METHOD FOR PROCESS CONTROL OF A 3D STEREOLITHOGRAPHY PROCESS

Description

[0001] The invention relates to a method for process control of a 3D printing process, according to the preamble of claim 1, and to a 3D printer with a build platform, according to the preamble of claim 17.

[0002] According to the current state of the art, in such a 3D printing process, stereolithography and / or a DLP process (digital light processing process), the components are produced from special printable plastic, which is in liquid form and is exposed to light and hardened by means of selective exposure in layers, usually with a thickness of 0.1 or 0.05 mm.

[0003] Typically, the light is passed through a transparent film, and after completion of the respective layer, the film is removed from the component or the component is removed from the film.

[0004] Recently, it has become known to use stereolithography devices and / or DLP processes (digital light processing processes) for dental technology, especially for the production of dental restoration parts.

[0005] It goes without saying that such components must be biocompatible on the one hand and, on the other hand, high demands are and must be placed on oral compatibility.

[0006] To make such a 3D printing process as efficient as possible, the process progress must be tracked. This enables minimal printing time with optimal curing and thus the best cost / effort-benefit ratio. Monitoring the surface during sintering processes of fine metal powders by measuring temperature with a thermal imaging camera is described in "Layerwise Monitoring of the Selective Laser Melting Process by Thermography" by Krauss. et al. known.

[0007] A stereolithographic process or a digital light processing process in which a photosensitive material is cured by radiation, the measuring radiation is coupled into a reference layer and remains predominantly within the reference layer due to internal reflection, and the measuring radiation is recorded by a sensor with spatial and temporal resolution, is known from WO 2017 / 106895 A1. In this process, the radiation is recorded with spatial and temporal resolution using a complex setup. This setup is very cost-intensive and therefore unsuitable for dental technology. Furthermore, with this solution, it is not possible to determine the optimal time to end the exposure. Another stereolithographic method is shown in WO 2020 / 069060 A1, in which the temperature of a light-sensitive material is controlled.

[0008] The invention is therefore based on the object of providing a simple method for process control of a 3D printing process according to the preamble of claim 1 and claim 17 as well as a 3D printer with a construction platform according to claim 20, which makes it possible to achieve the best possible printing result with minimal time expenditure.

[0009] According to the inventive method for controlling a 3D printing process, a thermal imaging camera is connected to a 3D printer or is connected to the control device of the 3D printer. Its output signal is transmitted to the control device.

[0010] According to the invention, the 3D printer consists of a build platform, a light source, a mounting device for printing material, a thermal imaging camera, and a control device. A preferred application includes stereolithography processes and digital light processing processes.

[0011] The mounting device consists of a tray, which can be open at the top or at least open to the construction platform.

[0012] In this printing material receiving device, a monomer or a monomer mixture and a photoinitiator are placed, for example. Other starting solutions, such as a photosensitive monomer / monomer mixture or a diluted monomer solution, are also possible.

[0013] The support device has a glass plate as its base, with a translucent layer, such as a film, applied thereon. An inhibitor layer can be applied to this film to prevent the printing material from adhering during curing or polymerization.

[0014] According to the invention, a build platform is inserted into the printing material so that it is immersed in the printing material and extends almost to the translucent film of the support device. To achieve a defined distance between the base film and the build platform, the build platform can be lowered until it contacts the base film and then raised by the desired, defined amount. Thus, a defined-thickness monomer layer is located between the base film and the build platform, which can then be exposed to specific locations. This layer can, for example, be between 50 and 100 µm thick.

[0015] A typical cycle of such a printing process takes about 10 s, with the exposure by the light source accounting for 2 s of this.

[0016] In a preferred embodiment of the invention, the object is created layer by layer. This means that after the exposure of a layer, the build platform is raised so that printing material can flow into the resulting gap, and this material is then exposed. For larger print objects, the build platform is first raised by more than one layer thickness, preferably by approximately 1 cm, so that the exposed layer can more easily detach from the film and printing material can flow into the resulting gap. The build platform is then lowered back to the desired height so that the desired layer thickness is achieved between the build platform and the film. Thus, in each exposure step, the object to be printed is enlarged by another layer of the cured printing material.

[0017] It is particularly advantageous if the build platform is raised asymmetrically. For example, you can start by lifting it from one side or corner. This allows the build material to detach more easily from the film. Shear forces are lower, so there is less likelihood of the film tearing. Furthermore, the film's service life is increased because the forces acting on it are less intense than if the build platform were raised vertically. When creating the object layer by layer, printing can be done at a build speed of approximately 0.5 mm / min.

[0018] To reduce shear forces, it is also conceivable for the device to be designed so that, during the detachment process of the built layer, the build platform moves vertically and the tank moves horizontally. Alternatively, they can also move at a slight angle to the horizontal.

[0019] In a further advantageous embodiment, the object can also be produced continuously. In this case, the translucent layer of the 3D printer's tank, which comes into contact with the printing material, is protected from the adhesion of the curing printing material by an inhibitor layer. One advantage of this embodiment is that individually formed layers do not have to be removed from a film; instead, the build platform is continuously raised at a defined speed, for example, 10 mm / min, during exposure. However, this requires a higher light source power than with layer-by-layer production. This creates an end product that does not reveal individual layers, but rather has a homogeneously smooth surface. Furthermore, this method allows for an acceleration of the printing process and thus a reduction in production time and the associated minimization of production costs.

[0020] The light source, which is mounted centrally below the translucent layer of the support device, can be a very precise laser emitting the optimal wavelength for the respective photoinitiator, or preferably an LED light source. This is directed at the translucent base of the tank and thus at the base film of the support device, and can thus expose the printing material from below / outside through the translucent film. This material cures preferentially between the build platform or the layer already hardened or partially hardened in the previous exposure process and the translucent film, adhering to the build platform or the previous layer, but not to the translucent film.

[0021] In this step, the layer is exposed to light in a location-selective manner. This can be done either across the entire surface of the layer or only in selected parts of the layer, such as two-dimensional patterns.

[0022] Due to the light energy of the light source, the site-selective, exothermic polymerization starts only at the exposed areas due to the photoinitiator or the photosensitive polymer.

[0023] According to the invention, during the exposure process and thus during polymerization, the temperature of the entire layer, i.e., the thermal radiation of the layer, is continuously monitored spatially using a thermal imaging camera at the exposed areas. The thermal imaging camera is preferably located laterally below the recording device and is directed at the translucent base film.

[0024] For this purpose, the thermal imaging camera is directed at the lower surface of the translucent film, specifically the exposed area of ​​this film, which, for example, for applications in dental technology, is on the order of a few square centimeters (1-10 cm²). Such thermal imaging cameras record the heat emission across the entire surface with a spatial resolution of a few hundredths of a square millimeter, time-resolved during (a few seconds) and between exposures (approximately 1 minute). The data obtained from such a time-resolved temperature measurement across an exposed surface correspond to a three-dimensional Gaussian distribution of the measured temperature curves.The control device can thus calculate a time-resolved 2-dimensional temperature distribution over the entire surface of the film from the values ​​measured by the thermal imaging camera, taking into account the thermal conductivity and heat capacity of the materials used (printing material and materials of the printer structure).

[0025] First, the thermal imaging camera captures the portion of the light energy that heats the surface. However, this portion remains approximately constant throughout the entire exposure process and is subtracted by the control device.

[0026] Furthermore, the thermal imaging camera records the energy generated during the exothermic polymerization of the material. The measured surface temperature thus corresponds to the emission of certain wavelengths and wavelength ranges from the exposed surface. This is recorded by the thermal imaging camera with time resolution.

[0027] The temperature change dT / dt thus corresponds to the first derivative of the temperature curve. A positive temperature change (dT / dt > 0) indicates heating (emission heat), while a negative temperature change (dT / dt < 0) indicates cooling (emission cooling) of the surface. A constant temperature, i.e., no temperature change (dT / dt = 0), means a constant surface temperature or a maximum / minimum value of the temperature curve (T max or T min ).

[0028] The temperature change across the film surface is recorded spatially resolved using the thermal imaging camera and transmitted to the control device.

[0029] In an advantageous embodiment, the integral of the temperature curve, i.e. the so-called heat quantity, is recorded with spatial resolution.

[0030] The end of the exposure is determined by reaching a predefined temperature T max or a predefined temperature change dT / dt. If this value or temperature change is measured by the thermal imaging camera and recorded by the control device, the control device emits a signal. The control device then terminates the build process for this layer by ending the exposure.

[0031] Furthermore, due to the measurement of an undesirable, inhomogeneous temperature distribution, it is possible that certain areas of the layer can continue to be exposed, while other parts are not further exposed because curing has already been satisfactorily completed in those areas. This allows the desired polymerization rate to be achieved in all areas.

[0032] The previously defined temperature T max is defined as the maximum desired heating temperature detected by the thermal imaging camera at the surface of the polymerizing layer. The temperature change dT / dt at this time converges towards zero, or more specifically, assumes the value zero. Since it is difficult to track the temperature change to zero over a finite period of time, a defined threshold value (e.g., dT / dt = 2 to 5 K / s) can also be used.

[0033] By monitoring the spatially resolved temperature profile, damage or unwanted polymerization of the surrounding material due to overexposure can be prevented. Exposure is preferably terminated as soon as a desired, predefined degree of curing is reached, since excessive or even complete polymerization of the monomers would reduce the adhesion of the next layer. Furthermore, the LEDs and LED-DMDs or other light sources used as light sources each have a limited, specific illumination duration. By monitoring the spatially resolved temperature profile, the optimal exposure duration can be determined and unnecessarily long exposure can be prevented, which would otherwise unnecessarily shorten the service life of the light source.

[0034] After exposure is complete, the layer must be removed from the film. This is done, for example, by raising the build platform a certain amount from the film.

[0035] This value must be at least the layer thickness of a print run, for example. However, it is preferable to first raise the build platform a greater amount to facilitate complete removal, and then lower it to the desired layer thickness (e.g., 50 to 100 mm). It is also conceivable to lower the film downwards. For this purpose, it is advantageous to lower the film with the tray at an angle, i.e., starting at one side, so that the film peels off the cured print material.

[0036] During a continuous printing process, the inhibitor layer prevents the cured printing material from adhering to the translucent film, thus eliminating the need for such a removal process.

[0037] Overexposure during a 3D stereolithography process or a digital light processing process also leads to excessive adhesion between the material and the film, as well as an unnecessarily prolonged printing process. This can be avoided by the method according to the invention, since the optimal exposure time is determined by temperature tracking with the thermal imaging camera. This enables—in addition to the aforementioned protection of the light source—an extended service life of the translucent film and saves time. Excessive adhesion can lead to damage, scratches, or detachment of the inhibitor layer and / or scratches, damage, or even tearing of the film, or both. Such damage can only be repaired by interrupting the printing process or even stopping it altogether, as otherwise structural defects remain in the final product.

[0038] During detachment, new material to be polymerized enters the resulting gap. This can be detected by the thermal imaging camera as a sharp drop in the measured heat emission, i.e., a drop in temperature (temperature change dT / dt << 0). After detachment and after the initial sudden drop in temperature when printing material initially flows in, any further material flow, with or without gas bubbles, between the build platform and the film can be detected by the thermal imaging camera as a temperature change (dT / dt > 0 or dT / dt < 0) and recognized in the control device.

[0039] Furthermore, it is advantageous that contamination on the film, with or without an inhibitor layer—such as sedimentation, contamination, the adhesion of residues from possible incorrect construction processes, or foreign particles—can be detected. Such cases can be detected, for example, by the thermal imaging camera detecting an insufficient temperature change during a movement of the build platform and recognizing this in the control device. This applies not only to insufficient temperature change across the entire film surface, but also to partial areas, i.e., if the thermal imaging camera detects that the temperature distribution / change is not plausible across the entire film surface. The control device then issues a warning signal.

[0040] This is especially important if gas bubbles or small contaminants remain on the film or inhibitor layer, especially between the object and the film or inhibitor layer. This is detected because the temperature at the location of a gas bubble is lower than in the surrounding area. The thermal imaging camera measures the temperature with spatial resolution and high precision, allowing the control device to detect such temperature differences and subsequently issue an error or warning signal.

[0041] The spatial resolution of the thermal imaging camera is preferably in the range of the layer thickness of the construction process, i.e. approximately 0.1 mm x 0.1 mm, but can also be considerably larger, e.g. ten times, i.e. 1 mm x 1 mm, or considerably smaller, e.g. 0.025 x 0.025 mm.

[0042] The control device only enables the exposure process for the next layer once these disturbances have been eliminated or the user has manually authorized exposure. The disturbances are recognized as having been eliminated when the thermal imaging camera registers and the control device recognizes that no gas bubbles or contaminants remain on the film. This can be seen because the thermal imaging camera measures a homogeneous temperature change across the entire surface during material flow. In this case, the control device detects no irregularities in the temperature change and therefore that no contaminants or gas bubbles are adhering to the surface of the film. Once the inflow of new printing material has stopped, i.e. once the material flow has ended, the thermal imaging camera measures a constant temperature across the entire surface of the film.The control device thus verifies its results during material flow and can check whether there are really no contaminants or gas bubbles left on the surface of the film.

[0043] In a further embodiment, it is provided that the entire printing process is aborted if interference factors are detected.

[0044] In addition, the control device only enables the exposure process for the next layer once the thermal imaging camera has registered and the control device has recognized that, after a temperature change exceeding a specified threshold—which corresponds to material flow—no further significant temperature change (dT / dt converges to 0) is measurable. This means that all of the cured material must have detached from the film surface, and the material flow is approaching a standstill.

[0045] In an advantageous development, the control device outputs a warning or error signal if the thermal imaging camera registers and the control device detects that the heat emission does not change within a predefined time, preferably 2 seconds, at the start of exposure. This would indicate that either the light source has failed, the exposed layer has already fully cured, or polymerization has not started due to another error source. It could also be that the layer has not completely detached from the film, the film has torn, or the printing material has been used up.

[0046] Thus, by monitoring the temperature profile during the polymerization process and also between the exposure of different layers, most errors during the 3D printing process, such as contamination of the film, deposits of foreign bodies, incomplete detachment of the object from the film or aging of the film, LED or LED-DM D, can be detected early on.

[0047] As a result, if a warning signal is issued, the exposure process is delayed, specifically paused, or aborted by the control device. The build process for the next layers can be adjusted to minimize damage, either automatically by the control device or through user intervention. If a warning signal is issued during an exposure process, it is also possible to restart the current exposure of the layer. This allows, for example, if the light source fails, the exposure process can be continued after it has been checked and repaired without resulting in a defective layer in the final product.

[0048] In an advantageous embodiment, the dimensions of the exposed layer of the 3D-printed object can be tracked during polymerization using multidimensional, in particular 2-dimensional, spatially resolved temperature measurement. For this purpose, the heat emission from the surface of the translucent film is tracked and recorded with a thermal imaging camera during the printing process, in particular during exposure and between exposures. The exposed areas emit more heat than the remaining areas. This is caused both by the incident light energy from the light source and by the heat generated during the exothermic polymerization process. The control device can thus calculate the actual dimensions of the exposed layer from the data determined by the thermal imaging camera and compare them with the theoretical values, i.e., the raw data before exposure that was fed to the light source.

[0049] To monitor and verify the printing process, the dimensions of the exposed parts of the film are determined with the thermal imaging camera during each exposure process and stored in the control device. This data can then be compared with each other and with the raw data.

[0050] If the dimensions of an exposed layer decrease compared to the previous layer, it can be assumed that the exposed area is reduced. Or, if the dimensions of the exposed layer increase, it is enlarged. This is especially important when there are very large differences between the layers. The large difference indicates an error in the build process. If such an unexpectedly large change in the exposed area is measured between two or more layers, the control device issues a warning signal, specifically an error signal.

[0051] Until now, 3D printing processes required a process known as distortion correction to compensate for the displacement of individual layers relative to one another during the printing process. According to the invention, this distortion correction is now unnecessary thanks to monitoring the increase or decrease in surface area of ​​the printed layer of a 3D object being produced. This is because the size of the object is tracked during polymerization, and the build process can be adjusted accordingly. When a warning signal is issued by the control device, the user—or, if necessary, the control device automatically—can decide whether the large increase or decrease in surface area was intentional or not. If it was unintentional, the source of the error can be eliminated. If the layer is too small, the desired area of ​​this defective layer can then be re-exposed to achieve optimal curing of the desired surface.If the exposure is unintentionally too high, the error is more difficult to correct. In this case, after removing the source of the error, the next layer can be printed without errors. After the printing process is complete, the user can remove the unwanted excess material by hand. This can be done, for example, with a standard hand sander.

[0052] The 3D printer according to the invention consists of a build platform, a light source, a mounting device for the printing material, a control device, and a connected thermal imaging camera. The thermal imaging camera transmits its output signal to the control device.

[0053] The control device can, for example, be a computer with special software that controls the 3D printing unit so that an object can be produced from the printing material layer by layer or continuously as described above.

[0054] In a further embodiment of the invention, a DMD unit is connected to the 3D printer. If the thermal imaging camera and the control device detect that a layer is curing unevenly, the radiation can be deflected away from the already fully cured areas using the DMD unit. This way, the not yet fully cured areas continue to be exposed, while simultaneously preventing overexposure of the already fully cured areas.

[0055] This process is based on the projection technology "Digital Light Processing" (DLP). Images, or in this case, light beams with a defined light energy, are generated by modulating a digital image, or wavelengths of defined energy, onto a light beam. The light beam is split into individual image points, or pixels, by an array of movable micromirrors and then individually reflected either into or out of the projection path. This allows for very precise and fine control of the light beams onto the desired areas of the lower surface of the translucent film, preventing overexposure of individual areas.

[0056] This arrangement of movable micromirrors, including their control technology, is referred to as a DMD unit (DMD = Digital Micromirror Device). The individual micromirrors of a DMD unit are preferably arranged in a square matrix and can be individually controlled, i.e., tilted, by the control device as needed. Such tilting between an "on" and an "off" position, which can be controlled autonomously for each individual micromirror, is induced by the application of electrostatic fields. This enables surprisingly fine and precise control of the light or laser radiation.

[0057] Preferably, two stable end states are defined for each individual micromirror, here an "on" and an "off" position, between which the micromirror can switch frequently, in particular up to 5000 times, within a second. The "on" position describes the state in which the light energy is directed to the area to be exposed, while the "off" position describes the state in which the light energy is directed away from the exposure surface.

[0058] The edge length of these individual micromirrors of a DMD unit is preferably a few micrometers, in particular approximately 16 µm. The number of mirrors within a DMD unit corresponds to the resolution of the projected image, i.e., in this case, the desired resolution of the exposed area. Preferably, several pixels can be represented by one micromirror.

[0059] In a preferred embodiment, the individual pixels can also be controlled at different, arbitrary brightness levels. This is achieved by controlling the individual micromirrors via binary pulse-width modulation, i.e., pulse density modulation. The brightness level of a pixel thus depends on how long a micromirror is switched on.

[0060] In a preferred embodiment, a color filter can be arranged in front of the DMD unit, i.e., in the projection path of the light beam. This can, for example, filter out a wavelength from the deflected light or limit the deflected light beam to a specific wavelength. It is also possible to use a monochromator instead of a color filter to spectrally isolate a specific wavelength from the incident beam.

[0061] A significant advantage of this DLP technology with a DMD unit is its increased precision. The light energy, i.e., the light radiation, can be very precisely directed away from the areas of the lower surface of the translucent film where the material to be polymerized has already reached the desired degree of cure, thus avoiding overexposure.

[0062] Further advantages, details and features will become apparent from the following description of several embodiments of the invention with reference to the drawings.

[0063] They show: Fig. 1 shows a schematic representation of the lateral cross-section of a 3D printer according to the invention in a first embodiment; Fig. 2 shows a schematic representation of the lateral cross-section of another embodiment of a 3D printer according to the invention; Fig. 3 shows a schematic representation of the light output, the temperature profile, and the z-position of the build platform during the 3D printing process; Fig. 4 shows a further schematic representation of the light output, the temperature profile, and the z-position of the build platform during the 3D printing process; and Fig. 5 shows a schematic representation of an optical structure of another embodiment of a 3D printer according to the invention.

[0064] Out of Figs. 1 is a schematic representation of the lateral cross-section of a 3D printer according to the invention 2 for a stereolithography process or a digital light processing process with a build platform 4,a recording device 10 for printing material 8, a thermal imaging camera 12 and a control device 14 In this embodiment, the underside of the construction platform is shown schematically 4 cured printing material 6 The cured printing material 6 can take on different configurations depending on the print object and is represented here by columns of varying thickness. Furthermore, the direction of movement of the build platform is indicated by an arrow. 4 indicated, which can be moved vertically up and down.

[0065] Below the construction platform 4 there is a recording device 10 for printing material 8, where the filling level of the printing material 8 is shown by a dashed line. The lower boundary 11 the recording device 10consists of a translucent material, preferably a translucent film.

[0066] On the outer and lower surface 9 this slide 11 is a thermal imaging camera 12 which in turn is connected to a control device 14 This thermal imaging camera 12 the temperature of the part of the surface to be checked 9 the foil 11 To do this, the thermal imaging camera is placed on the lower surface 9 the very thin film 11 and the heat emission is recorded during and between exposures. In this case, it is a dental restoration part and the area of ​​the foil to be monitored 11 is approximately 5 cm x 5 cm. The control device 14 can be determined from the thermal imaging camera 12recorded data taking into account the thermal conductivity and heat capacity of the materials used - the translucent film 11 and the printing material 8 - a 2-dimensional temperature distribution over the part of the surface to be tested 9 the foil 11 create.

[0067] In case of abnormalities in the 2-dimensional temperature distribution between two or more layers and / or other unexpected temperature changes, the control device 14A signal is emitted to warn of errors. Possible errors include, for example, the absence of a sudden increase in heat emission at the start of exposure or the absence of a sudden drop in temperature when the cured printing material is removed from the film. It is also possible that when comparing the 2-dimensional temperature tracking—and thus the comparison of the exposed and cured areas—of two or more layers, an excessive increase or decrease in the area of ​​the cured layer is registered.

[0068] If errors are registered in the printing process, this can be corrected by the control device 14 be adjusted automatically. However, it is more common that when a warning signal is issued, the control device 14 pauses the printing process until the user has corrected all errors and the printing process is released again by the user.

[0069] Out of Figs. 2is a schematic representation of the lateral cross section of another embodiment of a 3D printer according to the invention 2 This involves an active printing process - i.e. the exposure of a layer - using a 3D printer after Figs. 2 The columns of the cured printing material shown here 6, which are intended to schematically represent the already printed object, now appear in the printing material 8 Centrally below the mounting device 10 is schematically a light source 18 with a conical exposure area 20Such a light source can be a very precise laser, which emits the optimal wavelength to match the respective photoinitiator, or preferably an LED light source. For simplicity, the entire possible detection range is shown here, although the detection can be spatially resolved with a focus spot of, for example, 0.5 mm x 0.5 mm. Between the already cured printing material 6 and the translucent film 11 is schematically newly cured material 16 shown.

[0070] For simplification, a conical exposure 20 However, it would also be possible to use several light sources irradiating smaller areas or suitable deflection mirrors to only image the desired areas (1 pixel, e.g. 20 x 20 µm) between the already cured material 6 and the translucent film 11 -possibly with a small increase or decrease in area. The thermal imaging camera 12 In this embodiment, only the exposed area of ​​the film surface 9 directed – even if this is not shown in detail in the figures – and is focused, thus only recording the heat emission from this – small – area. For illustration purposes, this is shown using dot-dash lines. By concentrating on the important area, i.e., the precise area of ​​the 3D printing process, it is possible to avoid unnecessarily large amounts of data and thus slow processing by the control device. 14 to avoid.

[0071] Out of Figs. 3 is the light output, the temperature profile and the z-position of the build platform 4 - i.e. the internal height adjustment of the build platform 4 with regard to the translucent film 11 - graphically displayed during the 3D printing process.

[0072] The curve a) provides information about the light output of the light source 18 in [mW / cm 2< ]. When the light source is switched on 18 The light output increases linearly with a steep gradient, ideally in a stepwise manner. During the exposure, the light output is kept constant and then switched off, resulting in a sudden drop in light output.

[0073] The curve b) shows the room temperature during the 3D printing process. This should ideally be kept constant to avoid incorrect measurement of the temperature gradient on the film surface. 9 to avoid.

[0074] The curve c) shows an optimal temperature curve on the surface 9 the translucent film 11 during the printing process. This temperature profile is recorded by the thermal imaging camera 12recorded and by the control device 14 The temperature is measured at the time the light source is switched on 18 This is determined by the radiant light output of the light source 18, and the onset of exothermic polymerization of the printing material 8 If the temperature change dT / dt approaches a certain value as close as possible to zero, in particular the value zero, then the maximum temperature during the polymerization process T max is approximately reached. The value dT / dt = 0 cannot be reached in a finite time, therefore a value close to zero is used to keep the exposure time as short as possible while still ensuring adequate curing of the printing material. 8 At this point, the light source 18 switched off and the temperature of the surface 9 the translucent film 11slowly returns to its original value.

[0075] The curve d) provides information about the z-position of the build platform 4 - i.e. the internal height adjustment of the build platform 4 with regard to the translucent film 11 - during the course of a 3D printing process. The build platform 4 is placed near the translucent film before exposure 11 driven to achieve a defined gap of x mm (x can be adjusted according to requirements).

[0076] After the exposure process is completed, the build platform 4 again from the foil 11 moved away to prevent the hardened material from detaching 6 from the translucent film 11 to reach.

[0077] In addition, Figs. 3 the periods 1 until 5These indicate the individual stages of the 3D printing process. Area 1 is the initial value and represents the initial position and temperature without exposure. This point is optimally used for all curves a) until d) reached again after the printing process.

[0078] Area 2 shows the approach of the construction platform 4 to the translucent film 11. At this point the light source 18 switched off and the temperatures correspond to the initial value. At the end of the range 2, i.e. with optimal approach of the construction platform 4 to the foil 11, the light source 18 switched on and the polymerization starts. This can be seen from the beginning temperature increase of curve c) be recognized.

[0079] In area 3 is the maximum heating temperature T max of the film surface 9during exposure. Here, the temperature change dT / dt approaches a certain value, specifically zero. When this value is reached, the light source 18 switched off and the temperature of the surface 9 the translucent film 11 begins to fall again.

[0080] Area 4 refers to the cooling process of the film surface 9, after the exposure has ended and thus the polymerization is complete. When the initial temperature of the film surface is reached 9 begins period 5, on which the construction platform 4 returns to its starting position and the polymerized printing material 6 thus from the translucent film 11 At the end of area 5 all parameters return to their initial values ​​(range 1).

[0081] Out of Figs. 4is a modified embodiment compared to that according to Figs. 3 Here again the light output, the temperature profile and the Z-position of the build platform 4 - i.e. the internal height adjustment of the build platform 4 with regard to the translucent film 11 - graphically displayed during the 3D printing process.

[0082] Compared to Figs. 3 is Z-position of the build platform 4 different. Curve a) shows the Z-position of the build platform 4 - i.e. the internal height adjustment of the build platform 4 with regard to the translucent film 11 - during the course of a 3D printing process. The build platform 4 is removed from the translucent film after curing (exposure) 11moved away and then approached again to achieve a defined gap of x mm (x can be adjusted according to requirements).

[0083] The curve b) shows the light output curve of the light source 18 in [mW / cm 2< ]. When the light source is switched off 18 the light output increases dramatically. While the build platform 4 is moved, the light source is switched off and thus has an output of 0 mW / cm 2 . During exposure, the light output is kept constant.

[0084] The curve c) shows, similar to Figs. 3 , a possible temperature gradient on the surface 9 the translucent film 11 during the printing process. The temperature decreases at the moment the light source is switched off 18 and Z-shift of the build platform 4drops sharply and remains at a certain value, especially the temperature of the liquid printing material 8, constant. When the light source is switched on again, the temperature increases again. This is determined by the incoming light output of the light source 18, and the onset of exothermic polymerization of the printing material 8 After curing is complete, the temperature remains constant (temperature change dT / dt approaches zero). This corresponds to the maximum temperature during the polymerization process, T max .

[0085] The room temperature (here curve d)) During the 3D printing process, the temperature should also be kept as constant as possible to avoid incorrect measurement of the temperature profile on the film surface. 9 to avoid.

[0086] In addition, Figs. 4 the periods I until IIIThese indicate the individual stages of the 3D printing process.

[0087] Area I shows the Z-position of the build platform 4 during the exposure process. The light source 18 is switched on and the film surface 9 shows the maximum heating temperature T max reached during exposure. The temperature change dT / dt approaches zero here.

[0088] Area II shows the change in the Z position of the build platform 4 between two exposure processes. The build platform 4 is raised. At the beginning of the movement of the construction platform 4 the light source 18 switched off, which reduces the temperature on the surface 9 the translucent film 11 decreases sharply and when a certain temperature is reached, especially the temperature of the liquid printing material 8,to the end of this area II remains constant.

[0089] Area III shows the lowering of the Z-position of the build platform 4, at the start of an exposure process. The light source is switched on again so that the temperature on the surface 9 the translucent film 11 This is caused by the incoming light output of the light source 18, and the onset of exothermic polymerization of the printing material 8 caused.

[0090] After completion of the curing, the temperature returns to the constant value T max according to the range I (temperature change dT / dt approaches zero). This indicates the end of the polymerization process, so the cycle can begin again.

[0091] Out of Figs. 5 is a schematic representation of another embodiment of a 3D printer according to the invention2 The special feature here is the optics used to direct the light radiation to the print object or print material 6, so to the surface 9 the translucent film 11, and the heat or IR radiation from the print object / print material 6 to the thermal imaging camera 12 to lead.

[0092] The light source 18 In this embodiment, it is located next to the object to be printed and is shown in the illustration according to Figs. 5 shown above. Radiation emerging from the light source 18, used for exposure 19 is via a mirror 30 to a convex lens 28 distracted.

[0093] The mirror 30 In an advantageous embodiment, the lens is provided with an edge filter so that the radiation 19 is reliably reflected, but daylight can freely pass through the mirror. 28is suitable for the individual light rays of the exposure radiation 19, and, if necessary, the additional daylight radiation, to be collected and parallelized. The now parallel rays of the exposure radiation 19 are captured by the (collecting) lens 28 to two laminated prisms 25 and 26 and a DMD unit 24 forwarded.

[0094] In this embodiment, the two prisms form 25 and 26together a so-called "Total Internal Reflection Prism" (TIR Prism) 27. Total internal reflection ("Total Internal Reflection") is a physical phenomenon that occurs in waves, such as light rays, when light impinges flatly on an interface to another transparent medium, in which the propagation speed of the light is greater than in the original medium. If the angle of incidence is continuously varied, this effect occurs relatively suddenly at a certain value of the angle of incidence. This specific angle of incidence is called the critical angle of total internal reflection. The light no longer largely passes into the other medium, but is (more or less) totally reflected back into the original medium from this angle onwards. Thus, the TIR prism, i.e. the optical element consisting of the two laminated prisms 25 and 26composed of a mirror. If the refractive index of the TIR prism 27 is large enough, you get the total internal reflection (TIR) ​​and the TIR prism 27 acts like a mirror with 100% reflection.

[0095] As a combination of two laminated prisms 25 and 26, steers the TIR prism 27 the incident light on the DMD unit 24 and the image to be generated is compared with the DMD unit 24 reflected light beam is projected. The use of a TIR prism 27 This allows for considerable space savings, as the same effect could only be achieved with a very complex mirror combination. Furthermore, the contrast achieved by the system is greatly increased.

[0096] The radiation emerging from the TIR prism continues to a projection lens 22, which the parallel rays of the exposure radiation19 again to a conical exposure area 20 For simplicity, a possible exposure range is shown here 20 shown, although the exposure can be spatially resolved with a focus spot of, for example, 0.5 mm x 0.5 mm.

[0097] The radiation of the conical exposure area 20 is then converted into a semi-transparent mirror 32 a so-called splitter mirror, which directs the exposure radiation 19 can penetrate. The exposure radiation then hits 19 on a translucent film 11 and exposed between this and the already cured material 6 a desired area. The resulting heat radiation 13, i.e. the IR radiation, is emitted by the material 6 and through the translucent film 11 back to the semi-transparent mirror 32.

[0098] The mirror 32 In an advantageous embodiment, it is designed as a splitter mirror, i.e. in such a way that the heat radiation 13, i.e. the IR radiation, to the thermal imaging camera 12 is reflected, but the exposure radiation 19, for example UV radiation, can pass through the mirror unchanged.

[0099] For simplification, a conical exposure area 20 However, it is also possible to use several light sources irradiating smaller areas to only image the desired areas (1 pixel, e.g. 20 x 20 µm) between the already cured material 6 and the translucent film 11 - if necessary with a small increase or decrease in area - to be exposed.

[0100] In an advantageous embodiment, the DMD unit 24Furthermore, individual pixels of the exposure area can be switched on or off depending on the completeness of the curing. This prevents overexposure of already cured areas.

Claims

1. A process control method for a 3D-printing process using a 3D printer (2), said 3D printer (2) having a build platform (4), a light source (18), a receiving device (10) for printing material (8) and a control device (14) by means of which an item can be produced in layers or continuously from the printing material (8), characterised in that a thermal imaging camera (12), the output signal of which is transmitted to the control device (14), is connected to the 3D printer (2), and the method comprising 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 (12), • ending the building process of a layer by ending the illumination, the time for the end of the illumination being established by a predefined temperature Tmax or a predefined change in temperature dT / dt being reached.

2. The method according to claim 1, characterised in that the predefined temperature Tmax is defined as the maximum heating temperature detected at the surface of the polymerising layer by the thermal imaging camera (12), the change in temperature dT / dt at this time converging towards zero, in particular taking on a zero value.

3. The method according to any of the preceding claims, characterised in that the thermal radiation of the layer is detected by a thermal imaging camera (12), the change in temperature dT / dt corresponding to the first derivative of the temperature curve, a positive change in temperature dT / dt > 0 indicating heating of the surface, a negative change in temperature dT / dt < 0 indicating cooling of the surface, and no change in temperature dT / dt = 0 indicating a constant temperature or the maximum / minimum of a temperature curve Tmax or Tmin.

4. The method according to any of the preceding claims, characterised in that the change in temperature over the film surface is detected in a spatially resolved manner by the thermal imaging camera (12) and passed to the control device (14).

5. The method according to any of the preceding claims, characterised in that after the illumination ends the layer is detached from the film (11) and the build platform (4) is raised, new material to be polymerised arriving in the resulting gap, and the detachment of the item from the film (11) being recognised by the thermal imaging camera (12) as a major drop in temperature dT / dt << 0.

6. The method according to any of the preceding claims, characterised in that during the illumination the build platform (4) is raised continuously at a defined speed, in particular 0.5 mm / min, new material to be polymerised arriving in the resulting gap, and in that an inhibitor layer prevents adhesion to the film (11).

7. The method according to any of claims 1 to 5, characterised in that the build platform (4) is raised incrementally between the illumination times with a defined step height, in particular around 0.3 to 3 cm, and subsequently lowered to the desired layer thickness, new material to be polymerised arriving in the resulting gap and being cured in the next illumination step.

8. The method according to claim 7, characterised in that the build platform (4) is raised diagonally, in particular starting from one side or edge, to a height of 1 cm between the illumination times, and in that the build platform (4) is subsequently aligned substantially horizontally and subsequently lowered to the desired layer thickness.

9. The method according to any of the preceding claims, characterised in that the material flow with or without gas bubbles between the build platform (4) and the film (11) is detected by the thermal imaging camera (12) as a change in temperature dT / dt > 0 or dT / dt < 0 in each case and recognised in the control device (14).

10. The method according to any of the preceding claims, characterised in that impurities on the film (11) or inhibitor layer - for example due to sedimentation, contamination, adhesion of residues from any erroneous building process or of foreign particles - can be detected by the thermal imaging camera (12) and are recognised in the control device (14) on the basis of an excessively low detected change in temperature, optionally during movement of the build platform (4).

11. The method according to any of the preceding claims, characterised in that the illumination process of a next layer only starts when the thermal imaging camera (12) records and the control device (14) recognises that no gas bubble(s) or no impurities - for example due to sedimentation, contamination, adhesion of residues from any erroneous building process or of foreign particles - have been left behind on the film (11).

12. The method according to any of the preceding claims, characterised in that the illumination process of the next layer only starts when the thermal imaging camera (12) records and the control device (14) recognises that after a change in temperature exceeding a predetermined threshold no further change in temperature is measurable.

13. The method according to any of the preceding claims, characterised in that the control device (14) outputs an error signal or warning signal when the thermal imaging camera (12) records that the temperature distribution is not plausible over the entire film surface, in particular because gas bubbles or small impurities have been left behind on the film (11) or inhibitor layer, in particular between the item and the film (11) or inhibitor layer.

14. The method according to any of the preceding claims, characterised in that the control device outputs an error signal or warning signal when the thermal imaging camera (12) records and the control device (14) detects that the thermal emission does not change within a predefined time, preferably approximately 2 seconds, at the start of the illumination.

15. The method according to any of the preceding claims, characterised in that the illumination process is delayed, in particular paused, or aborted by the control device (14) if the control device (14) outputs a warning signal and subsequently the building process of the next layers is adapted or the previous illumination process is repeated.

16. The method according to any of the preceding claims, characterised in that the dimensions of each printed layer are tracked during polymerisation using multi-dimensional, in particular 2-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 item, the next layer being reduced if the size of the illuminated area decreases and enlarged if it increases.

17. A 3D printer (2) comprising a build platform (4), a light source (18), a receiving device (10) for printing material (8) and a control device (14) by means of which an item can be produced in layers or continuously from the printing material (8), wherein a thermal imaging camera (12), the output signal of which can be transmitted to the control device (14), is connected to the 3D printer (2), wherein the control device is configured to carry out the following steps: • illuminating a layer, • detecting the temperature of the layer during the polymerisation using the thermal imaging camera (12), • characterised in that the building process of a layer is ended by ending the illumination, the exact time for the end of the illumination being established by a predefined temperature or a predefined change in temperature being reached.