Additive manufacturing method including thermal modeling and control

Various embodiments described herein provide a method of making an object from a three-dimensional geometry file and a light polymerizable resin on a light-transmissive window by projection of light through the window in a bottom-up stereolithography process. The method may comprise: slicing the file into a series of sequential images. Temperature fluctuations in the resin may be calculated for at least some of the sequential images upon light polymerization thereof based on sequential exposure of the resin to light, the light corresponding to the series of sequential images. During producing of the object, the production may be modified based on the calculated temperature fluctuations by: (i) reducing production speed during at least a portion of the production; (ii) activating a window cooler during at least a portion of the production; or (iii) increasing production speed during at least a portion of the production.

TECHNICAL FIELD

The present invention concerns additive manufacturing, and particularly concerns methods, apparatus, and systems for controlling thermal effects created by an additive manufacturing process.

BACKGROUND

A group of additive manufacturing techniques sometimes referred to as “stereolithography” create a three-dimensional object by the sequential polymerization of a light polymerizable resin. Such techniques may be “bottom-up” techniques, where light is projected into the resin onto the bottom of the growing object through a light transmissive window, or “top down” techniques, where light is projected onto the resin on top of the growing object, which is then immersed downward into the pool of resin.

The recent introduction of a more rapid stereolithography technique known as continuous liquid interface production (CLIP), coupled with the introduction of “dual cure” resins for additive manufacturing, has expanded the usefulness of stereolithography from prototyping to manufacturing

SUMMARY

One artifact of increasing speed of production in additive manufacturing is that an excessive exothermic reaction may occur during the polymerization process. Significant exothermic reactions may create potential “hot-spots” in an object created using the thermosetting polymer reins, which may result in heat-induced shrinkage. The present application provides methods, devices, and systems for minimizing the effect of such potential hot-spots by modeling the thermal characteristics of an additive manufacturing session, and for controlling devices and systems to combat unwanted thermal excesses.

Various embodiments described herein provide a method of making an object from a three-dimensional geometry file (e.g., an .stl file) and a light polymerizable resin on a light-transmissive window by projection of light through the window in a bottom-up stereolithography process. The method may comprise: (a) slicing the file into a series of sequential images (e.g., .png images), where each sequential image corresponds to a part geometry; and (b) calculating temperature fluctuations in the resin for at least a subset of the sequential images upon light polymerization thereof based on sequential exposure of the resin to light, the light corresponding to the series of sequential images. The calculating may carried out based on resin properties and object geometry. The method may further include producing the object from the light polymerizable resin and the series of sequential images by bottom-up stereolithography. While producing the object from the light polymerizable resin, (d) the production may be modified based on the calculated temperature fluctuations by: (i) reducing production speed during at least a portion of the production that corresponds to at least one of the subset of the sequential images to avoid exceeding a predetermined temperature limit for the window or the resin (e.g., to avoid damaging the window); (ii) activating a window cooler to during at least a portion of the production that corresponds to at least one of the subset of the sequential images to cool the window and avoid exceeding a predetermined temperature limit for the window (e.g., to at least partially maintain, or increase, production speed while avoiding damaging the window); or (iii) increasing production speed during at least a portion of the production that corresponds to at least one of the subset of the sequential images based on a change in resin properties (e.g., reduced viscosity) caused by local temperature fluctuations.

In some embodiments, the subset of images includes images corresponding to portions of the object spaced apart from each other in a Z-direction in fixed increments.

In some embodiments, the calculating step is based on a solution of a partial differential equation (e.g., a Gaussian heat kernel).

In some embodiments, the calculating step (b) is carried out before the producing step (c) or concurrently with the producing step (c).

In some embodiments, the calculating step (b) is further based on planned or actual speed of the producing step (c).

In some embodiments, the window is heated, the calculating step is carried out based on window properties, and the window properties include a temperature profile for said heated window.

In some embodiments, the window cooler comprises a Peltier cooler.

In some embodiments, the window comprises a composite of at least two layers of different materials (e.g., a polymer layer, such as a fluoropolymer layer, on an inorganic supporting layer, such as a silicate glass, sapphire, or ALON (aluminum oxynitride) layer), optionally but in some embodiments preferably joined with an adhesive layer.

Various embodiments described also provide an additive manufacturing system including an additive manufacturing apparatus configured to construct an object from a light polymerizable resin on a light-transmissive window by projection of light through the window in a bottom-up stereolithography process; a processor; and memory storing non-transitory computer readable instructions that, when executed by the processor, cause the processor to carry out the method embodiments described herein.

The foregoing and other objects and aspects of the present invention are explained in greater detail in the drawings herein and the specification set forth below. The disclosures of all United States patent references cited herein are to be incorporated herein by reference.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

As used herein, the term “and/or” includes any and all possible combinations or one or more of the associated listed items, as well as the lack of combinations when interpreted in the alternative (“or”).

It will be understood that when an element is referred to as being “on,” “attached” to, “connected” to, “coupled” with, “contacting,” etc., another element, it can be directly on, attached to, connected to, coupled with and/or contacting the other element or intervening elements can also be present. In contrast, when an element is referred to as being, for example, “directly on,” “directly attached” to, “directly connected” to, “directly coupled” with or “directly contacting” another element, there are no intervening elements present. It will also be appreciated by those of skill in the art that references to a structure or feature that is disposed “adjacent” another feature can have portions that overlap or underlie the adjacent feature.

Numerous resins for use in additive manufacturing are known and can be used in carrying out the present invention. See, e.g., U.S. Pat. No. 9,205,601 to DeSimone et al. In some embodiments, the resin is a dual cure resin. Such resins are described in, for example, Rolland et al., U.S. Pat. Nos. 9,676,963; 9,598,606; and 9,453,142, and in Wu et al., US Patent Application Pub. No. US2017/0260418, the disclosures of which are incorporated herein by reference.

Resins may be in any suitable form, including “one pot” resins and “dual precursor” resins (where cross-reactive constituents are packaged separately and mixed together before use, and which may be identified as an “A” precursor resin and a “B” precursor resin).

Note that, in some embodiments employing “dual cure” polymerizable resins, the part, following manufacturing, may be contacted with a penetrant liquid, with the penetrant liquid carrying a further constituent of the dual cure system, such as a reactive monomer, into the part for participation in a subsequent cure. Such “partial” resins are intended to be included herein.

2. Production by Additive Manufacturing.

Polymerizable liquids or resins as described herein may be used to make three-dimensional objects, in a “light” cure (typically by additive manufacturing) which in some embodiments generates a “green” intermediate object, followed in some embodiments by a second (typically heat) cure of that intermediate object.

Techniques for additive manufacturing are known. Suitable techniques include bottom-up additive manufacturing, or bottom-up stereolithography. Such methods and techniques are known and described in, for example, U.S. Pat. No. 5,236,637 to Hull, U.S. Pat. Nos. 5,391,072 and 5,529,473 to Lawton, U.S. Pat. No. 7,438,846 to John, U.S. Pat. No. 7,892,474 to Shkolnik, U.S. Pat. No. 8,110,135 to El-Siblani, U.S. Patent Application Publication No. 2013/0292862 to Joyce, and US Patent Application Publication No. 2013/0295212 to Chen et al. The disclosures of these patents and applications are incorporated by reference herein in their entirety.

In some embodiments, the intermediate object is formed by continuous liquid interface production (CLIP). CLIP methods and apparatus are known and described in, for example, PCT Application Nos. PCT/US2014/015486 (published as U.S. Pat. No. 9,211,678 on Dec. 15, 2015); PCT/US2014/015506 (also published as U.S. Pat. No. 9,205,601 on Dec. 8, 2015), PCT/US2014/015497 (also published as U.S. Pat. No. 9,216,546 on Dec. 22, 2015), and in J. Tumbleston, D. Shirvanyants, N. Ermoshkin et al., Continuous liquid interface production of 3D Objects,Science347, 1349-1352 (published online 16 Mar. 2015). See also R. Janusziewcz et al., Layerless fabrication with continuous liquid interface production,Proc. Natl. Acad. Sci. USA113, 11703-11708 (Oct. 18, 2016). In some embodiments, CLIP employs features of a bottom-up three-dimensional fabrication as described above, but the irradiating and/or said advancing steps are carried out while also concurrently maintaining a stable or persistent liquid interface between the growing object and the build surface or window, such as by: (i) continuously maintaining a dead zone of polymerizable liquid in contact with said build surface, and (ii) continuously maintaining a gradient of polymerization zone (such as an active surface) between the dead zone and the solid polymer and in contact with each thereof, the gradient of polymerization zone comprising the first component in partially cured form. In some embodiments of CLIP, the optically transparent member comprises a semipermeable member (e.g., a fluoropolymer), and the continuously maintaining a dead zone is carried out by feeding an inhibitor of polymerization through the optically transparent member, thereby creating a gradient of inhibitor in the dead zone and optionally in at least a portion of the gradient of polymerization zone. Other approaches for carrying out CLIP that can be used in the present invention and potentially obviate the need for a semipermeable “window” or window structure include utilizing a liquid interface comprising an immiscible liquid (see L. Robeson et al., WO 2015/164234, published Oct. 29, 2015), generating oxygen as an inhibitor by electrolysis (see I Craven et al., WO 2016/133759, published Aug. 25, 2016), and incorporating magnetically positionable particles to which the photoactivator is coupled into the polymerizable liquid (see J. Rolland, WO 2016/145182, published Sep. 15, 2016).

After the intermediate three-dimensional object is formed, it is optionally cleaned, optionally dried (e.g., air dried) and/or rinsed (in any sequence). In some embodiments it is then further cured, preferably by heating (although further curing may in some embodiments be concurrent with the first cure, or may be by different mechanisms such as contacting to water, as described in U.S. Pat. No. 9,453,142 to Rolland et al.).

FIG. 1shows an example for a part (e.g., a model of the Eiffel Tower in Paris, France) that may be manufactured using an additive manufacturing technique.FIGS. 2A, 2B, 2C, and 2Dillustrate, on the right side of the Figures, cross-sectional areas of the tower model at identified points inFIG. 1, and on the left side of the Figures show simulated hot-spots that may result in unwanted or deleterious effects in the part when manufactured according to the additive manufacturing technique. For example, such hot-spots may result in heat-induced shrinkage at locations within the completed part. As disclosed herein, devices, systems, and methods may compensate for such deleterious effects without going through a trial and error process of generating ideal production part parameters, by modeling the temperature of the object during manufacturing, and by modifying production (e.g., modifying production parameters) during the production process.

By way of explanation of how to model the thermal or temperature properties of the additive manufacturing material (e.g., a thermosetting polymer resin), consider the function:
T(x,t)=(4πkt)−1/2exp(−x2/4kt)

In the equation above, k is the thermal diffusivity of the material (with example units of m2/s), t is time (with example units of seconds), x is the distance from the center of the heat pulse, and T is the temperature response for a unit impulse, with arbitrary units. The impulse response for a temperature spike is a Gaussian distribution, modeled as:

The convolution of the function may look like:

Where ξ is a spatial variable used for performing the convolution.

Hence, in performing the convolution, the units may become temperature units (e.g., Celsius or Kelvin units). This may be seen in that the impulse response has example units of [1/m], temperature has units of [K], the convolution adds a distance, which cancels out the [1/m] units, leaving temperature.

Accordingly, when an image is split into small pieces, such as pixels, localized temperature spikes due to curing may be calculated, and convolved with a Gaussian distribution, thus “blurring” the spike, enabling an examination into how temperature appears in a two-dimensional plane. This term may be further refined by inclusion of additional terms, such a heat loss term based on e.g., Newton's law of cooling and/or an exponential decay term). For example, heat loss to an environment may be modeled as:

Heat addition may be modeled as:
T(t1,x)=T(t0,x)+ΔT(x)

where ΔT is the expected temperature increase for curing a given slice thickness.

Lateral heat dissipation may be modeled as:

ξ is a spatial “dummy” variable for the convolution and k is thermal diffusivity in units of

L2T⁢⁢e.g.⁢[m2s]⁢⁢or⁢[µm2s],
depending on what x and ξ are in.
4. Thermal Control.

The above modeling technique may be used to calculate a piece-by-piece (e.g., pixel-by-pixel) approximation of temperature in a two-dimensional image with significantly high fidelity, available to a device or system with relatively low computational requirements.

FIG. 3illustrates a flowchart that may be used in accordance with the present disclosure to use in the calculating of the thermal model. For example, the flowchart ofFIG. 3may be used in a method of making an object from a three-dimensional geometry file (e.g., an .stl file) and a light polymerizable resin on a light-transmissive window by projection of light through the window in a bottom-up stereolithography process.

In a first operation, S100, the three-dimensional geometry file may be sliced into a series of sequential images (e.g., .png images). Each sequential image may correspond to a part geometry. In operation S200, temperature fluctuations in the resin may be calculated for at least a subset of said sequential images upon light polymerization thereof based on sequential exposure of the resin to light. The light may correspond to the series of sequential images. The calculating may be carried out based on resin properties and object geometry, using the thermal modeling technique discussed above. The series of sequential images may then be used to produce said object from said light polymerizable resin and said series of sequential images by bottom-up stereolithography, with each image used to generate a part geometry of the object (as seen in operation S500) until the series of images has been processed (as seen in operation S600). However, based on the calculated temperature fluctuations, the producing of said object may be modified (as seen in operations S300and S400). For example, a production speed may be reduced during at least a portion of the producing operation that corresponds to at least one of the subset of said sequential images to avoid exceeding a predetermined temperature limit for said window (e.g., to avoid damaging the window). As another example, a window cooler may be activated during at least a portion of said producing operation that corresponds to at least one of the subset of said sequential images to cool said window and avoid exceeding a predetermined temperature limit for said window (e.g., to at least partially maintain, or increase, production speed while avoiding damaging the window). As a third example, production speed may be increased during at least a portion of the producing operation that corresponds to at least one of the subset of said sequential images based on a change in resin properties (e.g., reduced viscosity) caused by local temperature fluctuations.

With respect to activating a window cooler to cool said window, any suitable cooler may be used to carry out the present invention. In some embodiments, the cooler can be simply a fan or blower positioned for blowing air that is cooler than the window (including air at ambient temperature) at the window bottom surface. In some embodiments, the cooler can comprise a compressor and a refrigerant system, or a heat exchanger, operatively associated with the window. In some embodiments, the cooler can comprise one or more thermoelectric devices (e.g., Peltier coolers) operatively associated with the window, and optionally but preferably also operatively associated with a heat sink (which heat sink may be the deck or support structure for the window, a separate heat sink with cooling fins, etc., including combinations thereof). When one or more thermoelectric devices is employed as the window, the window is preferably comprised of a sapphire, ALON, or other thermally conductive supporting member (upon which a semipermeable layer such as a fluoropolymer may be supported), and the thermoelectric devices are thermally coupled (directly or through one or more additional thermally conductive components) to peripheral edge portions of the window. Combinations of multiple different cooling systems, such as both blowers and thermoelectric devices, may also be used

According to aspects of the present disclosure, one or more computing devices may be deployed to perform the thermal modeling of the additive manufacturing session and/or to control the thermal properties of the resin or window to avoid unwanted effects in the final manufactured object. Nevertheless, it will be appreciated that any of a variety of different architectures can be employed. Controllers can be a general purpose computer dedicated to, or on board, a particular apparatus; a local general purpose computer operatively associated with a group of machines via a local area network (or metropolitan area network); a remote general purpose computer operatively associated with machines via a wide area network or internet connection; and combinations thereof (for example, organized in a client-server architecture and/or distributed architecture).

Peripheral devices for data entry and display can be implemented in any of a variety of ways known in the art, including typical keypad entry, video display, and printing apparatus, as well as graphical user interfaces such as touch-pads, touch-screens and the like, including smart-phone touch screens.

The modeling computing devices and/or controlling computing devices may use hardware, software implemented with hardware, firmware, tangible computer-readable storage media having instructions stored thereon, and/or a combination thereof, and may be implemented in one or more computer systems or other processing systems. The modeling computing devices and/or controlling computing devices may also utilize a virtual instance of a computer. As such, the devices and methods described herein may be embodied in any combination of hardware and software that may all generally be referred to herein as a “circuit,” “module,” “component,” and/or “system.” Furthermore, aspects of the present invention may take the form of a computer program product embodied in one or more computer readable media having computer readable program code embodied thereon.

In some embodiments, the modeling computing devices and/or controlling computing devices may include at least one processor. The at least one processor of the modeling computing devices and/or controlling computing devices may be configured to execute computer program code for carrying out operations for aspects of the present invention, which computer program code may be written in any combination of one or more programming languages, including an object oriented programming language such as Java, Scala, Smalltalk, Eiffel, JADE, Emerald, C++, C#, VB.NET, or the like, conventional procedural programming languages, such as the “C” programming language, Visual Basic, Fortran 2003, COBOL 2002, PHP, ABAP, dynamic programming languages such as Python, PERL, Ruby, and Groovy, or other programming languages.

The at least one processor may be, or may include, one or more programmable general purpose or special-purpose microprocessors, digital signal processors (DSPs), programmable controllers, application specific integrated circuits (ASICs), programmable logic devices (PLDs), field-programmable gate arrays (FPGAs), trusted platform modules (TPMs), or a combination of such or similar devices, which may be collocated or distributed across one or more data networks.

Data storage or memory of the modeling computing devices and/or controlling computing devices can be on separate (volatile and/or non-volatile) memory devices located locally or remotely, partitioned sections of a single memory device, etc., including combinations thereof (e.g., a remote back-up memory in addition to a local memory). For example, the database referred to herein may be one or more databases stored locally to the modeling computing devices and/or controlling computing devices or remote. In some embodiments, the database may be remotely accessible by the modeling computing devices and/or controlling computing devices.