Patent Description:
Stereolithography was originally conceived as a rapid prototyping technology. Rapid prototyping refers to a family of technologies that are used to create true-scale models of production components directly from computer aided design (CAD) in a rapid (faster than before) manner. Since its disclosure in <CIT>, stereolithography has greatly aided engineers in visualizing complex three-dimensional part geometries, detecting errors in prototype schematics, testing critical components, and verifying theoretical designs at relatively low costs and in a faster time frame than before.

During the past decades, continuous investments in the field of micro-electromechanical systems (MEMS) have led to the emergence of micro-stereolithography (µSL), which inherits basic principles from traditional stereolithography but with much higher spatial resolution e.g., <NPL>. Aided by single-photon polymerization and two-photon polymerization techniques, the resolution of µSL was further enhanced to be less than <NUM>, e.g., <NPL>;<NPL>; <NPL>.

The speed was dramatically increased with the invention of projection micro-stereolithography (PµSL), <NPL>; <NPL>. The core of this technology is a high resolution spatial light modulator, which is either a liquid crystal display (LCD) panel or a digital light processing (DLP) panel, each of which are available from micro-display industries.

While PµSL technology has been successful in delivering fast fabrication speeds with good resolution, further improvements are still wanted.

<CIT>, corresponding to the preamble of claim <NUM>, describes a 3D printing method, where elementary volumes, or voxels, of a material are sequentially transformed by irradiation, comprising the steps of:
breaking down the volume of a portion of an object to be printed which does not require a maximum resolution into identical blocks; for the printing, associating with each block a brick of same contour comprising hollow portions; and carrying out a succession of irradiations to print the voxels of the bricks, each irradiation providing an array of irradiation beams focused into an array of points distributed in the material in the same way for two successive irradiations, the array of points being offset in the material between two successive irradiations.

Due to the limitation of the physical size of the micro display chip, high resolution and large area printing are conflicting requirements. In the high-resolution printing, as the pixel size shrinks, the size of projection image proportionally scales down. Hence significantly reduce the printing speed.

In this invention a new method combines dual-projection lens of distinct pixel size with precision translation stage system to print faster than before over an area of 10cmX10cm.

In all 3D printing technologies, accuracy and efficiency in dimension replication is very important. Therefore, in the Multi-scale Projection Micro Stereolithography (<FIG>) of the invention, it is very important to have high accuracy and efficiency in dimension control of layers, so that the actual CAD model can be duplicated in a practical period of time.

The method of the present invention, as recited in claim <NUM>, provides more precise control, with greater speed and accuracy in a larger printing area, for example, 10cmX10cm printing area with a <NUM> optical resolution. In an embodiment part of the invention as claimed, the present method uses a projection lens complex, in particular a dual projection lens, combined with a precision translation stage system. The method not only maintains the dimensional accuracy of samples printed using, e.g., PµSL systems, but also significantly improves the printing speed by combining projection lenses with different imaging ratios for areas with different feature details.

The dual projection lens comprises a higher resolution lens and a lower resolution lens, for example, lenses having image pixel size of <NUM> and <NUM>. The image from the micro display chip is delivered to both lenses of a dual projection lens using beam splitter and mirror. But each time, only one lens projects the current image at selected pixel size by controlling the optical shutters in front of the dual lens.

For example, in many embodiments part of the invention as claimed, the method makes use of a system comprising: i) an optical light engine, it can be a DLP or LCD with a light source for projection micro stereolithography, ii) a high precision camera to monitor the printing interface, iii) three precision stages to control the motion of the substrate for supporting the printing sample or the printing projection system in the X, Y, and Z directions, iv) a resin vat under the membrane where the parts are printed and v) a laser displacement sensor for monitoring the membrane position and the printing substrate position to ensure one micron accuracy. The system is arranged relative to a surface of a substrate, i.e., sample holder, or sample so that the lens is situated between the surface of the substrate and the light engine and it is gravitationally above the substrate.

In one embodiment part of the invention as claimed, with the aid from the XY stages, in a configuration for multi-scale PµSL, this invention provides three printing modes. When only a single sample needed, which is smaller than the single exposure size, it is called single exposure mode. If multiple samples are needed, the XY stages will move stepwise and print the same sample in an array, which is called array exposure mode. As the sample size increases to exceed the size of the single exposure, the system will further divide one layer into multiple sections and stitch the adjacent sections into a whole layer by overlapping <NUM> to <NUM> on the shared edges. This is the stitching exposure mode. It is also possible to combine the stitching mode with array mode. In each layer, no matter its size, the image is analyzed and small features (gaps, holes, steps, sharps et al) are detected. A rectangular window equal to the size of the <NUM> resolution exposure is used to cover those tiny features. More windows can be added to cover all. The rest of the areas will be covered by <NUM> resolution exposures. The stitch happens not only among section images of same resolution, but also among section images of different resolutions.

In another embodiment part of the invention as claimed, the least square fitting error curves based on the measured data from actual samples will be fed into the translation of the XY stages to compensate the mechanical tolerances to ensure the accuracy of the stitching-printed sample is within the specifications.

In various embodiments part of the invention as claimed, the substrate holding the sample is translated in the XY plane for stitching and array printing, with optics (DLP or /LCD panel and lens) fixed, however, translating the optics, or translating both the substrate and optics will serve the same purpose.

According to the invention as claimed, the method is aided by a dual projection lens as in <FIG>, as part of the light engine/dual project lens/membrane/displacement system discussed above. The dual projection lens provides precision printing based on the local details of the printing part. It dramatically increases the precision printing time without sacrificing the resolution for very fine details by locally adapting the resolution as needed.

For the PµSL case, the printing process starts with generating a 3D model in the computer and then slicing the digital model into a sequence of images, wherein each image represents a layer (e.g., <NUM> to <NUM> micrometers) of the model. The control computer sends an image to the micro display chip, DLP or LCD, and the image is projected through the lens onto the bottom surface (the wet surface) of membrane. The bright areas of the projected image are polymerized whereas the dark areas remain liquid. As one layer is finished, the Z stage moves the sample substrate down about <NUM>-<NUM> to peel off the membrane from the sample. As soon as the membrane is separated from the sample, the sample again moves up to the flat membrane position less the thickness of next layer, during this movement different techniques are applied to flatten the membrane and defining the next layer of printing material, typically a resin, such as a photo curable resin. The above procedures are repeated for the number of the layers until the whole model is replicated in the resin vat.

Due to the size limit of either LCD or DLP chip, for example a DLP chip with 1920X1080 pixels at <NUM> printing optical resolution, a single exposure will only cover area of <NUM>. Therefore, if the cross-section of a sample is larger than <NUM>. <NUM>, it cannot be printed with single exposure method. In the present invention, a multiple-exposure stitching printing method is provided. By this method, an image representing a layer of the 3D model is further divided into multiple smaller sub-images with each image no larger than the DLP pixel resolution. For instance, an image of pixel resolution of 3800X2000 can be divided into four 1900X1000 sub-images with each one represents a quarter of this layer. As a result, a full layer of the model will be printed section by section based on the sub-images. To improve the mechanical strength of the shared edges of the adjacent sections, there is typically about a <NUM>-<NUM> micron overlap on the edges. The precise position and the amount of overlap are accurately controlled by the XY stage assembly. There are two coordinate systems: one is aligned with the DLP/LCD panel, the other one is the XY stage assembly. When these two coordinate systems are not parallel due to the assembly tolerance, there will be offset errors on the shared edges of adjacent sections. As shown in <FIG>, A (<NUM>) is the size of a single exposure; B (<NUM>) is the result of precise alignment on x direction; C (<NUM>) is the result with error offset on x direction; B' (<NUM>) is the result of precise alignment on y direction; C' (<NUM>) is the result with error offset on y direction. In precision printing, with error requirements less than <NUM>, stage assembly tolerance is usually off the allowed range; and the offset is not linear to the stage travel distance. Therefore, in the invention, offsets are measured at <NUM> or more evenly distributed points on both X and Y directions on a full-range printed square sample. At least second order polynomial error curves on both X and Y directions are fitted to the measure data by the least square method. <MAT> <MAT>.

Here Cs and Ds are polynomial coefficients calculated by the least square fitting method. These two error curves will be fed into the translation of the XY stages to compensate the offset thus ensure the accuracy of the stitching-printed sample is within the specifications. For example, the theoretical target is ( X<NUM>,Y<NUM> ) , then the actual executed translation commands are ( X<NUM>+XError ( X<NUM>,Y<NUM> ) ,Y<NUM>+YError ( X<NUM>,Y<NUM> ) ).

With the aid of the XY stages, the multi-scale PµSL provides basically three printing modes (<FIG>). When printing a single sample, which is smaller than the single exposure size of the finest lens, <NUM> in this invention, the XY stages will not move during printing. It is called single exposure mode. If multiple identical samples are needed, the XY states will move stepwise and print the same sample in an array. And this is called array exposure mode which is much faster for small volume production than repeating the single exposure mode. As the sample size increases to exceed the size of the single exposure or the sample needs multi-scale printing, the system will further divide one layer into multiple sections and stitch the adjacent sections into a whole layer by overlapping <NUM> to <NUM> on the shared edges. This is the stitching exposure mode. It is possible to combine the stitching mode with array mode when one needs multiple identical samples but needs stitching exposure as the sample is larger than single exposure. However, this case is usually treated as stitching exposure mode. Especially in the multi-scale printing (<FIG>), the layer image is analyzed and the small features ((gaps, holes, steps, sharps et al) are detected and isolated by windows of the <NUM> single exposure. For each layer, the printer first scans and prints using <NUM> exposures, then it alternates the shutter, reverses the scan and prints the isolated small features using <NUM> exposures. The projection images of the small features have <NUM>-<NUM> deep overlaps with the surrounding <NUM> images or <NUM> images all around. The overlaps happen not only among <NUM> images or <NUM> images, also happen between <NUM> and <NUM> images.

A high-resolution lens typically has a very small focus depth, for example the focus depth of the <NUM> lens is less than <NUM>. In printing, as the lens scans over the membrane, it is critical that the optical axis of both lenses are perpendicular to the membrane, such that the projected image will not be out of focus during the XY stage translation which impairs the printing resolution. Hence, a high accuracy laser displacement sensor with resolution of <NUM> is integrated with the dual lens. The displacement sensor serves two purposes. One is to align one surface parallel to another. The other is to precisely define the gap between two parallel surfaces, such as the membrane and the printing substrate in this invention, by placing the surfaces in the laser defined position.

For example, the methods herein can be used as part of multi-scale PµSL printing process to establish a resin free surface, membrane or hard window as parallel to the surface of a sample stage. As shown in <FIG> for a multi-scale PµSL printing system, three non-linear points, here forming the right-angle triangle shown, are selected on the sample stage surface and sequentially aligned with the displacement sensor emission vector by moving the XY stages. The minimum distances between the points should be <NUM> to guarantee good accuracy. The sample stage should be adjusted to make sure the distance readings between the displacement sensor and each point are the same. As the emission vector of the displacement sensor is parallel to the optical axis of both lens, proper controls of the system will provide a stages surface perpendicular to the emission vector of the displacement sensor and the optical axis of the lens. And it follows the same procedure to level the membrane such that the optical axis of both lenses is perpendicular to the membrane.

Referring now to <FIG>, it shows a schematic drawing of a multi-scale projection micro stereolithography system; including a digital light processing panel (DLP) and light source <NUM>; beam splitter <NUM>; charge coupled device (CCD) <NUM>; mirror <NUM>; shutter <NUM>; dual lenses 105A (<NUM>) and 105B (<NUM>); laser displacement sensor <NUM>; membrane <NUM>; resin vat <NUM>; video unit <NUM>; xyz stage assembly <NUM>; and sample substrate <NUM>; where z1 and z2 are the z directions and g is the direction of gravity.

Claim 1:
A method for fast, high resolution, 3D printing, the method comprising:
generating on a computer a 3D digital model of a sample to be printed, slicing the digital model into a sequence of images, wherein each image of the sequence represents a layer of the 3D digital model, and transferring an image from the sequence of images to a micro display chip of an optical light engine (<NUM>) comprising the micro display chip and a light source, wherein the micro display chip comprises a liquid crystal display or a digital light processing panel,
projecting the image along with light from the optical light engine (<NUM>) through a projection lens (105A) of a projection lens complex (105A, 105B) onto printing material (<NUM>),
causing the printing material (<NUM>) in the bright areas of the projected image are polymerized, while the dark areas remain liquid, and
the projection lens complex (105A, 105B) comprises two or more projection lenses with different imaging ratios, and wherein the image and light are projected through only one projection lens at a time, and
wherein the projection lens complex is a dual-projection lens comprising two projection lenses with different imaging ratios, one lens having higher resolution and a second lens having lower resolution, wherein both lenses share the same focus plane by design,
the method being characterized in that optical shutters (<NUM>) in front of the projection lenses (105A, 105B) are used to switch image projection from one projection lens to another projection lens.