Process Control Method For A 3D-Printing Process

Abstract

A process control method for a 3D-printing process using a 3D printer. The 3D printer has a build platform, a light source, a receiving device for printing material and a control device by which an object can be produced layer-wise or continuously from the printing material. The method includes using a thermal imaging camera, the output signal of which is transmitted to the control device, connected to the 3D printer, The method includes the following steps: illuminating a layer or parts of said layer positionally selectively, detecting the temperature of the layer during the polymerisation using the thermal imaging camera, ending the building process of a layer by ending the illumination, the time of the end of the illumination being established by a predefined temperature T.sub.max or a predefined change in temperature dT/dt being reached.

Claims

1. A process control method for a 3D-printing process using a 3D printer which comprises a build platform, a light source, a receiving device for printing material and a control device wherein an object is produced layer-wise or continuously from the printing material, wherein a thermal imaging camera, the output signal of which is transmitted to the control device, is connected to the 3D printer, and the method comprising the following steps: illuminating selective positions of a layer or parts of the layer, detecting the temperature of the layer during polymerisation using the thermal imaging camera, ending a building process of a layer by ending the illumination, the time of the end of the illumination being established by reaching a predefined temperature T.sub.max or a predefined change in temperature dT/dt.

2. The method according to claim 1, wherein the predefined temperature T.sub.max is established as the maximum heating temperature detected at a surface of the polymerising layer by the thermal imaging camera and the change in temperature dT/dt is established as converging towards zero.

3. The method according to claim 1, wherein thermal radiation of the layer is detected by a thermal imaging camera, the change in temperature dT/dt corresponding to a first derivative of a temperature curve, a positive change in temperature (dT/dt>0) indicating heating of a surface of the layer, a negative change in temperature (dT/dt<0) indicating cooling of the surface of the layer, and no change in temperature (dT/dt=0) indicating a constant temperature or a maximum/minimum of a temperature curve (T.sub.max or T.sub.min).

4. The method according to claim 1, wherein the receiving device comprises a film, wherein a change in temperature over a surface of the film is detected in a spatially resolved manner by the thermal imaging camera and passed to the control device.

5. The method according to claim 1, wherein the receiving device comprises a film, wherein, after the illumination ends, the layer is detached from the film and the build platform is raised, new material to be polymerised arriving in a resulting gap, and the detachment of the object from the film being recognised by the thermal imaging camera as a large decrease in temperature (dT/dt<<0).

6. The method according to claim 1, wherein the receiving device comprises a film with an inhibitor layer, wherein, during the illumination, the build platform is raised continuously at a defined speed, new material to be polymerised arriving in a resulting gap, and wherein an inhibitor layer prevents adhesion to the film.

7. The method according to claim 1, wherein the build platform is raised incrementally between the illumination times with a defined step height and subsequently lowered to a desired layer thickness, new material to be polymerised arriving in a resulting gap and being cured in the next illumination step.

8. The method according to claim 7, wherein the build platform is raised diagonally starting from one side or edge, to a height of 1 cm between the illumination times, and wherein the build platform is subsequently orientated substantially horizontally and subsequently lowered to the desired layer thickness.

9. The method according to claim 1, wherein the material flows with or without gas bubbles between the build platform and the film and is detected by the thermal imaging camera as a change in temperature (dT/dt>0 or dT/dt<0) in each case and recognised in the control device.

10. The method according to claim 1, wherein the receiving device comprises a film with an inhibitor layer, wherein impurities due to sedimentation, contamination, adhesion of residues from any erroneous building process or of foreign particles on the film or inhibitor layer are detected by the thermal imaging camera and are recognised in the control device by an excessively low detected change in temperature, optionally during movement of the build platform.

11. The method according to claim 1, wherein the receiving device comprises a film, wherein the illumination process of a next step only starts when the thermal imaging camera records and the control device recognises that no gas bubble(s) or impurities have been left behind on the film.

12. The method according to claim 11, wherein the illumination process of the next step only starts when the thermal imaging camera records and the control device recognises that after a change in temperature exceeding a predetermined threshold no further change in temperature is measurable.

13. The method according to claim 1, wherein the receiving device comprises a film, wherein the control device outputs an error signal or warning signal when the thermal imaging camera records that the temperature distribution is not plausible over the entire film surface because gas bubbles or small impurities have been left behind on the film or an inhibitor layer on the film, between the object and the film or inhibitor layer.

14. The method according to claim 1, wherein the control device outputs an error signal or warning signal when the thermal imaging camera records and the control device detects that the thermal emission does not change within a predefined time at the start of the illumination.

15. The method according to claim 1, wherein the illumination process is delayed, paused, or aborted by the control device if the control device outputs a warning signal and wherein subsequently the building process of the next layers is adapted or the previous illumination process is repeated.

16. The method according to claim 1, wherein the dimensions of each printed layer are tracked during polymerisation using multi-dimensional spatially resolved temperature measurement, the dimensions of the illuminated layer being compared against the real dimensions of the preceding layer and against the theoretical dimensions of the object, the next layer being reduced if the size of the illuminated area decreases and enlarged if it increases.

17. A process control method for a 3D-printing process comprising a stereolithography process and/or a DLP (digital light processing) process, using a 3D printer which comprises a build platform, a light source, a receiving device for printing material and a control device by which an object can be produced layer-wise or continuously from the printing material, wherein a thermal imaging camera and a digital micromirror device (DMD) unit are connected to the 3D printer, the output signal of the thermal imaging camera being transmitted to the control device and the DMD unit being controlled by the control device, and the method comprising the following steps: illuminating a layer or parts of the layer positionally selectively, detecting the temperature of the layer during the polymerisation using the thermal imaging camera.

18. The method according to claim 17, wherein radiation can be guided away from the layer to be illuminated and/or from already cured regions of the layer by the DMD unit controlled by the control device.

19. A computer program product comprising program code which is stored on a non-transitory machine-readable medium, the machine-readable medium comprising computer instructions executable by a processor, which computer instructions cause the processor to perform the method according to claim 1.

20. A 3D printer comprising a build platform, a light source, a receiving device for printing material, a control device by which an object can be produced layer-wise or continuously from the printing material, a thermal imaging camera, the output signal of which can be transmitted to the control device, whereby the 3D printer is configured to perform the following steps: illuminating a layer of printing material, detecting a temperature of the layer during polymerisation using the thermal imaging camera, ending the building process of the layer by ending the illumination, the exact time of the end of the illumination being established by a predefined temperature or a predefined change in temperature being reached.

21. The method according to claim 1, wherein the 3D-printing process comprises a stereolithography process and/or a DLP (digital light processing) process.

22. The method according to claim 2, wherein the change in temperature dT/dt is zero.

23. The method according to claim 6, wherein the defined speed comprises 0.5 mm/min.

24. The method according to claim 7, wherein the defined step height comprises around 0.3 to 3 cm.

25. The method according to claim 14, wherein the predefined time comprises approximately 2 seconds.

26. The method according to claim 16, wherein the multi-dimensional spatially resolved temperature measurement is 2-dimensional.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0082] Further advantages, details and features will be apparent from the following description of a plurality of embodiments of the invention with reference to the drawings, in which:

[0083] FIG. 1 is a schematic drawing of the lateral cross section of a 3D printer according to the invention in a first embodiment;

[0084] FIG. 2 is a schematic drawing of the lateral cross section of a further embodiment of a 3D printer according to the invention;

[0085] FIG. 3 is a schematic drawing of the light output, the temperature progression and the z-position of the build platform during the 3D-printing process;

[0086] FIG. 4 is a further schematic drawing of the light output, the temperature progression and the z-position of the build platform during the 3D-printing process; and

[0087] FIG. 5 is a schematic drawing of an optical construction of a further embodiment of a 3D printer according to the invention.

DETAILED DESCRIPTION

[0088] FIG. 1 is a schematic drawing of the lateral cross section of a 3D printer 2 according to the invention for a stereolithography process or digital light processing process, having a build platform 4, a receiving or holding device 10 for printing material 8, a thermal imaging camera 12 and a control device 14. In this embodiment, printing material 6 cured on the lower face of the build platform 4 is represented schematically. The cured printing material 6 can take on different forms depending on the printed object, and is represented here by columns of different thickness. Further, an arrow indicates the direction of movement of the build platform 4, which can be displaced vertically up and down.

[0089] Below the build platform 4, there is a receiving device 10 for printing material 8, the fill level of the printing material 8 being represented by a dashed line. The lower boundary 11 of the receiving device 10 consists of a transparent material, preferably a transparent film.

[0090] A thermal imaging camera 12 is directed onto the outer and lower surface 9 of this film 11, and is in turn connected to a control device 14. Using this thermal imaging camera 12, the temperature of the part of the surface 9 of the foil 11 to be inspected can be measured. For this purpose, the thermal imaging camera is directed onto or at the lower surface 9 of the very thin film 11, and the heat emission during and between illuminations is recorded. In this case, the part is a dental restoration and the area of the foil 11 to be monitored is approximately 5 cm×5 cm. The control device 14 can generate a 2-dimensional temperature distribution over the part of the surface 9 of the foil 11 to be checked from the data recorded by the thermal imaging camera 12, taking into account the thermal conductivity and thermal capacity of the materials used—the translucent foil 11 and the printing material 8.

[0091] In the event of abnormalities in the 2-dimensional temperature distribution between two or more layers and/or other unexpected changes in temperature, the control device 14 emits a signal to warn of errors. Possible errors include, for example, the absence of a sudden increase in heat emission when exposure is started, or the absence of a sudden decrease in temperature when the cured printing material is released from the film. It is also possible that upon comparison of the 2-dimensional temperature tracking, and thus the comparison of the illuminated and cured areas of two or more layers, an excessive area increase or decrease in the cured layer is recorded.

[0092] In the event that errors are recorded in the printing process, this can be optionally automatically adjusted by the control device 14. However, it is more common that if a warning signal is emitted, the control device 14 pauses the printing process until all errors have been eliminated by the user and the user triggers the printing process again.

[0093] FIG. 2 shows a schematic representation of the lateral cross-section of a further embodiment of a 3D printer 2 according to the invention. Here, an active printing process, i.e., a layer being exposed, is shown by means of a 3D printer according to FIG. 2. The columns shown herein of the cured printing material 6, which are intended to represent schematically the already printed object, are immersed or dipped into the printing material 8. Centrally below the receiving device 10, a light source 18 having a conical illumination region 20 is schematically shown. A light source of this type may be a high-precision laser which emits the optimum wavelength as appropriate for the respective photoinitiator, or else preferably an LED light source. For simplicity, the entire possible detection region is shown here, although the capture may take place in a spatially resolved manner using a focus mark of for example 0.5 mm×0.5 mm. Between the already cured printing material 6 and the transparent film 11, newly cured material 16 is schematically shown.

[0094] For simplicity, conical illumination 20 is shown. However, it would also be possible to illuminate only the desired regions (1 pixel for example 20×20 μm) between the already cured material 6 and the transparent film 11 using a plurality of light sources, which illuminate smaller regions or appropriate deflection mirrors, optionally with a slight increase or decrease in area. In this embodiment, the thermal imaging camera 12 is only directed onto the illuminated region or exposed region of the film surface 9, even if this is not shown in detail in the drawings, and is focused and therefore only receives or records the heat emission of this small region. By way of illustration, this is shown using dot-dash lines. As a result of this concentration on the important region, i.e., the exact region of the 3D-printing process, it is possible to avoid unnecessarily large datasets and thus slow processing by the control device 14.

[0095] FIG. 3 graphically displays the light output, the temperature progression and the z-position of the build platform 4, i.e., the internal vertical displacement of the build platform 4 with respect to the transparent film 11, during the 3D-printing process.

[0096] Line a) gives information about the light output of the light source 18 in [mW/cm.sup.2]. When the light source 18 is switched on, the light output increases linearly with a steep gradient, optimally jumping up. During the illumination, the light output or power is kept constant and subsequently switched off, which results in a sudden drop in the light power.

[0097] Line b) shows the progression of the room temperature during the 3D-printing process. This should optimally be kept constant so as to avoid incorrect measurement of the temperature progression at the film surface 9.

[0098] Line c) shows an optimum temperature profile at the surface 9 of the transparent film 11 during the printing process. This temperature progression is recorded by the thermal imaging camera 12 and evaluated by the control device 14. The temperature increases at the time when the light source 18 is switched on. This is caused by the incident light power of the light source 18, as well as the onset of exothermic polymerization of the printing material 8. When the change in temperature dT/dt approaches a particular value as close as possible to zero, in particular a zero value, the maximum of the temperature during the polymerisation process T.sub.max is approximately reached. The value dT/dt=0 cannot be reached within a finite time, and therefore a value close to zero is resorted to in order to keep the illumination duration as short as possible and nevertheless to achieve appropriate curing of the printing material 8. At this point, the light source 18 is switched off and the temperature of the surface 9 of the transparent film 11 slowly returns to its initial value.

[0099] Line d) provides information about the z-position of the build platform 4, i.e., the internal vertical displacement of the build platform 4 with respect to the transparent film 11, during the progression of a 3D-printing process. The build platform 4 is moved or displaced close to the light-transmissive/transparent film 11 prior to exposure or illumination in order to achieve a defined gap of x mm (where x may be adapted depending on the requirements).

[0100] After completion of the exposure process, the build platform 4 is moved away from the film 11 again to achieve a detachment of the cured material 6 from the light-transmissive film 11.

[0101] Moreover, time periods 1 to 5 are marked in FIG. 3. These denote the individual stages of the 3D-printing process. Region 1 is the initial value and represents the starting position and the temperature without illumination or exposure. This point is optimally reached again for all of curves a) to d) after the printing process.

[0102] Region 2 shows the approach of the build platform 4 towards the transparent film 11. At this point, the light source 18 is switched off and the temperatures correspond to the starting value. At the end of region 2, i.e., when the build platform 4 is optimally approaching the film 11, the light source 18 is switched on and the polymerisation starts. This can be recognized by the beginning temperature increase of line c).

[0103] In region 3, the maximum heating temperature T.sub.max of the film surface 9 is reached during exposure. Here, the temperature change dT/dt approaches a certain value, in particular the value zero. When this value is reached, the light source 18 is switched off and the temperature of the surface 9 of the transparent film 11 starts to fall again.

[0104] Region 4 denotes the cooling process of the film surface 9, after the illumination has terminated and the polymerisation is thus ended. When the starting temperature of the film surface 9 is reached, time period 5 starts, at which point the build platform 4 travels back to its starting position and the polymerised printing material 6 is thus detached from the transparent film 11. At the end of region 5, all the parameters return to their initial values (region 1).

[0105] FIG. 4 shows a modified embodiment compared to that shown in FIG. 3. In this context, the light output, the temperature profile and the Z-position of the build platform 4, i.e., the internal vertical displacement of the build platform 4 with respect to the transparent film 11, during the 3D-printing process, are again graphically shown.

[0106] Compared to FIG. 3, the Z-position of the build platform 4 is different. Line a) shows the Z-position of the build platform 4, i.e., the internal vertical displacement of the build platform 4 with respect to the transparent film 11, during the progression of a 3D-printing process. The build platform 4 is moved away from the light-transmissive film 11 after curing (exposure) and then approached again to achieve a defined gap of x mm (x can be adjusted according to the requirements).

[0107] Line b) shows the progression of the light output 18 in mW/cm.sup.2. When the light source 18 is switched on, the light output jumps up. While the build platform 4 is moving, the light source is switched off and thus has an output of 0 mW/cm.sup.2. During illumination, the light power is kept constant.

[0108] Line c) shows, similarly to in FIG. 3, an optimum temperature progression at the surface 9 of the transparent film 11 during the printing process. The temperature increases at the time when the light source 18 is switched on and the build platform 4 is displaced in the Z-direction, and remains constant when a particular value is reached, in particular the temperature of the liquid printing material 8. If the light source is switched on again, the temperature increases again. This is caused by the incoming light output of the light source 18 and the onset of the exothermic polymerisation of the printing material 8. After the curing has ended, the temperature remains constant again (change in temperature dT/dt approaches a zero value). This corresponds to the maximum of the temperature during the polymerisation process T.sub.max.

[0109] The room temperature (in this case line d)) during the 3D-printing process should be kept as constant as possible to avoid erroneous measurement of the temperature progression at the film surface 9.

[0110] Moreover, FIG. 4 shows time periods I to III. These denote the individual portions of the 3D-printing process.

[0111] Region I shows the Z-position of the build platform 4 during the illumination process. In this context, the light source 18 is switched on and the film surface 9 shows the maximum heating temperature T.sub.max reached during the illumination. The change in temperature dT/dt approaches a zero value here.

[0112] Region II shows the change in the Z-position of the build platform 4 between two illumination processes. The build platform 4 is raised. At the start of the movement of the build platform 4, the light source 18 is switched off, causing the temperature at the surface 9 of the transparent film 11 to decrease sharply and remain constant until the end of this region II when a certain temperature is reached, in particular the temperature of the liquid printing material 8.

[0113] Region III again shows the lowering of the Z-position of the build platform 4 at the start of an illumination progress. The light source is switched on again, in such a way that the temperature at the surface 9 of the transparent film 11 increases again. This is caused by the incoming light output of the light source 18 and the onset of the exothermic polymerisation of the printing material 8.

[0114] After the curing is complete, the temperature again takes on the constant value T.sub.max in accordance with region I (change in temperature dT/dt approaches a zero value). This indicates the end of the polymerisation process, so that the cycle can start again.

[0115] FIG. 5 is a schematic drawing of a further embodiment of a 3D printer 2 according to the invention. The special feature here is the optics or optical system used for guiding the light radiation towards the printed object or printing material 6, i.e., towards the surface 9 of the transparent film 11, and to guide the thermal or IR radiation from the printed object/printing material 6 towards the thermal imaging camera 12.

[0116] The light source 18 is located next to the object to be printed in this embodiment, and is shown above said object in FIG. 5. Radiation 19 exiting the light source 18 and used for illumination is deflected via a mirror 30 to a converging lens 28.

[0117] In an advantageous embodiment, the mirror 30 is designed with an edge filter, in such a way that the radiation 19 is reliably reflected but natural light can pass freely through the mirror. The lens 28 is suitable for converging and parallelizing the individual light rays of the illumination radiation 19, and optionally also the additional natural light radiation. The now parallel rays of the illumination radiation 19 are passed on from the (converging/collecting) lens 28 to two laminated prisms 25 and 26 and a DMD unit 24.

[0118] In this embodiment, the two prisms 25 and 26 together form a “Total Internal Reflection” prism (TIR prism) 27. Total Internal Reflection is a physical phenomenon occurring in waves, such as light rays, and occurs when light strikes a flat interface with another transparent medium in which the propagation speed of the light is greater than in the original medium. If the angle of incidence is varied continuously, this effect occurs relatively abruptly at a particular value of the angle of incidence. This specific angle of incidence is known as the critical angle of total internal reflection. The light mostly no longer passes over into the other medium, but instead is (more or less) totally reflected back into the starting medium from this angle onwards. The TIR prism, i.e., the optical element, which is composed of the two laminated prisms 25 and 26, can thus be used as a mirror. If the refractive index of the TIR prism 27 is high enough, total internal reflection (TIR) is achieved, and the TIR prism 27 acts like a mirror with 100% reflection.

[0119] As a combination of two laminated prisms 25 and 26, the TIR prism 27 deflects/directs the incident light onto the DMD unit 24, and the image to be produced is projected using the light ray reflected from the DMD unit 24. The use of a TIR prism 27 thus allows a considerable savings of space since the same effect could only be achieved with a highly complex combination of mirrors. Moreover, this greatly increases the contrast achieved by the system.

[0120] The radiation exiting the TIR prism continues onwards to a projection lens 22, which again splits the parallel rays of the illumination radiation 19 into a conical illumination region 20. For simplicity, a possible illumination region 20 is shown here, but the illumination may be spatially resolved using a focus mark of for example 0.5 mm×0.5 mm.

[0121] The radiation of the conical illumination region 20 is subsequently directed to a semi-transparent mirror 32, known as a splitter or divider mirror, through which the illumination radiation 19 can pass. Subsequently, the illumination radiation 19 is incident on a transparent film 11, and illuminates a desired region between said film and the already cured material 6. The resulting thermal radiation 13, i.e., the IR radiation, is emitted by the material 6 and guided back through the transparent film 11 to the semi-transparent mirror 32.

[0122] In an advantageous embodiment, the mirror 32 is formed as a splitter mirror, i.e., in such a way that the thermal radiation 13, i.e., the IR radiation, is re-reflected towards the thermal imaging camera 12, but the illumination radiation 19, for example UV radiation, can pass through the mirror unchanged.

[0123] For simplicity, a conical illumination region 20 is shown. However, by using a plurality of light sources which illuminate smaller regions, it is also possible to illuminate only the desired region or areas (1 pixel for example 20×20 μm) between the already cured material 6 and the transparent film 11, optionally with a slight increase or decrease in area.

[0124] In an advantageous embodiment, the DMD unit 24 can further switch individual pixels of the illumination region on or off depending on completeness of curing. This makes it possible to prevent overexposure of already cured regions.

[0125] In all places, the film can be light-transmissive, light transmitting, translucent or transparent. Illumination and exposure are interchangeable.

[0126] In some embodiments, the innovations may be implemented in diverse general-purpose or special-purpose computing systems. For example, the computing environment can be any of a variety of computing devices (e.g., desktop computer, laptop computer, server computer, tablet computer, gaming system, mobile device, programmable automation controller, etc.) that can be incorporated into a computing system comprising one or more computing devices.

[0127] In some embodiments, the computing environment includes one or more processing units and memory. The processing unit(s) execute computer-executable instructions. A processing unit can be a central processing unit (CPU), a processor in an application-specific integrated circuit (ASIC), or any other type of processor. In a multi-processing system, multiple processing units execute computer-executable instructions to increase processing power. A tangible memory may be volatile memory (e.g., registers, cache, RAM), non-volatile memory (e.g., ROM, EEPROM, flash memory, etc.), or some combination of the two, accessible by the processing unit(s). The memory stores software implementing one or more innovations described herein, in the form of computer-executable instructions suitable for execution by the processing unit(s).

[0128] A computing system may have additional features. For example, in some embodiments, the computing environment includes storage, one or more input devices, one or more output devices, and one or more communication connections. An interconnection mechanism such as a bus, controller, or network, interconnects the components of the computing environment. Typically, operating system software provides an operating environment for other software executing in the computing environment, and coordinates activities of the components of the computing environment.

[0129] The tangible storage may be removable or non-removable, and includes magnetic or optical media such as magnetic disks, magnetic tapes or cassettes, CD-ROMs, DVDs, or any other medium that can be used to store information in a non-transitory way and can be accessed within the computing environment. The storage stores instructions for the software implementing one or more innovations described herein.

[0130] Where used herein, the term “non-transitory” is a limitation on the computer-readable storage medium itself—that is, it is tangible and not a signal—as opposed to a limitation on the persistence of data storage. A non-transitory computer-readable storage medium does not necessarily store information permanently. Random access memory (which may be volatile, non-volatile, dynamic, static, etc.), read-only memory, flash memory, memory caches, or any other tangible, computer-readable storage medium, whether synchronous or asynchronous, embodies it.

[0131] The input device(s) may be, for example: a touch input device, such as a keyboard, mouse, pen, or trackball; a voice input device; a scanning device; any of various sensors; another device that provides input to the computing environment; or combinations thereof. The output device may be a display, printer, speaker, CD-writer, or another device that provides output from the computing environment.

[0132] The scope of protection of the present invention is given by the claims and is not limited by the features explained in the description or shown to the figures.