U.S. patent application number 15/222522 was filed with the patent office on 2016-11-17 for wall smoothness, feature accuracy and resolution in projected images via exposure levels in solid imaging.
The applicant listed for this patent is 3D Systems, Inc.. Invention is credited to Yong Chen, Charles W. Hull, Thomas Alan Kerekes, Jouni P. Partanen.
Application Number | 20160332368 15/222522 |
Document ID | / |
Family ID | 38859030 |
Filed Date | 2016-11-17 |
United States Patent
Application |
20160332368 |
Kind Code |
A1 |
Kerekes; Thomas Alan ; et
al. |
November 17, 2016 |
Wall Smoothness, Feature Accuracy and Resolution in Projected
Images via Exposure Levels in Solid Imaging
Abstract
A solid imaging apparatus and method employing levels of
exposure varied with gray scale or time or both of digitally light
projected image of a cross-section of a three-dimensional object on
a solidifiable photopolymer build material. The gray scale levels
of exposure of projected pixels permits the polymerization
boundaries in projected boundary pixels to be controlled to achieve
preserved image features in a three-dimensional object and smooth
out rough or uneven edges that would otherwise occur using digital
light projectors that are limited by the number of pixels in an
image projected over the size of the image. Software is used to
control intensity parameters applied to pixels to be illuminated in
the image projected in the cross-section being exposed in the image
plane.
Inventors: |
Kerekes; Thomas Alan;
(Calabasas, CA) ; Partanen; Jouni P.; (Palo Alto,
CA) ; Chen; Yong; (Valencia, CA) ; Hull;
Charles W.; (Santa Clara, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
3D Systems, Inc. |
Rock Hill |
SC |
US |
|
|
Family ID: |
38859030 |
Appl. No.: |
15/222522 |
Filed: |
July 28, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11468090 |
Aug 29, 2006 |
9415544 |
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15222522 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G05B 19/4099 20130101;
G05B 2219/49019 20130101; B33Y 70/00 20141201; B29C 64/135
20170801; B29K 2995/0073 20130101; B33Y 40/00 20141201; B33Y 30/00
20141201; B29C 64/393 20170801; B33Y 80/00 20141201; B29C 64/40
20170801; B33Y 50/02 20141201 |
International
Class: |
B29C 67/00 20060101
B29C067/00; B33Y 50/02 20060101 B33Y050/02; G05B 19/4099 20060101
G05B019/4099; B33Y 70/00 20060101 B33Y070/00; B33Y 80/00 20060101
B33Y080/00; B33Y 30/00 20060101 B33Y030/00; B33Y 40/00 20060101
B33Y040/00 |
Claims
1-15. (canceled)
16. An apparatus for forming a three-dimensional object
layer-by-layer by solid imaging comprising in combination: a. a
frame; b. a radiation transparent carrier moveably mounted to the
frame; c. a radiation source mounted to the frame for projecting an
image one cross-section at a time of a three-dimensional object to
be built through the radiation transparent carrier; d. a source of
solidifiable photopolymer build material in fluid flow
communication with the radiation transparent carrier effective to
apply the solidifiable photopolymer build material to the radiation
transparent carrier; e. a support platform moveably mounted to the
frame to support the three-dimensional object as it is built
layer-by-layer; f. a controller mounted to the frame effective to
control movement of the radiation transparent carrier and the
support platform and to control illumination of selected pixels by
the radiation source in an image exposure of a cross-section of the
three-dimensional object on the solidifiable photopolymer build
material; g. a computer in data flow communication with the
controller to receive the digital data relating to the
three-dimensional object to be built and convert the digital data
into data representative of cross-sections of the three-dimensional
object; wherein the radiation source projects an image
representative of cross-sectional digital data through the
radiation transparent carrier to selectively illuminate pixels in
an illumination pattern on an image area exposure and to
selectively solidify the photopolymer build material by applying
different exposure parameters to different portions of the image
area to control the polymerization boundary of the solidifiable
photopolymer build material to obtain a high resolution image and a
three-dimensional object that has all features properly positioned
and smooth walls.
17. The apparatus according to claim 16 wherein the radiation
source is selected one from the group consisting of a UV radiation
source, a visible light source and a combination thereof.
18. The apparatus according to claim 17 wherein the radiation
source is a digital light projector.
19. The apparatus according to claim 16 wherein the radiation
transparent carrier is an endless belt.
20. The apparatus according to claim 16 wherein the radiation
transparent carrier further is a flexible film.
21. The apparatus according to claim 16 wherein the radiation
transparent carrier is a plate.
22. The apparatus according to claim 16 wherein the controller is a
microprocessor that selects pixels to be illuminated in the
exposure of a cross-section of the three-dimensional object on the
solidifiable liquid by use of an algorithm.
23. The apparatus according to claim 22 wherein the microprocessor
uses software to control light intensity versus gray scale levels
across an image projected by the digital light projector onto the
solidifiable photopolymer build material in order to optimize the
image projection of the three-dimensional object.
24. The apparatus according to claim 22 wherein the microprocessor
uses software to apply light intensity levels of exposure,
illumination pattern width and time of illumination of pixels in
the image area as the exposure parameters applied to the image area
to position a boundary of a projected boundary pixel of the
three-dimensional object in a single exposure.
25. An apparatus for forming a three-dimensional object
layer-by-layer by solid imaging comprising in combination: a. a
frame; b. a source of solidifiable photopolymer build material
effective to supply the solidifiable photopolymer build material to
form a layer of build material to be solidified with an exposed
surface; c. a radiation source mounted to the frame for projecting
onto the surface of the solidifiable photopolymer build material a
cross-section at a time of a three-dimensional object to be built
effective to solidify each cross-section of the three-dimensional
object; d. a support platform moveably mounted to the frame to
support the three-dimensional object as it is built layer-by-layer;
e. a controller mounted to the frame effective to control recoating
of an exposed layer of the liquid medium with a fresh layer of the
solidifiable liquid medium, control movement of the support
platform, and to control illumination of selected pixels by the
radiation source in an image exposure of a cross-section of the
three-dimensional object on the solidifiable liquid medium; f. a
computer in data flow communication with the controller to receive
three-dimensional digital data in relation to the three-dimensional
object to be built and convert the digital data into data
representative of cross-sections of the three-dimensional object;
and g. software to control light intensity versus gray scale levels
across an image projected by the digital light projector onto the
solidifiable photopolymer build material in order to optimize the
image projection of a three-dimensional object to control the
polymerization boundary of the solidifiable photopolymer build
material in boundary pixels to obtain a high resolution image and a
three-dimensional object that has all features properly positioned
and smooth walls.
26. The apparatus according to claim 25 wherein the radiation
source is selected one from the group consisting of a UV radiation
source, a visible light source and a combination thereof.
27. The apparatus according to claim 25 wherein the radiation
source is a digital light projector.
28. The apparatus according to claim 25 further comprising a
recoater moveably mounted to the frame for recoating an exposed
layer of the photopolymer build material with a fresh layer of the
solidifiable photopolymer build material.
29. The apparatus according to claim 25 wherein the solidifiable
photopolymer build material selected from the group consisting of a
solidifiable photopolymer liquid resin formulation, a solidifiable
photopolymer paste, a solidifiable photopolymer gel, a solidifiable
photopolymer semi-liquid and combinations thereof.
30. The apparatus according to claim 25 wherein said software also
applies light intensity levels of exposure, illumination pattern
width and time of illumination of pixels in the image plane as
intensity parameters applied to the image plane medium by
controlling the polymerization boundary of a projected boundary
pixel of the three-dimensional object in a single exposure in order
to optimize the image projection of the three-dimensional object
and obtain a three-dimensional object that has all features
properly positioned and smooth walls.
31. The apparatus according claim 25 wherein the source of
solidifiable photopolymer build material being a vat mounted to the
frame containing the surface of the solidifiable photopolymer build
material onto which the image is projected.
32. The apparatus according to claim 25 wherein the solidifiable
photopolymer build material is image exposed on a radiation
transparent carrier.
Description
FIELD OF INVENTION
[0001] The present invention is directed to a technique to achieve
improved wall smoothness and feature accuracy in an image
projection system limited by the projected size of the pixels, such
as occurs with light valve projectors using DMD or LCD based
projectors, for use in an apparatus for forming three-dimensional
objects on a layer-by-layer basis. More particularly, it is
directed to an apparatus and method for forming three-dimensional
objects using exposure levels varied with gray scale or time or
both in a projected image to obtain enhanced smoothness in the x,
y-plane and on the z-axis edge, feature accuracy, and better
resolution in the three-dimensional object being formed from a
solidifiable photopolymerizable medium in response to exposure by
UV or visible light.
BACKGROUND OF THE INVENTION
[0002] In recent years, many different techniques for the fast
production of three-dimensional models have been developed for
industrial use. These solid imaging techniques are sometimes
referred to as rapid prototyping and manufacturing ("RP&M")
techniques. In general, rapid prototyping and manufacturing
techniques build three-dimensional objects layer-by-layer from a
working medium utilizing a sliced data set representing
cross-sections of the object to be formed. Typically, an object
representation is initially provided by a Computer Aided Design
(CAD) system.
[0003] Stereolithography, presently the most common RP&M
technique, was the first commercially successful solid imaging
technique to create three-dimensional objects from CAD data.
Stereolithography may be defined as a technique for the automated
fabrication of three-dimensional objects from a fluid-like
photopolymer build material utilizing selective exposure of layers
of the material at a working surface to solidify and adhere
successive layers of the object (i.e. laminae). In
stereolithography, data representing the three-dimensional object
is input as, or converted into, two-dimensional layer data
representing cross-sections of the object. Layers of photopolymer
build material are successively formed and selectively transformed
or solidified (i.e. cured) using a computer controlled laser beam
of ultraviolet (UV) radiation into successive laminae according to
the two-dimensional layer data. During transformation, the
successive laminae are bonded to previously formed laminae to allow
integral formation of the three-dimensional object. This is an
additive process. More recently, stereolithographic designs have
employed digital light-processing technology wherein visible light
initiates the polymerization reaction to cure the photopolymer
build material (i.e. also referred to as resin).
[0004] Stereolithography represents an unprecedented way to quickly
make complex or simple parts without tooling. Since this technology
depends on using a computer to generate its cross-sectional
patterns, there is a natural data link to CAD/CAM. Such systems
have encountered and had to overcome difficulties relating to
shrinkage, curl and other distortions, as well as resolution,
accuracy, and difficulties in producing certain object shapes.
While stereolithography has shown itself to be an effective
technique for forming three-dimensional objects, other solid
imaging technologies have been developed over time to address the
difficulties inherent in stereolithography and to provide other
RP&M advantages.
[0005] These alternate technologies, along with stereolithography,
have collectively been referred to as solid freeform fabrication or
solid imaging techniques. They include laminated object
manufacturing (LOM), laser sintering, fused deposition modeling
(FDM), and various ink jet based systems to deliver either a liquid
binder to a powder material or a build material that solidifies by
temperature change or photocuring. Each of these additive
technologies have brought various improvements in one or more of
accuracy, building speed, material properties, reduced cost, and
appearance of the build object.
[0006] During the same time period that solid imaging or solid
freeform fabrication has evolved, the two-dimensional imaging
industry evolved ways to displace the projected image on a screen
or, in the case of the printing industry, on a receiving substrate.
These approaches addressed the basic problem that digital light
projectors produce images with coarse resolution. Digital light
projectors typically project only 100 pixels per inch for an image
size of 10.24 inches by 7.68 inches, so their resolution is limited
by the pixel sizes. The photographic printing industry especially
has employed techniques to shift two-dimensional images to improve
resolution by a variety of techniques, including moving the light
source or light valve. Other approaches have included moving or
shifting the photographic paper, using polarizing and double
refracting plates, and, in the case of image projection systems,
using multiple spatial light modulators. All of these systems have
addressed the inherent limitation of image distortion when
projecting resized digital images or the problem of light valve
projectors, such as a liquid crystal display (LCD) or a digital
micro-mirror device (DMD), having a fixed number of pixels.
Attempting to utilize image displacement techniques with digital
image projections in solid imaging applications presents unique
problems because of the three-dimensional aspect of the object
being created. The problems of two-dimensional digital image
projection, when applied to three-dimensional solid imaging, cause
inaccurate feature placement, potential loss of feature details,
and smoothness of curves or edges on objects being built to be
roughened or uneven and poorly defined. Most recently, techniques
have been developed using pixel shifting to address this problem.
However, those approaches suffer from the deficiencies of requiring
multiple exposures of individual pixels, thereby inherently slowing
the process, and requiring mechanical hardware to accomplish the
pixel shifting. Additionally, when using multiple exposures with
techniques such as pixel shifting, there are alignment issues that
must be addressed to ensure the exposures are properly positioned
to obtain maximum resolution and the desired edge smoothness.
[0007] Lastly, none of the prior solid freeform fabrication
approaches, while making substantial improvements, achieve a truly
low cost system that produces highly accurate and visually
appealing three-dimensional objects in a short build time.
[0008] These problems are solved in the design of the present
invention by combining a new solid imaging technique with the use
of digital imaging projection in a manner that provides accurate
object features while achieving high resolution and object wall
smoothness in three-dimensional object fabrication while being
relatively low cost and not requiring additional hardware.
SUMMARY OF THE INVENTION
[0009] It is an aspect of the present invention that an apparatus
and method are provided that achieve improved wall smoothness,
feature accuracy and resolution imaging in three-dimensional
objects built using UV or visible light and a solidifiable
photopolymer build material.
[0010] It is another aspect of the present invention that either
gray scale exposure level for each projected pixel in a
cross-section or illumination time of each pixel within the image
area or both is used to vary the level of light energy during the
exposure of each image cross-section projected onto a solidifiable
photopolymer build material in order to control the polymerization
boundary of the solidifiable photopolymer build material so that
smooth edges and fine feature retention in the projected pixel
image via edge or wall placement is obtained as the
three-dimensional object is being built.
[0011] It is still another feature of the present invention that an
out of focus characteristic of a projected image is utilized with
exposure levels varied with gray scale or illumination time of each
pixel within the image area or both in order to better control the
polymerization boundary of the solidifiable photopolymer build
material that is exposed.
[0012] It is even another feature of the present invention that
software is used to characterize the light intensity versus gray
scale levels across the image projected by the digital light
projector onto the solidifiable photopolymer build material in
order to optimize the image projection of a three-dimensional
object to be built.
[0013] It is still another feature of the present invention that
the width of light distribution for each pixel can be controlled by
the degree of focus of a digital light projector.
[0014] It is yet another feature of the present invention that the
exposure levels for a radiation source such as digital light
projector can be characterized (i.e. controlled and finely
adjusted) either (1) by varying the gray scale levels of the
radiation source or (2) by controlling the illumination time of
different pixels differently or (3) by varying the degree of focus
to control the width of light distribution or each pixel
illumination or (4) by varying the light intensity versus age of
the radiation source (e.g. projector lamp), or (5) by combinations
thereof, so as to vary the energy level in the projected pixels so
that the polymerization boundary of the solidifiable photopolymer
build material is controlled by providing sufficient light energy
with respect to the critical energy E.sub.c to initiate
photopolymerization.
[0015] It is an advantage of the present invention that a low cost
solid imaging device is obtained that provides smooth object edges
in the projected image along with accurate features and good
resolution without the need for additional mechanical hardware.
[0016] It is another advantage of the present invention that
accurate feature placement in each cross-section in a
three-dimensional object being built layer-by-layer is obtained so
that accurate feature placement is not lost due to the fixed number
of pixels that can be projected in a single image projection by a
digital light projector.
[0017] It is still another advantage of the present invention that
a digital light projector can be characterized for light intensity
width in both the X and Y directions as a function of the pixel
location to achieve smooth image edges and less image granularity
with enhanced image resolution.
[0018] These and other aspects, features, and advantages are
obtained by the present invention through the use of a solid
imaging apparatus and method that employ gray scale exposure levels
for the boundary pixels in an image area of a cross-section and
software that determines the gray scale exposure levels for the
boundary pixels across the image projected by a digital light
projector onto a solidifiable photopolymer build material in order
to optimize the projected image by a projector that is
characterized for light intensity width and light intensity versus
gray scale controlling the polymerization boundary of the
solidifiable photopolymer build material in the boundary pixels
forming an object edge or wall to enable a three-dimensional object
to be built with no lost features or uneven or rough edges, while
retaining high resolution.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] These and other aspects, features and advantages of the
invention will become apparent upon consideration of the following
detailed disclosure of the invention, especially when taken in
conjunction with the following drawings wherein:
[0020] FIG. 1 is a diagrammatic illustration of the relation of
light intensity and gray scale levels;
[0021] FIG. 2 is a diagrammatic illustration of the accumulation of
light intensities at a point in an image plane;
[0022] FIG. 3 is a diagrammatic illustration of the relation of
light energy and photopolymer build material solidification using a
resin formulation as the solidifiable photopolymer build
material;
[0023] FIG. 4 is a diagrammatic illustration of the correlation
between gray scale values and the border location;
[0024] FIG. 5 is a graphical illustration of 4 adjacent pixels
modeled with a Gaussian distribution where the exposure intensity
of the first pixel is varied to 5 different levels illustrating the
control of the polymerization boundary of the photopolymer build
material (resin) in a projected border pixel as the gray scale
levels increase or decrease;
[0025] FIG. 6 is a graphical illustration showing the exposure
intensities of the 4 adjacent pixels modeled with a Gaussian
distribution for an exposure level of 1.0 and having a full width
of half maximum (fwhm) of 6.7 mils (0.0067 inch) and 5 mils (0.005
inch) peak to peak separation;
[0026] FIG. 7 is a graphical illustration showing the exposure
intensities of the 4 adjacent pixels modeled with a Gaussian
distribution for an exposure level of 0.8 for the boundary pixel
and having a full width at half maximum (fwhm) of 6.7 mils (0.0067
inch) and a 5 mil (0.005 inch) peak to peak separation;
[0027] FIG. 8 is a graphical illustration showing the exposure
intensities of the 4 adjacent pixels modeled with a Gaussian
distribution for an exposure level of 0.6 for the boundary pixel
and having a full width at half maximum (fwhm) of 6.7 mils (0.0067
inch) and a 5 mils (0.005) inch peak to peak separation;
[0028] FIG. 9 is a graphical illustration showing the exposure
intensities of the 4 adjacent pixels modeled with a Gaussian
distribution for an exposure level of 0.4 for the boundary pixel
and having a full width at half maximum (fwhm) of 6.7 mils (0.0067
inch) and a 5 mils (0.005) inch peak to peak separation;
[0029] FIG. 10 is a graphical illustration showing the exposure
intensities of the 4 adjacent pixels modeled with a Gaussian
distribution for an exposure level of 0.2 for the boundary pixel
and having a full width at half maximum (fwhm) of 6.7 mils (0.0067
inch) and a 5 mils (0.005) inch peak to peak separation;
[0030] FIG. 11 is a graphical illustration showing the exposure
intensities of the 4 adjacent pixels modeled with a Gaussian
distribution for an exposure level of 0.0 and having a full width
at half maximum (fwhm) of 6.7 mils (0.0067 inch) and a 5 mils
(0.005) inch peak to peak separation;
[0031] FIG. 12 is a diagrammatic illustration of pixels covered by
a straight line boundary of an object in an image plane showing
different gray scale levels for boundary pixels, interior pixels
and external pixels;
[0032] FIG. 13 is a diagrammatic illustration of pixels covered by
a curved line boundary of an object in an image plane showing
different gray scale levels for boundary pixels, interior pixels
and external pixels;
[0033] FIG. 14 is a photograph of a three-dimensional object
fabricated by an apparatus employing an imaging process employing
levels of exposure without gray scale levels showing rough edge
walls; and
[0034] FIG. 15 is a photograph of a three-dimensional object
fabricated by an apparatus employing an imaging process employing
levels of exposure with using gray scale levels showing smooth edge
walls.
DETAILED DESCRIPTION
[0035] The preferred flexible transport solid imaging of the type
disclosed herein involves the layer-by-layer build-up of articles
from a visible or UV solidifiable photopolymer build material that
is delivered by a radiation transparent flexible carrier, such as
an endless belt or reciprocating or extendable and retractable
sheet of transparent plastic film. The term "transparent" as used
in this context means any suitable material that allows the passage
of sufficient radiation (such as UV or visible light) through it to
effect the photopolymerization reaction. Because these transparent
materials may be very thin, the term would also include translucent
or partially transparent materials. The radiation transparent
flexible carrier works as a recoater that delivers fresh
solidifiable photopolymer build material to the imaging area for
subsequent layers that are formed after the initial layer.
Solidifiable photopolymer build material is applied to the
radiation transparent flexible carrier from a cartridge employing a
suitable mechanism to transfer the photopolymer to the flexible
transport device to provide a fresh material to create new layers
as the object is built. The build material positioned on the
carrier is then imaged one cross-section at a time. Each
cross-section may be either imaged over the full cross-sectional
image area or, more preferably, can be imaged in parts across that
image area (e.g. the interior portions of the part to be built are
imaged first for each cross-section and then the boundary portions
of the part to be built are imaged). Each layer of photopolymer for
the three-dimensional object to be built is preferably imaged by
radiation projected from a digital UV projector, a digital visible
light projector or a digital light projector providing both UV
radiation and visible light. The projector includes a spatial light
modulator, such as a digital micro-mirror device ("DMD") that
selectively illuminates pixels for imaging. UV radiation and
visible light projection is a preferred approach. These components
are appropriately mounted to the frame of an apparatus of the type
disclosed in U.S. patent application Ser. No. 11/416,812, assigned
to the assignee of the present invention. It should be noted that
the imaging steps of the present invention do not require multiple
images for boundary pixels as is normally needed for pixel shifting
operations.
[0036] Solid imaged parts are preferably built on an elevator
platform that moves the build object or part down or up into
contact with the solidifiable photopolymer build material and,
after exposure, up or down out of contact with the solidifiable
photopolymer build material as successive layers or laminae are
formed during the building process. The build object can be built
on structures known as supports rather than directly on the
elevator platform. Supports are used for more complex
three-dimensional objects being built that have unsupported or
partially unsupported surfaces.
[0037] Commercially available digital light projectors, optionally
modified to have a shorter focal length, may be employed, such as
those available from InFocus Corporation of Wilsonville, Oreg. and
BenQ America Corp. of Irvine, Calif. However, digital light
projectors inherently have limited resolution. For example, if a
1024 by 768 pixel image is projected over a 10.24 inch by 7.68 inch
area the effective resolution is 100 dpi (100 dots per inch). In
this case the minimum feature size is 0.010 inch, part features may
be located as far off their correct position as 0.005 inch, and a
wall drawn at 45.degree. to the X or Y axis will have a peak to
peak roughness of 0.007 inch.
[0038] Commercial digital light projectors produce images with
coarse resolution, as indicated above, because their resolution is
limited by the pixel sizes. The 1024.times.768 pixels in an image
generated by commercial digital light projectors limits the size of
a three-dimensional object that is able to be built on a support
platform because resolution decreases in proportion to an increase
in build platform size. The image size in a typical commercial
light projector limits resolution in the direction of the X axis to
1024 pixels and to 768 pixels in the direction of the Y axis,
thereby producing the coarse image. Additionally, commercial light
projectors cannot be perfectly focused so that inherent aberrations
in each projector result in fuzzy pixels. This "out of focus"
condition varies across a pixel image area. Further, there is a
different amount of "out of focus" condition for individual
projectors. However, utilizing the inherent fuzziness of pixels and
employing gray scale exposure levels for each boundary pixel to
vary the level of light intensity during exposure of a photocurable
solidifiable build material, such as a liquid resin formulation,
permits the gray scale value of a projected boundary pixel to
control the polymerization boundary of the solidifiable
photopolymer build material forming the object in that pixel in a
single exposure to achieve much greater build object accuracy and
wall smoothness of the solid image or three-dimensional part being
built. Being thus able to control the polymerization boundary of
the solidifiable build material corresponding to a projected
boundary pixel or multiple pixels in the X and Y planes, the
aforementioned 10.24 inch by 7.68 inch area would look like it had
a substantially greater effective resolution than merely 100 dpi.
Larger build platforms can also be utilized and larger objects can
be fabricated.
[0039] A digital light projector controls a set of very small
mirrors to reflect light into related pixels. By controlling the
reflecting or illumination time, the light intensity K of each
projected pixel in the image plane can vary from 0 (black) to 1
(white) with different gray scale levels from 0 to 255. FIG. 1
shows the relation of light intensity and the gray scale levels
input for a typical projector in a pixel. A preferred range of gray
scale levels is from about 60 to about 255. A more preferred range
of gray scale levels is from about 100 to about 255.
[0040] The inherent characteristic of commercial digital light
projectors that the pixels in an image are fuzzy or have slight
image blurring results in the light intensity overlapping
neighboring pixels. This light intensity distribution can be
approximated as a Gaussian distribution for purposes of further
explanation, but in actuality the light distribution can take many
different forms. The light intensity K at a point (x, y) is
actually the sum of the light intensity contributions of all the
neighboring pixels to the point (x, y). The accumulation of these
different light intensities is illustrated in FIG. 2. The present
invention utilizes the fact that the light intensity at point (x,y)
is the blended result of all of the light intensities of point (x,
y)'s neighboring pixels. The light intensity of a pixel decreases
the further it is away from the center of the pixel. Therefore, by
adjusting the optical system of a digital light projector to obtain
some image blurring of the pixels, the light intensity of a pixel
is spread to all of its neighboring pixels. This is seen as eight
neighboring pixels for a point located in an interior pixel in the
illustration shown in FIG. 2. Where the point is in a boundary
pixel forming the edge or wall of an object, there will be fewer
neighboring pixels, 3 neighboring pixels for a point located in a
corner wall pixel and 5 neighboring pixels for a point located in
an edge or wall pixel apart from a corner.
[0041] FIG. 4 shows the correlation between gray scale values and
the border location as the gray scale values are increased. The
graph shows of the position of the boundary as the intensity of the
boundary pixel goes from a gray scale value 0 to a gray scale value
255. The graph indicates a value of below zero initially because of
the scatter in the data. Nevertheless, the data does show a clear 1
pixel change in border position as the intensity increases over the
stated range.
[0042] The resolution of geometric features and object boundaries
can be improved using the gray scale technique based upon FIG. 2.
If the light intensities K.sub.1, K.sub.2, and K.sub.3 are all 1
and K.sub.7, K.sub.8, and K.sub.9 are all 0, then the boundaries
formed by the light energy from pixels 4, 5, and 6 can be located
with much higher resolution by setting light intensities K.sub.4,
K.sub.5, and K.sub.6 to different light intensities between 0 and
-1.
[0043] Controlling the exposure of an image area (also called an
image plane) can be achieved by controlling certain exposure
parameters. Energy delivered to a target substrate by a radiation
source can be expressed by the relationship, Exposure=Intensity of
the radiation.times.Time of exposure. This relationship is utilized
in controlling exposure parameters. One parameter is controlling
the illumination time of each pixel within the plane that
constitutes the cross-section of a three-dimensional object being
built. Alternatively, another parameter to control the level of
exposure can be varying the light intensity levels, for example by
varying the gray scale levels while holding the exposure time
constant for all boundary pixels. A third parameter is varying the
width of the light intensity of the projected pixels. This can be
achieved by varying the degree of focus of the projected pixels.
This parameter can vary at different locations across the image
area. All three techniques can be employed separately, jointly in
different combinations, or jointly all at the same time. An
individual projector can be characterized for its pixel intensity
width in both the X and Y planes as a function of the pixel
location by measuring the projected intensity with a digital camera
at selected pixels in the image area. This information can then be
stored in the solid imaging apparatus to be employed to obtain
optimal image performance across the image plane and accurate
control of the polymerization boundary in the object being formed.
A fourth additional parameter that may be controlled is the light
intensity versus the age of the lamp in the projector. Since a
lamp's intensity, and therefore the amount of light energy
delivered, decreases with time, the aging effect of the lamp on the
amount of energy delivered must be compensated for by increasing
the exposure time of the lamp to have the same number of photons be
delivered to the projected pixels over time. Intensity is routinely
measured with a radiometer. Lastly, the calibration of these four
parameters to the particular solidifiable photopolymerizable build
material, especially with a resin formulation, must be done since
each solidifiable photopolymer build material has its own
particular characteristics, such as photospeed, that affect
polymerization.
[0044] FIGS. 12 and 13 diagrammatically illustrate the use of
exposures with different levels of gray scale in border pixels,
interior pixels and external pixels in the image area of a
3-dimensional object being fabricated. FIG. 12 shows boundary
pixels covered by a straight line boundary of an object in an image
area with the levels of gray scale values being 0 outside of the
object boundary, ranging from 90 to 240 along the boundary pixels
and being 255 inside the object. FIG. 13 shows pixels covered by a
curved line boundary of an object in an image area with the levels
of gray scale values being again 0 outside of the object, ranging
from 60 to 200 along the boundary pixels and again being 255 inside
the object.
[0045] Regarding the light intensity or gray scale exposure levels
that must be tuned to the particular solidifiable photopolymer
build material being used, refer again to FIG. 2. This tuning is
achieved by adjusting the different light intensities
K.sub.1-K.sub.9 for the pixel at point (x, y) and its eight
neighboring pixels so that the desired accumulated light intensity
K is sufficient to solidify the solidifiable photopolymer build
material at that point based on the time of illumination of each
pixel in the image area forming an object cross-section. When the
accumulated light intensity K equals or exceeds the critical energy
E.sub.c needed to solidify the solidifiable photopolymer build
material, such as a liquid resin formulation, that build material
will solidify and add to the geometry of the object being formed.
Where the accumulated light intensity is less than the critical
energy E.sub.c, the build material will remain in the
non-solidified or liquid state. This relationship is illustrated in
FIG. 3 where the solidifiable photopolymer build material is a
resin formulation.
[0046] For a typical resin the critical energy E.sub.c is much less
that the energy used to expose the layer, perhaps 20% or less. The
E.sub.c for typical resins employed in apparatus of the present
inventor is preferably about 10% to 12% of the layer exposure
energy employed. Looking at FIG. 5, assuming E.sub.c is represented
by the intensity level of 0.2, the width of the region that is
solidified varies according to the light intensity of the edge
pixel. Since the light intensity from a pixel varies with gray
scale, the polymerization of the solidifiable build material
forming the edge or border of a region can be controlled by varying
the gray scale of the border pixels, as shown in FIG. 5.
[0047] In one application of the present invention, the
solidifiable photopolymer build material or resin formulation is
delivered to the imaging area via a radiation transparent flexible
carrier, such as a polypropylene or a polycarbonate film. The
photopolymer is applied in a thin layer to the flexible transport
film, which can be in the form of an endless belt, a reciprocating
film sheet or a retractable and extendible film sheet.
[0048] A digital light projector is the radiation or light source
that projects an image via a plurality of small movable mirrors
with selected pixels for illumination onto an image plane in the
exposure of a cross-section of a three-dimensional object being
formed on a support platform. The support platform is raised and
lowered to bring the cross-sectional layers being formed into
contact with the layer of build material that is deposited on
flexible transport film from a solidifiable photopolymer build
material cartridge. The build can be right side up with the object
being built on top of the support platform or upside down, being
suspended downwardly from the support platform. The cartridge
includes or is in fluid flow communication with a build material
supply reservoir of solidifiable photopolymer build material, such
as a liquid resin formulation, that is applied to the flexible
transport film. A transparent backing plate also can be employed.
The exposure of the image cross-section by illuminating selected
pixels creates a solidified portion of the cross-section of the
three-dimensional object being formed.
[0049] Looking now at FIG. 5, there is shown in a graphical
illustration the amount of polymerization boundary control
obtainable in a projected boundary pixel by combining the use of
varying exposure intensity levels and pixel intensity width or the
degree of projected pixel focus. This creates slight image blurring
and affects the amount of light energy delivered to boundary pixels
to control the polymerization boundary of the solidifiable
photopolymer build material to thereby define the effective
location of the edge or boundary of a feature in a boundary pixel
of an object with sub-pixel resolution. Varying the intensity
exposure levels is achieved by varying the illumination or exposure
time and/or the gray scale exposure levels. This technique achieves
much higher image boundary resolution than is present in just the
pixel resolution. FIG. 5 shows 4 adjacent pixels for which the
exposure intensity of the first pixel has been varied to 5
different levels with the pixels having a full width at half
maximum (fwhm) of 6.7 mils (0.0067 inch) and 5 mils (0.005 inch)
peak to peak separation. Approximately 1 mil (0.001 inch)
resolution is achieved at this spacing.
[0050] FIGS. 6-11 show the 4 adjacent pixels with exposure
intensities varying from 1.0 to 0.0 in 5 equal exposure intensity
decrements, as well as the sum or blended result of the
accumulation of the light intensities in each case. In each FIG.
6-11, the pixels have a full width at half maximum (fwhm) of 6.7
mils (0.0067 inch), which is the width of the Gaussian light
distribution, and a 5 mils (0.005 inch) peak to peak separation.
FIGS. 6-11 all have the pixels' light intensity modeled with a
Gaussian distribution.
[0051] Therefore, from FIGS. 5-11 and the preceding discussion it
can be seen that it is desired and possible to improve the
resolution of the boundaries of an object and features in an object
by using different gray scale values in boundary pixels. FIGS. 14
and 15 show the difference in the edge walls of a three-dimensional
object fabricated using apparatus employing a flexible transport
solid imaging system and process. FIG. 14 shows the rough edge
walls of the near vertical walls of an object fabricated with
exposure levels not employing gray scale. FIG. 15 shows the
improved geometry and smoother edge walls of the same
three-dimensional object fabricated with the same apparatus using
the same flexible transport solid imaging system and process, but
employing exposure levels with different levels of gray scale.
[0052] It should be noted that a projected image is a
two-dimensional image with edges in the X and Y directions. When a
three-dimensional object is fabricated by layering multiple
cross-sectional layers, the fabricated object extends in the Z
direction, or the third dimension. The present invention achieves
edge smoothness in the projected image in individual
two-dimensional cross-sectional image projections. When each
individual two-dimensional cross-sectional image projection
polymerizes the solidifiable photopolymer build material, a solid
layer is formed corresponding to the exposed projected pixel areas
on the solidifiable photopolymer build material. Adhering or
summing superimposed multiple cross-sectional layers one to another
in a build process, the present invention forms a three-dimensional
object with smooth walls that are formed from the plurality of
adhered smooth edges in the individual cross-sectional layers.
[0053] In operation, digital data for the three dimensional object
to be built is sent to the solid imaging system. This is preferably
from a CAD station (not shown) that converts the CAD data to a
suitable digital layer data format and feeds it to a computer
control system or host computer (also not shown) where the object
data is manipulated to optimize the data via an algorithm to
provide on/off instructions for the digital light projector.
Alternatively, this digital data can be received by the solid
imaging systems by digitizing a physical part or from pattern
files. The solid imaging layer data attained by the CAD data or by
digitizing a physical part or sometimes from pattern fills is
preferably processed by the host computer utilizing a slicing
program to create cross-sectional data representative of the
cross-sectional layers of the object to be built. The solid imaging
layer data obtained from pattern data is sometimes not sliced, but
patterns are directly utilized as bit-map cross-sections. These
steps of the present invention are conducted in the same fashion as
in standard stereolithography processes. The slice data obtained
from the slicing program is converted into bit map data. Using a
gray scale software program, the host computer calculates the gray
scale exposure levels required to control the polymerization
boundary of the solidifiable photopolymer build material when the
build material is exposed. A microprocessor controller or computer
in the solid imaging apparatus receives the bit map input of the
object to be formed. An algorithm is then applied to the bit mapped
cross-sectional data by the controller or computer in the solid
imaging apparatus to create the instructions for a controller, such
as a microchip, in the digital light projector. The digital light
projector has previously been characterized for its focus of the
projected light pixels by observing the degree of focus of pixels
across the image area (or plane). The projector has also been
characterized for its light intensity distribution by the use of a
radiometer to record light intensity at selected pixel locations on
the image area and the focus and illumination time have been
adjusted as needed. The illumination time is adjusted based on the
light intensity, age of the lamp in the digital light projector,
and the particular solidifiable photopolymer build material to be
utilized. A radiation transparent build material carrier delivers
the solidifiable photopolymer build material from a supply
reservoir or cartridge to the imaging area. The imaging area is
supported by the support platform that is movably mounted to the
frame of the solid imaging apparatus to extend and retract the
support platform on which the solidifiable photopolymer build
material is polymerized and formed into a three-dimensional object.
The digital light projector illuminates selected pixels in the
desired illumination pattern at desired exposure levels in the
image area within the boundary of the three-dimensional object by
projecting an image representative of the bit mapped
cross-sectional data in the cross-section being formed. The desired
exposure levels are obtained by having characterized the digital
light projector for one or more of the previously discussed
exposure parameters and adjusted the exposure levels as needed in
response to the characterization. The desired exposure levels from
the digital light projector in the image area permit the projected
image to selectively solidify the photopolymer build material and
control the polymerization boundary of the photopolymer build
material in the projected pixilated image through the application
of different exposure parameters to different areas of the image
plane. The support platform is then moved out of contact with the
radiation transparent build material carrier to prepare for the
receipt of fresh or uncured solidifiable photopolymer build
material prior to the next exposure. The steps of delivering the
solidifiable photopolymer build material, illuminating the image
area on the support platform and repositioning the support platform
to receive a fresh supply of solidifiable photopolymer build
material is repeated for each cross-sectional layer until the
three-dimensional object is completed.
[0054] The algorithm, based on experiential data, selects pixels
for a higher gray scale value exposure or illumination that have a
larger portion of the object's feature within a particular pixel.
Although the amount of photopolymer build material solidified at a
boundary will vary for different types of build materials and
different digital light projectors, the relationship between the
gray scale level and amount of photopolymer build material
solidified along a boundary pixel will generally follow the same
pattern. The gray scale value K of a pixel is set according to the
area of the pixel that is inside the part or feature. The gray
scale value K assigned to a pixel is a function of the ratio r
[K=f(r)], where r is the area A' of the portion of a pixel that is
inside the part or feature to the total area A of the pixel or
r=A'/A. However, a gray scale value is needed for boundary pixels
and their neighboring pixels since the ratio is 1 for pixels inside
the part or feature and 0 for pixels outside. The value of the area
of the portion of a boundary pixel that is inside the part or
feature is based on super-sampling and approximates the value of
the area A' of the portion of a pixel that is inside the part or
feature. This super-sampling approach is suitable for use with an
image-based slicing algorithm.
[0055] The algorithm divides a boundary pixel, through which some
portion of the object's feature passes, into a set of sub-pixels
P.sub.ij in a k.times.k matrix or subdivision of the sub-pixels
P.sub.ij. The algorithm then samples each of the sub-pixels
P.sub.ij at its center to determine if it is covered by some
portion of the object's feature. The total area of those sub-pixels
P.sub.ij covered by or within some portion of the object's features
approximates the area A' of the portion of the divided pixel that
is inside the object's feature. For a pixel having k.times.k
subdivisions, the ratio r can be expressed as the sum of the total
area of the sub-pixels P.sub.ij covered by or within some portion
of the object's features divided by k.times.k subdivisions, or
r = .SIGMA. P ij ( covered ) k .times. k . ##EQU00001##
[0056] Using this approach, the boundary resolution of an object
built is determined by the number of subdivisions described above
forming the k.times.k matrix or subdivision. For example, a
4.times.4 matrix will have 16 different gray scale values and the
boundary resolution determined by Pixel Size/4. Similarly, for an
8.times.8 matrix there will be 64 different gray scale values and
the boundary resolution determined by Pixel Size/8.
[0057] An alternative algorithmic approach to the super-sampling
technique just discussed for instructing which pixels for the
projector to illuminate can be based on boundary area calculations.
The area calculations within the boundaries of the projected object
or feature within each pixel use the actual or closely approximated
boundary segments to create distinct geometric shapes within the
boundaries of each pixel. The algorithm will calculate the exact or
approximate areas of each geometric shape to be illuminated within
the boundary segment or segments passing through each pixel and sum
the areas to generate an exact or nearly exact area within the
boundary segment or segments of the object or feature being
projected in each cross-section of the object.
[0058] There still may be visible systematic boundary errors in the
form of observable patterns on object boundaries of objects
fabricated using the described gray scale exposure levels.
Admittedly, these errors will be 1/k times smaller and less visible
by using different gray scale exposure levels for the boundary
pixels, but they may still be observable upon close inspection.
Introducing randomization into the gray scale values at the
boundary pixels can produce a fabricated object that does not have
any observable patterns on its boundaries. This can be accomplished
by adding a random number .DELTA.f within a given range and
assigning f(r)+.DELTA.f as the pixel's gray scale value or by using
a look-up table in which for any given area ratio r, the return
value f(r) from the look-up table varies within a given range.
[0059] As previously stated, an ultraviolet ("UV") digital
radiation projector or a visible digital light projector or a
digital light projector system using both UV radiation and visible
light may be used to cure the solidifiable photopolymer build
material. Any suitable flowable or spreadable medium capable of
solidification in response to the application of an appropriate
form of energy stimulation may be employed in the practice of the
present invention, for example liquids, gels, pastes, or
semi-liquid materials (e.g. mixtures of liquid materials with solid
materials suspended therein), and combinations thereof. Many liquid
state chemicals are known which can be induced to change to solid
state polymer plastic by irradiation with UV radiation or visible
light. A suitable curable photopolymer that may be employed as the
solidifiable photopolymer build material in the practice of the
present invention is Acrylate-24 shown in Table I below. This
formulation exhibited excellent resolution and photospeed when
utilized with a BenQ PB7220 projector. The parts created displayed
outstanding green strength with balanced stiffness and
toughness.
TABLE-US-00001 TABLE 1 Units of Weight Acrylate-24 Weight Percent
(%) PRO 6817 (from Sartomer Company) 4.8 23.02 SR 833S (from
Sartomer Company) 3.5 16.79 Ebecryl 83 (from UCB Chemicals 2.4
11.51 Corp.) PRO 6169 (from Sartomer Company) 5.2 24.94 SR 531
(from Sartomer Company) 3.6 17.27 Irgacure I-907 (From Ciba
Specialty 0.75 3.60 Chemicals, Inc.) Irgacure I-819 (From Ciba
Specialty 0.6 2.88 Chemicals, Inc.) Total 20.85 100.00
Additives can be incorporated into the formulation to promote
release ability from the transparent transport means, such as
silicone acrylate materials.
[0060] It should also be noted that the present invention also can
be employed to fabricate features in an object, such as a holes,
which are smaller than a projector's pixel size. This is
accomplished by adjusting the exposures of the neighboring pixels
so that the critical energy E.sub.c necessary to photopolymerize
the particular photopolymer being used is less than E.sub.c for the
area within a hole and greater than E.sub.c for area outside the
hole. Using this technique of controlling the gray scale values of
the neighboring pixels and that of the pixel containing the hole, a
projector with a resolution, for example, of 0.008 of an inch can
generate a hole with a diameter, for example, of 0.004 of an inch
since only the photopolymer outside the hole will be solidified
while the photopolymer within the hole will remain liquid.
[0061] While the invention has been described above with references
to specific embodiments thereof, it is apparent that many changes,
modifications and variations in the materials, arrangements of
parts and steps can be made without departing from the inventive
concept disclosed herein. For example, the image exposure
parameters control technique could equally well be employed in a
modified stereolithography apparatus having a vat containing the
working surface onto which an image is projected, but employing a
UV radiation, visible light, or combination UV radiation and
visible light digital image projector in place of a laser. An
example of such a stereolithography apparatus that could easily be
modified is a Viper si2.TM. SLA.RTM. system available commercially
from 3D Systems, Inc., the assignee of the present invention. The
image projector employs a controller device and software to control
exposure parameters as described herein to control the positioning
of the polymerization boundary in the boundary pixels in the image
of a projected cross-section of a three-dimensional object being
formed from a solidifiable liquid medium. The liquid medium forms a
fresh layer to be solidified either by a deep dip technique or by
use of a recoater that places a fresh layer of liquid material onto
an already exposed and solidified cross-section of the
three-dimensional object being formed, as described in U.S. Pat.
Nos. 5,902,537 and 6,048,487 both assigned to the assignee to the
present invention. Accordingly, the spirit and broad scope of the
appended claims are intended to embrace all such changes,
modifications and variations that may occur to one of skill in the
art upon a reading of the disclosure. All patent applications,
patents and other publications cited herein are incorporated by
reference in their entirety.
* * * * *