Patent Description:
The present invention relates to additive manufacturing processes and, in particular, to a multi-material membrane used in vat polymerization printers.

Additive manufacturing, or 3D printing as it is known, is a collection of different technologies that provide different means of direct production of various articles. One such technology is vat polymerization, which involves the selective curing of viscous resins contained in a vat using (typically) ultraviolet (UV) light sources. The resin is cured layer by layer so that the article under manufacture is created through a successive series of cross-sections that adhere to one another.

One issue of importance in vat polymerization printing is the makeup of the vat (or tank) in which the liquid polymer from which a printed three-dimensional object is obtained by photo-curing is collected. In order to avoid tearing newly-formed layers of polymer from other portions of the three-dimensional object under construction when an extraction plate is raised, the vat must permit detachment of that just-formed layer from its surface (typically, a transparent base that allows the passage of ultra-violet (UV) light for triggering the photo-curing process, e.g., quartz or borosilicate glass). Often, a non-stick coating is applied to the inside surface of the vat to allow the first layer of cured polymer to adhere to the extraction plate and successive layers to join together in sequence.

However, in conventional vat polymerization printers there exists a suction effect, which occurs between the surface of the object under construction and the non-stick material which covers the transparent base of the vat, and which imposes limiting effects on the speed with which the object can be printed. In effect, a newly-formed polymer layer remains immersed in the resin at a distance "s" (equal to the thickness of the next layer of the object being formed) from the non-stick surface of the vat (both surfaces being coplanar and flat to give precision to the layer which will be formed); and a new layer of the object is generated by photo-curing the resin within that space. The absence of air creates a vacuum between the two surfaces, which are surrounded by a highly viscous liquid, and when the newly formed layer is displaced away from the vat surface (to make room for yet a further layer of the object to be formed), mechanical stresses suffered by that new layer (which may be only a few tenths of a millimetre thick) may be significant. Thus, there is an attendant risk of tearing the newly formed layer if the previous layer to which it is adhered is displaced vertically away from the bottom surface of the vat in a rapid fashion.

In order to reduce this risk of tearing, conventional printing processes were performed in such a way that the extraction plate (and the objects adhered thereto) were raised slowly. This limited the speed of production of three-dimensional objects by vat polymerization to be on the order of hours per centimetre. Accordingly, techniques were developed to alleviate the mechanical stresses on newly formed polymer layers produced by such processes. One such technique was the introduction of flexible membranes between the bottom surface of the vat and the article undergoing fabrication. <CIT>, and assigned to the assignee of the present invention describes one such flexible membrane made of a clear, self-lubricating polymer. Other membrane-based approaches have also been employed. For example, <CIT> describes a membrane with an anti-stick surface made from an FEP fluoropolymer film. While flexible, such a film is not particularly elastic. Other materials contemplated by Elsey include nylon and mylar, or a laminated membrane having a layer of silicone bonded to a polyester film, with the silicone being the resin-facing side of the membrane and the polyester backing providing some elasticity.

While FEP fluoropolymer membranes do offer good anti-stick properties, they are relatively rigid and, therefore, do not afford much improvement of printing speeds over anti-stick coatings applied directly to vat surfaces. Furthermore, their rigidity can lead to the membrane being damaged during its installation in a vat polymerization printer. Silicone rubber membranes can provide improved flexibility over FEP fluoropolymer membranes, and thereby permit faster overall printing speeds, however, they suffer from susceptibility to wear and tear as they tend to degrade when exposed to high temperatures such as those produced due to the exothermic nature of the polymerization reaction within a printer's vat. They are also porous mediums and may offer little or no resistance to constituent components of some 3D printing resins.

<CIT>, corresponding to the preamble of the independent claims, describes a resin tank for a liquid-based 3D printer. The disclosed resin tank for a liquid-based 3D printer includes a bottom plate having light transmittance, and the bottom plate includes a photocurable resin layer.

<CIT> describes a stereolithography device having a trough for accommodating free-flowing, photopolymerizable material, a construction platform suspended above the trough bottom on a lifting unit, and having a heating unit for heating the photopolymerizable material in the trough. The heating unit has a transparent, electrically conductive layer, which covers the entire area of at least the exposure region above the trough bottom, and which is provided outside the exposure region on opposing sides of the layer with electrical contacts extended over the opposing sides, which are connected to a controlled electrical supplier to enable heating of the entire area of photopolymerizable material above the trough bottom in the exposure region by current flow through the layer.

<CIT> describes methods and systems for forming objects through photo-curing of a liquid resin in a tank by selective exposure (through a mask) to radiation, in which during printing operations the liquid resin in the tank is displaced relative to the build area along an axis orthogonal to that along which the object is extracted from the liquid resin in the tank. A volume of the photo-curing liquid resin may be cycled through a cooling arrangement by being extracted from the tank, cooled, and then reintroduced into the tank as printing of the object is taking place. The mask is preferably one in which charged colorant particles are dispersed in an optically transparent fluid within a plurality of bi-state cells.

The present invention is illustrated by way of example, and not limitation, in the figures of the accompanying drawings, in which:.

Disclosed herein are examples of multi-material membranes for use in vat polymerization printers.

<FIG> depicts a cross-section of 3D printing system <NUM> configured with a multi-material membrane in accordance with an embodiment of the present invention, in which electromagnetic radiation (e.g., UV light) is used to cure a photo-curing liquid resin (typically a liquid polymer) <NUM> in order to fabricate an object (e.g., a 3D object) <NUM>. Object <NUM> is fabricated layer by layer (i.e., a new layer of object <NUM> is be formed by photo-curing a layer of liquid polymer <NUM> adjacent to the bottom surface of object <NUM>), and as each new layer is formed the object may be raised by build plate <NUM>, allowing a next layer of photo-curing liquid resin <NUM> to be drawn under the newly formed layer. This process may be repeated multiple times to form additional layers until fabrication of the object is complete.

The 3D printing system <NUM> includes tank (or vat) <NUM> for containing the photo-curing liquid resin <NUM>. The bottom of tank <NUM> (or at least a portion thereof) is sealed (i.e., to prevent the photo-curing liquid polymer <NUM> from leaking out of tank <NUM>) by a flexible, multi-material membrane <NUM>, which is transparent (or nearly so) at wavelengths of interest for curing of the resin to allow electromagnetic radiation from a light source <NUM> to enter into tank <NUM>. A mask <NUM> (e.g., a liquid crystal layer) is disposed between light source <NUM> and the photo-curing liquid resin <NUM> to allow the selective curing of the liquid resin (which allows the formation of 3D objects into desired shapes/patterns). In various embodiments, collimation and diffusion elements such as lenses, reflectors, filters, and/or films may be positioned between mask <NUM> and light source <NUM>. These elements are not shown in the illustrations so as not to unnecessarily obscure the drawing.

A platen or backing member <NUM> formed of borosilicate glass or other material is disposed between the mask <NUM> and the flexible, multi-material membrane <NUM> and provides structural support. The platen is also transparent (or nearly so) at the one or more wavelengths of interest for curing the resin. In other instances, platen <NUM> may be metal or plastic and include a transparent window to allow electromagnetic radiation from light source <NUM> to enter into tank <NUM>. In other embodiments, the mask <NUM> itself may be used in place of a separate window and its perimeter sealed with a gasket. Note that although the mask <NUM>, platen <NUM>, and membrane <NUM> are shown as being displaced from one another by some distance, in practice these components may be positioned so as to touch one another, so as to prevent refraction at any air interfaces. Flexible, multi-material membrane <NUM> is secured to the edges of tank <NUM> or to a replaceable cartridge assembly (not shown) so as to maintain a liquid-tight perimeter at the edges of the tank or other opening ("liquid-tight" meaning that the tank does not leak during normal use).

When fabricating a layer of object <NUM> using 3D printing system <NUM>, electromagnetic radiation is emitted from radiation source <NUM> through mask <NUM>, platen <NUM>, and membrane <NUM> into tank <NUM>. The electromagnetic radiation forms an image on an image plane adjacent the bottom of object <NUM>. Areas of high (or moderate) intensity within the image cause curing of localized regions of the photo-curing liquid resin <NUM>. The newly cured layer adheres to the former bottom surface of object <NUM> and substantially does not adhere to the bottom surface of tank <NUM> due to the presence of flexible, multi-material membrane <NUM>. After the newly cured layer has been formed, the emission of electromagnetic radiation may temporarily be suspended (or not, in the case of "continuous printing") while the build plate <NUM> is raised away from the bottom of the tank so that another new layer of object <NUM> may be printed.

The build plate <NUM> may be raised and lowered by the action of a motor (M) <NUM>, which drives a lead screw <NUM> or other arrangement. Rotation of the lead screw <NUM> due to rotation of the motor shaft causes the build plate <NUM> to be raised or lowered with respect to the bottom of the tank <NUM>. In other embodiments, a linear actuator or other arrangement may be used to raise and lower the build plate <NUM>.

Aspects of the printing process are directed by a controller <NUM>, which may be implemented as a processor-based system with a processor-readable storage medium having processor-executable instructions stored thereon so that when the processor executes those instructions it performs operations to cause the actions described above. For example, among other things controller <NUM> may instruct raising/lowering of the build plate <NUM> via motor <NUM>, activation and deactivation of the light source <NUM>, and the projection of cross-sectional images of the object under fabrication via mask <NUM>. <FIG> provides an example of such a controller <NUM>, but not all such controllers need have all of the features of controller <NUM>. For example, certain controllers may not include a display inasmuch as the display function may be provided by a client computer communicatively coupled to the controller or a display function may be unnecessary. Such details are not critical to the present invention.

Controller <NUM> includes a bus <NUM>-<NUM> or other communication mechanism for communicating information, and a processor <NUM>-<NUM> (e.g., a microprocessor) coupled with the bus <NUM>-<NUM> for processing information. Controller <NUM> also includes a main memory <NUM>-<NUM>, such as a random access memory (RAM) or other dynamic storage device, coupled to the bus <NUM>-<NUM> for storing information and instructions (e.g., g-code) to be executed by processor <NUM>-<NUM>. Main memory <NUM>-<NUM> also may be used for storing temporary variables or other intermediate information during execution of instructions to be executed by processor <NUM>-<NUM>. Controller <NUM> further includes a read only memory (ROM) <NUM>-<NUM> or other static storage device coupled to the bus <NUM>-<NUM> for storing static information and instructions for the processor <NUM>-<NUM>. A storage device <NUM>-<NUM>, for example a hard disk, flash memory-based storage medium, or other storage medium from which processor <NUM>-<NUM> can read, is provided and coupled to the bus <NUM>-<NUM> for storing information and instructions (e.g., operating systems, applications programs such as a slicer application, and the like).

Controller <NUM> may be coupled via the bus <NUM>-<NUM> to a display <NUM>-<NUM>, such as a flat panel display, for displaying information to a computer user. An input device <NUM>-<NUM>, such as a keyboard including alphanumeric and other keys, may be coupled to the bus <NUM>-<NUM> for communicating information and command selections to the processor <NUM>-<NUM>. Another type of user input device is cursor control device <NUM>-<NUM>, such as a mouse, a trackpad, or similar input device for communicating direction information and command selections to processor <NUM>-<NUM> and for controlling cursor movement on the display <NUM>-<NUM>. Other user interface devices, such as microphones, speakers, etc. are not shown in detail but may be involved with the receipt of user input and/or presentation of output.

Controller <NUM> also includes a communication interface <NUM>-<NUM> coupled to the bus <NUM>-<NUM>. Communication interface <NUM>-<NUM> may provide a two-way data communication channel with a computer network, which provides connectivity to and among the various computer systems discussed above. For example, communication interface <NUM>-<NUM> may be a local area network (LAN) card to provide a data communication connection to a compatible LAN, which itself is communicatively coupled to the Internet through one or more Internet service provider networks. The precise details of such communication paths are not critical to the present invention. What is important is that controller <NUM> can send and receive messages and data, e.g., a digital file representing 3D articles to be produced using printer <NUM> through the communication interface <NUM>-<NUM> and in that way communicate with hosts accessible via the Internet. It is noted that the components of controller <NUM> may be located in a single device or located in a plurality of physically and/or geographically distributed devices.

<FIG> is a cross-sectional view of multi-material membrane <NUM>. The multi-material membrane <NUM> is made up of a fluorinated ethylene propylene (FEP) or polyolefin polymer film <NUM> bonded to a layer of silicone rubber <NUM>, with the FEP film <NUM> on the resin-facing side of the membrane and the layer of silicone rubber <NUM> on the vat-facing or light source facing side of the membrane. The layer of silicone rubber <NUM> is coated with coating <NUM> to reduce its surface energy and coefficient of friction. In various embodiments, each of the FEP film <NUM> and layer of silicone rubber <NUM> may have a respective thickness of approximately <NUM> to <NUM>. The multi-material makeup of membrane <NUM> provides both anti-stick properties (i.e., meaning that the membrane will allow for rapid printing by allowing newly formed polymer layers to separate from the FEP film with minimal tearing) as well as high heat resistance, chemical resistance, strength and flexibility.

The coating <NUM> applied to the silicone rubber layer <NUM> provides increased durability over untreated silicone rubber membranes used for 3D printing applications. Various coatings <NUM> may be used, for example chemical coatings such as silicone elastomers (e.g., silane acetates, silane ethyl acetates, silane triacetates, silane ethyl triacetates, silane methyl triacetates, octamethyltrisiloxane, methylhydosiloxane, siloxanes, and mixtures of two or more the foregoing, etc., with or without catalysts such as dibutyltindilaurate) dispersed in media such as xylene, tert-Butyl acetate, or similar solvents. These coatings are applied uniformly over the silicone rubber layer <NUM> and are allowed to cure, either at elevated temperature, e.g., <NUM>-<NUM>, or at room temperatures, for approximately <NUM> minutes to <NUM> hours (depending on the relative humidity of the curing environment) to form a thin silicone film and may be applied to the silicone rubber layer <NUM> of membrane <NUM> either by brushing, dipping, or, preferably, spraying on of the coating. Prior to coating, the silicone rubber layer <NUM> may be cleaned using an appropriate solvent (e.g., one which will not be absorbed by the silicone rubber layer), which should be allowed to completely evaporate before application of the coating. The coating is applied so as to completely (or nearly so) cover the silicone rubber layer <NUM> and is then allowed to cure, either at room temperature or by heating, so that the solvent in which the elastomer is dispersed is completely evaporated.

Alternatively, the coating <NUM> may be a physical coating such as a polytetrafluoroethylene (PTFE) -based dry lubricant, with particle sizes of a few microns, e.g., an emulsion of PTFE in a fluid propellant. Such lubricants are preferably sprayed on, although brushing or dipping applications may be used, to provide a uniform application to the silicone rubber layer <NUM>. These lubricants are sprayed on and typically dry as a thin layer adhering to the surface of the silicone rubber layer (by Van der Waals forces) at room temperatures. Prior to application, the silicone rubber layer <NUM> is cleaned with an appropriate solvent to remove any dirt or other surface coating. Other coatings that reduce the surface energy of the silicone rubber layer <NUM> may also be used.

Prior to the application of coating <NUM>, the silicone rubber layer <NUM> is bonded to the FEP film <NUM>. Any appropriate bonding technique may be used, for example using a plasma etching treatment as described in <CIT> or using a chemical etching treatment. After etching, the liquid silicone rubber is applied to the surface of the FEP film <NUM> and allowed to cure. During its application, the thickness of the liquid silicone rubber is controlled, e.g., using a roller arrangement with a well-defined gap between the rollers, or using a blade maintained at a well-defined distance from the surface of the FEP film to remove excess liquid. Once the liquid silicone rubber is cured, coating <NUM> is applied to it. The service life of the coated multi-material membrane <NUM> has been found to be very long as compared to other membranes, even where the other membranes are similarly coated (e.g., on the order of <NUM> times longer than a coated silicone rubber membrane) but it is possible that the multi-material membrane will need to be reconditioned at some point in its service life. To do so, the multi-material membrane <NUM> is removed from the tank <NUM>, cleaned, and a fresh coating <NUM> is applied (e.g., by spraying, dipping, or brushing). Depending on the area of the membrane being coated, a coating layer of between <NUM> grams - <NUM> grams, and preferably <NUM> grams - <NUM> grams, may be applied.

While the refurbishment may be offered as a service by vendors of the multi-material membrane <NUM> and/or 3D printing system <NUM>, it may also be performed by users of the 3D printing system with the aid of a refurbishment kit. Such a kit <NUM>, as illustrated in <FIG>, may include a supply of coating material <NUM>, an applicator (e.g., a spray bottle, brush or roller, or vat for dipping) <NUM>, safety apparatus (such as gloves, goggles, and a mask) <NUM>, and, optionally, a drying rack <NUM> for the membrane for use after the fresh coating is applied. Cleaning solvent <NUM> may also be included.

As mentioned above, the multi-material membrane may be part of a replaceable cartridge assembly. <FIG> depicts a perspective view of a membrane assembly <NUM> for a 3D printing system in accordance with an embodiment of the present invention. Membrane assembly <NUM> may include radiation-transparent, flexible, multi-material membrane <NUM>, the perimeter of which is secured to a frame <NUM>. Frame <NUM> may be configured to stretch membrane <NUM> along a first plane. Frame <NUM> may comprise lip <NUM> that extends in a direction perpendicular to the first plane. Lip <NUM> may be secured to a bottom rim of a tank sidewall (as discussed below). Membrane assembly <NUM>, when secured to the bottom rim of the tank sidewall, forms a bottom of a tank configured to contain a photo-curing liquid resin. In <FIG>, frame <NUM> is depicted to have a rectangular shape, however, other shapes for frame <NUM> are possible, including square, oval, circular, etc..

<FIG> depicts a perspective view of tank sidewall <NUM> for a 3D printing system. The tank sidewall <NUM> includes bottom rim <NUM> with groove <NUM>. Lip <NUM> of frame <NUM> may be inserted within groove <NUM> so as to secure membrane assembly <NUM> onto the base of tank sidewall <NUM>. The shape and dimensions of tank sidewall <NUM> must match the shape and dimensions of frame <NUM>. For instance, if frame <NUM> were rectangular, a tank sidewall <NUM> must also be rectangular (i.e., when viewed from above).

<FIG> depict cross-sectional views of membrane assembly <NUM> (with frame <NUM> and membrane <NUM>) and tank sidewall <NUM> and show how membrane assembly <NUM> is secured to bottom rim <NUM> of tank sidewall <NUM>. <FIG> depicts lip <NUM> of frame <NUM> aligned under groove <NUM> of tank sidewall <NUM>. <FIG> depicts lip <NUM> of frame <NUM> inserted within groove <NUM> of tank sidewall <NUM>. Lip <NUM> and groove <NUM> may interlock with one another (e.g., in a snap-fit attachment), may snugly fit so that surfaces of lip <NUM> and groove <NUM> contact one another (e.g., in a friction-fit attachment), etc. In one embodiment, membrane assembly <NUM> may be a "consumable" product, in that it is disposed of or refurbished at the end of its useful lifetime. As such, membrane assembly <NUM> may play a similar role as printer cartridges in a printer; razor blades in a razor; etc..

<FIG> depict perspective views of a frame assembly <NUM> and LCD assembly <NUM>, showing how frame assembly <NUM> may be secured to LCD assembly <NUM>. Frame assembly <NUM> may include frame <NUM> and radiation-transparent, flexible, multi-material membrane <NUM>, with frame <NUM> configured to hold membrane <NUM> at its perimeter. In other embodiments, the frame assembly <NUM> may support both membrane <NUM> and a transparent glass plate. Frame <NUM> may comprise through holes 510a and magnetized portions 512a distributed about a bottom surface of frame <NUM>. LCD assembly <NUM> may include frame <NUM> and LCD <NUM>, in which frame <NUM> is configured to hold LCD <NUM>. Frame <NUM> may comprise through holes 510b and magnetized portions 512b distributed about a top surface of frame <NUM>.

As depicted in <FIG>, a pattern in which through holes 510a are distributed about the bottom surface of frame <NUM> may be a mirror image of a pattern in which through holes 510b are distributed about the top surface of frame <NUM>. As further depicted in Figure 5A, a pattern in which magnetized portions 512a are distributed about the bottom surface of the frame <NUM> may be a mirror image of a pattern in which magnetized portions 512b are distributed about the top surface of frame <NUM>. Each one of magnetized portions 512a may be attracted to a corresponding one of magnetized portions 512b such that when frame <NUM> is disposed in proximity to frame <NUM>, the bottom surface of the frame <NUM> automatically contacts the top surface of frame <NUM>, and each one of the through holes 510a automatically aligns with a corresponding one of through holes 510b. Gasket <NUM> may be disposed at or near a perimeter of LCD <NUM>. The purpose of gasket <NUM> will be explained below with respect to <FIG>.

<FIG> depicts a perspective view of frame <NUM> affixed to LCD frame <NUM>. Frame <NUM> surrounds radiation-transparent, flexible, multi-material membrane <NUM> and (optionally) a glass plate. LCD <NUM> is not visible in <FIG> but is located directly beneath membrane <NUM>. Small screws or pins may be inserted through aligned pairs of through holes 510a and 510b to secure this arrangement. Openings for such screws or pins may be located in a bottom surface of frame <NUM> (not depicted).

<FIG> depicts a cross-sectional view along line I-I of <FIG>. As shown in <FIG>, frame assembly <NUM> is affixed to the LCD assembly <NUM>. More particularly, a bottom surface of frame <NUM> contacts a top surface of frame <NUM>, and membrane <NUM> is disposed above LCD <NUM>. Gasket <NUM> is disposed within or near a boundary region between the bottom surface of frame <NUM> and the top surface of frame <NUM>. In the event that resin (or another fluid) is able to penetrate the boundary region between the bottom surface of frame <NUM> and the top surface of frame <NUM>, gasket <NUM> may prevent the resin from flowing between LCD <NUM> and membrane <NUM> (which may lead to undesirable distortion in images projected from LCD <NUM>).

As described above, magnets (or magnetized portions of the frames) were used to automatically align through holes 510a with through holes 510b. In addition or alternatively, grooves (e.g., saw tooth grooves) disposed on both the bottom surface of frame <NUM> and the top surface of frame <NUM> (and particularly grooves in the bottom surface that are complementary to grooves in the top surface,) may also be used as a self-alignment mechanism.

Claim 1:
A multi-material membrane (<NUM>, <NUM>, <NUM>) for a three-dimensional printing system (<NUM>), the membrane (<NUM>, <NUM>, <NUM>) comprising a fluorinated ethylene propylene (FEP) or polyolefin polymer film (<NUM>) bonded to a layer of silicone rubber (<NUM>) at a first side of the layer of silicone rubber (<NUM>), characterized in that the layer of silicone rubber (<NUM>) has a coating (<NUM>) on a second side thereof, the coating (<NUM>) configured to reduce surface energy of the layer of silicone rubber (<NUM>).