Method and system for determining the viscosity of a photocurable resin for a water bath type photopolymerization printer

Abstract

A method and system for forming a 3D article by a water bath photopolymerization process are disclosed, wherein the resin viscosity is determined by measuring the torque required to raise and lower a build plate within the resin in the water bath before the start of the build process and optionally during the build process. The resin may be heated by heating the resin in the water bath using a light engine used to produce the 3D article, and the viscosity of the resin may be changed accordingly.

Description

Technology Field

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

[0002] The present invention relates to a method and system for determining the viscosity of a photo-curing resin used in additive manufacturing processes, particularly in vat polymerization printers. Background Technology

[0003] As previously announced, additive manufacturing or 3D printing is a collection of different technologies that provide different means for the direct manufacturing of various articles. One such technology is bath-based photopolymerization, which includes stereolithography (SLA), direct light processing (DLP), and liquid crystal display (LCD) direct printing. These technologies generally involve the selective curing of a resin contained in a bath using a (typically) ultraviolet (UV) light source. The resin is cured layer by layer to create a continuous series of cross-sections through which the article being manufactured adheres to one another.

[0004] It is known that resin viscosity is an important parameter in these layer-by-layer printing processes. Viscosity represents the internal resistance of a fluid to motion, that is, the fluid's resistance to deformation. Thicker fluids have higher viscosity. For example, the viscosity of oil is higher than that of water. In the case of bath-type photopolymerization processes, low-viscosity resins are generally preferred because they allow for relatively rapid replenishment in the build area between layer formations. However, articles formed from low-viscosity resins tend to undergo shrinkage and warping during curing and post-solidification. Therefore, high-viscosity resins are preferred because they do not suffer from such undesirable side effects (at least not to the same extent as low-viscosity resins). Higher-viscosity resins can also produce more desirable article properties equivalent to those formed from injection molding processes.

[0005] Viscosity is caused by the cohesive forces between liquid molecules, which vary with temperature. For liquids, viscosity (μ) can be approximated as μ = a10b / (T - c), where T is the absolute temperature and a, b, and c are experimentally determined constants. Therefore, it is known that heating high-viscosity resins, which would otherwise be difficult to process in a vat photopolymerization printer, lowers their viscosity, making such resins more suitable for use in such devices. WO 2015 / 074088 proposes heating the resin using a resistive heating element at the edge of the exposed area. WO 2016 / 078838 proposes heating the resin using a transparent electrically conductive coating on the bottom of the vat. US 2019 / 0202112 proposes heating the resin using an electromagnetic radiation source independent of the light source. In addition, a water tank type photopolymerization device is disclosed in which a water tank is placed in a furnace-like enclosure where the entire build process takes place.

[0006] In one embodiment, the present invention provides a method for determining the viscosity of a photocurable resin used in a water bath photopolymerization printer. Before starting the build process using the water bath photopolymerization printer, the tank of the water bath photopolymerization printer is filled with resin, and the build plate of the water bath photopolymerization printer is lowered into the resin. A motor is engaged to raise the build plate within the resin, and a measurement of the torque required to raise the build plate is recorded by a torque meter. Using the torque measurement, a table of torque measurements is indexed using known resin viscosities to determine the viscosity of the resin in the water bath photopolymerization printer tank. The torque meter may be integrated into the motor and configured to convert the torque into an electrical signal. In some cases, the torque meter may include a rotary torque sensor connected in-line with a lead screw adjusted to raise and lower the build plate under operation by the motor. Such a rotary torque sensor may be an optical or surface acoustic wave (SAW) torque sensor. Additionally, in some cases, the controller may be partially composed of a torque meter. When the measured torque is between two index values ​​of resin viscosity in a table, the viscosity of the resin in the tank of the water bath photopolymerization printer may be determined as the nearest index value for the viscosity for torque measurement, or alternatively, as the interpolated value of viscosity for torque measurement.

[0007] During the build process, the current viscosity of the resin in the tank of the water-tub photopolymerization printer can be determined by using a motor to raise and / or lower the resin build plate, recording the current measurement of the torque required to raise and / or lower the build plate by a torque meter, and determining the current viscosity of the resin in the tank using a current table lookup. Additionally, during the build process, the current viscosity of the resin in the tank of the water-tub photopolymerization printer can be changed or controlled using the light source of the photopolymerization printer (e.g., to heat the resin). While changing the current viscosity of the resin in the tank of the water-tub photopolymerization printer, the current viscosity of the resin can be measured by using a motor to raise and / or lower the resin build plate, recording the current measurement of the torque required to raise and / or lower the build plate by a torque meter, and determining the current viscosity of the resin in the tank using a current table lookup.

[0008] In one embodiment of the present invention, a system for determining the viscosity of a photocurable resin used in a photopolymerization printer may include: a tank-type photopolymerization printer having a tank configured to maintain the volume of the resin; a build plate configured to rise and fall 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 configured to determine the viscosity of the resin within the tank of the tank-type photopolymerization printer by operating the motor to raise and / or lower the build plate within the resin, receiving the measured value from the torque meter, and indexing a table of the measured torque values ​​using the measured torque values ​​with the known viscosity of the resin. The system may also have a light engine. Brief explanation of the drawing

[0009] The present invention is illustrated without limitation by way of example in the drawings of the following attached drawings. FIG. 1 is a schematic cross-sectional view of a 3D printing system configured according to an embodiment of the present invention in which an object is manufactured in a tank containing a photocurable liquid resin. FIG. 2 illustrates an example of a controller for a 3D printing system exemplified in FIG. 1. Specific details for implementing the invention

[0010] A method and system for forming a 3D article by a water bath photopolymerization process is disclosed herein, wherein the resin viscosity is determined by measuring the torque required to raise and lower a build plate within the resin in the water bath before the start of the build process and optionally during the build process. The resin may be heated in the water bath using a light engine used to produce the 3D article, by an independent heater (of a light source), or by other means, and its viscosity may be changed accordingly.

[0011] FIG. 1 illustrates a cross-section of a 3D printing system (100) configured according to an embodiment of the present invention, wherein electromagnetic radiation (e.g., UV light) is used to cure a photocurable liquid resin (typically, liquid polymer) (18) to produce an object (e.g., a 3D object) (22). The object (22) is produced layer by layer (i.e., a new layer of the object (22) is formed by photocuring a layer of liquid polymer (18) adjacent to the bottom surface of the object (22), and when each new layer is formed, the object is raised by a build plate (20) so that the next layer of photocurable liquid resin (18) is drawn under the newly formed layer. This process may be repeated several times to form additional layers until the production of the object is completed.

[0012] A 3D printing system (100) comprises a tank (10) for containing a photocurable liquid resin (18). The bottom (or at least part thereof) of the tank (10) is sealed by a flexible membrane (14) that is transparent (or nearly similar) at the wavelength of interest for curing the resin so that electromagnetic radiation from a light source (26) can enter the tank (10) (i.e., to prevent the photocurable liquid polymer (18) from leaking out of the tank (10). A mask (24) (e.g., a liquid crystal layer) is placed between the light source (26) and the photocurable liquid resin (18) to allow selective curing of the liquid resin (which allows the formation of a 3D object in a desired shape / pattern). In various embodiments, collimation and diffusion elements such as lenses, reflectors, filters, and / or films may be placed between the mask (24) and the light source (26). These elements were not illustrated in the examples to avoid unnecessarily obscuring the drawings.

[0013] A platen or backing member (16) formed of borosilicate glass or other material is placed between the mask (24) and the flexible membrane (14) and provides structural support. The platen is also transparent (or nearly transparent) at one or more wavelengths of interest for resin curing. In other examples, the platen (16) may be metal or plastic and may 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 instead of a separate window and its periphery is sealed with a gasket. Note that although the mask (24), platen (16), and membrane (14) are depicted as being displaced from each other by a certain distance, in practice these components may be positioned to be in contact with each other to prevent refraction at any air interface. A flexible membrane (14) is secured to the edge of the tank (10) or to a replaceable cartridge assembly (not shown) to maintain a liquid-tight perimeter at the edge of the tank or other openings ("liquid-tight" means that the tank does not leak during normal use).

[0014] When fabricating a layer of an object (22) using a 3D printing system (100), electromagnetic radiation is emitted from a radiation source (26) into a tank (10) through a mask (24), a platen (16), and a membrane (14). The electromagnetic radiation forms an image on an image plane adjacent to the bottom of the object (22). Areas of high (or medium) intensity within the image cause localized curing of the photocurable liquid resin (18). The newly cured layer adheres to the previous bottom surface of the object (22) and is substantially not adhered to the bottom surface of the tank (10) due to the presence of the flexible membrane (14). After the newly cured layer is formed, the emission of electromagnetic radiation may be temporarily interrupted (or not interrupted, as in the case of "continuous printing") while the build plate (20) is raised from the bottom of the tank so that another new layer of the object (22) can be printed.

[0015] The build plate (20) can be raised and lowered by the action of a motor (M) (30) driving a lead screw (12) or other array. The rotation of the lead screw (12) caused by the rotation of the motor shaft causes the build plate (20) to be raised or lowered relative to the bottom of the tank (10). In another embodiment, a linear actuator or other array may be used to raise and lower the build plate (20).

[0016] The mode of the printing process is directed by a controller (28), which is implemented as a processor-based system having a processor-readable storage medium in which processor-executable instructions are stored, and which performs work when the processor executes these instructions to trigger the aforementioned measures. For example, among other things, the controller (28) may direct the raising / lowering of the build plate (20) via the motor (30), the activation and deactivation of the light source (26), and the projection of a cross-sectional image of the object being manufactured via the mask (24). FIG. 2 provides an example of such a controller (28), but not all such controllers need to have all the features of the controller (28). For example, a particular controller may not include a display, as the display function may be provided by a client computer that is communicationally coupled to the controller, or the display function may be unnecessary. Such details are not important in the present invention.

[0017] 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 random access memory (RAM) or other dynamic storage device, coupled to the bus (202) to store 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 the 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 (302) to store static information and instructions for the processor (204). A storage device (210), such as a hard disk, a flash memory-based storage medium, or other storage medium readable by a processor (204), is provided and coupled to a bus (202) to store information and instructions (such as an operating system, an application program such as a slicer application, etc.).

[0018] The controller (28) may be coupled via a bus (202) to a display (212), such as a flat panel display, to display information to a computer user. An input device (214), such as a keyboard containing alphanumeric and other keys, may be coupled to the bus (202) to transmit information and command selections to the processor (204). Another type of user input device is a cursor control device (216), such as a mouse, trackpad, or similar input device, to transmit directional information and command selections to the processor (204) and to control cursor movement on the display (212). Other user interface devices, such as a microphone, speaker, etc., are not illustrated in detail but may be involved in receiving user input and / or displaying output.

[0019] The controller (28) also includes a communication interface (218) coupled to the bus (202). The communication interface (218) may provide a two-way data communication channel with a computer network, which provides a connection to the various computer systems discussed above. For example, the communication interface (218) may be a Local Area Network (LAN) card for providing a data communication connection to a compatible LAN, which itself is coupled to communicate with the Internet through one or more Internet Service Provider networks. The exact details of such communication paths are not important in the present invention. What is important is that the controller (28) can transmit and receive messages and data, for example, digital files representing 3D articles to be manufactured using the printer (100) via the communication interface (218), and thus communicate with a host accessible via the Internet. It should be noted that the components of the controller (28) may be located in a single device or in multiple physically and / or geographically distributed devices.

[0020] According to an embodiment of the present invention, before starting the build process using a printing system (100), a tank (10) is filled with resin and a build plate (20) is lowered into the resin. A controller (28) engages with a motor (30) to raise the build plate (20) within the resin, and the torque required to raise the build plate is recorded by a torque meter (32). In one embodiment, the torque meter (32) includes a sensor or transducer integrated with the motor (30) that converts the torque into an electrical signal. For example, the sensor may be a rotary torque sensor connected in-line with a lead screw (12) that provides a direct measurement of the torque required to rotate the lead screw to move the build plate (20) within the resin. Optical or surface acoustic wave (SAW) torque sensors are two types of sensors that are very suitable for this application. In another embodiment, the torque meter may be a function of the controller (38) or may be a standalone unit that is processor-based like the controller (38) and operates under the control of processor execution instructions stored in memory or other storage devices.

[0021] The torque required to displace the build plate within the resin will be proportional to the viscosity of the resin. Less torque is required for low-viscosity resin, while greater torque is required for high-viscosity resin. The absolute torque required for a given combination of motor and build plate can be determined and tabulated for various resins of known viscosities. The tabulated results can be stored in the form of a table in the non-volatile memory of, for example, the controller (28) or the torque meter (32). Then, when the controller (28) engages with the motor (30) to raise the build plate (20) within the resin before the build process, the measured torque required to raise the build plate can be used as an index to the table to determine the viscosity of the resin within the tank (10). If the measured torque falls between two tabulated resin viscosity values, the closest value of viscosity for the measured torque may be provided, or an interpolated value of viscosity for the measured torque may be provided.

[0022] A similar process may be used during the build process. That is, the viscosity of the resin may be determined using the above table lookup procedure at a time or at other desired times. During the build process, as long as the resin is heated during the build process by being exposed to UV light from a light source (26), the viscosity of the resin is highly likely to change. The photopolymerization process is exothermic and generates heat, and the heat is transferred throughout the resin in the bath (though not necessarily uniformly). In some embodiments, a resin circulation system such as that disclosed in U.S. Patent Application No. 16 / 676,940 assigned to the assignee of the present invention may be used to maintain the temperature of the resin relatively constant throughout the entire build process.

[0023] Before the build process, the resin temperature may be changed to achieve the desired resin viscosity. For example, the resin circulation system disclosed in the patent application referenced above may be used for such purposes. Alternatively, the resin may be heated by activating the light source (26) while keeping the mask (24) in a dark state. This dark state is necessary to prevent UV light from entering the tank (10) and curing the resin. Heat from the light source (26) and the mask (24) itself is transferred to the resin in the tank, and as the resin is heated, its viscosity will change. The above procedure for measuring resin viscosity by measuring the torque required to move the build plate through the resin may be used during this heating process until the desired viscosity is reached and the build process begins.

[0024] As an example of the use of torque to determine resin viscosity, the torque required to raise and lower the build plate in the resin can be expressed as follows:

[0025] Torque = Force x [Length x sin(angle)]

[0026] Here, "length" is the vertical displacement of the lead screw, and "angle" is the angle of rotation at which the lead screw is driven for a specified "length" that can be measured. As shown above, since torque can be determined from measurements provided by a torque meter, the "force" can be determined as follows:

[0027] Force = [Length x sin(angle)] / Torque

[0028] This "force" can be considered as the drag experienced when the build plate rises or falls in the resin. The drag (FD) is related to the density (ρ) of the resin as follows:

[0029]

[0030] Here, "v" is the velocity of the build plate relative to the resin (for the purposes of the present invention, it may be considered stationary during the movement of the build plate), "A" is the cross-sectional area of ​​the build plate, and CD is the dimensionless drag coefficient of the resin. Resin densities are generally available from the relevant manufacturers and are typically 1.05 to 1.25 g / cm³. 3 The drag coefficient of individual build plates for various printers can be determined empirically for various resins and tabulated for use. Furthermore, general drag coefficients for rectangular plates with a specific aspect ratio (length:depth) are available in various commercial publications and generally vary from 1.5 to 2; if no determined drag coefficient is available for a specific build plate, a value of 1.8 can be used as a good approximation, assuming the build plate has a smooth rectangular surface. Since resin density can vary with temperature, drag measurements must be performed across a temperature spectrum that mimics the operating environment when attempting to tabulate drag coefficients for various build plates and resin combinations.

[0031] As expected in 3D printing applications, when resin is processed as a non-compressible fluid at low speeds, the flow of the resin over the moving build plate is assumed to be laminar or nearly laminar. Additionally, the size of the build plate relative to the object being fabricated (if any) is assumed to be dominant. Therefore, drag (even during object fabrication) is related to the resin viscosity (η) as follows:

[0032]

[0033] Here, "a" is the "size" of the build plate and "v" is the speed of the build plate when raised or lowered in the resin. The "size" of a given build plate can be determined empirically. For example, for a given printer / build plate / resin combination, drag can be calculated from the measured torque as specified above. Resin manufacturers generally specify the resin viscosity at a reference temperature, typically 25°C. Therefore, by measuring the torque using the resin at the reference temperature, the "size" of the build plate can be calculated as follows:

[0034]

[0035] As mentioned above, since these dimensions can be considered invariant for a typical object assembly during a printing operation, various viscosity values ​​for various temperatures can be tabled as a function of torque.

[0036] Thus, a method and system for measuring the viscosity of a photocurable resin used in a water bath type polymerization printer have been described.

Claims

Claim 1 A method for forming 3D articles from a photo-curing resin (18) using a vat photopolymerization printer (100), comprising: engaging a motor (30) to raise and / or lower a build plate (20) of the vat photopolymerization printer (100) located within the resin (18) contained in the tank (10) of the printer (100); recording a measure of torque applied to the build plate (20) by the motor (30) during the engaging step; determining the viscosity of the resin (18) in the tank (10) of the vat photopolymerization printer (100) using the measured torque; and controlling the viscosity of the resin (18) by changing the temperature of the resin (18) according to the determination. A method for forming a three-dimensional article, comprising the step of operating a water tank type photopolymerization printer (100) using the resin to form the three-dimensional article. Claim 2 A method for forming a three-dimensional article according to claim 1, wherein the step of determining the viscosity of the resin (18) includes the step of calculating the viscosity according to a proportional relationship from the measured value of torque. Claim 3 A method for forming a three-dimensional article according to claim 1, wherein the torque measurement value is measured using a torque meter (32) integrated into a motor (30) and configured to convert the torque measurement value into an electrical signal. Claim 4 A method for forming a three-dimensional article, wherein the torque measurement value is measured using a torque meter (32) comprising a rotational torque sensor connected inline with a lead screw (12) adjusted to raise and lower the build plate (20) under the action of a motor (30). Claim 5 A method for forming a three-dimensional article according to claim 4, wherein the rotational torque sensor comprises an optical or surface acoustic wave (SAW) torque sensor. Claim 6 A method for forming a three-dimensional article, wherein the step of determining the viscosity of the resin (18) in claim 1 includes the step of using a table look-up procedure. Claim 7 In claim 6, when the measured torque is between two tabulated values ​​of resin viscosity in a table, the determining step uses the tabulated viscosity value closest to the measured torque, a method for forming a three-dimensional article. Claim 8 In claim 6, when the measured torque is between the values ​​of two tables of resin viscosity in the table, the determining step uses an interpolated value of viscosity for the measured torque, a method for forming a three-dimensional article. Claim 9 A method for forming a three-dimensional article, wherein the temperature of the resin (18) is changed using a light source (26) of a water bath type photopolymerization printer (100). Claim 10 A method for forming a three-dimensional article, wherein, in any one of claims 1 to 9, the controlling step is performed before starting the formation of the three-dimensional article. Claim 11 A system for forming a three-dimensional article from a photo-curing resin (18) using a vat photopolymerization printer (100), comprising: a vat photopolymerization printer (100) having a tank (10) configured to maintain the volume of the resin (18); a heat source configured to heat the resin (18) maintained in the tank (10); a build plate (20) configured to rise and fall within the tank (10); a motor (30) coupled to raise and / or lower the build plate (20); and a torque meter (32) configured to measure the torque applied by the motor (30) to raise and / or lower the build plate (20) within the resin (18). A three-dimensional article forming system comprising a controller (28) configured to operate the motor (30) to raise and / or lower the build plate (20) within the resin (18), receive a torque measurement value from the torque meter (32), determine the viscosity of the resin (18) in the tank (10) of the water tank photopolymerization printer (100), and control the viscosity of the resin by operating the heat source to change the temperature of the resin (18) in the tank (10) according to the determined viscosity during and / or before the operation of the system for forming a three-dimensional article. Claim 12 In claim 11, the torque meter (32) is integrated into the motor (30) and configured to convert torque into an electrical signal, a three-dimensional article forming system. Claim 13 In claim 11, the torque meter (32) comprises a rotary torque sensor connected inline with a lead screw (12) coupled to raise and lower the build plate (20) under the action of a motor (30), in a three-dimensional article forming system. Claim 14 In claim 13, the rotational torque sensor comprises an optical or surface acoustic wave (SAW) torque sensor, a three-dimensional article forming system. Claim 15 In claim 11, the controller (28) is partially composed of a torque meter (32), a three-dimensional article forming system. Claim 16 In claim 11, the controller (28) is further configured to determine the viscosity of the resin (18) in the tank (10) of the water bath type photopolymerization printer (100) to the nearest tabulated viscosity value for measuring torque when the measured value of torque is between two tabulated values ​​of resin viscosity in a tabulated table with known resin viscosity. Claim 17 In claim 11, the controller (28) is further configured to determine the viscosity of the resin (18) in the tank (10) of the water bath type photopolymerization printer (100) as an interpolated viscosity value for measuring torque when the measured torque value is between two tabulated values ​​of resin viscosity in a tabulated table with known resin viscosity.

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