Method for additively structuring a shaped body by stereolithography

Abstract

The present invention relates to a method for constructing a molded body by solidifying a viscous photopolymerizable building material layer by layer by means of stereolithography, wherein: a) the building material is dispensed to the bottom of a barrel, b) the barrel is moved relative to a scraper, the scraper being suspended above the barrel bottom so that the dispensed building material moves under the scraper to form a smooth layer, the smooth layer having a uniform layer thickness preset by the positioning of the scraper relative to the barrel bottom, c) the smooth layer is brought to an area between an exposure unit and a building platform, d) the building platform is controllably lowered relative to the barrel bottom, forming the remaining layer in the gap to a predetermined layer thickness when the building material is displaced, e) the layer is cured, f) the building platform is raised, the building material is dispensed to the barrel bottom, steps b) to f) are repeated until a molded body is constructed by a plurality of layers solidified on top of each other, the position of the scraper relative to the barrel bottom being adjusted so that the resulting predetermined uniform layer thickness is greater than the predetermined layer thickness set by lowering the building platform, but does not exceed the predetermined layer thickness by more than 50%.

Description

Technical Field

[0001] The invention relates to a method for the additive construction of shaped bodies by means of stereolithography, by layer-by-layer solidification of a viscous, photopolymerizable building material. Background Art

[0002] The building material can be a ceramic slurry with an organic matrix—a flowable, photopolymerizable material filled with ceramic particles, where the viscosity increases with the number of ceramic particles—or it can be composed of a high-viscosity, light-curable composite material or a photopolymer. When using a ceramic slurry as the building material, a green body is produced as a molded body, which is then further processed into a ceramic body by degreasing and sintering. The present invention is particularly suitable for the production of dental restorations.

[0003] To achieve aesthetically pleasing results for dental restorations, it is often desirable to employ a variety of building materials and / or colors, depending on the location. Using conventional stereolithography, a green body is constructed layer by layer from a ceramic slurry. After the current layer is cured, the green body is removed from the build area and a pigmented photopolymer is selectively applied to the final cured layer using an inkjet printing method. Such a process is known, for example, from WO 2013 / 182547 A1, which employs a drum-shaped carrier with four building platforms arranged 90° apart around its circumference. Processing stations are also spaced 90° apart around the drum. Located between the processing stations is a barrel with a transparent bottom and an exposure unit positioned below it; an inkjet printer for printing the pigmented photopolymer onto the final cured layer; and another exposure unit for curing the spatially selectively imprinted pigmented photopolymer. The drum is rotatably mounted so that one of the building platforms is positioned within the area of ​​one of the processing stations and processing takes place there. After completing a work step, the drum carrier is rotated 90° at the corresponding processing station so that the next work step can be performed on the corresponding part on the build platform at the next processing station. Although work steps can be performed in parallel on parts on several build platforms in this process, it is very time-consuming, as much of the time is spent mechanically moving the build platform between processing stations.

[0004] Alternatively, a variety of different materials can be used to form the layer, and these different materials can be stored in different barrels. The build platform is then lowered into different barrels to successively solidify multiple sub-areas of the layer to be solidified using different build materials in different barrels. Such a process is described in DE 10 2007 010 624 B4. To avoid cross-contamination (transferring a certain amount of build material to a barrel containing a different build material), it is necessary to clean the component after the build platform has been raised and the component suspended on it, but before it is lowered into the next barrel into another build material. Consequently, a cleaning process must be performed every time the build material is changed, which makes this method time-consuming. Furthermore, the transportation required to change between different barrels is also time-consuming.

[0005] DE 10 2011 11 7 005 B4 relates to a method for manufacturing ceramic dental restorations based on additive manufacturing, in which individual slurry layers are sequentially deposited and cured layer by layer. After the slurry layer is deposited, its thickness is reduced by a doctor blade, also creating a smooth layer, after which spatially selective deposition of ink is performed. In addition to a colorant, the ink also contains an initiator that triggers a chemical reaction that causes the slurry layer to cure, resulting in simultaneous coloring and curing.

[0006] US Pat. No. 5,975,323 B2 generally relates to additive 3D printing methods, and also mentions an additive method in which a liquid in a vat is solidified spatially selectively by a laser or other energy source. The method described particularly emphasizes that a plurality of layers are selectively formed on top of each other, wherein the volume formed by the layers on top of each other consists of a plurality of columns adjacent to each other, each column comprising a plurality of voxel elements or voxels (voxels) located on top of each other, wherein the coloring / transparency of each voxel element is selectively created in each column. Specific details regarding the individual coloring of the individual voxel elements and the application of individual colors to each voxel element are not described.

[0007] EP2337667B1 discloses a method according to the preamble of claim 1. In this method, a viscous, photopolymerizable build material is dispensed onto a flat, transparent barrel bottom. A scraper is suspended above the barrel bottom in an adjustable position. The barrel is moved relative to the scraper in a direction parallel to the barrel bottom plane, forcing the dispensed build material beneath and past the scraper, thereby forming a smooth layer with a uniform layer thickness predetermined by the scraper's position relative to the barrel bottom. This can be accomplished, for example, by rotating the barrel about a rotation axis perpendicular to the barrel bottom so that the barrel bottom moves beneath the non-rotating scraper. The dispensed build material accumulates upstream of the scraper, and only some of the accumulated build material passes through the gap beneath the scraper, resulting in a smooth layer with a predetermined, uniform layer thickness. This smooth layer is moved by the relative movement of the barrel to an area between an exposure unit located below the barrel bottom and a build platform suspended above the barrel in an adjustable height. Next, while displacing the build material from the smooth layer, the build platform is lowered relative to the barrel bottom in a precisely controlled manner so that the remaining layer within the gap between the build platform and the barrel bottom is set to a predetermined layer thickness determined by the distance between the lower surface of the build platform (or the lower surface of the last cured layer) and the barrel bottom. This allows the predetermined layer thickness to be set with high precision. The layer with the predetermined layer thickness is then cured by spatially selective exposure through controlled operation of the exposure unit to achieve exposure within the desired contour of the current layer to be cured. Finally, the build platform is raised, the build material is dispensed to the barrel bottom, and the steps described above are repeated until a molded object has been formed by selectively curing multiple layers on top of each other. Summary of the Invention

[0008] The object of the present invention is to provide a method of the type described above which can be carried out in such a way that it can be carried out quickly and precisely even with highly viscous building materials. It would also be desirable if successive solidified layers of a shaped body could be colored in a spatially selective manner.

[0009] This object is achieved by a method comprising the features of claim 1. Preferred embodiments are set forth in the dependent claims.

[0010] Accordingly, a method for building a shaped body by solidifying layer by layer a viscous, photopolymerizable building material by means of stereolithography, wherein:

[0011] a) Dispense building material onto the flat, transparent bottom of the bucket,

[0012] b) the barrel is moved in a direction parallel to the plane of the barrel bottom relative to a scraper, the scraper being suspended above the barrel bottom in a positionally adjustable manner, so that the dispensed building material moves under the scraper to thereby form a smooth layer having a uniform layer thickness predetermined by the position of the scraper relative to the barrel bottom,

[0013] c) the smooth layer is brought to the area between the exposure unit located below the bottom of the barrel and the building platform suspended above the barrel in an adjustable height by the relative movement of the barrel,

[0014] d) the building platform is lowered relative to the barrel bottom in a controlled manner so that when the building material is displaced, the remaining layer in the gap is formed to a predetermined layer thickness,

[0015] e) The layer is cured in a spatially selective manner by controlled operation of the exposure unit within the profile required for the current layer.

[0016] f), the building platform is raised, the building material is distributed to the bottom of the barrel, and steps b) to f) are repeated until a molded body is built by stacking and solidifying multiple layers.

[0017] According to the present invention, the position of the scraper relative to the barrel bottom is adjusted such that the resulting predetermined uniform layer thickness is greater than the predetermined layer thickness set by lowering the building platform, but does not exceed the predetermined layer thickness by more than 50%. In other words, the building material disposed above the barrel bottom is formed into a layer having a predetermined, uniform layer thickness, which is greater than but close to (at most 50% greater than) the predetermined layer thickness to be set by lowering the building platform, by the relative movement of the scraper, which is positionally adjustable and suspended above the barrel bottom.

[0018] It is advantageous if the layer thickness determined by the position of the scraper relative to the barrel bottom exceeds the predetermined layer thickness set by the build platform to some extent. This ensures that sufficient build material is always available at all locations within the area of ​​the layer to be defined, so that the predetermined layer thickness can still be set by lowering the build platform, even if errors or tolerances in the scraper's definition of the predetermined layer thickness (particularly localized deficiencies in the predetermined layer thickness) occur. In other words, sufficient build material is present at all locations so that the lower surface of the build platform (or the lower surface of the last cured layer) over the entire area contacts the build material when the gap from the barrel bottom is set to the predetermined layer thickness. For many build materials, particularly those with low or medium viscosity values, a predetermined, uniform layer thickness can be easily achieved across the entire area of ​​the layer. For build materials with higher viscosity values, there is a certain variation in the actual layer thickness across the entire area of ​​the layer, so that the actual layer thickness value associated with a location within the layer area is actually a distribution of layer thickness values, which is very narrow and has a very small full width at half the maximum value of the average layer thickness. In this case, a "predetermined, uniform layer thickness" is considered to be the average layer thickness of the thickness distribution; in this case, the term "uniform layer thickness" is justified and technically meaningful, since the standard deviation of the thickness distribution is always less than the average layer thickness. In this case, the predetermined, uniform (average) layer thickness is preferably set slightly above the predetermined layer thickness to be set by lowering the build platform, for example, by three standard deviations of the distribution, so that at virtually all locations across the layer area, the build platform comes into contact with the build material when it is lowered to the predetermined layer thickness to be set. Alternatively, the predetermined layer thickness can also be close to the predetermined layer thickness, and any variations in the layer thickness over the entire area can be compensated by laterally shifting the build material when the build platform is lowered.

[0019] Because the build material is formed by the scraper into a layer with a very small, predetermined layer thickness (at most exceeding 50% of the predetermined thickness), the predetermined, uniform layer thickness formed by the scraper is ensured to be close to the predetermined layer thickness set by the build platform. Therefore, when the build platform is lowered toward the barrel bottom, only a small amount of build material must be removed from the gap. In this regard, it is important to consider that when using build materials with high viscosity, a greater force is required to lower the build platform and displace the build material from the remaining gap, from which the high-viscosity build material must be displaced. If the maximum force that can be applied is limited by the barrel material (for example, to avoid damage or other failure of the barrel bottom), the build platform must be lowered slowly to limit this force. For this reason, lowering the build platform to set the predetermined layer thickness takes a longer time for high-viscosity build materials. Conversely, reducing the maximum amount of build material to be displaced reduces the required time. Because the scraper forms the dispensed build material into a layer with a uniform, preset layer thickness (up to 150% of the predetermined layer thickness set by the build platform), the maximum amount of build material to be displaced is strictly limited, allowing rapid layer setting by lowering the build platform and thereby allowing shorter cycle times.

[0020] The minimal amount of building material to be displaced when setting a predetermined layer thickness by the building platform has the additional advantage that, after the layer has solidified, a smaller separating force is required to raise the building platform than in a situation where a larger amount of building material has been displaced to set the predetermined layer thickness. The required separating force must overcome the negative pressure when raising the building platform, because the volume created between the lower surface of the component being built and the barrel bottom when the building platform is raised must be filled with inflowing air. In the case of a large amount of displaced building material, this displaced material forms an obstruction around the building platform and the component being built, which obstructs the flow of ambient gas into the growing volume above the barrel bottom when the building platform is raised. By minimizing the amount of building material displaced, the inflow of air into the growing volume above the barrel bottom is increased, thereby reducing the separating force required to raise the building material.

[0021] If lowering the build platform to set a predetermined layer thickness results in a displacement of a significant amount of build material, this also adversely affects the dimensional accuracy of the component built by stereolithography, particularly in the z-direction (perpendicular to the plane of the barrel bottom). In a "bottom-up" process, as in the example of the present invention, the layer to be cured is clamped between the build platform (or the lower surface of the component being built, if one or more layers have already been cured) and the barrel bottom surface. The height of this gap determines the predetermined layer thickness of the layer to be cured. In this region, the maximum cure depth is determined by the gap height (predetermined layer thickness), even if the penetration depth of the light under the selected exposure parameters (intensity and exposure time) and depending on the build material would result in a deeper cure depth. If the layer to be cured currently protrudes laterally beyond the last previously cured layer, the build material removed during the process of setting the layer thickness of the currently cured layer also reaches those parts where the currently cured layer protrudes beyond the last cured layer, which results in two material stacks and two higher layer thicknesses in the laterally protruding parts of the currently cured layer. Since the actual curing depth of the exposure is always greater than the predetermined layer thickness, the material in these parts undergoes curing exceeding the predetermined layer thickness in the z-direction in the depth region of the last cured layer, which can result in a loss of precision in the z-direction (oversizing) of the order of several layer thicknesses. Minimizing the amount of building material removed, or in other words, ensuring that the predetermined layer thickness by the scraper is optimally approximated to the predetermined layer thickness to be set by the build platform, also results in improved precision of the part to be built. Of course, it is advantageous to keep the amount of displaced building material as low as possible by ensuring that the predetermined layer thickness, as determined by the position of the scraper, is close to the predetermined layer thickness to be set by the build platform.

[0022] At the end of the build process, some unremoved, excess, and uncured build material is always present on the built part. This necessitates a cleaning step, particularly if the part will undergo thermal post-processing steps such as degreasing and sintering. Cleaning is crucial in additive manufacturing. For complex-shaped parts, using liquids to clean tiny gaps or cavities is only possible with considerable effort. Furthermore, solvents that are good at removing monomer mixtures can, in some cases, damage the part's surface, and in the case of suspensions (slurries), particulate fillers may remain on the surface. By keeping the amount of build material displaced during the build process as small as possible, the amount of excess build material ultimately adhering to the part can also be kept as low as possible, which reduces the complexity of the cleaning procedure. These aspects become particularly important if the part is built from different materials and is switched between barrels of different build materials during the build process, as cleaning is essentially required at each material change before the part being built is transferred to the next barrel containing a different build material. In cases where the amount of build material that is displaced when setting the layer thickness is greatly reduced, cleaning work during material changes can be eliminated if slight contamination by a small amount of adhering build material residue on the part, which then comes into contact with another build material in the next vat, is acceptable.

[0023] Preferably, the position of the doctor blade relative to the barrel bottom is adjusted such that the resulting preset, uniform layer thickness lies in the range of 110% to 130% of the predetermined layer thickness to be set by lowering the building platform.

[0024] In a preferred embodiment, the relative motion of the barrel and scraper blade relative to each other is achieved by rotating the barrel about a rotation axis centered and perpendicular to the barrel base while the scraper blade is held stationary, or by rotating the scraper blade about a barrel held separately. In the case of a stationary scraper and a rotatable barrel, the barrel base can be disc-shaped with the rotation axis extending through the center of the disc. The stationary blade has a directional component pointing radially with respect to the rotation axis and extends radially outward from a point radially closest to the rotation axis.

[0025] In a preferred embodiment, the position of the scraper above the barrel bottom is defined by a straight line coinciding with the scraper's lower edge. This straight line has a minimum distance from the barrel bottom at the point of the scraper's lower edge radially closest to the axis of rotation. The position of the straight line is further defined by an inclination angle defined between the straight line and a plane parallel to the barrel bottom that intersects the straight line, the inclination angle being greater than 0° but less than 15°. An inclination angle greater than 0° causes the vertical (perpendicular to the barrel bottom) distance between the lower edge and the barrel bottom to increase from the minimum distance at the point radially closest to the axis of rotation and to increase with increasing radial distance from the axis of rotation.

[0026] The scraper may include a planar scraper defining a plane oriented along an inclination angle relative to the barrel bottom, the inclination angle being between 0° and 90°. Preferably, the planar scraper is inclined relative to the barrel bottom, wherein the inclination angle is preferably within a range of 30° to 75° and is defined relative to the direction of movement between the barrel bottom and the scraper such that a lower edge of the scraper trails behind an upper edge of the scraper in the direction of relative movement.

[0027] Instead of a relative rotational movement of the barrel and the scraper, the relative movement of the barrel and the scraper can also be achieved by linearly displacing the barrel or the scraper. In this embodiment, the position of the scraper above the barrel bottom is preferably defined by a straight line coinciding with the lower edge of the scraper, the straight line extending at a constant distance and parallel to the barrel bottom.

[0028] It can be useful to adjust the build material and the barrel bottom surface with respect to interfacial tension to ensure that the entire barrel bottom is wetted by the build material without gaps, thereby ensuring that the build material contacts the entire barrel bottom. This can be achieved by using additives such as defoamers or surfactants to adjust the surface tension of the build material and / or by modifying the barrel bottom surface, for example, by silanization.

[0029] In a preferred embodiment, the smooth, thin-layer surface is colored by position-dependent application of a selected colorant in an intermediate step before the lowering of the building platform and before the position-dependent exposure.

[0030] Preferably, the colorants used to adjust the color and translucency of the part are selected in a manner that is suitable for the build material currently being used:

[0031] a) in the case where the building material is a photopolymer: a solution comprising dye molecules and / or a suspension comprising pigments,

[0032] b) in the case of a paste comprising glass ceramics: colour pigments, in particular oxides, tin oxide or zirconium oxide, dispersed in an organic medium,

[0033] c) In the case where the building material is a ZrO2 slurry: a nitrate solution (aqueous or based on another solvent) or acetylacetonate dissolved in ethanol.

[0034] In a preferred embodiment, the colorant is dissolved and / or dispersed in ink and applied to the smoothing layer by an inkjet printer.

[0035] In a preferred embodiment, the colorant is photocurable or thermally cured and, after spatially selective application to the smoothing layer, is fixed by electromagnetic radiation, wherein the electromagnetic radiation used for fixing is outside the absorption spectrum of the photoinitiator of the build material.

[0036] In a preferred embodiment, the scraper is made of polytetrafluoroethylene, and the circumferential side wall of the barrel is made of polytetrafluoroethylene.

[0037] It is preferred that a panel made of glass or polymethyl methacrylate (PMMA) is used as the barrel bottom, and the ethylene-tetrafluoroethylene copolymer film is bonded to the barrel bottom on its surface facing the barrel bottom. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The present invention will now be described with reference to embodiments shown in the accompanying drawings, in which:

[0039] Figure 1 shows a schematic perspective view of components of an apparatus for carrying out the method according to the invention;

[0040] Figure 2 A schematic top view of a device for carrying out the method according to the invention is shown in the order of four subsequent steps in the course of carrying out the method according to the invention;

[0041] Figure 3 The corresponding top views are shown in the order of four subsequent steps in the process of carrying out the method according to the invention;

[0042] Figure 4 A detailed view showing a cross section of the barrel bottom and the scraper of an apparatus for carrying out the method according to the invention;

[0043] Figure 5 From the bottom of the barrel and the scraper Figure 4 The right side is a partial cross-sectional plan view of these components as viewed from plane AA;

[0044] Figure 6 An alternative embodiment for influencing the relative motion between the scraper and the bucket is shown. Figure 5 The corresponding view of

[0045] Figure 7 shows a cross-sectional view of a portion of the barrel bottom with a smooth layer of build material on top;

[0046] Figure 8 Shown is the corresponding Figure 7 a cross section in which a portion of the barrel bottom has been moved into position beneath the inkjet printer;

[0047] Figure 9 Shown is the corresponding Figure 7 A cross-sectional view of the barrel bottom, wherein a portion of the barrel bottom has left the area of ​​the inkjet printer and is in a state of moving toward the building area, a building platform is arranged above the barrel, and an exposure unit is arranged below the barrel bottom;

[0048] Figure 10corresponds to Figure 7 A cross-sectional view of the barrel, wherein the bottom portion of the barrel has been moved to the build area below the build platform;

[0049] Figure 11 is after lowering the build platform and during exposure of a defined layer of build material to the applied colorant corresponding to Figure 10 Cross-sectional view of

[0050] Figure 12 After the exposure is completed and the build platform is raised, the Figure 10 and 11 Cross-sectional view of . DETAILED DESCRIPTION

[0051] Figure 1 A highly simplified schematic perspective view shows the essential components of an apparatus for carrying out the method according to the present invention. The apparatus comprises a rotatable barrel 2, which, for simplicity, is shown as a disk-shaped barrel with a barrel bottom 3 without the side walls that would actually enclose a disk. This barrel bottom 3 is transparent, at least in the area where an exposure unit 6 is exposed to the build zone. A vertically movable building platform 8 is arranged above barrel bottom 2, opposite exposure unit 6. A component 10 being constructed is suspended on building platform 10.

[0052] The scraper 4 is suspended above the barrel bottom 3 in an adjustable manner. The barrel 2 is rotatable about a vertically extending rotation axis extending from the center of the circular disc of the horizontal barrel bottom 3. A rotary drive (not shown) is provided, which rotates the barrel 2 under the control of a control unit (not shown) and stops the barrel at a position determined by the control unit. A dispensing device ( ) for viscous building material is arranged upstream of the scraper 4 in a direction opposite to the direction of rotation of the barrel 2 indicated by the arrow. Figure 1 (not shown) This dispensing device dispenses a viscous build material. This can be, for example, an ink cartridge, from which a driven piston delivers the build material through a delivery nozzle. As barrel 2 rotates, the build material accumulates in front of scraper 4. Due to the rotation of barrel 2, a portion of the dispensed build material moves beneath scraper 4 and passes through it, forming a smooth layer 20 with a predetermined, uniform layer thickness determined by the position of scraper 4 relative to barrel bottom 3.

[0053] The inkjet printer 12 is movably supported above the barrel 2, displaced approximately 90° in the circumferential direction relative to the doctor blade 4. After the rotation of the barrel 2 has been stopped, the inkjet printer 12 is used to apply colorant in a spatially selective manner to specific areas of the smoothing layer 20, so that the layer to be cured subsequently obtains the desired, position-dependent color.

[0054] After the printing process is completed by inkjet printer 12, barrel 2 is rotated further 90°, so that the area of ​​smoothing layer 20 to which the colorant has been applied by the inkjet printer is moved between exposure unit 6 and build platform 8. Next, build platform 8 is lowered relative to the surface of barrel bottom 3 under the control of the control unit to such an extent that the lower surface of the build platform (in the case of the first layer to be cured onto the build platform) or the last cured layer of the component 10 being built is at a distance from the barrel bottom equal to the predetermined layer thickness. This displaces the build material from the gap and creates a remaining layer having the predetermined layer thickness. According to the present invention, the position of doctor blade 4 above barrel bottom 3 is set such that smoothing layer 20 approaches, and preferably only slightly exceeds, the predetermined layer thickness. To this end, the position of the doctor blade relative to the barrel bottom is adjusted so that, after passing the doctor blade, the resulting predetermined, uniform layer thickness lies within a range of 100% to 150% of the predetermined layer thickness to be set by lowering the build platform. As explained above, there are several advantages if the amount of build material displaced during the layer thickness setting by lowering the build platform is small, ideally negligible.

[0055] After a predetermined layer thickness is set by controlled lowering of the building platform relative to the drum bottom, the layer of building material printed with colorant defined in the gap is exposed in a spatially selective manner by exposure unit 6 through the drum bottom and thereby cured. The building platform is then raised with the component 10 to be built suspended thereon, so that the currently cured layer is detached from drum bottom 3 and raised.

[0056] The operation of the inkjet printer 12 and the exposure unit 6 is controlled by a control unit (not shown), which stores the three-dimensional shape data of the molded body to be constructed, and in particular also stores the contour shape data of each layer to be subsequently cured and the distribution of the colorant in the area of ​​the corresponding layer to be cured.

[0057] Figure 2 For example, in Figure 1 The schematic plan view from above on the device shown in FIG shows the sequence of method steps, wherein the sequence shows a series of four working steps in carrying out the process according to the invention. Figure 2In the first step shown on the far left, the barrel rotates counterclockwise about a vertical axis of rotation perpendicular to the barrel bottom 3. At the same time, a building material, for example a photopolymer material filled with a granular ceramic material, for example, is distributed to the barrel bottom in the direction of rotation from an ink cartridge located upstream of the scraper 4, so that a certain length of material 18 is extended at the scraper 4. Due to the rotation of the barrel bottom 3, the building material moves under the scraper 4 and passes through the scraper, wherein the scraper 4 is adjustable in position and suspended above the barrel bottom, so that a smooth layer 20 is formed by the lower edge of the scraper, which has a preset, uniform layer thickness determined by the position of the scraper 4 relative to the barrel bottom 3. The smooth layer 20 is moved by rotating the barrel to the area below the inkjet printer 12, which is turned 90° in the counterclockwise direction with respect to the scraper 4. Once the area of ​​the smooth layer 20, which will later be the building area for curing another layer, has reached the area below the inkjet printer 12, the barrel is stopped. At this stage, the layer of building material is printed with colorant in a spatially selective manner. The application of colorant results in Figure 2 The second figure from the left shows a schematic diagram, in which the printed letters DLP are intended to indicate the applied colorant that has been printed in a spatially selective manner (of course, in methods for processing dental products, discrete color structures such as letters are usually not applied, but rather continuously varying colors). The movable suspension of the inkjet printer 12 is indicated by crossed arrows, wherein the inkjet printer moves in a controlled manner controlled by a control unit (not shown) to achieve the spatially selective application of color in the building area.

[0058] Afterwards, the barrel is rotated 90° counterclockwise again and then stopped again, wherein this state is Figure 2 As shown in the second figure from the right in FIG. Due to this rotation, the areas to which the colorant has already been applied in the previous step (indicated by DLP) reach the exposure unit 6 opposite the doctor blade 4 and are cured there in a spatially selective manner by exposure after the building platform has been lowered to set a predetermined layer thickness. This is shown in FIG. Figure 2 The grid is represented by the grid opposite the scraper 4, which represents the picture elements (pixels) of the exposure unit. (The construction platform is in Figure 2 (omitted from the illustration so that the build area below the build platform remains visible). In sync with the exposure, the subsequent build area of ​​the wet build material layer at the 6 o'clock position is printed with colorant, which is once again labeled DLP. During the rotation phase, the doctor blade 4 continues to form a smooth layer 20 with a preset, uniform layer thickness. Note again that Figure 2 The building platform which is actually arranged on the exposure unit above the barrel bottom has been omitted in the illustration for illustrative reasons, so that the exposure area of ​​the exposure unit is visible.

[0059] In transition to Figure 2In the state shown on the right side of the figure, the barrel bottom 3 is further rotated 90°, wherein the scraper 4 continues to continuously form a smooth layer 20 with a preset, uniform layer thickness. After the 90° rotation and the rotation of the barrel bottom is stopped, the inkjet printer 12 once again applies colorant in the next building area, while the previous Figure 2 The build area printed with colorant in the second image from the right is now located in Figure 2 In the rightmost diagram of FIG, the exposure unit is located in the area of ​​the exposure unit at the 3 o'clock position and is exposed after the building platform (not shown) is lowered. In this example, it is assumed that the area corresponding to the letter DLP is exposed and cured.

[0060] The exposed area DLP in the second image from the right has already been rotated to the subsequent method step (in Figure 2 12 o'clock position shown in the right figure) and is shown there as an area with the letter sequence DLP, in which the barrel bottom is visible because the building platform has been raised again after curing of this area, thus Figure 2 As shown in FIG. 1 , a residual reverse image of the cured layer in layer 20 remains, ie, after raising the building platform and the just-cured layer in the shape of the letter sequence DLP, the area remains in layer 20 as a reverse image or blank.

[0061] Figure 3 It shows that in the process of executing the method according to the present invention, Figure 2 A sequence of method stages, wherein the apparatus for carrying out the method is connected at the following points Figure 2 The equipment shown varies. Figure 3 Shown in Figure 2 , wherein the building material dispensing device 30 is shown, wherein the building material dispensing device includes two ink cartridges with different building materials. The ink cartridges are connected to a common mixing device, which prepares a selected mixture of building materials and distributes the mixture. The scraper 4 is constructed as two or a pair of scrapers in this example, that is, it includes two parallel scrapers, and a cavity open at the bottom is formed between the two blades. The building material is delivered directly to the cavity between the two scrapers by the mixing device. The position of the scraper located downstream with respect to the direction of rotation is once again set so that a smooth layer 20 is formed, which has a preset, uniform layer thickness determined by the position of the scraper 4 with respect to the bottom of the barrel.

[0062] At the 12 o'clock position, a vacuum scraper 34 is mounted. It is also formed as two or a pair of scrapers with a cavity open at the bottom between them. The cavity of the vacuum scraper 34 is kept under negative pressure to allow the residual build material left after the exposure step at the 3 o'clock position and the raising of the build platform to be sucked away.

[0063] See also Figure 4 and Figure 5 , describes an embodiment of an adjustable scraper for forming a smooth layer 20 having a preset, uniform layer thickness determined by the position of the scraper 4 relative to the barrel bottom. Figure 5 The right-hand side of the figure shows a schematic plan view of the barrel bottom 3 from above. The longitudinal direction of the scraper 4 extends here from a starting point close to the rotation axis of the circular barrel bottom 3 in the radially outward direction to an end point close to the outer circumference of the circular barrel bottom 3. Figure 4 A cross-sectional view of a portion of the barrel bottom 3 and a portion of the scraper 4 is shown in FIG. Figure 5 The radial longitudinal extension of the scraper 4 is visible in FIG. Figure 4 As can be seen in the diagram, the blade of the scraper 4 is not oriented perpendicularly to the barrel bottom 3, but is tilted relative to the barrel bottom 3, forming an inclination angle κ relative to the barrel bottom. This inclination angle is an acute angle, preferably in the range of 30° to 75°, wherein the scraper 4 is tilted at this inclination angle κ so that the inclination angle is arranged in relation to the direction of movement of the scraper 4 relative to the barrel bottom such that the lower edge of the scraper 4 trails behind the upper edge of the scraper 4 in the direction of movement of the scraper 4 relative to the barrel bottom. Figure 4 , which means that the direction of movement of the scraper 4 relative to the barrel bottom 3 points to the right, with the lower edge of the scraper 4 subsequently trailing behind the upper edge during the movement of the scraper 4 relative to the barrel bottom 3. In the direction of movement of the scraper 4 relative to the barrel bottom 3, the building material scattered on the barrel bottom is accumulated in front of the scraper 4. The position of the scraper relative to the barrel bottom is adjusted so that the portion of the building material that passes through the gap between the barrel bottom and the lower edge of the scraper forms a smooth layer 20 with a predetermined, uniform layer thickness D determined by the position of the scraper relative to the barrel bottom 3. N The described inclination angle causes a funnel effect, ie the building material is forced towards the gap between the lower edge of the scraper and the barrel bottom due to the relative movement of the inclined scraper.

[0064] For the embodiment of the apparatus described herein with a rotatable barrel 2 and a non-rotatable scraper 4, it is advantageous to adjust another angle, which will now be combined with Figure 5 Provide a description. Figure 5 Shown from Figure 4The plan view is taken along plane AA, with the view in the x-direction and directed toward one side of scraper blade 4. The position of scraper blade 4 relative to the barrel bottom is defined by a straight line profile coincident with the lower edge of scraper blade 4. This straight line coincident with the lower edge of scraper blade 4 extends substantially radially relative to the rotatable barrel 2, but is not parallel to the surface of barrel bottom 3. Instead, it is tilted at an angle α relative to the surface of the barrel bottom 3, which is greater than 0° but less than 15°. This angle α increases the distance between the lower edge of scraper blade 4 and the barrel bottom 3, starting from a minimum distance, as the radial distance from the axis of rotation increases. Adjusting this angle α is necessary for rotating barrels because the amount of build material required increases with increasing radial distance from the axis of rotation, as the area over which the build material is distributed increases with increasing radial distance. In other words, the relative speed of the lower edge of the scraper blade relative to the barrel bottom increases linearly with radial distance from the axis of rotation, resulting in a correspondingly larger demand for build material at greater radial distances. This build material is then distributed over a larger area due to the higher relative rotational speed of the barrel bottom relative to the scraper blade. It has been observed that upstream of the scraper, the material accumulates at the scraper along its entire length, with the build material distributed radially over the entire length of the scraper. Furthermore, it has been found that in this embodiment comprising a rotating barrel, even at greater axial distances from the axis of rotation, the layer thickness is determined by the distance between the scraper's lower edge and the barrel bottom. However, the gap width, which depends on the radial distance from the axis of rotation, is not equal to the resulting layer thickness; the layer thickness is less than the gap width in areas radially outward of the scraper. Instead, material properties, such as the build material's viscoelasticity associated with surface tension and its adhesion to the barrel bottom, have the effect that in areas with greater radial distances, the build material is pulled further apart and forms a thinner layer than would be determined by the gap width between the scraper's lower edge and the barrel bottom. This necessitates adjusting the scraper's position relative to the barrel bottom when switching to a different build material with different viscosity, surface tension, and adhesion properties, in order to achieve the desired, predetermined, and uniform layer thickness across the entire area where the build material is being dispensed. In this regard, it was found that the optimal position of the scraper relative to the bottom of the barrel can be found quite quickly when using an adjustable suspension of the scraper, which suspension allows the height and inclination angle of the lower edge of the scraper to be continuously adjusted using an actuating drive to change the position of the scraper, which allows the position of the scraper to be quickly adjusted by changing its position until the desired layer is formed.

[0065] The predetermined layer thickness set by the displacement of the building material caused by lowering the building platform lies in the range of 20 micrometers (μm) to 100 μm during a typical building process, and can be, for example, 50 μm. In order to generate a predetermined, uniform layer thickness by means of a doctor blade that is greater than the predetermined layer thickness but not more than 50% greater than the predetermined layer thickness, it is necessary to realize an adjustable suspension of the doctor blade, which allows adjusting the minimum distance of the lower edge of the doctor blade 4 relative to the barrel bottom 3 and in combination with the above in a precise and reproducible manner. Figure 4 and Figure 5 The two angles described above can be adjusted without the risk of changing the adjustment position during operation. This can be achieved, for example, by using two positioning tables that are designed to adjust the table position with high precision and form a coupling suspension for the scraper. The positioning tables are connected to each other. One of the positioning tables is used to adjust the minimum distance of the lower edge of the scraper from the bottom of the barrel, and the other is used to adjust the inclination angle α of the lower edge of the scraper from the bottom of the barrel using an angle meter (see Figure 4 ).

[0066] Figure 6 An alternative embodiment for achieving the relative movement of the scraper 4 and the barrel bottom 3 is shown. In this example, a rectangular barrel bottom 3 is used. Figure 6 The right hand side of the barrel is shown in a top view (viewed in the z direction). In this example, the barrel can be Figure 6 The right-hand side diagram shows a linear movement in the x-direction indicated by the arrow. Figure 6 The left-hand side of the figure shows a plan view of the scraper and barrel bottom, viewed from the side (viewing direction = x-direction). In this example, the scraper 4 is suspended so that its lower edge extends parallel to the surface of the barrel bottom 3. The distance between the lower edge of the scraper 4 and the barrel bottom 3 is adjustable. In this example, the distance between the lower edge of the scraper and the barrel bottom is the same everywhere because the relative speed between the lower edge of the scraper and the barrel bottom is the same everywhere. The distance between the lower edge of the scraper and the barrel bottom is equal to the predetermined, uniform layer thickness to be formed by the scraper 4.

[0067] As in Figure 6 On the right-hand side of the drawing, it can be seen that a long strip 18 of dispensed building material is formed in front of the scraper 4 in the direction of movement of the barrel, even though a smooth layer 20 with a predetermined, uniform layer thickness has already been formed behind the scraper 4 in the direction of movement. It has been observed that for many types of building materials, the thickness of the smooth layer actually produced behind the scraper is not exactly equal to the gap width between the lower edge of the scraper and the barrel bottom, but rather is in many cases slightly less. In this regard, it has also been discovered that by changing the position of the lower edge of the scraper, for example by means of an actuating drive controlled by a control unit, the correct position of the scraper relative to the barrel bottom, which produces the desired predetermined, uniform layer thickness, can be quickly found.

[0068] exist Figure 7-12 Another description of the sequence of method steps in the process of performing the method of the present invention is shown in FIG. Figure 7 A schematic plan view of a portion of the barrel bottom 3 is shown, on which the doctor blade 4 has formed a smoothing layer 20 with a predetermined, uniform layer thickness. This smoothing layer is moved by the barrel rotation to the area below the inkjet printer 12, in which the barrel rotation is stopped. At this stage Figure 8, which illustrates the movement and operation of the inkjet printer 12 for applying colorant 22 in a spatially selective manner. After the inkjet printer stops operating, the barrel is further rotated, wherein Figure 9 A plan view of the barrel at this stage of rotation is shown, wherein the area of ​​the layer 20 previously provided with colorant is shown. The barrel continues to rotate until the printed layer area reaches the building area between the building platform 8 and the exposure unit. This state is Figure 10 In this method step, the building platform 18 is lowered by a drive controlled by the control unit until the distance between the lower surface of the last solidified layer of the component 10 currently being built and the surface of the barrel bottom is equal to the predetermined layer thickness. Figure 11 The currently defined layer located above the barrel bottom 3 is then cured in a spatially selective manner by controlled operation of the exposure unit. The layer cured in this way by exposure is firmly attached to the previously last cured layer by an ongoing polymerization reaction, which results in a strong bond between the two layers and ultimately forms a molded body of layers firmly connected to each other.

[0069] After stopping the exposure step, the building platform 8 is Figure 12 rises again in order to allow the Figure 11 The part 10 of the last layer previously cured is lifted off the bottom of the barrel. Figure 12 It can be seen that in the region of the last solidified layer, empty areas or holes are still located on the barrel bottom. When the barrel is rotated and the empty areas reach the dispensing area of ​​the building material in front of the scraper, these areas are filled up again.

[0070] The following factors must be taken into consideration when selecting the materials for the scraper, bucket bottom, and bucket sidewalls. PTFE (polytetrafluoroethylene) is the most suitable material for the scraper and bucket sidewalls. Due to its low surface energy, PTFE is beneficial for all components that come into direct contact with the build material being processed. The build material being processed does not adhere to the scraper or the bucket sidewalls. On the other hand, when using scrapers made of other plastic materials (such as polycarbonate or polyamide), voids in the coating can be observed.

[0071] When the scraper is in direct contact with the barrel sidewall, PTFE's excellent sliding properties come into play, so the barrel's rotation mechanism is generally not blocked. PTFE is chemically inert to solvents, reactive components, and colorants in the suspension. PTFE possesses sufficient rigidity and wear resistance for grinding ceramic suspensions, so that tearing and abrasion effects are not observed in practice.

[0072] When selecting a material for the barrel bottom, two aspects must be taken into consideration. First, the barrel bottom must have sufficient rigidity. Second, the surface must be very smooth and flat. Third, the barrel bottom surface must be wetted by the build material suspension. For this purpose, the contact angle and viscosity of the build material are important. These requirements are best met by the combination of PMMA (polymethyl methacrylate, thickness: 3 mm) and an ETFE (ethylene tetrafluoroethylene copolymer, thickness: 80 μm) film located thereon. Glass or similar materials could also be considered as an alternative to PMMA for the barrel bottom. Further limitations on the material choice for the barrel bottom arise from the additional requirements for transparency during exposure and the pull-off force during the stereolithographic build process. The ETFE film used in conjunction with the present invention is provided with a self-adhesive side, which allows the ETFE film to be bonded to the PMMA support without bubbles and in a flat manner. The highly flat barrel bottom formed in this way is one of the prerequisites for forming thin build material layers with high precision using a doctor blade. Like PTFE, ETFE is also inert to chemicals that come into contact with the ETFE film. In combination with the construction material formulations with ETFE suitable in combination with the present invention, smaller contact angles and thus better wettability can be achieved compared to FEP (fluorinated ethylene propylene).

[0073] A low contact angle (considered an indicator of wettability) is associated with a well-defined thinness of the build material layer. If the wettability is insufficient, this can lead to "ablation" of the build material layer and "island formation" due to localized retraction or contraction of the build material layer. This can result in the formation of pores in the coating and potentially droplet formation. These effects can be significantly reduced or eliminated by increasing the viscosity of the build material suspension. Experiments have found that viscosities between 10 Pa·s and 50 Pa·s are suitable for build material contact angles typically between 50° and 60°.

[0074] In the method according to the invention, the still wet building material can be imprinted with a colorant in a spatially selective manner before the building platform is lowered onto the layer and the layer is exposed and cured by an exposure unit. The colorant must be selected depending on the type of building material, with the following distribution being preferred.

[0075] For both unfilled and filled photopolymers, build material solutions including dye molecules and / or suspensions containing pigments can be used.

[0076] When using glass ceramic slurries as building materials, preference is given to using color pigments dispersed in an organic medium, in particular oxides, tin oxide or zirconium oxide, as colorants.

[0077] In the case where a ZrO2 slurry is used as the building material, it is best to use a nitrate solution (aqueous or based on another solvent) or acetylacetonate dissolved in ethanol.

Claims

1. A method for constructing a shaped body by layer-by-layer solidification of a viscous, photopolymerizable building material by means of stereolithography, wherein: a) dispensing the building material into the flat transparent bottom of the bucket (2), b) the barrel (2) is moved relative to a scraper (4) in a direction parallel to the plane of the barrel bottom (3), the scraper being adjustably suspended above the barrel bottom (3) so that the dispensed building material moves under the scraper (4) to form a smooth layer (20), the smooth layer having a uniform layer thickness predetermined by the positioning of the scraper (4) relative to the barrel bottom (3), c) the smoothing layer (20) is brought to the area between the exposure unit (6) located below the barrel bottom (3) and the building platform (8) suspended above the barrel in an adjustable height by the relative movement of the barrel (2), d) the building platform (8) is lowered relative to the barrel bottom in a controlled manner so that the remaining layer in the gap between the building platform (8) and the barrel bottom (3) is formed into a predetermined layer thickness when the building material is displaced, e) said layer is cured in a spatially selective manner by controlled operation of said exposure unit (6) within the desired profile of the current layer, f), the building platform (8) is raised, the building material is dispensed to the bottom of the barrel, and steps b) to f) are repeated until a molded body is built up by a plurality of layers that are solidified on top of each other, The invention is characterized in that the position of the scraper (4) relative to the barrel bottom (3) is adjusted so that the obtained preset uniform layer thickness is greater than the predetermined layer thickness set by lowering the building platform (8), but does not exceed the predetermined layer thickness by more than 50%; and In an intermediate step before lowering the building platform (8) and before the spatially selective exposure, the surface of the smooth layer is colored by applying a selected colorant in a spatially selective manner, The colorants used to set the color and translucency are selected in the following manner as appropriate for the building material currently being used: a) in the case of a photopolymer as the building material, a solution comprising dye molecules and / or a suspension comprising pigments, b) in the case where the building material is a paste comprising glass ceramics, color pigments, oxides, tin oxide or zirconium oxide dispersed in an organic medium, c) In the case where the building material is a ZrO2 slurry, it is a nitrate solution or acetylacetonate dissolved in ethanol.

2. The method according to claim 1, wherein: The position of the scraper (4) relative to the barrel bottom (3) is adjusted in such a way that the resulting preset uniform layer thickness is in the range of 110% to 130% of the predetermined layer thickness.

3. The method according to any one of the preceding claims, characterized in that The relative movement of the barrel (2) and the scraper (4) is achieved by rotating the barrel (2) around a central axis perpendicular to the barrel bottom (3) relative to the scraper (4) suspended in a stationary manner, or by rotating the scraper (4) around the axis relative to the stationary barrel (2).

4. The method according to claim 3, wherein: The position of the scraper (4) above the barrel bottom (3) is defined by a straight line coinciding with the straight lower edge of the scraper, by a minimum distance from the point of the lower edge of the scraper closest to the axis of rotation to the barrel bottom (3), and by an inclination angle (α) defined by the straight line of the lower edge and a plane parallel to the barrel bottom (3) intersecting the straight line, the inclination angle being greater than 0° and less than 15°, so that the distance of the lower edge from the barrel bottom (3) increases from the minimum distance with increasing radial distance from the axis of rotation.

5. The method according to claim 4, wherein: The scraper (4) comprises a planar blade, and the scraper is inclined relative to the barrel bottom (3) at an inclination angle (κ), wherein the inclination angle (κ) is greater than 0° and less than 90°, and the inclination angle is defined with respect to the movement direction of the barrel bottom (3) relative to the scraper (4) so ​​that the lower edge of the scraper (4) trails behind the upper edge of the scraper (4) in the relative movement direction of the scraper (4) relative to the barrel bottom.

6. The method according to claim 5, wherein: The tilt angle (κ) is in the range of 30° to 75°.

7. The method according to any one of claims 1 to 2, characterized in that: The relative movement of the barrel (2) and the scraper (4) is achieved by linearly displacing the barrel (2) or linearly displacing the scraper (4).

8. The method according to claim 7, wherein: The position of the scraper (4) above the barrel bottom is defined by an extended straight line coinciding with the lower edge of the scraper (4), the straight line extending at a constant distance and parallel to the barrel bottom (3).

9. The method according to claim 1, wherein: The colorant is dissolved and / or dispersed in ink and applied to the smoothing layer in a spatially selective manner by an inkjet printer.

10. The method according to claim 1 or 9, wherein: The colorant is photocured or thermally cured and, after spatially selective application to the smoothing layer, is fixed by electromagnetic radiation, wherein the electromagnetic radiation used for fixing is outside the absorption spectrum of the photoinitiator of the build material.

11. The method according to claim 1, wherein: As the scraper, a scraper made of polytetrafluoroethylene is used, and the circumferential side wall of the barrel (2) made of polytetrafluoroethylene is used.

12. The method according to claim 1, wherein: A panel made of glass or polymethyl methacrylate is used as the barrel bottom, and an ethylene-tetrafluoroethylene copolymer film is bonded to the barrel bottom with its surface facing the barrel bottom.

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