Sliding window screen for reducing resin refilling time in stereolithography

A sliding window is used in a projection-based stereolithographic process to more quickly deliver uncured resin after each curing pass. The sliding window may be configured in different patterns, and includes features for delivering resin and exposing resin to curing radiation. The window screen divides the stereolithographic building area divided into two portions, a light exposure portion for resin curing and a liquid resin refilling portion. The light exposure portion is used to selectively solidify liquid resin, while the liquid resin refilling portion is used to quickly refill liquid resin in order to build additional layers. During the layer fabrication process, a mask image is projected to the tank; however, the projection light only passes through the light exposure portion of the window screen.

FIELD

The present disclosure relates to methods and apparatus for additive manufacturing, and in particular, a three dimensional (3D) printing approach using projection-based stereolithography.

BACKGROUND

Material refilling is a critical issue in stereolithography (SL), including both laser-based SL and projection based SL. Fabrication speed is slowed by the extra time required for refilling liquid resin. Although several methods have been developed to reduce this extra refilling time, they are designed to be effective for shapes that have small building areas. The building speed will be significantly reduced when the shape under construction has large cross-section areas, because liquid refilling for large building areas is much more complex than for small building areas.

In one stereolithography process (Hull, 1986) a liquid photo sensitive polymer is solidified by absorbing a certain amount of photons. In each build layer, the photo sensitive polymer is made to cover previously solidified layers, sometimes referred to as “refilling.” By successively solidifying the liquid resin and refilling, a three dimensional object is fabricated layer-by-layer. The SL process can be divided into three main steps: 1) resin refilling, 2) light exposure, and 3) separating newly cured layer that is attached to the previous layers from the resin interlace. Due to inefficient material refilling and separation processes, most conventional SL processes have slow fabrication speed, for example, less than about 5 μm/s in the building direction.

Several prior approaches have been taken with refilling. These methods can be classed as “sequential” or “simultaneous.” In the sequential methods, the material is refilled by a relative motion between a tank containing the resin, or a recoating blade, and a platform, sometimes called a “carrier.” The carrier carries the newly cured part after one layer is selectively solidified. Due to the viscosity of uncured resin, the time required for the uncured resin to flow from an outer area into the center of the building area slows down the build process.

Conventional projection-based, sequential SL also adds an extra move: an up-and-down motion of the carrier to refill liquid resin between each cure cycle. The time required for the up-and-down motion may cause additional delay. To speed up the material refilling process, Pan et al. (Pan, Zhou, and Chen, 2012) disclose refilling liquid resin by sliding the tank horizontally over the carrier, allowing the platform to reach the adjacent fresh material more quickly. However, all these sequential methods require a long resin refilling time, which slows down the fabrication speed of the projection-based SL process.

Recently, a simultaneous method to refill the resin together with the light exposure has been proposed (John, et al., 2015). In John's method, a liquid dead zone is formed where the resin curing is inhibited by oxygen and this dead zone acts as conduits to refill the resin from the outer to the center of building area. However, the dead zone has limited height (˜50 micron) due to process stability. Such a height is still too shallow for liquid resin to quickly refill into a large building area, such as for example over about 50×50 mm square (250 mm2) or larger. A related simultaneous refilling method also proposed by John opens an array of conduits in the carrier and internal tubes in the targeted part. However, use of this second method may require redesign of the parts' geometry, which is not desirable for users in most cases.

The inventors hereof disclosed a simultaneous refilling method in U.S. Pat. No. 9,120,270 issued on Sep. 1, 2015, in which a moving zone is formed to refill liquid resin by continuously rotating the tank. This “moving zone” method can successfully improve the refilling speed while separating newly cured layers from the resin interface. However, for a very large building area, the method will still take a relatively long time due to the long moving distance and the limitation on the moving speed.

It would be desirable, therefore, to provide a method and apparatus for refilling in SL, that overcomes these and other limitations of the prior art.

SUMMARY

This summary and the following detailed description should be interpreted as complementary parts of an integrated disclosure, which parts may include redundant subject matter and/or supplemental subject matter.

The present disclosure introduces a novel SL process using a sliding window screen that significantly reduces the required moving distance for refilling liquid resin regardless of the shape and size of a given 3D model, relative to the prior moving zone method referred to above. Simply put, a window screen is an apparatus that can simultaneously deliver light and refill liquid resin into a very large building area. Thus the window screen enables an SL process to fabricate a large 3D object with faster building speed than prior approaches.

In an aspect of the disclosure, a system for additive manufacturing of an object in three dimensions (consisting of an X dimension, a Y dimension, and a Z dimension) includes various elements. The elements include a tank configured to contain a liquid resin, a window screen configured to simultaneously deliver light and refill liquid resin into a build area in the tank, and a build platform configured to be located within the tank for at least an initial portion of building the object. One or more translation/rotation stages are operatively coupled to the build platform, the window screen and/or the tank. The one or more translation/rotation stages move the tank and/or the build platform in the Z dimension, and the build platform and/or the window screen in the X dimension, the Y dimension, by translation, rotation, or a combination of translation and rotation.

As used herein, the ‘Z’ direction or dimension refers to the build direction, and is usually perpendicular to the plane of the platform on which the part is built the “build plane.” The build plane is defined by the ‘X’ and ‘Y’ dimensions. As used herein in connection with the accompanying figures, the ‘X’ and ‘Z’ directions are coplanar with the plane of the figure on the page, while the ‘Y’ direction is perpendicular to the page.

The system may further include a light projection device configured to emit light through the window and into the tank to cure the liquid resin. Transparent areas of the window screen in conjunction with the projection mask determine which portions of the liquid resin are cured. The window may be moved in conjunction with varying the projection mask to deliver uncured resin and cure resin only in desired areas. The window screen includes a pattern of slots for delivering resin and windows for light transmission. The pattern may be designed to optimize refilling and curing of liquid resin for the article being manufactured.

The system may further include a computer control system including at least one hardware processor and a storage device coupled with the hardware processor. The computer control system may be coupled with the one or more translation stages to control movement of the tank, the build platform and/or the window screen. The computer control system may be coupled to the light projection device to control emission of the light, and to the storage device. The storage device or other computer memory may hold an encoded program, including instructions that when executed by the at least one processor, cause the computer control system to control the light projection device to emit the light through the window screen into the tank to cure the liquid resin to form a first portion of the object in a current layer. The instructions may further includes instructions for controlling the one or more translation stages to move the build platform and/or the window screen in the X dimension, the Y dimension, or both, including by rotation. The instructions may further includes instructions for controlling the light projection device to emit the light through the window screen into the tank to cure the liquid resin to form a second portion of the object in the current layer. The instructions may further includes instructions for controlling the one or more translation stages to move the tank and/or the build platform in the Z dimension for a curing a next layer of the object.

In an aspect of the system, the window screen is integral with the tank, and the one or more translation stages move the window screen by moving the tank in the X dimension, the Y dimension, or both, including by rotation. The window screen may be, or may include, a mesh window. The window screen may be, or may include, a parallel pattern of opaque slots for refilling liquid resin and transparent slots for exposing projection light to selectively solidify liquid resin. In an alternative, the window screen may be, or may include, a grid pattern of opaque areas for refilling liquid resin and transparent areas for exposing projection light to selectively solidify liquid resin. In another alternative, the window screen may be, or may include, a radial pattern of opaque areas for refilling liquid resin and transparent areas for exposing projection light to selectively solidify liquid resin. In another alternative, the window screen may be, or may include, a branch pattern of opaque areas for refilling liquid resin and transparent areas for exposing projection light to selectively solidify liquid resin.

In another aspect, a method for stereolithography for additive manufacturing using a sliding window screen may include fabricating an article from liquid resin by layer-based stereolithography, including performing certain operations in forming each layer. The certain operations may include simultaneously photopolymerizing and resin refilling a portion of the layer by projecting a mask through a window screen having distinct resin curing and refilling regions, sliding the window screen to a different portion of the layer; and repeating the simultaneous photopolymerizing and resin refilling for distinct portions of the layer until fabrication of the layer is completed.

The method may further include determining one or more projection masks for each layer based in part on a pattern in which the distinct resin curing and refilling regions of the sliding window are arranged. In an aspect, for example, the distinct resin curing and refilling regions of the window screen are arranged in a pattern selected from: parallel, gridded, radial, or branched. In another aspect, the projection mask may be configured with black pixels correlated to all refilling regions of the window screen, and light emitting pixels correlated one or more resin curing regions of the window screen. In some embodiments, the window screen is opaque in the refilling regions. In others, the resin refilling regions are transparent. The method may include determining one or more projection masks for each layer based in part on whether the window screen is opaque in the refilling regions. In an aspect, the repeating step is performed only once for each layer.

The method may further include, for example, setting a layer thickness based on the relation h=ηCd, wherein h is the layer thickness, η is an overlap ratio between layers, and Cd is a light penetration depth. Setting other parameters may be performed as described in the detailed description.

The method may include separating the window screen from the portion of the layer at least in part by exerting a separation force in a direction perpendicular to a plane of sliding motion of the window screen. The method may include other operations and aspects, as described in the detailed description.

An apparatus for projection-based stereolithography may include least one processor coupled to a memory, a motor controller, and a light source capable of emitting light in a masked pattern. The memory holds program instructions that when executed by the processor causes the apparatus to perform the operations of the method described above, and in the detailed description.

A window screen for additive manufacturing by projection-based stereolithography may include one or more of resin curing regions comprising raised transparent windows aligned in a plane, and one or more resin refilling regions arranged around the resin curing regions. The resin refilling regions may be, or may include slots configured for enabling resin to flow to a layer under cur while light is projected through the resin curing regions. A working plane of the window screen may consist essentially of the resin curing regions and resin refilling regions. In some embodiments, the resin refilling regions are opaque to light from the projection mask, acting as light blockers to prevent curing. In other embodiments, the resin refilling regions are transparent or translucent, and the light in the refilling region is blocked by a projection mask. The window screen may include a wall around a periphery of the window screen for retaining liquid resin. In an aspect, the resin curing and refilling regions of the window screen may be equal in area. In another aspect, the resin curing and refilling regions of the window screen are arranged in a pattern selected from: parallel, gridded, radial, or branched. Further details of the window screen may be as described in the detailed description below.

To the accomplishment of the foregoing and related ends, one or more examples comprise the features hereinafter particularly pointed out in the claims and fully described in the detailed description following the brief description of the drawings.

DETAILED DESCRIPTION

Referring toFIG.1, an apparatus for stereolithography may include a platform110positioned relative to a sliding window screen100for use in a projection-based SL process. To solve the material refilling issue in SL, the incorporated window screen100is configured to minimize the sliding distance that is required for every position of a layer to access liquid resin during the fabrication process. Unlike the tank used in the conventional SL process, the tank in our process is defined by the wall108and incorporates the window screen100. The window screen100includes both opaque slots102(one of six indicated inFIG.1, as examples) for refilling liquid resin, and transparent slots106(one of six indicated) sealed by a transparent material and surrounded around a periphery thereof by a blocking wall104(one of six indicated). The transparent slots (e.g., slot106) are provided for exposing projected light to selectively solidify liquid resin.

Referring toFIGS.2A and2B, operation of the sliding window screen in an apparatus or system200for projection-based SL process is illustrated at different times of the process. In bothFIGS.2A and2B, a tank wall202encloses uncured resin214and includes in its lower surface an array or other arrangement of transparent windows204(one of three indicated for illustrative purposes) surrounded by opaque areas making up a remainder of the tank wall202. In other words, a working surface of the apparatus200is divided into transparent and opaque areas divided by boundary walls according to some pattern. The pattern may be selected to minimize refilling time and/or total fabrication time. At any given moment, due to the use of the window screen, the building area on the platform208is divided into two portions, a light exposure portion (also called a resin curing region) made up of the windows204, and a liquid resin refilling portion (also called a resin refilling region) recessed in the Z direction relative to the resin curing portion, thereby allowing resin to flow to the surface of the part under fabrication. The light exposure portion is used to selectively solidify liquid resin, while the liquid resin refilling portion is used to quickly refill liquid resin in order to build additional layers, e.g., second layer212. In the illustrated example200, the platform208moves up in the direction of the vertical arrow230, while the sliding window screen moves back-and-forth as indicated by the horizontal double-headed arrow220.

During the layer fabrication process, a mask image is projected to the tank; however, the projection light (indicated by the vertical rising arrows) can only pass through the light exposure portion204of the window screen.FIG.2Ashows an SL process building a second layer212on top of a first layer210at a first (earlier) point in time. Uncured resin214flows to portions of the layer212that are not being cured while the exposure is made. After liquid resin in the portion receiving light is selectively cured, the window screen including the opaque wall202and transparent windows204is moved in an XY plane for a small distance that is determined by the pattern of the exposure and refilling portions in the window screen, as shown inFIG.2Bat a second (later) time. Consequently, the portion of the second layer212that has refilled liquid resin is positioned over the windows204, while the previously cured portion of the layer212that was cured at the time shown inFIG.2Ais positioned over the liquid resin refilling portion to expose to a sufficient supply of liquid resin. Although not shown inFIGS.2A and2B, a small overlapping region exists between the second and first exposures.

After projecting two mask images related to a given cross section of a 3D model, the whole layer212is formed. The platform that carries the previously cured layers moves up (in the Z axis) the distance of a layer thickness. At the same time, the window screen is moved (in the X axis) back to its original position. Thus, the building process can be repeated until all the layers of the 3D object are constructed.

FIGS.3A-Billustrate bottom up and top down embodiments, respectively, of systems for a projection-based SL process using a sliding window screen.FIG.3Aillustrates a bottom up embodiment, in which the projection light302is shown upwards and the platform312also moves upwards during fabrication. In the bottom up SL embodiment, an SL apparatus300may include a computer304including a processor, memory, and other components for motion and projection control, a motion controller306, the platform312, a window screen308(also called a mesh window), liquid photocurable material310, and a light source302. The platform312may be, or may include, a plate or similar support component that is used to carry the built part314. The motion controller is coupled to the platform312via a drive train or the like, and is used to control the motion of the platform312in the Z axis and the window screen308in the X and/or Y axes. The motion controller306may be coupled to the computer304, which provides higher-level control signals to the motion controller coordinated with control of the light source302also coupled to the computer304. The light source302is controlled by the computer304to project mask images that selectively solidify the liquid resin310and build up the layers314forming the built part. The computer304is programmed via encoded instructions in a non-transitory computer-readable medium to synchronize the masks and operation of the light source302with movements of the platform312and window screen308controlled by the motion controller. It should be apparent that the operation of the light source and motion controller will depend on both the physical configuration of the window screen308and the geometry of the part to be manufactured.

FIG.3Bshows a related top down embodiment including an apparatus350, including a computer354for motion and projection control, a motion controller356, the platform362, a window screen358(also called a mesh window), liquid photocurable material360, and a light source352. Configuration of the top down apparatus350is analogous to the bottom up apparatus300, with adjustments for the top-mounted light source352and platform362that moves downwards in the tank366as the article under manufacture is built up from the layers364.

A window screen is used in the SL system to achieve both light exposure and liquid resin delivery simultaneously. Consequently, a window screen contains three key components: exposure sections, material delivery slots, and light blocking walls unless specially configured projection masks are used in the light source instead. Exposure sections are transparent openings in the window screen that allows light to pass in order to cure liquid resin in the section. Material delivery slots serve to refill liquid resin for the next projection image exposure. Blocking walls protect the material in the material delivery slots from being cured by the projection light, but may be omitted in some embodiments as explained below.FIG.4Ashows an embodiment400using opaque blocking walls at the bottom of the material delivery slots in the window screen404to block unmasked light406from reaching the uncured resin408in the material delivery slots. The light406passes only through the light exposure sections of the window screen404to cure resin between the platform402and light-transmissive windows of the light exposure sections,

FIG.4Bshows an embodiment450without opaque blocking walls at the bottom of the material delivery slots in the window screen454. Instead, the light456is masked in regions corresponding to the material delivery slots of the window screen454(e.g., by setting source pixels to black in those regions), thus preserving the liquid resin458in the material delivery slots uncured. The light406passes only through the light exposure sections of the window screen404to cure resin between the platform452and light-transmissive windows of the light exposure sections. The window screen454may be formed entirely of a light transmissive material without need for configuring the screen into opaque and transmissive areas, provided appropriate light masks are used at the light source or between the source and the window screen454. Notice that at any time, only the exposure sections of a window screen can solidify resin to form the part, in both embodiments400and450. Therefore, to fabricate a whole layer, a window screen should move repeatedly in the X axis, Y axis, or X-Y plane to cover a whole layer uniformly or wherever it is needed to cure resin.

Window screen pattern and sliding motion are critical elements of the present window-screen-based SL process. Window screen pattern means the shape and arrangement of the liquid resin refilling slots.FIGS.5A-Deach show a different type of pattern for configuring a sliding window screen.FIG.5Ashows a window screen500configured in a parallel pattern, wherein transmissive regions506are interspersed with material delivery regions (slots)502arranged in parallel bars separated by parallel sidewalls504and all surrounded by tank walls508.FIG.5Bshows a window screen520configured in a grid pattern, wherein transmissive regions522are interspersed with material delivery regions526arranged in a grid separated by sidewalls524and all surrounded by surrounding walls528.FIG.5Cshows a window screen540configured in a radial pattern, wherein transmissive regions546are interspersed with material delivery regions542arranged in a radial pattern separated by sidewalls544, with all surrounded by walls548.FIG.5Dshows a window screen560configured in a branching pattern, wherein transmissive regions566are interspersed with material delivery regions562arranged in a grid separated by sidewalls564, with all surrounded by surrounding walls568. These patterns are merely examples, and other patterns for window screens may also be useful.

Corresponding to the pattern design, the window screen needs to move in such a way that the remaining exposure portion of the window screen will cover the whole building area. There are at least three useful window screen movements as shown inFIGS.6A-C: a back and forth movement602,604shown inFIG.6A, with unequal increments in the forward direction602and reverse direction604: rotation606as shown inFIG.6B; and a rectangular or other polygonal cycle608as shown inFIG.6C. For example, the parallel pattern as shown inFIG.5Awill use the sliding motion of back and forth shown inFIG.6A. The radial and branch patterns as shown inFIGS.5C and5Dwill use the rotation motion shown inFIG.6B. The grid pattern as shown inFIG.5Bwill use the cycles pattern illustrated byFIG.6C. In the following parts of the detailed description, examples are given of the back and forth motion using a window screen based on the parallel pattern, and rotation motion using a window screen based on the radial pattern.

Additional window screen using a combination of the motions and patterns can also be used as long as the window screen satisfies the following requirements: 1) the liquid resin refilling portion is blocked from the light source, 2) the light exposure portion is transparent and non-stick, and 3) the refilling slot should be deep (>1 mm) such that the resin can be effectively delivered to any position of the building area without slowing the build process.

The apparatus and systems as described above may be used to perform a novel method of refilling liquid resin in an SL process. Referring toFIG.7, the window screen700divides the window into two mutually exclusive portions, (i) an exposure portion702for selectively solidifying liquid resin using projection light, and (ii) a liquid resin refilling portion704for recoating liquid resin to previously cured layers. With such an apparatus, an entire large build area (e.g., >250 mm2) may be refilled with liquid resin by moving the window screen for a much smaller distance. For example, when using a parallel configured window700, the smaller distance may be as small as the sum of the refilling and exposure portions (e.g., We+Wr=Wp). Hence the building process including both light exposure and resin recoating can be accomplished quickly.

Using the window screen with desired pattern and motion, the material refilling in the present system is a force refilling method rather than the slow self-refilling in the conventional SL process and CLIP process. The material refilling time in the current process may be represented by:

tforce-refill=Wp/2vm
where vmis the moving speed of the window screen, and Wpis the pitch of window screen, which is a fixed value (e.g. 5 mm to 20 mm in our experiment). Such a moving distance is fixed regardless the shape or size of given 3D models, and can be much smaller than the size of a building object (e.g. 200 mm). In comparison, the liquid resin refilling in the conventional SL and CLIP processes is driven by the pressure difference. The required refilling time is determined as (John, et al., 2015):

tself-refill∝ud2Phi2
where μ is the material viscosity, d is the feature's diameter, P is the pressure, and hiis the height of inhibition zone. This equation indicates that the fabrication speed is largely mitigated by the building feature's diameter d and material viscosity.

Compared to the pressure-based self-refilling, the force-refilling used in the present process is only governed by the window screen, and is not dependent on the liquid resin or the given 3D models. Hence, the present material refilling method has advantages over prior SL processes. That is, first, the refilling time is independent of the shapes and sizes of input 3D models, because the material is refilled by the motion of window screen. The refilling time is only limited by the sliding speed of the window screen. Second, the refilling time is not subject to the material's viscosity, which means the present process can fabricate photocurable materials that may have a much larger range of viscosity.

Once liquid resin is refilled, the window screen moves to a position where the controlled projection light can be exposed onto the areas that have recoated resin. The present technology is built on the conventional projection-based SL processes, in which input 3D models are sliced into many 2D layers, and the correspondingly prepared mask images are projected to selectively solidify liquid resin for building each layer. For the present process, a similar slicing procedure may be used. That is, given an input 3D model, it may be sliced into a sequence of mask images. And for each mask image, a pattern may be applied based on the window screen's position such that only the exposure portion of the window screen will have light exposure to solidify liquid resin. That is, all the pixels that fall into the material delivery slots should be replaced with black pixels (i.e. no light exposure for such portions).

Overlapping areas between two or more light exposures of a layer are designed to enhance the mechanical strength of the built layers. As shown inFIG.8, suppose the whole layer800is covered by two light exposures804,802using the parallel pattern (refer toFIG.5A). The solidified portion by light exposure802needs to have overlapping areas806with the neighboring solidified portions by light exposure804. To enhance the bonding between different light exposure area, reduced energy can be used for in each exposure. For example, only 50% energy is used in the overlapping areas while 100% energy is used in other areas. In some embodiment, the overlap areas could be covered by more than 2 exposures. In such cases the light exposure can be set as 100/r % energy, where r is the total number of light exposures. Generally speaking, light exposure should be configured to expose the whole area to be cured with uniform light energy,

Fabrication Speed Analysis: In the projection-based SL processes, the fabrication speed is bounded by two factors: 1) the liquid resin curing speed, and 2) the liquid resin refilling speed. In this disclosed process, if neglecting the resin refilling time, the resin curing speed is

vcure=htcure=ϕEc⁢h⁢⁢e-hη⁢⁢Dp≤e-1⁢η⁢⁢ϕ⁢⁢DpEc,
where Dpis the light penetration depth, η is overlapping ratio between two layers (e.g., 0.7 may be set in the present process), and the equality holds when the layer thickness h=ηDp. This equation indicates that the building speed is proportional to the input energy power ϕ, building resolution h and material critical dosage Ec. Similar results can be found in “continuous liquid interface production” method (John, et al., 2015).

However, another speed bound is the material refilling speed. In the conventional SL processes, the fabricated height is h and material refilling time tself-refill. Hence, the bound of the material self-refilling speed in the conventional SL processes is

vself-refill=htself-refill∝Pμ⁢h3d2.
This equation indicates that the refilling speed in conventional SL process is inversely proportional to the square of the fabrication features' diameter d, multiplied by the viscosity μ of liquid resin. And limited by this bound, any SL process relying on material self-refilling is subjected to the feature's diameter d, and material's viscosity μ.

In the present technology, however, the material refilling speed is

vforce-refill=htforce-refill=2⁢hvmwp,
which is usually larger than the curing speed. For each layer, the present process needs two exposures to cure the whole layer. Thus the total time for each layer is the summation of two exposure times and two window screen transition times. Hence, the building speed in the present process is

vbuild=h2⁢tcure+2⁢tforce-refill=12⁢11vcure+1vforce-refill.
In some embodiments, the speed of forced refill may be greater than the speed of cure. In such cases, part accumulation speed lies in the range of
0.25vcure≤vbuild≤0.5vcure
That is,

0.25e-1⁢ηϕ⁢DpEc≤vbuild≤0.5e-1⁢ηϕ⁢DpEc.
The part accumulation in the present process has better material refilling performance, which can achieve a fast speed for large area fabrication.

FIG.9illustrates aspects of layer separation in a projection-based SL process apparatus900using a sliding window screen904. After each light exposure, the newly cured material is accumulated between the previous cured layers906(shown attached to platform902) and the window screen904in liquid resin908. A separation of the layer906and window screen904is needed for the newly cured layers to be detached from the window screen and liquid resin908to recoat for building the next layer. Both the sliding motion in the X axis and the pulling up motion in the Z axis are incorporated such that the separation force and the fabrication time are largely reduced.

During the fabrication of a layer, the window screen moves from one side to another side along the X axis as diagramed inFIG.9. After one light exposure, an inhibition zone hi(˜2.5 microns for PDMS) exists between the window screen and the newly cured layers. Hence when the window screen is sliding in the X axis, the liquid resin in the inhibition zone will follow the movement. Thus, the sliding separation force Fx can be modeled as a friction between the material and the sliding window screen as:

Fx=μ⁢vmhi⁢We⁢AWe+Wr,
Where μ is the dynamic viscosity of material, vmis the window screen's speed, hiis the inhibition zone between the window screen and the newly cured part, A is the whole area, and a ratio of light exposure area to whole area is given by

WeWe+Wr,
wherein Weis the light exposure area and Wris the non-exposure refilling area. Comparing with the force in conventional SL process

Fx=μ⁢vmhi⁢A,
the sliding force in the present process is

When the platform moves in the Z axis for one layer thickness, it generates a pulling up separation force Fz. As shown inFIG.9, the force is a suction force that drives the resin to flow into the gap between the window screen and the cured layers, which is determined as

Four∝μ⁢vz⁢d3⁢Lhi3⁢N2,
wherein μ is the liquid resin viscosity, Vzis the moving speed, d is the width of the cured part, L is the length of the cured part, hiis the inhibition height, and N is the number of slots covered by the cured part. Compared with the separation force in the convention SL processes

Fconventional∝μ⁢vz⁢d3⁢Lhi3,
the pulling up force in the present process is much smaller, for example, about 50 times smaller when N=7 (about N2times smaller).

Separation Sequences: Layer separation in the present process has the following properties: 1) a sliding force is much smaller than a pulling up force, 2) the window screen largely reduces the pulling up force, which is in cubic relationship with part's diameter in the conventional SL processes. To make full use of these relationships, the separation sequence may be designed such that the separation method can combine the advantages of both sliding and pulling up separations.

Coupled with the planned projecting images, the separation method can have several alternatives illustrated byFIGS.10A-D. There are three basic controls in our process, i.e. projecting image, sliding window screen, and moving-up (or down) platform that carries cured layers. In our process, the sequence of these three steps may be organized in variously as shown inFIGS.10A-D.FIGS.10A-Cshow sequential configurations, whileFIG.10Dshows a simultaneous configuration. For the sequential configurations, extra refilling time is required to refill the material, while the simultaneous configuration refills liquid resin together with light exposure, reducing or eliminating dedicated refilling time.

The sequence1000shown inFIG.10Aincludes at1002moving the platform an increment dz in the Z direction, moving the mesh window1004, projecting the mask image for a time increment dt1006, testing for completion and repeating blocks1004,1006as necessary1008until finishing all layers1009. The sequence1020shown inFIG.10Bincludes moving the platform an increment dz in the Z direction1022, moving the mesh window1024, projecting the mask image for a time increment dt1026, testing for completion and repeating all prior blocks as necessary1028until finishing all layers1029. The sequence1030shown inFIG.10Cincludes moving the mesh window1032, moving the platform an increment dz in the Z direction1034, projecting the mask image for a time increment dt1036, testing for completion and repeating all prior blocks as necessary1038until finishing all layers1039. The simultaneous process1040shown inFIG.10Dincludes synchronizing all operations1042, performing the following operations contemporaneously (i.e., simultaneously) and continuously: moving the mesh window1044, moving the platform in the Z direction1046, and projecting the mask image1048, until all layers are finished1049.

System Control and Parameter Setting: Different from the conventional SL processes, the sliding of a window screen is incorporated in the fabrication of each layer in the present process. Given an input 3D model, the system follows the method1100as shown inFIG.11to fabricate a related 3D object. The input 3D model can be a standard STL file or in other formats. First, a computer program algorithmically slices1102the input 3D model into a sequence of mask images and applies the masks to each image1104. Then the system forms a whole layer1112, including refilling/sliding the window screen1114projecting the planned mask images1116, separating the window screen from the formed layer1118, and determining layer completion1120. Then, the system moves up (or down) the platform1106until all the mask images have been exposed1108. After the part has been fabricated, a post processing1110may be performed to remove supports and clean the fabricated part.

To improve the system performance, many parameters in the foregoing process should be controlled including parameters determining material photocuring and material refilling using the window screen. According to the curing characteristics (Jacobs, 1992), the exposure time for a given layer thickness should be set as

t=Ecϕ⁢e-hη⁢Cd
where Ec is the critical energy dose, ϕ is the light energy, h is the layer thickness, η is the overlap ratio between layers, and cdis the light penetration depth. In order to maximize the fabrication speed in our process, the layer thickness should be set as
h=ηCd.
Hence, the corresponding exposure time for the fastest speed is

t=e-1⁢Ecϕ.
In experimental results, a time 2 of 2.2 seconds for h=100 microns was found.

Window Screen Slot Size and Moving Speed: The material delivery slots in the window screen are designed such that the resin can quickly reach the center of the building area. The refilling flow rate in the slots is

Q∝PWr4μ⁢L
assuming the slots are a tube with diameter Wr. In the extreme situation, the input 3D model is a solid block that can cover the whole window screen. In this limiting case, the fabrication speed vzis governed by

vz=QLWr∝PWr3μ⁢L2.
Thus, the slot size should be set according to the relation:

FIG.12shows how a small feature1204behaves when the window screen1206moves at speed v, relative to the platform1202in a system1200. Assuming the shear stress of a printed part is σ, the moving speed of the window screen should be set within

vm∝σ⁢hi⁢dμ⁢H
wherein hiis the inhibition gap, μ is the resin viscosity, H is the feature's height, σ is the cured part's shear stress, and d is the feature's diameter. In one experimental setup, the moving speed was set as vm=5 to 15 mm/s.

The foregoing methods and apparatus were tested using circular radial and parallel rectangular window screens. A cylinder of diameter 90 mm was fabricated at a rate of 25 μm/s using a parallel window screen. Using a window screen speed of 5 mm/s, it was possible to fabricate small features, as well. The XY resolution of the process is determined by the pixel size. The Z axial resolution is determined by the related Z stage, which could be 10-200 μm, using a parallel window screen. A solid freeform object (3D model of human teeth as arranged in jaw) was fabricated at a speed of 16 μm/s at a size of 50×70×15 mm, using a parallel window screen. A parallel window screen was used to make an arbitrary geometric shape, including a hang-on angle of around 85°. Generally, the process could directly fabricate hang-on structure within the hang-on angle range 0-85°, especially for features that are aligned along the sliding direction,FIG.13shows a diagram1300illustrating a hang-on angle in an overhanging portion1304of a feature1302. The direction of platform movement is indicated by the vertical arrow1310. A radial-patterned sliding window was also successfully used to fabricate a hemisphere with a rectangular hole.

In summary of and consistent with the foregoing,FIG.14illustrates further aspects of a projection-based SL method1400using a sliding window screen. The method1400for stereolithography for additive manufacturing using a sliding window screen may include, at1402, fabricating an article from liquid resin by layer-based stereolithography, including performing certain operations in forming each layer. The certain operations may include, at1404simultaneously photopolymerizing and resin refilling a portion of the layer by projecting a mask through a window screen having distinct resin curing and refilling regions, at1406sliding the window screen to a different portion of the layer; and at1408repeating the simultaneous photopolymerizing and resin refilling for distinct portions of the layer until fabrication of the layer is completed.

The method may further include determining one or more projection masks for each layer based in part on a pattern in which the distinct resin curing and refilling regions of the sliding window are arranged. In an aspect, for example, the distinct resin curing and refilling regions of the window screen are arranged in a pattern selected from: parallel, gridded, radial, or branched. In another aspect, the projection mask may be configured with black pixels correlated to all refilling regions of the window screen, and light emitting pixels correlated one or more resin curing regions of the window screen. In some embodiments, the window screen is opaque in the refilling regions. In others, the resin refilling regions are transparent. The method may include determining one or more projection masks for each layer based in part on whether the window screen is opaque in the refilling regions. In an aspect, the repeating step is performed only once for each layer.

The method may further include, for example, setting a layer thickness based on the relation h=ηCd, wherein h is the layer thickness, η is an overlap ratio between layers, and Cd is a light penetration depth. Setting other parameters may be performed as described in the detailed description above.

The method may include separating the window screen from the portion of the layer at least in part by exerting a separation force in a direction perpendicular to a plane of sliding motion of the window screen. The method may include other operations and aspects, as described in the detailed description above.

Consistent with method1400and other disclosures above, and as further illustrated byFIG.15, an apparatus1500may function as an improved apparatus for additive manufacturing by stereolithography. The apparatus1500may comprise an electronic component or module1502for fabricating an article from liquid resin by layer-based stereolithography, including performing certain operations in forming each layer. The component1502may be, or may include, a means for said fabricating. Said means may comprise an algorithm executable by the processor to cause the apparatus1500to perform the fabricating function, for example, as described herein above in connection withFIG.11and elsewhere.

The apparatus1500may comprise an electronic component or module1503for simultaneously photopolymerizing and resin refilling a portion of the layer by projecting a mask through a window screen having distinct resin curing and refilling regions. The component1503may be, or may include, a means for said simultaneously photopolymerizing and resin refilling. Said means may comprise a window screen as described herein, in conjunction with an algorithm executable by the processor to cause the apparatus1500to perform the function of simultaneously photopolymerizing and resin refilling, with projection of a mask image. For example, the algorithm may include selecting a mask image based on the part geometry slice corresponding to the layer and configuration of window screen, and controlling a projector so that the mask image is projected through the window screen, excluding the resin refilling areas.

The apparatus1500may comprise an electronic component or module1504for sliding or otherwise repositioning the window screen to a different portion of the layer. The component1504may be, or may include, a means for said sliding or repositioning. Said means may comprise an algorithm executable by the processor to cause the apparatus1500to perform the sliding function, for example, selecting parameters for motor control based on conditions of the SL process as described above, separating the window screen from the cured portion of the layer, and moving the window screen to that its resin curing region aligns with a portion of the layer that remains to be cured.

In addition, the apparatus1500may comprise an electronic component or module1506for repeating the simultaneous photopolymerizing and resin refilling for distinct portions of the layer until fabrication of the layer is completed. The component1506may be, or may include, a means for said repeating. Said means may comprise an algorithm executable by the processor to cause the apparatus1500to perform the repeating function, for example, any one of the algorithms diagramed inFIGS.10A-Dherein above.

The apparatus1500may include a processor module1518having at least one processor; in the case of the apparatus1500this may be configured as a special purpose controller, rather than as a general purpose microprocessor. The processor1518, in such case, may be in operative communication with the modules1502-1506via a bus1512or similar communication coupling. The processor1518may effect initiation and scheduling of the processes or functions performed by electrical components1502-1506.

In related aspects, the apparatus1500may include a motor controller module1514through under higher level control of the processor1518that controls Z axis motion of the platform and X or Y axis movement of the window screen. In addition, the apparatus may include a projecting light source1520under control of the processor1518. The projecting light source may operate by processor control in the manner described herein above to cause the resin curing regions of the window screen to perform curing, while preventing curing in the resin refilling regions. In further related aspects, the apparatus1500may include a module for storing data and executable instructions, such as, for example, a memory device/module1516. The computer readable medium or the memory module1516may be operatively coupled to the other components of the apparatus1500via the bus1512or the like. The memory module1516may be adapted to store computer readable instructions and data for effecting the processes and behavior of the modules1502-1506, and subcomponents thereof, or the processor1518, or the methods disclosed herein, and other operations for content identification, playing, copying, and other use. The memory module1516may retain instructions for executing functions associated with the modules1502-1508. While shown as being external to the memory1516, it is to be understood that the modules1502-1506may exist at least partly within the memory1516.

Various aspects will be presented in terms of systems that may include a number of components, modules, and the like. It is to be understood and appreciated that the various systems may include additional components, modules, etc. and/or may not include all of the components, modules, etc. discussed in connection with the figures. A combination of these approaches may also be used. The various aspects disclosed herein can be performed on electrical devices including devices that utilize touch screen display technologies, microphone inputs, and/or mouse-and-keyboard type interfaces. Examples of such devices include computers (desktop and mobile), smart phones, personal digital assistants (PDAs), voice-activated terminals and other electronic devices both wired and wireless.

In view of the exemplary systems described supra, methodologies that may be implemented in accordance with the disclosed subject matter have been described with reference to several flow diagrams. While for purposes of simplicity of explanation, the methodologies are shown and described as a series of blocks, it is to be understood and appreciated that the claimed subject matter is not limited by the order of the blocks, as some blocks may occur in different orders and/or concurrently with other blocks from what is depicted and described herein. Moreover, not all illustrated blocks may be required to implement the methodologies described herein. Additionally, it should be further appreciated that the methodologies disclosed herein are capable of being stored on an article of manufacture to facilitate transporting and transferring such methodologies to computers. The term article of manufacture, as used herein, is intended to encompass a computer program accessible from any computer-readable device, carrier, or medium.

Having thus described a preferred embodiment of an apparatus, system and method for projection-based SL using a sliding window screen, it should be apparent to those skilled in the art that certain advantages of the within system have been achieved. The previous description of the disclosed aspects is provided to enable any person skilled in the art to make or use the present disclosure. Various modifications to these aspects will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the disclosure. For example, a method and system using particular window screen patterns has been described, but it should be apparent that the novel concepts described above may be applied by one of ordinary skill to other window screen patterns without departing from the novel teachings of the disclosure.