A method of 3D printing

Abstract

The present invention provides a method of 3D printing for reducing, minimising or eliminating print artefacts caused by improper coalescence of ink or printing material deposited by a nozzle of a 3D printing system. The method is optimised to encourage or ensure proper or intended coalescence between two adjacent printed lines by the determination of a predicted dynamic overlap either between the adjacent lines, or between a first of the adjacent lines from a first of the two laterally adjacent nozzles and a subsequently deposited droplet (8) from the other of the two laterally adjacent nozzles and causing the control unit to adjust a parameter of the print process to ensure that the predicted dynamic overlap remains positive for the length of the two adjacent lines.

Description

[0001] A method of 3D printing

[0002] FIELD

[0003] The present invention relates to a method of 3D printing for reducing, minimising or eliminating print artefacts.

[0004] BACKGROUND

[0005] In 3D printing technologies, such as 3D inkjet printing or material jetting, the printing process involves a deposition of droplets or lines of ink, or printing material, onto a build platform, or a previously printed print layer to build up and form a product with a desired shape.

[0006] In some forms of 3D printing, there can be multiple nozzles all printing simultaneously to form a wider line of ink or printing material, or a patchwork of lines of ink or printing material. That wider line or patchwork of lines will typically be formed of multiple staggered lines of droplets arranged laterally side by side. The lines of droplets are staggered because in a multi-nozzle print head, the nozzles are commonly arranged in a plurality of arrays arranged longitudinally side by side, with the nozzles in each array staggered relative to the nozzles in the other arrays. This is so that all the nozzles can be laterally distributed along the print head. It is to be noted that the term “array” is commonly used in the 3D printing sector for a line of nozzles, rather than a block of nozzles.

[0007] The nozzles are fitted to or are part of a print head, and the build platform is usually positioned below the nozzles and the print head.

[0008] During printing, the print head, or the build platform, moves back and forth relative to the other. The print head, for example, may be mounted on or under a print head bridge, and the build platform may be underneath the print head and print head bridge. The print head bridge, and one or the other of the print head and the build platform, can then be configured for movement relative to the other of the print head and the print head bridge for printing a layer of the product as defined in a print strategy for the product. The printer thus follows the print strategy when forming the product. This print strategy controls the printer during each pass (multiple passes may be needed to form each layer) and it also defines a predetermined pattern for each layer, and more specifically for each printing pass.

[0009] In material jetting, the print head may be fixed relative to the print head bridge while the build platform moves underneath it, and thus just the build platform is moved for achieving the relative movement between it and the nozzles during a pass. If multiple passes are needed, then further passes are carried out.

[0010] In some prior art products the print head may be smaller than the depth of the build platform. The multiple passes may then be needed to print over the whole build platform. For this, the build platform might move also in a transverse direction for enabling this, or the print head might instead be moved along the print head bridge to a different position, before the printer then performs one or more further passes over the build platform. This can be repeated until the whole build platform has been printed over. More commonly for multi-nozzle 3D inkjet printers, however, either the print head has a length that extends across the full depth of the print platform, or there are multiple print heads along the print head bridge, whereby the printer prints a complete depth of the build platform in one pass. Thus, the print head(s) usually always remain(s) fixed relative to the print head bridge in the transverse direction.

[0011] There are also 3D printers where instead it is the print head that moves while the build platform remains stationary.

[0012] There are even 3D printers where both the build platform and the print head move at the same time across the width (or depth) of the apparatus. However, the present invention primarily concerns printers where print head is effectively stationary in the transverse direction on the print head bridge during movements of the build platform (i.e. during printing), and thus the printer is creating substantially straight and parallel printing lines when printing the product, i.e. during the movement of the print head relative to the build platform (albeit with the to-be- described inherent wobble).

[0013] During inkjet 3D printing, for example within a material jetting process, multiple printing passes are often required over the same part of the build platform for forming a given layer of printing material. These passes can be to ensure that all the droplets or lines correctly coalesce to create the requisite layer, or may enable the printing of different colours or materials - for example using additional print heads. It is known, however, to achieve different colours in one pass - for example by filling pixels / nozzles with different materials.

[0014] A printing pass refers to a single traversal (i.e. a movement from one end to the other of the build platform, or the printed product). This may be the movement of the 3D printer's print head over the build platform, or the movement of the build platform under the print head, or potentially both along a common main axis direction, although that is unusual. The deposited printing material is commonly a liquid, and it will generally remain in a liquid form during each pass. Thus, it can spread or flow, or coalesce with adjacent droplets or lines when touching, or if it spreads to the point of touching such adjacent droplets. Indeed, the printing material will usually remain in a liquid form until it is cured or otherwise solidified. That is because in a material jetting process, the printing material that is deposited during these passes needs to be cured or otherwise solidified before the ink or printing material will become set. This is typically a curing step and it may be, and usually is, carried out after each pass, and certainly needs to be done before the printing machine starts to form the next print layer (i.e. after a Z-axis displacement is undertaken).

[0015] Typically the ink or printing material is a UV curable liquid. It is cured with UV light at a curing step - usually under a curing lamp at the end of the travel of the build platform. Other solidifying mechanisms can be processes such as freezing or cooling, which are steps typically used for, for example, wax printing.

[0016] In some prior art systems, in order to avoid or mask undesirable coalescence patterns being formed by the lines or droplets of deposited printing material, four or more printing passes may be needed on top of each other in order to form a full layer. The curing can be done for each of these passes to prevent or control the coalescence of the droplets between the passes. The multiple passes per layer means that a considerable amount of time can be required to form a product, which product may be formed of hundreds of layers (i.e. hundreds of slices). For example, for a 15mm tall object, each layer may be 20pm thick and they would then be 750 full layers, and thus potentially 3000 passes if there are 4 passes per layer.

[0017] The fact that the printing material is in a liquid form can be further utilised in some print processes: a flattening roller can be used in some prior art systems, when needed, to flatten the layer formed by the multiple passes prior to curing the layer. It is noted, however, that this can lead to excessive pattern bleed, or other errors or print artefacts, particularly when printing with more than one colour of ink or printing material.

[0018] It is well known that these multiple passes, and the many layers thereof that form any given object, mean that 3D inkjet additive manufacturing of some objects is slow, and can result in the objects being expensive to manufacture in bulk in this manner.

[0019] The inventors have also realised that the movement(s) of the print head or build platform, although typically a linear movement along a main axis, is / are not perfectly straight. The inventors note that the movement can have an inherent wobble relative to the printer’s main axis. Commonly the wobble is only of a few tens of microns in amplitude, but that can still result in the droplets or lines being deposited with a certain waviness. The present inventors have identified this wobble, or the resulting waviness, as being a cause of an issue that can arise during 3D printing, which can also lead to print artefacts.

[0020] An object of the present invention is to try to avoid, reduce, minimise or eliminate any one or more of the above issues, or to improve the print quality or print speed from a 3D printer, such as a 3D inkjet printer, also known as a material jetting 3D printer.

[0021] The following prior art publications provide examples on how the prior art sought to mitigate against print artefacts: WO21247352 and US2023035868.

[0022] Regarding the above-mentioned prior art, it is remarked that any discussion of documents, acts, materials, devices, articles or the like included in the present specification is for the purpose of providing a context for the present invention, and is not to be taken as an admission that any such matters form part of the prior art or were common general knowledge in the field relevant to the present invention before the priority date of each claim of this application.

[0023] SUMMARY

[0024] According to the present invention there is provided a method of 3D printing for reducing, minimising or eliminating print artefacts caused by improper coalescence of ink or printing material deposited by a nozzle of a 3D printing system, the 3D printing system comprising: a discharge unit with at least one print head that has a plurality of nozzles for discharging ink or printing material; a build platform for supporting a 3D object as it is being printed by the discharge unit; one or more motor for moving the discharge unit, or the build platform, relative to the other of the discharge unit and the build platform, along a main axis direction; and a control unit for controlling the one or more motor and the discharge unit; wherein the plurality of nozzles are spaced apart and are distributed on the print head in a lateral direction; and two laterally adjacent nozzles of the plurality of nozzles are configured with a spacing there between in the main axis direction; and wherein the method comprises: controlling the discharge unit to deposit ink or printing material onto the build platform, or a previously printed print layer, in a series of passes to build up or generate one or more layers of the ink or printing material, each pass that deposits the ink or printing material depositing that ink or printing material according to a predefined print pass pattern by following a pre-configured print strategy for the control unit; and during at least one of the passes, two laterally adjacent nozzles of the plurality of nozzles will at least once each deposit droplets or lines of ink or printing material to form two adjacent lines of ink or printing material extending generally in the main axis direction, which two adjacent lines are intended to coalesce, those adjacent lines inherently having a degree of waviness relative to the main axis direction caused by wobble of the moving discharge unit or build platform; characterised in that the method comprises the step of generating the pre-configured print strategy to encourage or ensure proper or intended coalescence between the two adjacent lines, that step comprising a determination of a predicted dynamic overlap either between the adjacent lines, or between a first of the adjacent lines from a first of the two laterally adjacent nozzles and a subsequently deposited droplet from the other of the two laterally adjacent nozzles, and the print strategy causes the control unit to adjust a parameter of the print process to ensure that the predicted dynamic overlap remains positive for the length of the two adjacent lines.

[0025] When the predicted dynamic overlap remains positive, the lines will partially overlap and thus the lines will coalesce as intended. This ensures that unintended gaps between adjacent lines are not generated by the printer. Furthermore, this ensures that the top surface of the two lines properly coalesce to avoid creating excessively high peaks or ink or printing material during the pass.

[0026] In some embodiments, the determination of a predicted dynamic overlap takes into account both an anticipated waviness of the first or both of the adjacent lines, and different phasing of the waviness for the two adjacent lines. There will be a different phasing of the adjacent lines due to the spacing between the laterally adjacent nozzles in the main axis direction.

[0027] In some embodiments, the determination of the predicted dynamic overlap between the adjacent lines takes into account the spacing between the laterally adjacent nozzles in the main axis direction. However, in other embodiments it may instead take into account a longitudinal spacing between respective arrays within the print head that contain the two laterally adjacent nozzles and the alignment angle of the print head relative to the main axis direction. That is because a misalignment of the print head relative to the main axis direction of the printer will cause a variation of the relative phasing between the adjacent lines across the whole print head. That is because there will then effectively be an asymmetric overlap between the left and right neighbours throughout all the arrays, which thus increases the chance of an unwanted gap.

[0028] In some embodiments, the width of the first line or both lines, or the width of the droplets, or the width of the droplets when they have been deposited on the build platform (or a previously printed print layer), is taken into account, or predetermined, to ensure that unwanted gapping does not occur. For example, a width can be chosen, based upon the known phase shifts and the known amplitude of oscillation of the wobble, to ensure that no gaps are created.

[0029] In some embodiments, the print strategy varies the ink or printing material’s rate of flow to enable the droplet size, and thus the line widths, to be varied. In some embodiments this may result in the printed lines being wider or narrower at different times - wider when a greater extent of divergence is expected between the adjacent lines to close any otherwise potential gapping between the lines, and narrower when the lines are converging. More typically, however, a fixed width is chosen.

[0030] The anticipated waviness will also vary depending upon the movement speed of the print head or the movement speed of the build platform. Faster movements (usually with the intention of creating faster print speeds) tend to lead to a greater amplitude for the waviness. The period of the waves may also vary dependent upon the movement speed. The greater amplitude can in particular lead to a greater likelihood of a failure in the intended coalescence at some point along the two adjacent lines by virtue of the predicted dynamic overlap becoming negative due to the greater potential for a wider divergence of the respective lines i.e. when the phases oppose one another.

[0031] In some embodiments, the print strategy slows the movement speed to reduce the amplitude of the wobble, thus reducing the degree of divergence between the lines.

[0032] In some embodiments, the print strategy varies the degree of curing on a preceding layer to vary the extent of spread of the lines thereon. For example, a lower amount of curing (e.g. using a lower curing power) may lead to better spreading and more overlap with a subsequent pass.

[0033] In some embodiments, the print strategy varies the rate of depositing droplets for a given movement distance to again vary the width of the lines. In some embodiments, the 3D printing system is a 3D inkjet printing system for printing droplets of ink or printing material. Such a 3D printing system may alternatively be referred to as a material jetting 3D printer.

[0034] In some embodiments, the movement is powered by just one motor. For example, it may be underneath the platform, or to an end - for driving a belt drive or a worm-gear drive. In others, the movement may be powered by at least two motors - one to each side of the moving component - e.g. one at each side of the build platform, or one at each side of the bridge.

[0035] In some embodiments, the plurality of nozzles form at least two arrays of nozzles, each adjacent pair of nozzles being in a different array of nozzles. As indicated above, an array of nozzles is typically a linearly aligned set of nozzles.

[0036] In some embodiments, there are only two arrays of nozzles, or three or more arrays of nozzles. Typically there are four or more arrays of nozzles.

[0037] The arrays are typically configured parallel to one another, and spaced or distributed relative to one another in the longitudinal direction.

[0038] Adjacent arrays are typically located on the print head with a relative displacement versus its adjacent array both in the longitudinal direction and the lateral direction.

[0039] It is to be understood that the lateral direction extends substantially perpendicular to the main axis direction and the longitudinal direction extends perpendicular to the lateral direction. The angle of variance between the longitudinal direction and the main axis direction corresponds to a head alignment angle, often referred to as the Rz rotation.

[0040] In some embodiments, the print strategy instructs the control unit to reduce an ink volume in a region where the adjacent lines are converging and to increase an ink volume in a region where the adjacent lines are diverging.

[0041] In some embodiments, there is a third laterally adjacent nozzle that is configured within the print head to form a triangle of nozzles with the first two laterally adjacent nozzles, with a central one of those three laterally adjacent nozzles being spaced from the other two of those three laterally adjacent nozzles in the main axis direction. With the three laterally adjacent nozzles, a further advantageous method can occur - the predicted dynamic overlap is determined between the lines of the outer two nozzles and either the line of the central nozzle or a subsequently deposited droplet from the central nozzle, and the print strategy causes the control unit to adjust a parameter of the print process to ensure that the predicted dynamic overlap on both sides of the central line remains positive for the length of the three adjacent lines. In this manner, the correct coalescence can be ensured between all three lines.

[0042] The importance of correct coalescence can be significant. With proper coalescence, the liquid ink or printing material correctly levels between lines. When there are two lines, this is important for the creation of a substantially level print height between lines as the second line in the main movement direction will kick towards the first line when coalescence occurs. With a third line, it is even more important, as with three lines, the middle line can coalesce in both lateral directions. With an improper coalescence, or with one or the other side inadvertently forming a gap, the liquid can only coalesce one way, and thus kicks in that direction, preventing or reducing the amount of flow in the other direction, whereby the likelihood of such post-deposit flow triggering coalescence being significantly reduced. The gap thus will never fill. Irregular line heights and irregular print characteristics will thus result.

[0043] The liquid can also incorrectly pool within the non-coalesced line and start to flow or settle in the main axis direction, which can lead to further print issues, such as shape errors or poor print boundaries.

[0044] The present invention therefore seeks to avoid improper line or droplet coalescence between adjacent droplets and lines by ensuring the wobble is compensated for.

[0045] In some embodiments, the three laterally adjacent nozzles are each spaced from the other two of those three laterally adjacent nozzles in the main axis direction.

[0046] In some embodiments, all the nozzles are configured such that they form a set of three laterally adjacent nozzles with two other nozzles, and those sets of three laterally adjacent nozzles are all configured to form a triangle of nozzles, with at least a central one of those three laterally adjacent nozzles being spaced from the other two of those three laterally adjacent nozzles in the main axis direction.

[0047] Optionally, in these sets of three laterally adjacent nozzles, all three nozzles are positioned within different arrays of nozzles. To minimise artefacts in inkjet-based 3D printing technology several further measures can be taken. A proper print head calibration may contribute to an accurate droplet placement and precise layer alignment. This can help to reduce the extent of divergence between the lines as a result of the inherent waviness in the lines. Also a material and process optimisation may contribute in minimising the effect of that waviness - for example choosing a material with a suitable amount of post-print spreading (to correctly coalesce in the period between printing and curing). Process optimisation may also include a proper selection of printing parameters, like discharge timing, printing speed and resolution, which fine tuning may further reduce the effect of that waviness on the final print quality. Preferably therefore the method comprises a head calibration step where wobble data is collected and stored in a memory for use in generating the print strategy.

[0048] In some embodiments, the build platform is configured to move also in a z direction (up and down) to change its height between subsequent layers by a vertical movement relative to the print head. In other embodiments the change in vertical height occurs to the discharge unit.

[0049] In some embodiments, the main axis direction is referred to as an X-axis of the 3D printing system. A Y-axis is defined perpendicular to the main axis direction and is substantially parallel to the direction along which the arrays extend. There can be a misalignment between these directions - in practice this misalignment corresponds to the previously described Rz rotation to the axis pairs and direction pairs being all orthogonally arranged. The Z-axis is orthogonal to the plane formed by the X-axis and Y-axis.

[0050] In some embodiments, the wobble occurs due to a hobbling or rocking movement of the build platform as it moves along a rail, or a pair of rails. In some embodiments, the method involves a hardware calibration step in which irregularities in stage motion are detected and stored in the memory for use in generating the print strategy. Ideally, the build platform would move with a perfectly linear motion along its travel path (following, for example, the x axis). However, due to irregularities in the rail(s) and the build platform, and any gearing or drive systems, including the or each motor, in practice, the movement deviates from the perfect linear path along the X- axis with the above-mentioned wobble, which can be an oscillatory wobble defining a waviness. The waviness may not be regular, but typically it is substantially regular and predictable. In other words it is usually fairly repeatable, as it occurs similarly for subsequent passes. As such, it is relatively predictable upon carrying out the above hardware calibration step. The waviness is also reproduced simultaneously by the droplets and lines formed by the nozzles during the printing process, albeit with a phase displacement between adjacent lines due to the longitudinal spacing between the arrays.

[0051] Typically, the waviness repeated in the lines occurs as a Y shift from straight line extending in the x axis direction, that Y shift having a positive and negative amplitude. It can also have a Z shift, but that tends to even out due to the ink or printing material being a liquid. It is thus not seen to be particularly problematic. Nevertheless, there can be times when it might be. For example, a z-shift may actually lead to an x error: due to a z-shift, the time of flight of the droplet would potentially change if the z shift does not even out fast enough. That is because the droplet would then land too late (for a negative or downward z shift) or too early (for a positive or upward z shift). However, this error is nearly always going to be small, and typically insignificant. That is because usually the droplet speed is 6 m / s or more and the platform speed is 1 m / s or lower. That means, that for these numbers any z-error leads to a x-error that is at least 6x smaller. In a situation where the platform speed is instead 0.25m / s, it is at least 24x times smaller! For a 1.5m / s platform speed, it is at least 4x smaller. Therefore, the consequence of z-errors are considered negligible for the determination or avoidance of waviness for most scenarios, although different droplet speeds are also known - e.g. from 4m / s to 10m / s in common 3D inkjet printers.

[0052] From measured wobble data acquired from the hardware calibration step, the Y shift can be estimated for future movements of the build platform (or the print head if that is instead the component being moved during the passes).

[0053] The discharge unit is controlled to discharge simultaneously a neighbouring first and second line of inkjet droplets being spaced in Y-direction to obtain a printed surface. The first line is printed by the first nozzle and the second line is printed by the second nozzle. Each deposited line usually has a certain line width. A print of the first and second line by the first and second nozzle are both generally straight - corresponding with the direction of movement. Due to the offset of the nozzles, the lines are printed in a staggered manner. When looking at the printed surface, the offset in the main axis direction between the first and second nozzle causes a shift between these first and second lines in the travel direction. This is the phase shift.

[0054] As the lines include positive and negative amplitudes, due to the wobble, the phase shift results in the waveforms being shifted along the main axis direction relative to one another. As a result there can be areas in which the lines converge with each other and other areas in which the lines diverge from each other. If the printer makes no adjustments, then when the lines converge, relatively more ink is deposited in that area, which may result in a local elevation of the printed surface, while when the lines instead diverge, relatively less ink is deposited, which may result in a local deepening of the printed surface. Like gapping (i.e. areas where coalescence fails), these can also be undesirable if the ink is too viscous as these localised deepenings and elevations can create irregularities in the printed surface.. According to the invention, the print strategy can also be generated with commands for the control unit to compensate for these by increasing or decreasing the ink (or printing material) flow rate for these regions - increasing it to compensate for a localised deepening and decreasing it to compensate for a localised elevation. This will then also improve a surface uniformity for a given layer. However, more commonly the viscosity of the ink is chosen to enable such localised deepenings and elevations to be only temporary, with the liquid of the ink redistributing prior to curing, thus flattening the print. Indeed, this redistribution can be very quick - measured potentially in microseconds.

[0055] It is to be recognised that an excessive local divergence can also lead to gapping between the lines, which is important to avoid if possible when coalescence is required as such gapping can lead to an absence of coalescence - or improper flow of the ink or printing material, as will be explained below in greater detail.

[0056] With the present invention, therefore, the print strategy ensures that the control unit of the 3D printing system controls at least one 3D inkjet process parameter. This may be to reduce an ink volume in areas of line convergence on the printed surface, although that may be optional due to the natural redistribution resolving this issue, but to increase an ink volume in areas of line divergence on the printed surface to potentially mitigate the occurrence of deepenings, but to certainly try to avoid the instance of gapping in the printed surface.

[0057] As mentioned above, in a worst print scenario, in a print layer, a gap occurs in a diverging area of the printed surface. A gap occurs when neighbouring droplets do not coalesce - i.e. they do not flow into each other. In particular, gaps can appear when there is asymmetric coalescence: if the new line touches e.g. the left neighbour, but not the right neighbour, it will tend to immediately snap to the left neighbour, it thus pulling away from the right neighbour. The ink is then redistributed by the coalescence and the gap will not be closed. Therefore, the control unit is instructed by the print strategy to set at least one 3D inkjet process parameter for preventing such a possible gap from occurring between deposited lines of droplets where coalescence is intended. In an embodiment of the 3D printing system according to the invention, the at least one 3D inkjet process parameter may include at least one firing parameter for firing an ink droplet from a nozzle. The at least one firing parameter may comprise any one or more of a firing frequency, a firing timing and a firing droplet size. In particular, a firing parameter of each nozzle is controlled by the control unit by the instructions within the print strategy.

[0058] The control unit may be arranged to control the discharge unit in adapting a droplet size for each nozzle independently. Due to the use of the wobble data acquired at the hardware calibration step, it is possible to accurately predict the positions where the lines converge or diverge, based upon the phase shift resulting from the stagger in the longitudinal direction and the Rz rotation, and thus correctly control the droplet size for each nozzle accordingly when coalescence is required to be maintained. By adapting a droplet size to compensate for the phase shift an optimised surface finish can be achieved for the layer.

[0059] The firing parameter may include a control of a discharge timing, also called firing timing, for each nozzle. Instead of just simultaneously firing all nozzles at a fixed frequency, a discharge timing of adjacent nozzles may be adapted differently. For example, when based on the wobble data it can be foreseen that a second printed line converges towards a first printed line, so a discharge timing for the second printed line may be temporarily advanced, such that the ink droplets will be deposited further away from the first printed line (i.e. before the full convergence). The opposite may also be possible - to defer a discharge timing for the second nozzle when it is foreseen by using the wobble data that a second printed line diverges away from a first printed line (to increase the discharge later on - i.e. during the divergence). Beneficially, therefore, by providing particular discharge timings, a positioning of a droplet can be adapted to mitigate surface irregularities or to eliminate undesirable gapping.

[0060] It may also be similarly beneficial to adapt the speed of the movement during printing for controlling the amplitude of the wobble - or even simply to speed it up when no wobble (or only minimal wobble is anticipated, as then no or only minimal convergence or divergence will be occurring.

[0061] In some embodiments, the print strategy may define a printing material selection, with particular inks or printing materials having relatively faster or slower spreading capabilities. A fast spreading capability may ensure a correct coalescence in circumstances where a slower spreading capability may not. For example, a printer may comprise multiple different print heads along the x axis, and these may be provided for different materials, with the print strategy dictating use of one of these print heads rather than the other(s) for certain areas of the pass. This can then ensure proper infill of gaps in between deposited neighbouring droplets or lines, which can also be beneficial in mitigating artefacts caused by gapping, or deepenings or elevations. In other embodiments, the selected ink instead defines the print strategy, as the ink has known print qualities and thus its flow can be predicted and accounted for by the print strategy.

[0062] Where multiple print heads are provided, they may collectively be referred to as a print head system.

[0063] It is to be noted that a given line width can be defined for all the printing, which line width can be confirmed by the processor and software to be enough to compensate for the predicted gapping that would occur had the lines been narrower.

[0064] In some embodiments of the 3D inkjet printing process according to the invention, the print strategy requires a step of controlling a droplet temperature, or an environmental temperature around the build platform, for example to control the viscosity of the ink. By controlling the droplet temperature of the ink or printing material, flow properties on the build platform or a preceding ink layer may be controlled, and thus a coalescence of neighbouring droplets in the printing surface can be controlled to smoothen the printing surface. More usually, however, the printer works best at a fixed temperature for a given ink.

[0065] In some embodiments the print head, the print bridge or the build platform incorporates a driver for providing mechanical wobble compensation by counter wobbling against the inherent wobble, the method comprising driving the driver under the instructions of the print strategy. This may thus serve to cancel (or partially cancel) the inherent wobble from the use of the motor(s). In some embodiments, this alone can adequately provide wobble compensation to avoid gapping or improper coalescence due to the inherent (measured) wobble.

[0066] In another aspect of the present invention there is provided a computer program comprising computer executable instructions that, when executed by a processor, cause the processor to perform the method as defined above with the 3D printing system.

[0067] In another aspect of the present invention there is provided a computer-implemented method for controlling a 3D inkjet printing system for printing a 3D object by jetting inkjet droplets from at least two arrays of nozzles, the method being as defined above. In another aspect of the present invention there is provided a computer program product comprising a computer readable medium, the computer readable medium having computer readable code embodied therein, the computer readable code being configured such that, on execution by a suitable computer or processor, the computer or processor is caused to perform the computer-implemented method as defined above. The computer program product may for example be a computer, a USB stick or a cloud-storage provided with the computer readable medium.

[0068] In another aspect of the present invention there is provided a 3D printer comprising control electronics which are programmed to carry out the method as defined above or elsewhere herein. In particular, the present invention may provide a material jetting 3D printer comprising a build platform and a print head having at least one array of nozzles, wherein the material jetting 3D printer further comprises a controller programmed to cause the material jetting 3D printer to perform the method as defined above, or elsewhere herein.

[0069] BRIEF DESCRIPTION OF THE DRAWINGS

[0070] The invention will now be explained in more detail with reference to the appended drawings. The drawings show a practical embodiment according to the invention, which may not be interpreted as limiting the scope of the invention. Specific features may also be considered apart from the shown embodiment and may be taken into account in a broader context as a delimiting feature, not only for the shown embodiment but as a common feature for all embodiments falling within the scope of the appended claims, in which:

[0071] Fig. 1 schematically shows a side view of a printer for performing the method of the present invention;

[0072] Fig. 2 schematically shows 10 nozzles from a print head that comprises four arrays of nozzles, printing theoretically straight lines of ink;

[0073] Fig. 3 shows how three of those lines might actually print in the real world due to imperfections in the equipment, albeit noting the asymmetric axes (mm versus pm).

[0074] Fig. 4 shows how the real world print, ignoring sub-oscillations or micro-ticks, or noise, might more accurately be represented in the schematic form of Fig. 2.

[0075] Fig. 5 shows how thicker print lines fill the majority of the gaps between the lines; Figure 6 is similar to Figure 5, but shows how an Rz rotation (a misalignment of the print head) may affect the gapping between the lines;

[0076] Figure 7 schematically illustrates the potential overlap between a droplet and a first of two already printed lines on the build platform (or a previous print layer);

[0077] Figure 8 schematically illustrates the larger potential overlap between that droplet and the first of two already printed lines once the droplet has been deposited on the build platform (or a previous print layer) as an alternative characteristic to promote; and

[0078] Fig. 9 shows a newly printed droplet in the process of coalescence with a previously printed line (or lines).

[0079] DETAILED DESCRIPTION

[0080] For ease of annotation, throughout the text and drawings the term “pm” (i.e. the length measurement of “micron” or “micrometer”) may be shorthanded by “urn”.

[0081] Referring first of all to Figure 1 , there is schematically shown a side view of a printer 1 with a build platform 2, a print head bridge 3, three discharge units 4, 5, 6, each comprising a print head, and a UV lamp 7 for curing the ink or printing material after a pass or layer has been printed. In the illustrated example, each discharge unit 4, 5, 6 prints with a different material, and each prints multiple droplets 8. The print heads 4, 5, 6 and the build platform 2 are controlled by a control unit 9 that is connected by wires 10, 11 to the print head bridge 3 and a motor 12 for the build platform. During printing the build platform will move 13 relative to the print heads 4, 5, 6 and the UV lamp 7 and the droplets 8 will gradually build up the product 16 layer by layer through gradually decreasing the Z height of the build platform upon each layer completion 14 (or raising the bridge 3).

[0082] As previously mentioned, during normal printing operations it has been observed that print artefacts occasionally form on the upper surface of the print. These artefacts can be called a waviness of the print. The waves are generally perpendicular to the printing direction (the main axis direction or the x-axis), and generally parallel to the arrays of nozzles within the print heads 4, 5, 6. This lateral or transverse direction is the x-axis. The inventors noted that the period of the waviness along the x axis would usually be pretty constant, with test prints showing either approximately a 4 mm period or approximately an 8 mm, the period depending on the print head alignment. In the samples it was (about 4 mm for print heads that were substantially perfectly aligned with respect to the Rz rotation, and about 8 mm for print heads that have a slight RZ rotation. Upon much experimentation it was determined that a root cause of this phenomenon was a combination of two features. The first was “table wobble” - i.e. the wobble of the build platform as it moved along the x-axis under the power of the motor. The second was a lack of dynamic ink overlap caused by rather slow lateral spreading of the ink.

[0083] Regarding the first root cause, the inventors found that in a typical 3D inkjet printer where the table moves via a linear stage, and where the stage slid over two rails, driven by a spindle in the middle, due to slight imperfections of the spindle (which are always there in the real world), the table would be pushed alternatingly towards the left or the right rail. That resulted in an oscillating translation in y and / or a cyclical rotation around Rz during the movement 13 of the build platform 2. This wobbling motion was found to be very reproducible on a variety of movement mechanisms in 3D printers. Typically, however, the amount of wobble was small - the amplitude in y was found usually to be just a few tens of microns.

[0084] The second root cause was a lack of dynamic ink overlap. A schematic exaggeration of this is shown in Figure 5. The cause of this, however, needed some study, which revealed that it was caused by the presence of multiple arrays 20, 21 , 22, 23 in a typical print head 4, 5, 6, each having a longitudinal displacement from the other as they are fitted together side by side.

[0085] Figure 2 shows in part an example of a typical print head 4 as used in a 3d inkjet printer 1 , showing a typical print head layout. As shown in this Figure, this print head 4 has four arrays 20, 21 , 22, 23 of nozzles 24. Furthermore, all the nozzles are distributed laterally along the print head so as to have even spacing 25 therebetween in that lateral direction. The nozzles 24 in each array are not laterally adjacent with respect to the print head, but instead each laterally adjacent nozzle is in a different array to its laterally adjacent nozzle. This enables the nozzles to be configured at a closer lateral displacement with respect to one another, even though individual arrays have a minimum spacing between adjacent nozzles that is larger than that lateral spacing (due to the manufacturing constraints of the arrays or the desire to retain a commercially viable cost for the arrays).

[0086] The print heads 4, 5, 6 will be mounted to the print head bridge 3 using a mounting guide to help ensure good alignment. However, getting a perfect alignment of the arrays 20, 21 , 22, 23 within the printer 1 (i.e. relative to the main axis direction x - to which they will lie substantially perpendicular) is in practice very difficult. Figure 2 also shows lines representing the intended printed lines from the nozzles 24. These print lines are intentionally shown to be narrower than needed for the lines to naturally coalesce with one another just to ensure that the separation of the lines is clear to see.

[0087] Figure 4 instead shows schematically how these lines may actually appear on the print surface due to the wobble of the moving build platform (Figure 3 shows actual plots of the wobble, with various sub-oscillations or micro-ticks, or noise, therealong; these are excluded from consideration as they are less predictable and only brief, and they rarely exceed the amplitudes of the main waviness of the plot lines; they may even just be measurement errors).

[0088] As can be seen in Figure 4, rather than a series of substantially parallel lines, as shown in Figure 2, the print lines are actually a series of oscillating lines that have different x-positions for their starting point, even though they commenced their printing at the same time. This varying start point is due to the longitudinal spacing between the arrays of nozzles. Those varying start points effectively create a phase shift in the respective waveforms formed by those lines.

[0089] A consequence of those phase shifts is that there are positions along the x axis where the lines are all approximately equidistant from one another in y (e.g. at around x = 3 mm, around x = 11 , etc). However, there are also x-positions where two neighbouring lines are touching. Furthermore, there are positions in x where there are much larger gaps between neighbouring lines (e.g. at around x = 5 mm and x = 13mm, etc). The inventors noted that it is those wider gaps that lead to issues in the print quality. The reasoning behind this is better understandable by making similar plots, but then including a wider line width for each line. This is plotted in Figures 5 and 6. In these plots, the line width is 90 pm - more than wide enough to bridge between adjacent print lines if the print lines were as straight as in Figure 2.

[0090] In these plots, the period of the table wobble is 7.8 mm and the amplitude is 30 urn. The graph of Figure 5 is for a perfectly aligned print head with respect to Rz, whereas Figure 6 is for a print head with a misalignment (Rz rotation) of 0.5°.

[0091] As can be seen in Figure 5, despite the width of the line being supposedly adequate to bridge between adjacent lines, and thus achieve good coalescence between adjacent print lines, the wobble creates gaps every 3.9 mm. This is half the period of the table wobble. Figure 6, however, instead has some very small, and likely insignificant gaps, plus also much wider gaps every 7.8 mm (the same period as the table wobble). At the x-positions where the gaps appear, there are also regions amongst other print lines with essentially 2 lines on top of each other.

[0092] It can be seen in practice that the printed ink in the latter scenario will redistribute away from the positions where there are two lines on top of each other, leaving an acceptable print surface laterally. However, because of the nearby gaps, and the nature of the fluid to follow lines of coalescence or the existing print lines, that redistribution will not or cannot fill those gaps, resulting also in a transportation of a volume of the ink along the x direction, rather than just the y direction. This ink transportation along the x direction, and the absence of ink in the gaps, leads to the surface waviness in the final cured print, which waviness follows the period of the gaps, i.e. 3.9mm or 7.8mm, as witnessed at the start.

[0093] Another characteristic was also noted during the testing process: the amplitude of the wobble depends on the print (movement of build platform) speed. A factor of 2 decrease in amplitude was noted when moving at 270 mm / s compared to 300 mm / s. This was found to be in line with waviness experiments, where smaller waviness amplitudes in the print surface were seen for samples printed at 270 mm / s compared to samples printed at 300 mm / s.

[0094] From these tests it was determined that line thicknesses that are wider than first thought are necessary to eliminate the problematic waviness on the print surface, and it is possible to determine an optimum line thickness for eliminating the gaps once the amplitude and period of the oscillations are known, along with the off-sets in the main axis direction of the adjacent nozzles. Furthermore it was determined that controlling the speed of printing can reduce the occurrence of gapping, and thus allow narrower lines to be used. The present invention implements this in the manner as claimed, which considers the dynamic overlap between neighbouring print lines.

[0095] Figure 9 simply shows the coalescing droplet 8 before it has redistributed Q its liquid with the neighbouring line 30. However, it also shows that the coalescing droplet 8 has a contact angle 0 with the surface on which it has been deposited. That angle can determine the extent to which the droplet will tend to flow as a steeper angle will normally encourage lateral flow and a shallower angle will tend to resist lateral flow. Whether that flow is “spreading” will depend on whether the liquid is already snapping towards the other direction due to coalescence, along with well know fluid dynamic principles.

[0096] The leading edge of the line will likewise have a contact angle, which can vary upon the speed of the movement, the speed of the droplet, and other well know fluid dynamic principles. As these contact angles are not fixed, and will vary both over time and dependent upon the print strategy and printing parameters, they can be referred to as dynamic contact angles. The print parameters can thus be set to encourage or dissuade flow - particularly spreading and coalescence speed.

[0097] The main question relevant for determining whether waviness will occur is whether gaps between neighbouring lines appear or not. If there are gaps, the ink transport mentioned before will be for a significant part along the x-direction. Whereas in case there are no gaps, the ink transport will be dominantly in the y-direction, actually smoothening all kinds of errors. Therefore, the present inventors sought to calculate whether gaps appear or not and to ensure that a print strategy prevents such gaps from occurring unintentionally.

[0098] The existence of gaps in a print depends on the amplitude of the wobble on the one hand and on the line width on the other hand. Determining the correct line width, or better ensuring a positive dynamic overlap, is the aim.

[0099] Due to the high x-resolution of droplets during printing it is safe to assume that lines, rather than droplets, are being deposited. A cross-section along the y-direction of a three line print is schematically shown in Figures 8 and 9. These show two slightly different scenarios. However, the main rationale is the same for both scenarios and will be explained first: they both show a cross-section along the y-direction at the position of three adjacent arrays. The two lower arcs represent lines that are printed with nozzles of a first and third array (or possibly the same array), and are already on the substrate (i.e. the build platform or a preceding layer). They each have a certain width, depending on the dynamic contact angle, the drop volume, the viscosity of the liquid, and the x-resolution of the print. They can be measured from test data for a given ink, or they can be calculated using known techniques. The circular “line” is a new line or droplet printed with a nozzle of a second array - which nozzle will be spaced in the main axis direction (x axis) from the earlier nozzles, and hence is encountering already printed lines.

[0100] As can be seen in Figure 8, this droplet, with a given and known drop radius, has a certain overlap with the printed line from the other nozzles (which themselves have a known spacing between them, as the nozzle pitch is known). It is also known that the droplet will be nominally centred between those neighbouring lines, assuming all nozzles are printing in their intended directions, and are not blocked or otherwise damaged. However, due to the table wobble, this droplet will mostly not be centered in the middle of the gap between the two printed lines as it is at a different phase to those earlier lines. Furthermore, if the table wobble amplitude is big enough to negate that nominal overlap, a gap will appear and hence waviness will occur. The droplet diameter thus needs to be wide enough to compensate for the wobble to avoid gaps appearing in the print surface, assuming that the droplet - upon being deposited on the print surface, does not spread laterally.

[0101] Whether it spreads laterally - and to what degree - will depend upon a few characteristics of the droplet and the surface on which it is landing. However, if it lands in a manner where it is contacting one of the other lines but not the other, a lower chance of it spreading laterally towards the non-contacted line will occur due to the snap-effect of coalescence with the first- contacted line. Nevertheless, some degree of spread in both directions is likely. Figure 9 considers or illustrates a theoretical maximum likely spread (corresponding to the spread of the earlier printed lines).

[0102] As mentioned above, the actual spread that occurs will be dependent upon whether it contacts one or both of the lines to either side of it, and also the time between contact and curing as the ink can continue to spread until it is cured. This time period can vary dependent upon print speed as well - again with longer (and thus slower) passes encouraging wider spreads. Nevertheless, the typical relevant time scale is usually only a few ms up to a few tens of ms, depending on the print speed. For this reason, it is a dynamic overlap, instead of just an overlap, as it varies over time.

[0103] The inventors realise, nevertheless, that as soon as a gap appears, ink transport in the x direction occurs, and the damage may be done, so the present invention serves to avoid those gaps by controlling the print parameters - most usually the line widths and thus the droplet size or frequency, and print speed, and thus spread amount and wobble amplitude.

[0104] Theoretically, it is possible for gaps to refill due to ink spread and secondary coalescence (once the spread is enough to close the gaps), provided that the final wet volume and viscosity of the ink is sufficient and appropriate, and that sufficient time is allowed between printing and curing. These parameters thus may also be controlled.

[0105] It is to be appreciated that waiting for ink spread and secondary coalescence can be a relatively slow process versus printing with proper coalescence in the first place, with little energy savings. Therefore, in order to prevent waviness, and to print efficiently in terms of time spent, the present invention prefers to prevent the creation of gaps instead of trying to allow any gaps to refill with time. The present invention therefore aims to ensure a dynamic overlap that is bigger than the table wobble to prevent waviness in the final print surface, with multiple ways to achieve this, including at least a) using fast spreading inks or variable curing powers applied to previous passes or layers, b) using an adequately large droplet volume to ensure that a printed line is wide enough to eliminate gaps, c) using a high x resolution again to ensure that a printed line is wide enough to eliminate gaps, but also to slow the print down and thus to reduce the amplitude of the wobble or to increase the amount of spread of the line, and d) using a movement mechanism for the build platform relative to the print head that has less table wobble!

[0106] It is of course to be appreciated that 3D inkjet additive manufacturing as used by embodiments of the invention may fabricate objects based on three-dimensional (3D) information, for example a three-dimensional computer model (or design file), of the object. Accordingly, examples described herein not only include objects as described herein, but also methods of manufacturing such objects via additive manufacturing and computer software, firmware or hardware for controlling the manufacture of such products via additive manufacturing.

[0107] The structure of one or more objects may be represented digitally in the form of a design file. A design file, or computer aided design (CAD) file, is a configuration file that encodes one or more of the surface or volumetric configuration of the shape of the object. That is, a design file represents the geometrical arrangement or shape of the object.

[0108] Design files can take any now known or later developed file format. For example, design files may be in the Stereolithography or “Standard Tessellation Language” (.stl) format which was created for stereolithography CAD programs of 3D Systems, or the Additive Manufacturing File (.amf) format, which is an American Society of Mechanical Engineers (ASME) standard that is an extensible markup-language (XML) based format designed to allow any CAD software to describe the shape and composition of any three-dimensional object to be fabricated on any additive manufacturing printer.

[0109] Further examples of design file formats include AutoCAD (.dwg) files, Blender (.blend) files, Parasolid (,x_t) files, 3D Manufacturing Format (,3mf) files, Autodesk (3ds) files, Collada (.dae) files and Wavefront ( obj) files, although many other file formats exist.

[0110] Design files can be produced using modelling (e.g. CAD modelling) software and / or through scanning the surface of a product to measure the surface configuration of the product. Once obtained, a design file may be converted into a set of computer executable instructions that, once executed by a processer, cause the processor to control an additive manufacturing apparatus to produce an object according to the geometrical arrangement specified in the design file. These instructions typically provide the print strategy. The conversion may convert the design file into slices or layers that are to be formed sequentially by the additive manufacturing apparatus. The instructions (otherwise known as geometric code or “G-code”) may be calibrated to the specific additive manufacturing apparatus and may specify the location and amount of material that is to be formed at each stage in the manufacturing process. The instructions may be according to an embodiment of the invention.

[0111] The code or instructions may be translated between different formats, converted into a set of data signals and transmitted, received as a set of data signals and converted to code, stored, etc., as necessary. The instructions may be an input to the printer 1 and may come from a part designer, an intellectual property (IP) provider, a design company, the operator or owner of the additive manufacturing system, or from other sources. The printer may execute the instructions to fabricate an object according to an embodiment of the invention.

[0112] Design files or computer executable instructions may be stored in a (transitory or non-transitory) computer readable storage medium (e.g., memory, storage system, etc.) storing code, or computer readable instructions, representative of the object to be produced. As noted, the code or computer readable instructions define the object and can be used to physically generate the object, upon execution of the code or instructions by an additive manufacturing system. For example, the instructions may include a precisely defined 3D model of the object and can be generated from any of a large variety of well-known computer aided design (CAD) software systems such as AutoCAD®, TurboCAD®, DesignCAD, 3D Max, etc.. Alternatively, a model or prototype of the object may be scanned to determine the three-dimensional information of the component.

[0113] Accordingly, by controlling the printer according to the computer executable instructions, the printer can be instructed to print the object in accordance with a method of the invention.

[0114] In light of the above, embodiments include methods of manufacture via material jetting 3D printing. This includes the steps of obtaining a design file representing the object and instructing a printer to print the object according to the design file. The printer may include a processor that is configured to automatically convert the design file into computer executable instructions for controlling the manufacture of the object. In these embodiments, the design file itself can automatically cause the production of the object once input into the printer. Accordingly, in this embodiment, the design file itself may be considered computer executable instructions that cause the printer to manufacture the object. Alternatively, the design file may be converted into instructions by an external computing system, with the resulting computer executable instructions being provided to the printer.

[0115] Given the above, the design and manufacture of implementations of the subject matter and the operations described in this specification can be realized using digital electronic circuitry, or in computer software, firmware, or hardware, including the structures disclosed in this specification and their structural equivalents, or in combinations of one or more of them. For instance, hardware may include processors, microprocessors, electronic circuitry, electronic components, integrated circuits, etc. Implementations of the subject matter described in this specification can be realized using one or more computer programs, i.e. , one or more modules of computer program instructions, encoded on computer storage medium for execution by, or to control the operation of, data processing apparatus. Alternatively or in addition, the program instructions can be encoded on an artificially-generated propagated signal, e.g., a machinegenerated electrical, optical, or electromagnetic signal that is generated to encode information for transmission to suitable receiver apparatus for execution by a data processing apparatus. A computer storage medium can be, or be included in, a computer-readable storage device, a computer-readable storage substrate, a random or serial access memory array or device, or a combination of one or more of them. Moreover, while a computer storage medium is not a propagated signal, a computer storage medium can be a source or destination of computer program instructions encoded in an artificially-generated propagated signal. The computer storage medium can also be, or be included in, one or more separate physical components or media (e.g., multiple CDs, disks, or other storage devices).

[0116] While specific embodiments of the invention have been described above, the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to a person skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

CLAIMS1 . A method of 3D printing for reducing, minimising or eliminating print artefacts caused by improper coalescence of ink or printing material deposited by a nozzle of a 3D printing system, the 3D printing system comprising: a discharge unit with at least one print head that has a plurality of nozzles for discharging ink or printing material; a build platform for supporting a 3D object as it is being printed by the discharge unit; one or more motor for moving the discharge unit, or the build platform, relative to the other of the discharge unit and the build platform, along a main axis direction; and a control unit for controlling the one or more motor and the discharge unit; wherein the plurality of nozzles are spaced apart and are distributed on the print head in a lateral direction; and two laterally adjacent nozzles of the plurality of nozzles are configured with a spacing there between in the main axis direction; and wherein the method comprises: controlling the discharge unit to deposit ink or printing material onto the build platform, or a previously printed print layer, in a series of passes to build up or generate one or more layers of the ink or printing material, each pass that deposits the ink or printing material depositing that ink or printing material according to a predefined print pass pattern by following a pre-configured print strategy for the control unit; and during at least one of the passes, two laterally adjacent nozzles of the plurality of nozzles will at least once each deposit droplets or lines of ink or printing material to form two adjacent lines of ink or printing material extending generally in the main axis direction, which two adjacent lines are intended to coalesce, those adjacent lines inherently having a degree of waviness relative to the main axis direction caused by wobble of the moving discharge unit or build platform; characterised in that the method comprises the step of generating the pre-configured print strategy to encourage or ensure proper or intended coalescence between the two adjacent lines, that step comprising a determination of a predicted dynamic overlap either between the adjacent lines, or between a first of the adjacent lines from a first of the two laterally adjacent nozzles and a subsequently deposited droplet from the other of the two laterally adjacent nozzles, and the print strategy causes the control unit to adjust a parameter of the print process to ensure that the predicted dynamic overlap remains positive for the length of the two adjacent lines.

2. The method of claim 1 , wherein the determination of a predicted dynamic overlap takes into account both an anticipated waviness of the first or both of the adjacent lines, and different phasing of the waviness for the two adjacent lines.

3. The method of claim 1 or claim 2, wherein the determination of the predicted dynamic overlap between the adjacent lines takes into account the spacing between the laterally adjacent nozzles in the main axis direction.

4. The method of claim 1 or claim 2, wherein the determination of the predicted overlap takes into account a longitudinal spacing between respective arrays within the print head that contain the two laterally adjacent nozzles and the alignment angle of the print head relative to the main axis direction.

5. The method of any one of the preceding claims, wherein the width of the first line or both lines, or the width of the droplets, or the width of the droplets when they have been deposited on the build platform (or a previously printed print layer), is taken into account6. The method of any one of the preceding claims, wherein the print strategy varies the ink or printing material’s rate of flow to enable the droplet size, and thus the line widths, to be varied.

7. The method of any one of the preceding claims, wherein the print strategy slows the movement speed.

8. The method of any one of the preceding claims, wherein the print strategy varies the degree of curing on a preceding layer.

9. The method of any one of the preceding claims, wherein the print strategy varies the rate of depositing droplets for a given movement distance.

10. The method of any one of the preceding claims, wherein the 3D printing system is a 3D inkjet printing system for printing droplets of ink or printing material.11 . The method of any one of the preceding claims, wherein the movement is powered by just one motor.

12. The method of any one of the preceding claims, wherein the plurality of nozzles form at least two arrays of nozzles, each adjacent pair of nozzles being in a different array of nozzles.

13. The method of any one of the preceding claims, wherein there are only two arrays of nozzles.

14. The method of any one of claims 1 to 12, wherein there are three or more arrays of nozzles.

15. The method of any one of the preceding claims, wherein the arrays are configured parallel to one another, and spaced or distributed relative to one another in the longitudinal direction.

16. The method of any one of the preceding claims, wherein adjacent arrays are located on the print head with a relative displacement versus its adjacent array both in the longitudinal direction and the lateral direction.

17. The method of any one of the preceding claims, wherein the print strategy instructs the control unit to reduce an ink volume in a region where the adjacent lines are converging and to increase an ink volume in a region where the adjacent lines are diverging.

18. The method of any one of the preceding claims, wherein there is a third laterally adjacent nozzle that is configured within the print head to form a triangle of nozzles with the first two laterally adjacent nozzles, with a central one of those three laterally adjacent nozzles being spaced from the other two of those three laterally adjacent nozzles in the main axis direction.

19. The method of claim 18, wherein the predicted dynamic overlap is determined between the lines of the outer two nozzles and either the line of the central nozzle or a subsequently deposited droplet from the central nozzle, and the print strategy causes the control unit to adjust a parameter of the print process to ensure that the predicted dynamic overlap on both sides of the central line remains positive for the length of the three adjacent lines.

20. The method of claim 18 or claim 19, wherein the three laterally adjacent nozzles are each spaced from the other two of those three laterally adjacent nozzles in the main axis direction.

21. The method of any one of the preceding claims, wherein all the nozzles are configured such that they form a set of three laterally adjacent nozzles with two other nozzles, and those sets of three laterally adjacent nozzles are all configured to form a triangle of nozzles, with at least a central one of those three laterally adjacent nozzles being spaced from the other two of those three laterally adjacent nozzles in the main axis direction.

22. The method of any one of the preceding claims, wherein the method comprises a head calibration step where wobble data is collected and stored in a memory for use in generating the print strategy.

23. The method of any one of the preceding claims, wherein the method involves a hardware calibration step in which irregularities in stage motion are detected and stored in the memory for use in generating the print strategy.

24. The method of any one of the preceding claims, wherein the print strategy is generated with commands for the control unit to compensate for localised deepenings and elevations by increasing or decreasing the ink or printing material flow rate for these regions - increasing it to compensate for a localised deepening and decreasing it to compensate for a localised elevation.

25. The method of any one of the preceding claims, wherein the print strategy ensures that the control unit of the 3D printing system controls at least one 3D inkjet process parameter for at least one of these purposes: a) to reduce an ink volume in areas of line convergence on the printed surface, or b) to increase an ink volume in areas of line divergence on the printed surface; or c) to try to avoid the instance of gapping in the printed surface.

26. The method of any one of the preceding claims, wherein the control unit is instructed by the print strategy to set at least one 3D inkjet process parameter for preventing a possible gap from occurring between deposited lines of droplets where coalescence is intended.

27. The method of claim 25 or claim 26, wherein the at least one 3D inkjet process parameter include at least one firing parameter for firing an ink droplet from a nozzle.

28. The method of claim 27, wherein firing parameter may include a control of a discharge timing, also called firing timing, for each nozzle.

29. The method of any one of the preceding claims, wherein the control unit is arranged to control the discharge unit in adapting a droplet size for each nozzle independently.

30. A computer-implemented method for controlling a 3D inkjet printing system for printing a 3D object by jetting inkjet droplets from at least two arrays of nozzles, the method being as defined in any one of the preceding claims.31 . A computer program comprising computer executable instructions that, when executed by a processor, cause the processor to control a 3D inkjet printing system to perform the method as defined in any one of the preceding claims.

32. A computer readable medium, the computer readable medium having computer readable code embodied therein, the computer readable code being configured such that, on execution by a suitable controller, computer or processor, the controller, computer or processor is caused to perform the computer-implemented method of claim 30.

33. A 3D printer comprising control electronics which are programmed to carry out the method as defined in any one of claims 1 to 30.

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