. The word “exemplary” is used herein to mean “serving as an example, instance or illustration”. Any embodiment described as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments and / or to exclude the incorporation of features from other embodiments. As used herein, unless otherwise stated, adjectives such as “substantially”, “approximately” and “about” that modify a condition or relationship characteristic of a feature or features of an embodiment of the presently disclosed subject matter, are to be understood to mean that the condition or characteristic is defined to within tolerances that are acceptable for operation of the embodiment for an application for which it is intended, or within variations expected from the measurement being performed and / or from the measuring instrument being used. For example, when the term “about” or “approximately” precedes a numerical value, it may indicate + / -15%, or + / -10%, or even only + / -5%, or any other suitable + / - variation within such ranges, and in some instances may indicate the precise value. Furthermore, unless otherwise stated, the terms (e.g., numbers) used in an embodiment of the presently disclosed subject matter, even without such adjectives, should be construed as having tolerances which may depart from the precise meaning of the relevant term but would enable the embodiment or a relevant portion thereof to operate and function as described, and / or as understood by a person skilled in the art. To the extent necessary to understand or complete the present disclosure, all publications, patents, and patent applications mentioned herein, including in particular the applications of the Applicant, are expressly incorporated by reference in their entirety as is fully set forth herein. Certain marks referenced herein may be common law or registered trademarks of third parties. Use of these marks is by way of example and shall not be construed as descriptive or limit the scope of this disclosure to material associated only with such marks.

Claims

1. A method of 3D printing which comprises forming a plurality of superposed layers within which at least each intermediate layer formed on an underlying layer is composed, within an area bounded only by an outer perimeter, of one or more materials, wherein each intermediate layer that comprises more than one material is formed by:a) depositing at least one coating of a first material on selected first regions within the outer perimeter by:(i) selectively applying an adhesive to, or selectively activating an adhesive in, the selected first regions, and(ii) adhering particles to the adhesive in the selected first regions,b) subsequently depositing a coating having a different composition from the previously applied coating(s) on all remaining regions within the outer perimeter of the intermediate layer not occupied by the first material;c) compacting at least the particles of the first material of each layer prior to the deposition of an overlying layer; andd) following application of the coatings over all the regions within the outer perimeter, ensuring that the coatings constituting the intermediate layer provide an even continuous surface onto which the overlying layer can be deposited.

2. The method as claimed in claim 1, wherein coating of the remaining regions of each intermediate layer is achieved by application or activation of an adhesive within said remaining regions and adhesion of particles thereto, at least one of the adhesive and the particles of the coating applied to said remaining regions being different from the adhesive and the particles of coating applied to the first regions.

3. The method as claimed in claim 2, wherein the particles of the remaining regions are compacted during and / or after compaction of the particles of the selected first regions, the compaction of all particles serving to provide the even continuous surface.

4. The method as claimed in claim 1, wherein coating of the remaining regions of each intermediate layer is achieved by application of a second material as a liquid within the perimeter, the liquid second material being applied after one or more cycles of deposition of the first material.

5. The method as claimed in claim 4, wherein following application of the liquid second material, the second material is solidified.

6. The method as claimed in claim 5, wherein the liquid second material is a curable resin that is solidified by curing.

7. The method as claimed in any one of claim 1 to claim 6, wherein the step of ensuring that the coatings constituting the intermediate layer provide an even continuous surface includes removing at least part of any coating of remaining regions overlying the selected first regions, by milling, grinding, and / or abrasion.

8. The method as claimed in any one of claim 1 to claim 7, wherein the coating of the regions is deposited in one cycle and at least the particles of the first material are adhered to a selectively activated adhesive applied to the entire surface of the underlying layer, and wherein the selective activation is a chemical and / or a physical activation, the latter comprising application of energy by heat conduction or radiation.

9. The method as claimed in any one of claim 1 to claim 7, wherein at least the particles of the first material deposited in the selected first regions of each cycle are adhered to an adhesive that is selectively applied to the underlying layer or an underlying coating by jetting, by use of a mask, or by transfer of a selective adhesive image.

10. The method as claimed in any one of claim 1 to claim 9, wherein following application of particles to an activated adhesive, surplus particles not in direct contact with the adhesive are removed.

11. The method as claimed in any one of claim 1 to claim 10, wherein compacting of regions formed by application of particles includes applying a pressure in a range of 1 to 300 MegaPascal (MPa), the applied pressure being optionally controllably modified for each coating or layer being compacted.

12. The method as claimed in claim 11, wherein energy is applied during compaction to raise the temperature of the particles to be compacted to at least one temperature in the range of 20°C to 500°C, the applied heat being optionally controllably modified for each coating or layer being compacted.

13. The method as claimed in any one of claim 1 to claim 12, wherein compacting is effected by a rotating cylinder controllably urged into rolling contact with the coating(s) to be compacted, the compressive force applied to the particles being optionally sufficient to causeplastic deformation of the particles in at least a part of the intermediate layer and the rotating cylinder being further optionally vibrated during compaction.

14. The method as claimed in any one of claim 1 to claim 13, wherein any adhesive applied to an underlying layer to adhere particles thereto is applied in a thickness of 10 pm or less, 5 pm or less, 2 pm or less, 1 pm or less, or 0.5 pm or less15. Themethod as claimed in any one of claim 1 to claim 14, wherein any adhesive applied to any underlying layer or any underlying coating to adhere particles thereto, is heat activatable and comprises a thermoplastic polymer and optionally an agent adapted to absorb EM radiation and / or an agent adapted to facilitate consolidation of the particles.

16. The method as claimed in any one of claim 1 to claim 15, wherein the materials applied in the form of particles are selected from the group comprising metals, ceramics, carbides, salts, and plastics materials.

17. The method as claimed in any one of claim 1 to claim 16, wherein materials applied in the form of particles have a particle size with an average longest dimension not exceeding 200 pm or 100 pm.

18. The method as claimed in any one of claim 1 to claim 17, wherein materials applied in the form of particles comprise clusters of individual particles including nanoparticles having an average longest dimension no greater than 200 nm and a binder.

19. The method as claimed in any one of claim 1 to claim 18, wherein the particles of at least one of the materials applied in particle form are composite particles, including two or more materials agglomerated, encapsulated, or coated one within the other.

20. The method as claimed in any one of claim 1 to claim 19, further comprising applying a sacrificial first layer onto a surface prior to forming a first intermediate layer.

21. The method as claimed in any one of claim 1 to claim 20, wherein following formation of all the layers, one of the materials is removed to leave a three-dimensional shape composed of the remaining materials and having voids corresponding to regions previously occupied by the removed material.

22. The method as claimed in claim 21, wherein the one material is removed by at least one of:a) a chemical treatment, the chemical treatment optionally adapted to solubilize the said one material; andb) a physical treatment, the physical treatment optionally including a mechanical treatment to dislodge particles of the said one material, or a thermal treatment to melt the said one material.

23. The method as claimed in any one of claim 1 to claim 22, further comprising consolidating the particles of the first material one with the others, the consolidation being by application of energy, said energy being optionally applied following the compacting of only some of the superposed layers.

24. The method as claimed in claim 23, wherein the first particles are consolidated whilst attached to a backing surface and the backing surface is optionally a rigid and / or flexible substrate adapted for the manufacturing of electronic components selected from a group including printed circuit boards (PCBs), radio frequency identification (RFID) tags, capacitors, batteries, Liquid Crystal Displays (LCDs), wave guides and solar cells.

25. A printing system for printing of a three-dimensional object by superposition of compacted layers, the system comprising:a) a transport mechanism comprising a movable carrier adapted to displace a backing surface along a direction of travel;b) an adhesive application module adapted to render tacky desired regions of the backing surface, or of a compacted layer previously formed thereon;c) a first dispensing module for applying particles of a first material to adhere selectively to the regions rendered tacky;d) a second dispensing module for applying another material having a composition different from the previously adhered particles to coat at least regions of the backing surface, or of a compacted layer previously formed thereon, not coated with the particles of the first material; ande) at least one compacting module for compressing the particles and / or the other material to form thereon a compacted layer or a part thereof;the transport mechanism serving to displace the movable carrier to pass cyclically through the adhesive application, dispensing and compacting modules so as to form superposed compacted layers on the movable carrier.

Images