Method and system for determining viscosity of photocurable resin for vat photopolymerization printer

Abstract

To provide a method and system for forming 3D articles by a vat photopolymerization process.SOLUTION: In such a method and system, a torque required to raise and lower a molding plate through a resin in a vat is measured both before the molding process begins and, optionally, during the molding process, thereby determining a viscosity of the resin. The resin is preferably heated, and the viscosity of the resin is altered by heating the resin in the vat using a light engine employed to manufacture a 3D article.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This application claims priority to U.S. Provisional Patent Application No. 62 / 941,653, filed November 27, 2019.

[0002] The present invention relates to additive manufacturing processes, and more particularly to a method and system for determining the viscosity of photocurable resins used in vat polymerization printers. [Background technology]

[0003] Additive manufacturing, or 3D printing, as it is known, is a collection of various technologies that offer various means for the direct production of various articles. One such technology is vat photopolymerization, which includes stereolithography (SLA), direct light processing (DLP), and liquid crystal display (LCD) direct printing. These technologies generally involve selectively curing a resin contained in a vat, typically using an ultraviolet (UV) light source. The resin is cured layer by layer, and the article under manufacture is formed by a series of cross sections that are successive and adhered to one another. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. WO2015 / 074088 [Patent Document 2] International Publication No. WO2016 / 078838 [Patent Document 3] U.S. Patent Application Publication No. US2019 / 0202112 Summary of the Invention [Problem to be solved by the invention]

[0005] Resin viscosity is known to be an important parameter in these layer-by-layer printing processes. Viscosity represents the fluid's internal resistance to movement, i.e., its resistance to deformation. A viscous fluid has a high viscosity. For example, oil has a higher viscosity than water. In vat photopolymerization processes, low-viscosity resins are generally desirable because they allow for relatively rapid replenishment of the build area between layers. However, products formed with low-viscosity resins tend to suffer from shrinkage and warping during solidification after curing. Therefore, high-viscosity resins are desirable because they do not suffer from such undesirable side effects (at least not to the same extent as low-viscosity resins). High-viscosity resins also produce more desirable product characteristics comparable to those of products formed using injection molding processes.

[0006] Viscosity is caused by cohesive forces between molecules in a liquid and varies with temperature. For liquids, viscosity (μ) is approximated as μ = a10b / (Tc), where T is the absolute temperature and a, b, and c are experimentally determined constants. Thus, it is known that heating a highly viscous resin reduces its viscosity, thereby making it suitable for use in vat photopolymerization printers, even when high viscosity resins would otherwise be difficult to process in such printers. U.S. Patent No. 5,629,999 proposes heating the resin by using resistive heating elements on the edges of the exposed zone. U.S. Patent No. 5,629,999 proposes using a transparent, electrically conductive coating on the bottom of the vat to heat the resin. U.S. Patent No. 5,629,999 proposes using an independent electromagnetic radiation source to heat the resin. Also known are vat photopolymerization devices in which the vat is placed within an enclosure, such as a furnace, and the entire build process takes place within the enclosure. [Means for solving the problem]

[0007] In one embodiment, the present invention provides a method for determining the viscosity of a photocurable resin used in a vat photopolymerization printer. Prior to initiating a build process using the vat photopolymerization printer, the vat photopolymerization printer's tank is filled with resin and the vat photopolymerization printer's build plate is lowered into the resin. A motor is activated to raise the build plate in the resin, and a torque meter records a measurement of the torque required to raise the build plate. The torque measurement is used to look up a table listing torque measurements for known resin viscosities to determine the viscosity of the resin in the vat photopolymerization printer's tank. The torque meter may be integral with the motor or may be configured to convert torque into an electrical signal. In some cases, the torque meter may include a rotary torque sensor aligned with a lead screw configured to raise or lower the build plate under actuation by the motor. Such a rotary torque sensor may be an optical torque sensor or a surface acoustic wave (SAW) torque sensor. In some examples, a controller may be configured as part of the torque meter. When the torque measurement falls between two resin viscosity values ​​listed in the table, the viscosity of the resin in the vat photopolymerization printer tank is determined as the listed viscosity value that is closest to the torque measurement, or alternatively, as the viscosity value interpolated with respect to the torque measurement.

[0008] During the build process, the current viscosity of the resin in the tank of the vat photopolymerization printer can be determined by using a motor to raise and / or lower a build plate in the resin, recording current measurements of the torque required to raise and / or lower the build plate with a torque meter, and using a lookup table to determine the current viscosity of the resin in the tank. Also during the build process, a light source (e.g., for heating the resin) of the photopolymerization printer can be used to change or control the current viscosity of the resin in the tank of the vat photopolymerization printer. While changing the current viscosity of the resin in the tank of the vat photopolymerization printer, the current viscosity of the resin can be measured by using a motor to raise and / or lower a build plate in the resin, recording current measurements of the torque required to raise and / or lower the build plate with a torque meter, and using a lookup table to determine the current viscosity of the resin in the tank.

[0009] In one embodiment of the present invention, a system for determining the viscosity of photocurable resin used in a photopolymerization printer thus includes a vat photopolymerization printer having a tank configured to hold a volume of resin, a build plate configured to raise and lower within the tank, a motor coupled to raise and / or lower the build plate, a torque meter configured to measure the torque required to raise and / or lower the build plate within the resin, and a controller, the controller configured to operate the motor to raise and / or lower the build plate within the resin, receive torque measurements from the torque meter, and use the torque measurements to determine the viscosity of the resin within the tank of the vat photopolymerization printer by looking up a table listing torque measurements for known viscosities of resin. The system may also include a light engine.

[0010] The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a schematic cross-sectional view of a 3D printing system configured in accordance with an embodiment of the present invention in which the fabrication of an object takes place in a tank (vat) containing a light-curable liquid resin. [Figure 2] An example of a controller for the 3D printing system shown in Figure 1 is shown. DETAILED DESCRIPTION OF THE INVENTION

[0012] Disclosed herein are methods and systems for forming 3D articles by a vat photopolymerization process, in which the viscosity of the resin is determined both before the start of the build process and, optionally, during the build process, by measuring the torque required to raise and lower a build plate through the resin in the vat. The resin may be heated to alter its viscosity, which may be achieved by heating the resin in the vat using a light engine, a separate heater, or other means employed to fabricate the 3D article.

[0013] 1 shows a cross-section of a 3D printing system 100 configured in accordance with an embodiment of the present invention, which uses electromagnetic radiation (e.g., UV light) to cure a photo-curable liquid resin (typically a liquid polymer) 18 to fabricate an object (e.g., a 3D object) 22. The object 22 is fabricated layer by layer (i.e., a new layer of the object 22 is formed by photo-curing a layer of liquid polymer 18 adjacent the bottom surface of the object 22), and as a new layer is formed, the object is raised by a build plate 20 to allow the next layer of photo-curable liquid resin 18 to be drawn underneath the newly formed layer. This process of forming additional layers is repeated multiple times until fabrication of the object is complete.

[0014] The 3D printing system 100 includes a tank 10 for containing a photo-curable liquid resin 18. The bottom of the tank 10 (or at least a portion of the tank 10) is sealed (i.e., preventing the photo-curable liquid polymer 18 from leaking out of the tank 10) by a flexible membrane 14, which is transparent (or nearly transparent) at wavelengths of interest for curing the resin to allow electromagnetic radiation from a light source 26 to enter the tank 10. A mask 24 (e.g., a liquid crystal layer) that enables selective curing of the liquid resin (enabling the fabrication of a 3D object in a desired shape / pattern) is positioned between the light source 26 and the photo-curable liquid resin 18. In various embodiments, collimating and diverging elements, such as lenses, reflectors, filters, and / or films, may be positioned between the mask 24 and the light source 26. These elements are not shown to avoid unnecessarily obscuring the drawings.

[0015] A platen or backing member 16, formed of borosilicate glass or other material, is positioned between the mask 24 and the flexible membrane 14 to provide structural support. The platen is also transparent (or nearly transparent) at one or more wavelengths of interest for curing the resin. Alternatively, the platen 16 may be metal or plastic and include a transparent window that allows electromagnetic radiation from the light source 26 to enter the tank 10. In other embodiments, the mask 24 itself may be used in place of a separate window and sealed around its perimeter with a gasket. It should be noted that while the mask 24, platen 16, and flexible membrane 14 are shown spaced a distance apart, in practice these components may be positioned to contact one another and prevent refraction at any air interface. The flexible membrane 14 is affixed to the rim of the tank 10 or to a replaceable cartridge assembly (not shown) to maintain a fluid-tight perimeter at the tank's rim or other opening ("fluid-tight" meaning that the tank will not leak during normal use).

[0016] When using 3D printing system 100 to fabricate one layer of object 22, electromagnetic radiation is emitted from radiation source 26 and passes through mask 24, platen 16, and flexible membrane 14 into tank 10. The electromagnetic radiation forms an image on an image plane adjacent the bottom of object 22. Areas of high (or medium) intensity in the image cause localized areas of photocurable liquid resin 18 to harden. The newly hardened layer adheres to the previous bottom surface of object 22 but, due to the presence of flexible membrane 14, is substantially free from adhesion to the bottom surface of tank 10. After the newly hardened layer is formed, the emission of electromagnetic radiation may be temporarily interrupted (or not, in the case of “continuous printing”) while build plate 20 is raised away from the bottom of the tank, thereby printing another new layer of object 22.

[0017] The build plate 20 is raised and lowered by the operation of a motor (M) 30 which drives a lead screw 12 or other structure. Rotation of the lead screw 12, via rotation of the motor shaft, raises and lowers the build plate 20 relative to the bottom of the tank 10. In another embodiment, a linear actuator or other structure may be used to raise and lower the build plate 20.

[0018] Some aspects of the printing process are directed by a controller 28, implemented as a processor-based system, having a processor-readable storage medium storing processor-executable instructions such that, when executed by the processor, the processor performs operations that cause the actions described above. For example, among other things, the controller 28 directs the raising and lowering of the build plate 20 by the motor 30, the activation and deactivation of the light source 26, and the projection of a cross-sectional image of the object in production through the mask 24. While FIG. 2 provides examples of such controllers 28, not all such controllers need have all of the features of the controller 28. For example, some controllers may not include a display if the display functionality is provided by a client computer communicatively connected to the controller, or if the display functionality is not needed. Such details are not important to the present invention.

[0019] The controller 28 includes a bus 202 or other communication mechanism for communicating information and a processor 204 (e.g., a microprocessor) coupled to the bus 202 for processing information. The controller 28 also includes a main memory 206, such as a random access memory (RAM) or other dynamic storage device, coupled to the bus 202 for storing information and instructions (e.g., G-code) to be executed by the processor 204. The main memory 206 may also be used to store temporary variables or other intermediate information during execution of instructions to be executed by the processor 204. The controller 28 further includes a read-only memory (ROM) 208 or other static storage device coupled to the bus 202 for storing static information and instructions for the processor 204. A storage device 210, such as a hard disk, flash memory-based storage medium, or other storage medium readable by the processor 204, is provided and coupled to the bus 202 for storing information and instructions (e.g., an operating system, application programs such as a slicer application, etc.).

[0020] Controller 28 is connected via bus 202 to a display 212, such as a flat panel display, for displaying information to a computer user. An input device 214, such as a keyboard including alphanumeric and other keys, is connected to bus 202 for communicating information and command selections to processor 204. Another type of user input device is a cursor control device 216, such as a mouse, trackpad, or similar input device, for communicating command information and command selections to processor 204 and for controlling cursor movement on display 212. Other user interface devices, such as a microphone, speaker, etc., not shown in detail, may be associated with accepting user input and / or presenting output.

[0021] Controller 28 also includes a communications interface 218 connected to bus 202. Communications interface 218 may provide a bidirectional data communications channel with a computer network providing connections to and within the various computer systems described above. For example, communications interface 218 may be a local area network (LAN) card providing a data communications connection to a compatible LAN, which is itself communicatively connected to the Internet via one or more Internet service provider networks. The precise details of such communications paths are not important to the present invention. What is important is that controller 28 can send and receive messages and data, e.g., digital files representing 3D articles to be produced using printer 100, via communications interface 218 and thus communicate with hosts accessible via the Internet. It should be noted that the components of controller 28 may be located within a single device or multiple devices that are physically and / or geographically separated.

[0022] According to aspects of the present invention, before starting a build process using the printing system 100, the tank 10 is filled with resin and the build plate 20 is lowered into the resin. The controller 28 activates the motor 30 to raise the build plate 20 through the resin, and the torque required to raise the build plate is recorded by the torque meter 32. In one embodiment, the torque meter 32 is integral with the motor 30 and includes a sensor or transducer that converts torque into an electrical signal. The sensor is, for example, a rotary torque sensor aligned with the lead screw 12, which provides a direct measurement of the torque required to rotate the lead screw to move the build plate 20 through the resin. Optical torque sensors or surface acoustic wave (SAW) torque sensors are two types of sensors well suited to this application. In other embodiments, the torque meter may be a function of the controller 38 or may be a stand-alone unit that, like the controller 38, is processor-based and operates under the control of processor-executable instructions stored in memory or other storage.

[0023] The torque required to move the build plate through the resin is proportional to the viscosity of the resin. Low-viscosity resins require relatively small torques, while high-viscosity resins require relatively large torques. The absolute torque required for a given combination of torque motor and build plate can be determined and tabulated for various resins of known viscosity. The tabulated results can be stored in table form, for example, in the non-volatile memory of the controller 28 or torque meter 32. In this case, when the controller 28 activates the motor 30 to raise the build plate 20 through the resin prior to the build process, the measured torque required to raise the build plate can be used to look up the table to determine the viscosity of the resin in the tank 10. If the measured torque falls between two of the listed resin viscosities, the viscosity value closest to the measured torque can be provided, or an interpolated viscosity value can be provided for the measured torque.

[0024] A similar process may be used during the build process. That is, the viscosity of the resin may be determined periodically or at other desired times using the look-up table procedure described above. Because the resin during the build process heats due to exposure to UV light from light source 26, it is quite likely that the viscosity of the resin will change during the build process. The photopolymerization process is exothermic, generating heat that is transferred (although not necessarily uniformly) throughout the resin in the vat. In some embodiments, a resin circulation system, such as that disclosed in commonly assigned U.S. patent application Ser. No. 16 / 676,940, may be employed to maintain the temperature of the resin relatively constant throughout the build process.

[0025] Prior to the build process, the temperature of the resin can be varied to achieve the desired viscosity. For example, the resin circulation system disclosed in the above-referenced patent application can be used for this purpose. Alternatively, the resin can be heated by activating the light source 26 while keeping the mask 24 in a dark state. This dark state requires that no UV light, which cures the resin, enters the tank 10. Heat from the light source 26 and the mask 24 itself is transferred to the resin in the tank, and as the resin warms, its viscosity changes. The procedure described above for measuring the viscosity of the resin via measuring the torque required to move the build plate through the resin can be used during this heating process until the desired viscosity is reached, allowing the build process to begin.

[0026] As an example of using torque to determine resin viscosity, the torque required to raise or lower a build plate through resin is: Torque = Force x (Length x sin(Angle)) where "length" is the distance traveled up or down by the lead screw, and "angle" is the angle of rotation while the lead screw is driven through the specified "length," and each of the "length" and "angle" may be measured. As noted above, torque may be determined from measurements provided by a torque meter, and thus "force" may be expressed as: Force = (length x sin(angle)) / torque is determined as follows.

[0027] It is best to think of this "force" as the drag experienced when raising and lowering the build plate through the resin. D )teeth, F D = 1 / 2×ρv 2 C D A where v is the velocity of the build plate relative to the resin (which for purposes of this invention is considered stationary during the build plate's movement), "A" is the cross-sectional area of ​​the build plate, and C D is the dimensionless drag coefficient of the resin. The density of the resin is usually available from the manufacturer and is typically between 1.05 and 1.25 g / cm 3 The drag coefficients for individual build plates for different printers and different resins should be experimentally determined and tabulated for use. Additionally, common drag coefficients for rectangular, flat plates with specific aspect ratios (length:depth) are used in various commercial publications and generally vary between 1.5 and 2. In the absence of a determined drag coefficient for a specific build plate, a value of 1.8 may be used as a good approximation, assuming the build plate has a smooth, rectangular surface. Because resin density varies with temperature, drag measurements should be performed across a temperature spectrum that mimics the operating environment when attempting to tabulate drag coefficients for various build plate and resin combinations.

[0028] At low velocities, when treating resin as an incompressible fluid, as expected in 3D printing applications, we assume that the flow of resin over a moving build plate is laminar or nearly laminar. We further assume that the dimensions of the build plate relative to the object being produced (if any) are dominant. Thus, the drag forces (even during the production of the object) are: F D = aηv where "a" is the "dimension" of the build plate and "v" is the velocity of the build plate as it is raised / lowered through the resin. The "dimension" of a given build plate is best determined experimentally. For example, for a given printer / build plate / resin combination, the drag force is calculated from the measured torque as specified above. Resin manufacturers usually specify the viscosity of their resin at a reference temperature, typically 25°C. Thus, if torque measurements for this resin are taken at that reference temperature, the "dimension" of the build plate can be calculated as a = F D / ηv = ρvC D A / 2η As mentioned above, this dimension is assumed to be constant across a common object assembly during a printing run, thus allowing different viscosity values ​​for different temperatures to be tabulated as a function of torque.

[0029] Thus, a method and system for determining the viscosity of photocurable resins used in vat polymerization printers has been described.

Claims

1. 1. A method for determining the viscosity of a photocurable resin (18) used in a vat photopolymerization printer (100), comprising: Before starting a modeling process using the vat photopolymerization printer (100), a tank (10) of the vat photopolymerization printer (100) is filled with resin (18) and a modeling plate (20) of the vat photopolymerization printer (100) is lowered into the resin (18); activating a motor (30) to raise the build plate (20) within the resin (18) and recording a measurement of the torque required to raise the build plate (20) with a torque meter (32); and using the torque measurement to look up a table listing torque measurements at known viscosities of resin to determine the viscosity of the resin (18) in the tank (10) of the vat photopolymerization printer (100).

2. The method of claim 1, wherein the torque meter (32) is integral with the motor (30) and configured to convert the torque measurement into an electrical signal.

3. 2. The method of claim 1, wherein the torque meter (32) comprises a rotary torque sensor aligned with a lead screw (12) configured to raise and lower the build plate (20) under actuation by the motor (30).

4. The method of claim 3 , wherein the rotary torque sensor comprises an optical torque sensor or a surface acoustic wave (SAW) torque sensor.

5. The method of claim 1, wherein the controller (28) is configured as part of the torque meter (32).

6. 2. The method of claim 1, wherein when the torque measurement is between listed resin viscosity values ​​in the table, the viscosity of the resin in the tank of the vat photopolymerization printer is determined as the listed viscosity value that is closest to the torque measurement.

7. 2. The method of claim 1, wherein when the torque measurement is between two resin viscosity values ​​listed in the table, the viscosity of the resin in the tank of the vat photopolymerization printer is determined as the viscosity value interpolated from the torque measurement.

8. 10. The method of claim 1, further comprising: determining the viscosity of the resin (18) in the tank (10) by using a motor (30) to raise and / or lower a build plate (20) in the resin (18) during the build process; recording with a torque meter (32) a current measurement of the torque required to raise and / or lower the build plate (20); and determining the current viscosity of the resin (18) using a current lookup table.

9. 10. The method of claim 1, further comprising using a light source (26) of the vat photopolymerization printer (100) to modify the current viscosity of the resin (18) in the tank (10) of the vat photopolymerization printer (100) during the build process.

10. 10. The method of claim 9, further comprising: measuring the current viscosity of the resin (18) by using a motor (30) to raise and / or lower a build plate (20) in the resin (18) while changing the current viscosity of the resin (18) in the tank (10) of the vat photopolymerization printer (100); recording a current measurement of the torque required to raise and / or lower the build plate (20) with a torque meter (32); and determining the current viscosity of the resin (18) in the tank (10) using a current lookup table.

11. 1. A system for determining the viscosity of a photocurable resin (18) used in a vat photopolymerization printer (100), comprising: a vat photopolymerization printer (100) having a tank (10) holding a volume of resin (18); a shaping plate (20) configured to be raised and lowered within the tank (10); a motor (30) coupled to raise and / or lower the build plate (20); a torque meter (32) configured to measure the torque required to raise and / or lower the build plate (20) within the resin (18); a controller (28); The controller (28) is configured to operate a motor (30) to raise and / or lower a build plate (20) within the resin (18), and is configured to receive torque measurements from a torque meter (32), and to use the torque measurements to determine the viscosity of the resin (18) within the tank (10) of the vat photopolymerization printer (100) by searching a table listing torque measurements for known viscosities of the resin.

12. The system of claim 11, wherein the torque meter (32) is integral with the motor (30) and configured to convert torque into an electrical signal.

13. 12. The system of claim 11, wherein the torque meter (32) comprises a rotary torque sensor aligned with a lead screw (12) coupled to raise and lower the build plate (20) under actuation by the motor (30).

14. 14. The system of claim 13, wherein the rotational torque sensor comprises an optical torque sensor or a surface acoustic wave (SAW) torque sensor.

15. The system of claim 11, wherein the controller (28) is configured in part as a torque meter (32).

16. 12. The system of claim 11, wherein the controller is further configured to determine the viscosity of the resin in the tank of the vat photopolymerization printer as the listed viscosity value that is closest to the torque measurement when the torque measurement is between two listed viscosity values ​​of the resin in the table.

17. 12. The system of claim 11, wherein the controller is further configured to determine the viscosity of the resin in the tank of the vat photopolymerization printer as an interpolated viscosity value relative to the torque measurement when the torque measurement is between two of the resin viscosity values ​​listed in the table.

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