U.S. patent application number 11/171888 was filed with the patent office on 2005-11-17 for ceramic article and method of manufacture therefor.
This patent application is currently assigned to Rapid Ceramic Technologies Ltd. Invention is credited to Bardes, Bruce Paul, Dzugan, Robert.
Application Number | 20050252631 11/171888 |
Document ID | / |
Family ID | 32770939 |
Filed Date | 2005-11-17 |
United States Patent
Application |
20050252631 |
Kind Code |
A1 |
Bardes, Bruce Paul ; et
al. |
November 17, 2005 |
Ceramic article and method of manufacture therefor
Abstract
A ceramic article resulting from a chemical interaction between
a particulate ceramic material and a ceramic matrix material is
described. The ceramic matrix results from at least partial
chemical transformation of a precursor material. A chemical bond
between the ceramic matrix and the particulate ceramic material is
developed during manufacture. The configuration of the ceramic
article is developed through use of a rapid prototyping process. A
ceramic article comprising different compositions in two or more
regions of the article is described. A manufacturing process
comprising the steps employed to produce such a ceramic article is
also described. The ceramic article described herein is
particularly suited for use as a mold for metal casting. The
manufacturing process disclosed herein enables production of such a
mold within a matter of hours, rather than days, as required by
prior art casting technologies.
Inventors: |
Bardes, Bruce Paul;
(Montgomery, OH) ; Dzugan, Robert; (Cincinnati,
OH) |
Correspondence
Address: |
Bruce P. Bardes
7651 Cornell Road
Montgomery
OH
45242
US
|
Assignee: |
Rapid Ceramic Technologies
Ltd
|
Family ID: |
32770939 |
Appl. No.: |
11/171888 |
Filed: |
June 30, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11171888 |
Jun 30, 2005 |
|
|
|
10357053 |
Feb 3, 2003 |
|
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Current U.S.
Class: |
164/349 ;
164/4.1 |
Current CPC
Class: |
C04B 35/185 20130101;
C04B 2235/3418 20130101; C04B 2235/48 20130101; C04B 2235/483
20130101; C04B 2235/5436 20130101; C04B 2235/665 20130101; C04B
2235/80 20130101; C04B 2235/85 20130101; C04B 2235/6028 20130101;
B33Y 80/00 20141201; C04B 2235/6026 20130101; C04B 35/64 20130101;
C04B 35/117 20130101; C04B 35/14 20130101; B33Y 10/00 20141201;
C04B 2235/3217 20130101; C04B 35/78 20130101 |
Class at
Publication: |
164/349 ;
164/004.1 |
International
Class: |
B22C 009/02 |
Claims
We claim:
1. A ceramic article resulting from a chemical interaction between
a particulate ceramic material and a ceramic matrix material;
wherein: the particulate ceramic material and a precursor material
are intermixed, so that particles of the particulate ceramic
material are in intimate contact with the precursor material; at
least a portion of the precursor material is chemically transformed
to form the ceramic matrix material; the chemical interaction
between the particulate ceramic material and the ceramic matrix
material produces a chemical bond therebetween; and the ceramic
article has a configuration developed through use of a rapid
prototyping process.
2. The ceramic article as recited in claim 1, wherein the chemical
interaction produces a new chemical species, consuming at least a
portion of the particulate ceramic material and at least a portion
of the ceramic matrix material in the chemical interaction.
3. The ceramic article as recited in claim 1, wherein the precursor
material is provided in liquid form.
4. The ceramic article as recited in claim 3, wherein the precursor
material comprises a silicone resin.
5. The ceramic article as recited in claim 4, wherein the precursor
material is chemically transformed by oxidation, and wherein the
ceramic matrix material comprises at least one member of a group
consisting of silica and silicates.
6. The ceramic article as recited in claim 1, wherein the precursor
material is provided in gaseous form.
7. The ceramic article as recited in claim 1, wherein the
particulate ceramic material comprises a plurality of chemical
species.
8. The ceramic article as recited in claim 1, wherein the precursor
material comprises a plurality of chemical species.
9. The ceramic article as recited in claim 1, wherein the rapid
prototyping process comprises stereolithography.
10. The ceramic article as recited in claim 1, wherein the rapid
prototyping process comprises three-dimensional printing.
11. The ceramic article as recited in claim 1, wherein the rapid
prototyping process comprises fused deposition modeling.
12. The ceramic article as recited in claim 1, wherein the rapid
prototyping process comprises selective laser sintering.
13. The ceramic article as recited in claim 1, wherein the ceramic
article is a core for a metal casting process.
14. The ceramic article as recited in claim 1, wherein the ceramic
article is a mold for a metal casting process.
15. The mold for a metal casting process as recited in claim 14,
wherein the mold comprises an integral core.
16. A ceramic article resulting from chemical interactions between
at least one particulate ceramic material and at least one ceramic
matrix material; wherein: a first particulate ceramic material and
a first precursor material are intermixed, so that particles of the
first particulate ceramic material are in intimate contact with the
first precursor material, thereby creating a first intermixed
material; at least a portion of the first precursor material is
chemically transformed to form a first ceramic matrix material; the
chemical interaction between the first particulate ceramic material
and the first ceramic matrix material produces a chemical bond
therebetween; the first intermixed material is employed to
manufacture a portion of the ceramic article; a second particulate
ceramic material and a second precursor material are intermixed, so
that particles of the second particulate ceramic material are in
intimate contact with the second precursor material, thereby
creating a second intermixed material; at least a portion of the
second precursor material is chemically transformed to form a
second ceramic matrix material; the chemical interaction between
the second particulate ceramic material and the second ceramic
matrix material produces a chemical bond therebetween; the second
intermixed material is employed to manufacture a portion of the
ceramic article; and the ceramic article has a configuration
developed through use of a rapid prototyping process.
17. The ceramic article as recited in claim 16, wherein the first
precursor material and the second precursor material are
identical.
18. The ceramic article as recited in claim 16, wherein the first
ceramic matrix material and the second ceramic matrix material are
identical.
19. The ceramic article as recited in claim 16, wherein first
particulate ceramic material and the second particulate ceramic
material are identical.
20. The ceramic article as recited in claim 16, wherein the first
intermixed material and the second intermixed material comprise the
same components, in different proportions.
Description
RELATED PATENT DOCUMENT
[0001] This application is a divisional application of pending
application Ser. No. 10/357,053, filed Feb. 3, 2003. Priority is
claimed for this application, based on the filing date of said
pending application.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates generally to ceramic articles and
methods for the manufacture thereof. The invention is particularly
suited for use in manufacturing ceramic molds for metal casting
processes.
[0004] 2. Description of Related Art
[0005] The origins of many of the commonly used methods of
manufacturing ceramic articles have been lost in antiquity.
Naturally, many of these methods have been updated over the years,
but the essential elements of many methods of manufacturing ceramic
articles haven't changed very much in centuries. Most of the common
methods employ a vehicle, typically water, for facilitating
manipulation of ceramic particles into whatever configuration is
appropriate to a particular application. In their textbook,
"Manufacturing Engineering and Technology" (Fourth Edition),
Kalpakjian and Schmid identify three groups of ceramic
manufacturing methods: casting, plastic forming and pressing. All
of the manufacturing methods in the casting and plastic forming
groups, and about half of the methods in the pressing group employ
a vehicle such as water; these may be termed wet methods. In terms
of the tonnage of ceramic articles produced by these methods, the
overwhelming majority is produced by wet methods. The processing
methods that do not employ a vehicle, or dry methods, are analogous
to powder metallurgy methods.
[0006] Wet methods for manufacturing ceramic articles contain steps
that remove the vehicle essential to the manufacturing process.
Those steps may include drying at a relatively low temperature to
evaporate most of the vehicle, and baking at a higher temperature
to evaporate the remaining vehicle. Where water is the vehicle,
drying typically occurs below 212.degree. F. (100.degree. C.), so
that the water does not boil, which could cause a ceramic article
to literally explode. Water present in ceramic articles may be
present as a vehicle, or as water of hydration. Removal of such
water is a slow process, particularly if the ceramic article has
substantial thickness. In a subsequent manufacturing step, a
ceramic article is typically fired at a much higher temperature,
thereby creating strong bonds between adjacent ceramic
particles.
[0007] Dry methods for manufacturing ceramic articles contain steps
that simply press the ceramic particles together under high
pressure. The pressing can be done at room temperature, or at
temperatures high enough for diffusion of the atomic or ionic
species present in the ceramic to be appreciable. In the latter
case, no subsequent firing is necessary. Where pressing is done at
room temperature, it is followed by sintering, which consolidates
the powder into a dense article.
[0008] During drying, firing or hot pressing operations, a ceramic
article can shrink as much as 20 percent. Such shrinkage can be a
significant problem if the nature of the ceramic article mandates
close dimensional tolerances.
[0009] Although it has much broader utility, the present invention
is particularly applicable to ceramic molds for metal casting
operations. In such applications, the slow process of removing
water from the ceramic can significantly lengthen the process of
developing a new product. Someone designing a cast product may have
to wait several days to see a pilot model of whatever he has
designed. The same lengthy cycle is required for each iteration in
the development of a product.
[0010] Kalpakjian and Schmid describe many casting processes, but
three such processes are germane to the present discussion:
plaster-mold casting, ceramic-mold casting and investment casting.
In plaster-mold casting, a slurry of plaster of Paris and other
ingredients is poured over a pattern representing the configuration
of one half of the finished part. The resulting half-mold is
allowed to dry. The pattern is then removed from the half-mold. A
mating half-mold is made in a similar fashion. The manufacture of a
mold for ceramic-mold casting is similar, except that the slurry
contains ceramic particles selected for their resistance to the
high temperatures characteristic of metal casting operations, plus
an organic binder that holds the ceramic particles in position
until firing. Note that in either process, casting design is
limited by the requirement for removing the pattern, which is
typically made from aluminum, brass, plastic and the like.
[0011] In the investment casting process, a disposable pattern is
made from a material such as wax or plastic. The pattern is then
embedded in a ceramic investment. The investment can be in the form
of a plaster mold, or it can be a shell of multiple layers of
ceramic material. The latter version of the process provides higher
temperature capability than a plaster investment. It is therefore
preferred for many applications, even though a period of ten days
can be required to make a mold. The investment casting process has
been known in the Far East for more than 4000 years.
[0012] Given the desirability of manufacturing articles as quickly
as possible, particularly in the contexts of product development
and custom product design, there has been a continuing search for
more rapid methods for making articles. Such methods can be
collectively identified as rapid prototyping (RP) methods.
Kalpakjian and Schmid have identified seven such RP methods, each
involving the addition of material to the object being
manufactured.
[0013] One example of how rapid prototyping can be employed to
accelerate development of new products is the use of
stereolithography (SLA) to make patterns for investment casting.
SLA was developed by Hull (U.S. Pat. No. 4,575,330). In a common
embodiment of the SLA process, a thin film of liquid photosensitive
polymer resin is spread on a build table. A localized spot of
light, preferably a laser beam, is moved over the film of resin,
causing polymerization of the resin wherever the light strikes it.
After achieving polymerization in all desired regions of this layer
of resin, the build table is lowered into a vat of resin, a new
layer of resin is spread over the first layer, and the process is
repeated. Movement of the spot of light is controlled by a computer
system, which causes beam movements corresponding to the
configuration of the desired workpiece, as defined in a
computer-aided drafting (CAD) file. While SLA is useful in making a
pattern for investment casting, it does not address the matter of
making a mold for investment casting. In an alternate form of the
SLA process, the workpiece is lifted from a shallow bath of resin;
polymerization occurs by directing the spot of light through a
window at the bottom of the bath.
[0014] Crump (U.S. Pat. Nos. 5,121,329 and 5,340,433) has developed
an RP process, which he termed fused deposition modeling (FDM). In
the FDM process, a thin filament of thermoplastic or wax material
is heated and extruded through a small orifice in a movable
deposition head. Molten (or nearly molten) material extruded
through the deposition head impinges previously deposited material,
and solidifies upon contact therewith. Movement of the deposition
head is controlled by a computerized control system.
[0015] Deckard (U.S. Pat. No. 5,639,070) has developed another RP
process, which he terned selective laser sintering (SLS). In the
SLS process, a thin layer of powder is spread over a build table. A
laser beam is moved over the layer of powder so that the powder
particles are sintered together wherever the laser beam has been
aimed. After the desired localized sintering is achieved on the
first layer of powder, a second layer of powder is spread over the
first, and the process is repeated. Ceramic articles can be made by
the SLS process. However, the localized heating to cause sintering
can also cause sufficient thermal shock to crack the workpiece. In
a variation of the SLS process, Langer et al (U.S. Pat. Nos.
5,460,758 and 6,155,331) have taught the use of powder particles
coated with a resin layer. Their teachings indicate that the green
strength of a fabricated article can be increased without the
thermal stresses that often exist in an article made by the SLS
process. However, both the Deckard method and the variation
described by Langer et al are vulnerable to considerable shrinkage
during manufacture.
[0016] Sachs et al (U.S. Pat. No. 5,204,055) have developed yet
another RP process, which they termed three-dimensional printing
(3D printing). In the 3D printing process, a thin layer of powder
is spread over a build table. A liquid binder material is
selectively deposited over designated regions of the layer of
powder. A print head generally similar in function to a computer
ink jet printer is useful for depositing the binder. After
deposition of binder on the first layer of powder has been
completed, a second layer of powder is spread over the first, and
the process is repeated. After the entire article has been thusly
created, it is sintered to achieve whatever densification is
appropriate. Ceramic articles can be made by the 3D printing
process. However, considerable shrinkage can occur during
sintering, so that an article that is dense enough to have useful
strength can be too distorted to serve its intended function.
Kalpakjian and Schmid disclose that in a variation on the 3D
printing process, the binder liquid can contain colloidal silica
particles. However, this variation is still subject to shrinkage
during sintering.
[0017] Notwithstanding advocates of SLS and 3D printing, none of
the above-referenced RP technologies, nor any other known RP
technology, is fully appropriate for the manufacture of ceramic
metal casting molds.
[0018] Szweda, Millard and Harrison (U.S. Pat. Nos. 5,306,554,
5,488,017 and 5,601,674) teach a method for developing a ceramic
matrix for a composite material comprising ceramic reinforcing
fibers in a ceramic matrix. Specifically, Szweda et al teach the
use of a silicone resin precursor as a means of achieving a ceramic
matrix that is substantially silica and/or silicates. For their
application, it was desirable that the entire composite article
would be laid up in its intended configuration before the silicone
resin precursor was transformed to a ceramic matrix. The context of
the present invention, namely, fabricating a ceramic mold for metal
casting, preferably by RP technology, working from a CAD file of
the finished part, presented process requirements that were
significantly contrary to the problems addressed by Szweda et
al.
SUMMARY OF THE INVENTION
[0019] Briefly, the present invention provides a ceramic article
that can be produced in a short time, employing a novel combination
of chemical transformations and rapid prototyping processes. The
key feature of the invention is the use of a precursor material
that is amenable to processing by one of a variety of rapid
prototyping processes. The precursor material is typically provided
as a liquid that can be transformed to a solid, either during or
immediately following fabrication of an article by a rapid
prototyping process. This attribute of the precursor material can
be achieved by employing a monomeric resin that is polymerized
during processing. The polymerized resin is subsequently
transformed into a ceramic matrix material, preferably by
oxidation. Particulate ceramic material that had been intermixed
with the liquid precursor material becomes embedded in the ceramic
matrix material. Further chemical interaction between the
particulate ceramic material and the ceramic matrix material
develops a chemical bond therebetween. Still further chemical
interaction therebetween can create a new chemical species. The
sequence of chemical interactions typically results in
transformation of substantially all of the precursor material into
ceramic matrix material. Depending upon the nature of the specific
materials selected for a particular application, formation of the
new chemical species may consume part, or all, of either the
particulate ceramic material or the ceramic matrix material.
[0020] In one embodiment of the present invention, the precursor
material is a photosensitive silicone resin. The preferred resin
has low enough viscosity that is can be conveniently processed as a
liquid, prior to polymerization. Polymerization of the resin is
initiated by exposure to ultraviolet light. Upon exposure of the
polymerized silicone resin to air at a moderately elevated
temperature, it is oxidized to form silica and/or silicates, and
gaseous byproducts such as water vapor and carbon dioxide. The
resulting silica exists as a matrix that surrounds and supports
particulate ceramic material that had been intermixed with the
monomeric form of the precursor resin material. Further treatment
of this product at an elevated temperature can cause the silica
matrix to interact with the particulate ceramic material. A variety
of particulate ceramic materials, and even mixtures of two or more
particulate ceramic materials can be employed, depending on the
nature of the intended application of the resulting ceramic
article.
[0021] The process disclosed herein may use several rapid
prototyping processes, and variations thereof. Several embodiments
of the present invention, each relating to a particular RP process,
are described hereinbelow. Each of these embodiments of the present
invention offers certain advantages. However, the same fundamental
chemical interactions and transformations occur during each
embodiment of the process. Thus, the process of the present
invention must be viewed broadly, to encompass these, and other,
rapid prototyping processes.
[0022] One particularly useful application of the ceramic article
of the present invention is as a mold for casting metallic
articles. Through the technology disclosed herein, a ceramic mold
can be produced in a matter of hours, rather than the several days
that might be required to produce such a mold by conventional
investment casting technology. Further, a mold made by the process
of the present invention can have a core as an integral part
thereof, thus avoiding the need for a separate core, as is required
in conventional investment casting technology.
[0023] Specific features of the ceramic article of the present
invention are detailed in the following Detailed Description of the
Invention and the accompanying drawings. Several embodiments of the
present invention are also described therein. Those having ordinary
skill in the ceramic and metal casting arts will recognize
alternative means for practicing the present invention, all of
which are deemed to be equivalent to and to fall within the scope
of the present invention.
DESCRIPTION OF THE DRAWING
[0024] FIG. 1 is a schematic representation of the process of the
present invention, showing particulate ceramic material and
precursor material, transformation of the precursor material to a
ceramic matrix material, and chemical interaction between the
particulate ceramic material and the ceramic matrix material to
form a new chemical species.
DETAILED DESCRIPTION OF THE INVENTION
[0025] Several prior art casting and rapid prototyping processes
have been described hereinabove. Understanding these examples of
prior art is deemed useful in understanding the present invention.
Note that the prior art casting processes described hereinabove
include drying operations, where water contained in ceramic molds
made by wet ceramic processes is removed. Such drying operations
are very time-consuming, typically requiring several days. The
processes of the present invention eliminate or significantly
reduce the drying time associated with these prior-art casting
processes.
[0026] The ceramic article of the present invention is
advantageously described with reference to the Figure described
hereinabove. The manufacturing process for that ceramic article is
likewise advantageously described with reference to the Figure.
[0027] The chemical transformations that typically occur during
practice of the present invention are illustrated in FIG. 1, which
comprises four schematic micrographs of the same region in a
material as it might exist at various stages of the process. FIG.
1a illustrates the intermixed combination of particulate ceramic
material 10 and monomeric precursor material 20. Note that the
precursor material is in intimate contact with the particulate
ceramic material. FIG. 1b illustrates the effect of polymerizing
the precursor material 20 shown in FIG. 1a to a polymer matrix
material 21. FIG. 1c illustrates the chemical transformation of the
polymer matrix material 21 shown in FIG. 1b to a ceramic matrix
material 22. Using the process and materials described herein, and
perhaps other materials as well, a chemical bond between the
ceramic matrix material 22 and the particulate ceramic material 10
is achieved thereby. The transformation typically produces gaseous
byproducts such as water vapor and carbon dioxide; pores 30 are
typically formed during the transformation. FIG. 1d illustrates the
formation of a new chemical species 35 from reaction of the
particulate ceramic material 10 with the ceramic matrix material
22. In FIG. 1d; pockets of unreacted particulate ceramic material
are shown at 11 and pockets of unreacted ceramic matrix material
are shown at 23. For simplicity, it assumed that the size, shape
and distribution of pores 30 are unaffected by the formation of the
new chemical species 35. This assumption is an oversimplification,
for the chemical diffusion necessary to achieve the formation of a
new chemical species is quite sufficient to achieve movement, shape
change and even consolidation of the pores.
[0028] In one presently preferred embodiment of the present
invention, the monomeric precursor material 20 is a low-viscosity
silicone resin. The resin also contains a photosensitive substance
that initiates polymerization of the resin when it is exposed to
light, preferably ultraviolet light. Even though the resin is
initially intermixed with particulate ceramic material at the
outset of the present manufacturing process, the low viscosity of
the resin makes the resulting mixture amenable to processing by SLA
technology, in a manner similar to that described by Hull. Because
monomeric silicone resins are typically produced as two separate
components, which are mixed together shortly before use, the
premixed resin is perishable, having a rather short working life.
Accordingly, the inverted embodiment of the SLA process is deemed
preferable for the present invention, because a much smaller volume
of the perishable precursor material is required in this
configuration.
[0029] Polymerizing the silicone resin 20 results in a solid
substance 21 that has a modest amount of structural strength, at
least enough to hold the article being made together for further
processing. In the next step of processing, the solid silicone
substance 21 is oxidized to form silica and/or silicates, shown at
22. Heating the solid silicone substance 21 in air at temperatures
in the range of 1100-1400.degree. F. (550-750.degree. C.) is
generally sufficient for this purpose. The preferred temperature
depends upon many factors, including the specific silicone resin
employed in the process, size of the workpiece, and acceptable
distribution of porosity 30 in the workpiece. Temperatures at the
high end of this range favor rapid oxidation, but at the risk of
rapid formation of gaseous byproducts such as steam and carbon
dioxide, and such rapid formation may cause unacceptably large
pores and/or damage to the ceramic article. Further heating, at a
higher temperature, can cause the silica 22 to react with the
particulate ceramic material 10 to form a new chemical species 35.
If the particulate ceramic material is alumina, the new chemical
species will be inullite. The appropriate temperature for this
reaction depends on what material(s) comprises the particulate
ceramic material 10. According to Szweda et al, temperatures as
high as 2550.degree. F. (1400.degree. C.) may be appropriate. It
should be noted that if the intended application of the ceramic
article admits to a structure comprising ceramic particles in a
matrix of silica, this last step could be omitted from the process.
However, silica softens at relatively low temperatures, much lower
than mullite, for example, so that interaction between alumina
particles with a silica matrix to produce mullite is useful in
extending the high temperature capability of the completed ceramic
article.
[0030] In the context of the present invention, it is contemplated
that substances other than silicone resin can be incorporated in
the precursor material. For example, Szweda et al have shown the
utility of mixing a moderate percentage of an epoxy resin into the
silicone resin. Other precursor materials that transform to ceramic
materials such as alumina can be mixed into the silicone resin.
[0031] As indicated above, and illustrated in FIG. 1, the monomeric
precursor material 20 is intermixed with particulate ceramic
material. That ceramic material can be a mixture of two or more
chemical species. Under most circumstances, the particulate ceramic
material will be comprised primarily of the species that will
become an essential component of the completed ceramic article. In
a presently preferred embodiment of the present invention, that
essential component is alumina. However, a wide variety of other
particulate materials can be employed. For example, the use of
silica particles can result in a ceramic article that is
predominantly silica.
[0032] The preferred size of the particulate ceramic material
depends on several factors. It is essential that the individual
ceramic particles must be smaller than the thickness of the layer
of precursor material applied to the build table. That thickness is
typically about 5 mils or less. [One mil is 0.001 inch, or 25
micrometers, or microns.] Unduly small ceramic particles create
problems in handling. For the purposes of the present invention, it
is believed that a preferred particle size lies between about 0.03
mil (0.75 micron) in diameter and about 3 mils (75 microns) in
diameter. It is believed that a more preferred particle size lies
between about 0.04 mil (1 micron) and about 2 mils (50
microns).
[0033] Other species of particulate ceramic material can be
employed in the method of the present invention. For some purposes,
it can be useful to provide multiple chemical species in the
particulate ceramic material 10 that subsequently react with
silica, specifically to produce a final structure that is a
three-component ceramic compound, or a structure comprising two or
more distinct phases. Also, Szweda et al have taught that minerals
having a lathy-type structure, notably pyrophyllite, are very
useful in controlling shrinkage that can occur during high
temperature processing.
[0034] Another embodiment of the present invention incorporates an
RP process similar to 3D printing. In this embodiment, particulate
ceramic material is spread on a build table, and droplets of
silicone resin are "printed" wherever needed to create solid
material in the finished part. The intermixing of precursor
material and particulate ceramic material occurs at this point. The
silicone resin is then polymerized by flooding the entire printed
layer with light, preferably ultraviolet light. The method of the
present invention differs from that of Sachs et al, in that Sachs
et al teach the use of a binder that is largely evaporated or
burned up in subsequent processing, leaving little or no useful
material to be incorporated into the ceramic article, whereas the
present process employs a precursor that becomes an integral
component of the ceramic article.
[0035] In another embodiment of the present invention, the
particulate ceramic materials are provided as very small particles,
betveen about 0.0004 mil (0.01 micron) and 0.4 mil (10 microns) in
diameter. These particles are intermixed with the precursor
material, and the mixture is deposited onto a build table (or a
previously printed layer) by a printer that is generally similar to
an ink jet computer printer. After each layer is deposited, it is
bathed in light, preferably ultraviolet light, to polymerize the
precursor material. Although this embodiment permits the use of
rapid printing technology, for computer printers routinely provide
printing rates of 10 pages per minute, the effective build rate of
this embodiment is limited by the thickness of each deposited
layer. This embodiment bears some similarity to both FDM and 3D
printing processes, but is distinct from either.
[0036] In the context of the present invention, the precursor
material is intermixed with the particulate ceramic material, to
bring the two substances into intimate contact. In theory, the term
"intimate contact" would imply that each individual ceramic
particle would be completely coated with precursor material.
However, achieving such a condition in a production manufacturing
process is, practically speaking, impossible. Thus, the term
"intimate contact" must be interpreted broadly, to indicate that
reasonable efforts to intermix the precursor material and
particulate ceramic material are taken. It is assumed that intimate
contact is achieved in the FDM and 3D printing process, as
described above. The term is also taken to include the possibility
that wetting agents to facilitate such intimate contact can be
included in the precursor material.
[0037] Where the porosity in the ceramic article made by the
process of the present invention is deemed objectionable, such
porosity can be filled using a variety of methods known to those
skilled in the ceramic arts. Likewise, a selected portion of the
surface of the ceramic article can be coated, either to minimize
surface-connected porosity, or to impart some special properties to
that portion of the surface. A variety of appropriate methods for
surface coating are known to those skilled in the ceramic arts.
Although the method mentioned herein are known, the application
thereof to the novel ceramic articles of the present invention is
novel.
[0038] The present invention also contemplates the possibility of
depositing at least two different substances during the RP
processing. The substances might differ in the combination of
chemical species included in the particulate ceramic material, or
they might differ in the nature and/or chemical composition of the
precursor material. In such an embodiment of the present invention,
a variation in mechanical and/or physical properties between
different regions of the resulting ceramic article can be achieved.
The two different substances can be delivered through a deposition
apparatus analogous to the print head of a computer printer capable
of color printing. The substances might also be completely
different in chemical nature, i.e., one substance can be a mixture
of particulate ceramic material and precursor material, as
described herein, and the other can be a polymeric material that
would be burned away during subsequent processing. The latter would
be useful in building a ceramic article that comprises overhanging
features that would be unsupported during deposition, but for the
presence of a disposable support deposited during the manufacturing
process.
[0039] One particular class of ceramic articles, namely molds for
metal casting, represents a useful embodiment of the present
invention. As noted above, several RP technologies can be used for
making patterns for investment casting from a polymeric material.
The time-consuming process of making a ceramic mold from such a
pattern is no different than making a similar mold from a pattern
made by any other process. The slow layer-by-layer process for
building a mold is still commonly employed. With the process of the
present invention, a ceramic mold can be produced directly from a
CAD file describing the configuration of the finished metal
casting. Current computer technology permits enlarging the size of
the finished metal casting to compensate for metal shrinkage during
casting. It also permits reversing the sense of the part, so that
the exterior surface of the metal casting defines the interior
surface of a ceramic mold, and also adding an appropriate thickness
to that surface, to define the configuration of a ceramic mold. The
process of the present invention is employed to create a ceramic
article having a configuration defined by the CAD file containing
the aforementioned modifications.
[0040] If the configuration of the finished metal casting comprises
holes and/or internal passages, conventional metal casting
processes typically employ cores to achieve such features.
Coremaking is a relatively slow process, and it necessarily
precedes the manufacture of any mold with which a core is used. In
the course of making a casting of a new design, and that casting
requires a core, one must first make a core box, then make a core
therefrom. Making a core box is typically expensive and
time-consuming. An embodiment of the present invention is a core
for virtually any casting process. Although a ceramic core is
typically avoided in production casting operations, due to the cost
of such a core, making a core in accordance with the present
invention within a few hours may outweigh cost considerations,
particularly in the context of product development. Also, a core
made in accordance with the present invention can be hollow, or
comprised of two or more ceramic materials, wherein the surface
material could be harder and/or stronger than the interior
material. Either of these alternative embodiments can facilitate
removal of the core from a finished casting.
[0041] In conventional investment casting operations, a wax pattern
is typically made by injecting wax into a die cavity that contains
a previously fabricated ceramic core. Whenever a core is employed
in making a casting, provisions for properly situating that core
within the mold cavity must be made. For most casting processes,
this typically requires the use of projections on the core itself
and mating core prints in the mold cavity. For investment casting,
the mating core prints are situated in the die cavity used to make
the wax pattern. The process of the current invention admits to the
manufacture of mold features corresponding to such holes or
internal features simultaneously with manufacture of the mold
itself. By permitting in situ fabrication of a core during
manufacture of the ceramic mold, the process of the present
invention eliminates the time needed to fabricate a separate core,
and eliminates the dimensional inaccuracies inherent in situating a
core within the mold cavity.
[0042] Although the idea of producing a ceramic mold for metal
casting by RP technology has been previously disclosed by Sachs et
al, and by Kalpakjian and Schmid, the concept of employing a
precursor material in lieu of a disposable binder is novel. Note
that the use of a precursor material, according to the process of
the present invention, materially contributes to controlling
dimensions of the completed ceramic mold.
[0043] In one embodiment of the present invention, an article to be
cast in metal is described in a CAD file. That CAD file is
converted into a second computer file, wherein the sense of the
second file is reversed, relative to the CAD file. In other words,
the second file describes an article containing a cavity that
corresponds exactly to the article to be cast. The second file is
used to control an RP machine, in which a ceramic article is made.
The raw materials for the ceramic article are particulate alumina
and a low-viscosity silicone resin. The operation of the RP machine
causes the silicone resin to be polymerized, yielding a
self-supporting object comprising alumina particles in a silicone
matrix. In a first subsequent process, the object is heated in air
to about 1250.degree. F., for a time long enough to oxidize the
silicone matrix to a silica matrix. In a second subsequent process,
the object is heated to a higher temperature, about 2500.degree. F.
(1370.degree. C.) for a time sufficient to react the alumina
particles with the silica matrix. The result of this chemical
interaction is a ceramic structure that is principally comprised of
mullite, a reaction product of alumina and silica. The mullite
object is appropriate for use as a mold for casting the desired
metal article. The entire process is completed within a few days.
There is no need to machine a die for molding a wax pattern, and no
need to build layer upon layer of ceramic material around the wax
pattern. Thus, two time-consumilg steps in the conventional
investment process are avoided.
[0044] While several embodiments of the present invention have been
described herein in order to better illustrate the principles and
applications thereof, it is understood that various modifications
or alterations can be made to the present invention without
departing from the true scope of the invention set forth in the
appended claims.
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