U.S. patent application number 09/802274 was filed with the patent office on 2001-10-25 for method and apparatus for fabrication of casting from patterns.
Invention is credited to Penn, Steven M., Wright, Tommy D..
Application Number | 20010032713 09/802274 |
Document ID | / |
Family ID | 22423935 |
Filed Date | 2001-10-25 |
United States Patent
Application |
20010032713 |
Kind Code |
A1 |
Penn, Steven M. ; et
al. |
October 25, 2001 |
Method and apparatus for fabrication of casting from patterns
Abstract
A stereolithography machine is employed to solidify a pattern
including a series of stacked grids forming the pattern having an
outer skin and inner labyrinth and any number of gates. The hollow
pattern is mounted on a central sprue through runners at the gate
of the pattern. The pattern is dipped in ceramic slurry and sifted
in a refractor grain in order to coat the hollow pattern. The
hollow pattern is melted to form a mold.
Inventors: |
Penn, Steven M.; (Plano,
TX) ; Wright, Tommy D.; (Irving, TX) |
Correspondence
Address: |
TEXAS INSTRUMENTS INCORPORATED
P O BOX 655474, M/S 3999
DALLAS
TX
75265
|
Family ID: |
22423935 |
Appl. No.: |
09/802274 |
Filed: |
March 8, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09802274 |
Mar 8, 2001 |
|
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08126272 |
Sep 24, 1993 |
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Current U.S.
Class: |
164/34 ;
164/45 |
Current CPC
Class: |
B33Y 10/00 20141201;
Y02P 10/25 20151101; Y02P 10/292 20151101; B29C 33/3842 20130101;
B33Y 80/00 20141201; B22C 9/043 20130101; B33Y 30/00 20141201; B29C
64/135 20170801; B22C 7/023 20130101 |
Class at
Publication: |
164/34 ;
164/45 |
International
Class: |
B22C 007/02; B22C
009/04 |
Claims
What is claimed is:
1. A method for fabrication of casting patterns comprising the
steps of: receiving an investment pattern having an outer skin, a
fill riser and an inner labyrinth; curing said investment pattern
in an ultraviolet oven; positioning said investment pattern on a
support; coating said investment pattern in a coating solution;
sifting said coated investment pattern in a refractory medium to
form a mold; removing said investment pattern; filling said mold
with casting material in order to form a casting; and removing said
mold to release said casting.
2. A method for fabrication of casting patterns as in claim 1,
wherein said receiving step includes the step of draining said
inner labyrinth of ultraviolet material.
3. A method for fabrication of casting patterns as in claim 1,
wherein said method further comprises the step of forming the
investment pattern from at least two stacks.
4. A method for fabrication of casting patterns as in claim 1,
wherein said method step comprises the step of forming the
investment pattern from a first stack having a first predetermined
pattern and a second stack having a second predetermined
pattern.
5. A method for fabrication of casting patterns as in claim 1,
wherein said method further comprises the step of flushing said
mold.
6. A method for fabrication of casting patterns as in claim 1,
wherein said method further comprises the step of flushing said
mold -with air and water.
7. A method for fabrication of casting patterns as in claim 3,
wherein said forming step further comprises the step of forming
said investment pattern with two stacks, each of said stacks having
a different cross section.
8. A method for forming a pattern adapted to be used in the
production of casting patterns, comprising the steps of:
positioning a first grid to form a first portion of said pattern,
said grid having a first cross section; positioning a second grid
to form a second portion of said pattern, said grid having a second
cross section; arranging said first and said second grid so that
said second cross section is not aligned with said first cross
section; and curing said pattern in an ultraviolet light
source.
9. A method for forming a pattern as in Claim 8, wherein said
curing step includes the step of curing said pattern in an
ultraviolet oven.
10. A investment pattern adapted to be used in the production of
casting patterns, said investment pattern comprises: a first stack
forming a portion of investment pattern, said first stack having a
first cross section; a second stack forming a portion of the
investment pattern, said second stack having a second cross
section; a gate to drain said investment pattern, said first stack
and said second stack being positioned adjacent so that liquid
ultraviolet solution flows through said gate.
11. An investment pattern as in claim 10, wherein each of said
grids comprises at least two elements coupling at least two sides
of each of said grids.
12. An investment pattern as in claim 11, wherein each of said at
least two elements are substantially perpendicular to the other of
said at least two elements.
13. An investment pattern as in claim 12, wherein a first element
of said first grid and a second element of said second grid forms
an angle of substantially 45.degree..
Description
TECHNICAL FIELD OF THE INVENTION
[0001] The present invention relates to investment casting
techniques, and more particularly, to methods and apparatus for
stereolithography, or providing the rapid fabrication of casting
prototype patterns.
BACKGROUND OF THE INVENTION
[0002] The casting industry has long used heat-disposable solid
patterns in the making of metal objects. The process of investment
casting is historically known as the "lost wax" process. This
process is performed by supplying a wax pattern of the item to be
cast. A geometric cavity results after the solid pattern is
encapsulated in ceramic and then removed by melting of the wax of
the pattern. The cavity is filled with metal or other casting
material which then takes the exact shape of the original pattern.
Other pattern materials have been tried such as wood, foam, and
plastic, but these types of pattern materials are more difficult to
remove by heating than the wax. A requirement of any casting method
is that the ceramic encapsulant or mold remains intact without
cracking during the pattern melt or burn-out.
[0003] Stereolithography, or rapid prototyping, enables
construction of plastic patterns in a short time period and without
tooling. The stereolithography patterns are made of polymer which
is solidified on the exterior of the pattern by scanning a UV
(ultraviolet) laser according to a three dimensional data in a
computer database. The pattern is produced one layer at a time by
the laser. The polymer remains liquid in the interior of the
pattern until the pattern is further cured, for example by curing
the pattern in a UV oven. Solid patterns produced in this manner
have been used in the casting process with great difficulty,
primarily due to the high burn-out temperatures of resin and high
thermal expansion of the pattern. The ceramic shell typically
cracks or breaks as stereolithography patterns expand outward
during the bum-out phase. The cracking or breaking is a result of
weak or incomplete portion of the pattern being produced one layer
at a time. This cracking or breaking normally occurs at the
position where two layers are joined together. In addition, the
polymer does not burn cleanly or completely, leaving the end
product casting contaminated or of low quality due to the ash of
the burned polymer.
SUMMARY OF THE INVENTION
[0004] The hollow patterns generated with this invention are
compatible with existing casting techniques. During the burn-out
process, the pattern's skin is evacuated at temperature, for
example from 200.degree. F. to 800.degree. F. The polymer shell is
then able to collapse toward the pattern interior as thermal
stresses increase. This collapsing toward the interior of the
pattern results in less outward expansion of the pattern, and the
ceramic shell is undamaged. Polymer burn-out is then successfully
completed (at approximately, for example 650.degree. C.) without
damage to the ceramic mold and with minimal residual ash.
[0005] In the present invention, a stereolithography machine
manufactured by 3D Systems, Inc. or any other lazer/polymer rapid
prototype machine, is used to solidify a pattern including a series
of stacked grids of varying cross section that forms a pattern
having an outer skin and inner labyrinth and any number of gates.
The labyrinth or grid allows the unsolidified polymer to drain
completely from the inside of the pattern via the gates, resulting
in a pattern that is not solid polymer. The resulting three
dimensional pattern provides the contiguous surface geometry
adequately supported by an interior framework.
[0006] The pattern is completed by attaching the gates of the
pattern to a runner which is attached to a central sprue. Several
of these patterns are so attached via the runner to the central
sprue to form a pattern cluster. The pattern cluster is
subsequently dipped in ceramic slurry, and the patterns of the
pattern cluster are sifted by refractory grain to form a shell
layer. These two steps are repeated to form a plurality of shell
layers. The mold material is dried, and the patterns are melted out
of the mold; the mold is cooled, and carbon and trash found within
the mold if found, can be flushed with air and water. The molds are
heated and filled with liquid metal, the filling being aided by
either gravity, pressure/vacuum or centrifugal force. The mold is
broken away from the castings, and the castings are removed from
the sprue; gate stubs are ground off to achieve a finished
product.
[0007] The present invention can be used in conjunction with
existing polymer methods. The production of the hollow pattern is
achieved in half of the amount of time as previously required. The
present invention can be easily integrated with existing processes
employed by shell foundries. The present invention produces the
pattern molds at less cost than previous pattern molds.
[0008] These and other features of the invention that will be
apparent to those skilled in the art from the following detailed
description of the invention, taken together with the accompanying
drawings.
DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is an illustration depicting grid forms that are
stacked to provide a pattern;
[0010] FIG. 2 is an isometric drawing illustrating a typical
pattern resulting from stacking grids such as those illustrated in
FIG. 1;
[0011] FIGS. 3a and 3b illustrate various steps in the process to
produce an investment of the present invention; and
[0012] FIGS. 4a-4h illustrate various steps in the process to
produce a casting of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0013] The present invention uses a stereolithography machine
manufactured by 3D Systems, Inc. or other lazer/polymer rapid
prototype machine to solidify a pattern comprising a series of
stacked A-grids having a first predetermined cross section 8 and
B-grids 10 having a second predetermined cross section such as
those illustrated in FIG. 1. When grids 8 and 10 are stacked one on
top of the other to form a pattern 11, which is a combination of
the first and second predetermined cross section, a labyrinth 12
results as illustrated in FIG. 1. Each of the A and B grids have
elements that couple sides of the grids. The element of A-grid and
an element of B-grid form, for example substantially a 45.degree.,
angle or any other angle will achieve satisfactory results. FIG. 2
illustrates pattern 11 having an outer skin 14 and an inner
labyrinth 12 (see FIG. 1) and a gate 16 such that the outer skin 14
includes A-outer skin 14a and B-outer skin 14b. Labyrinth 12 allows
any unsolidified polymer to drain completely from the inside of
pattern 11 via gate 16. Gate 16 also operates as a vent for the
filling process. In the preferred embodiment, one or more drains
and one or more vents are recommended. The preferred embodiment is
illustrated as having one gate for simplification, however, any
number of gates, drains and vents may be provided. The resulting
pattern 11 of three dimensions provides an essential contiguous
surface geometry adequately supported by an interior framework.
[0014] Referring now to FIGS. 3a and 3b, the various steps of the
process of the present invention are illustrated. In FIG. 3a,
investment 30 is completed by curing the x-y cross sections of the
polymer by using an ultraviolet light source. Layers of A-grid and
B-grid are constructed in sequential manner as platform 34 is
lowered into vessel 32. Investment 30 is completed by placing it
within vessel 32 through the use of lowering platform 34. Vessel 32
is filled with an ultraviolet curable liquid 36. Investment 30 is
formed by ultraviolet light source 38 which is moveable in the x-y
directions, curing the ultraviolet curable liquid 36. This process
may be completed by any method currently known in the art, such as
stereolithography.
[0015] In FIG. 3b, investment 30 is placed on platform 50, which is
oriented for maximum drainage of remaining resin 52 of ultraviolet
curable liquid 36 from within investment 30. The drainage of the
remaining resin 52 is facilitated by the labyrinth 12. Several
orientations of the investment 30 may be required depending on the
placement of a riser or gate 16. Investment 30 may be drained
inside a heated chamber (not shown) to take advantage of the
elevated temperature. Use of low viscosity resins as the
ultraviolet curable liquid is preferred but not required.
Investment 30 may be rinsed thoroughly, for example, in IPA, making
sure that remaining ultraviolet curable liquid 52 is not trapped in
corners or pockets within investment 30. Investment 30 can be
soaked in, for example, clean EPA for several hours if required.
Preferably, investment 30 should not be exposed to ultrasonic
cleaning for more than ten minutes. Any ultraviolet curable liquid
36 on the exterior should be removed from the exterior, for example
by wiping the exterior of investment 30. Then, in the preferred
embodiment, investment 30 may be allowed to dry, for example for
approximately one hour and wiped again. Investment 30 is preferably
not soaked in alcohol after the last drying cycle, or the skin may
stretch and split on removal. After drying, investment 30 is cured
in an ultraviolet oven (not shown) for one and a half minutes at
full power. The short postcure of the oven will assure a non-porous
skin without warpage. Investment 30 is wiped again if any
ultraviolet curable liquid 36 remains on the exterior of investment
30.
[0016] Referring to FIG. 4a, the investments 30 are mounted at gate
16 to runner 60, which is coupled to central sprue 62. A plurality
of similar investments 30 are mounted to other runners 60, which
are all coupled to central sprue 62.
[0017] Referring to FIG. 4b, ceramic slurry 64 is prepared in tank
66. The investments 30 are dipped or immersed in the ceramic slurry
64 so that all the investments 30 are completely covered by ceramic
slurry 64. Although, the dipping of the investments 30 may be
accomplished by various methods, for example by automatic machine,
or FIG. 4b illustrates that the investments 30 are dipped by the
hand of an operator. The investments 30 may be agitated by rotating
and elevating investments 30 so as to completely cover investments
30 with ceramic slurry 64.
[0018] As illustrated in FIG. 4c, investments 30 are sifted through
refractory grain 72 to produce a layer of refractory grain 72 on
the exterior surface of the ceramic slurry coated investments 30.
The central sprue 60 is positioned for the sifting refractory grain
72, for example, by hand to achieve a uniform coating of refractory
grain 72. The central sprue 60 may be rotated to ensure that
refractory grain 72 is uniformly coated on investment 30.
[0019] Multiple layers of refractory grain 72 are desired to assure
a complete and relatively thick coating and achieved by repeating
the above dipping step illustrated in FIG. 4b and then by sifting
investments 30 in refractory grain 72. In the preferred embodiment,
at least nine shell layers are obtained by repeating the dipping
and sifting steps illustrated in 4b and 4c, respectively.
[0020] However, additional layers could be formed in a similar
manner.
[0021] Referring to FIG. 4d, the mold material formed from the
shell layers is set and dried. Subsequently, heat is applied to the
mold and investment 30 in order to melt the investment from the
mold. Referring to FIG. 4e, molds 68 are, for example, inverted and
a supply of water and a supply of air are independently coupled to
central sprue 62 so that the water and air may enter central sprue
62, runners 60, and the interior of molds 68 in order to flush
molds 68 to remove carbon and other trash. The flushing step may be
eliminated if the carbon or other trash is not present. The source
of water and the source of air are removed from central sprue 62,
and molds 68 are positioned upright to prepare for the reception of
heated metal.
[0022] Referring to FIG. 4f, molds 68 are heated, and hot molds 68
are filled with hot metal through the central sprue 62 and runner
60. A filling of the hot metal is facilitated, for example by
gravity, pressure/vacuum or centrifugal force to remove any trapped
bubbles, obtaining an accurate casting from the mold 68. The
material of mold 68 is broken away from the casting as illustrated
in FIG. 4g. Subsequently, the castings are removed from the central
sprue 62. And as illustrated in FIG. 4h, the solidified metal
corresponding to the gate 16 of the investment is removed, for
example by grinding.
[0023] One unique quality of the hollow investments 30 is that the
expansion of the investments 30 while heating the investment does
not crack mold 68 since the hollow investments 30 expand both
inwardly and outwardly, reducing the outward expansion of
investment 30.
[0024] The operation of the present investment follows:
[0025] The ultraviolet light source 38 forms investment 30 on
platform 35 positioned in vessel 32 by reacting with the
ultraviolet curable liquid 36. The investment 30 is drained of the
remaining ultraviolet curable liquid 36. The investment 30 is dried
and cured in an ultraviolet oven.
[0026] The investment 30 is mounted on runner 60, which is coupled
to central sprue 62. The central sprue 62 including the investments
30 is dipped in a ceramic slurry. The ceramic slurry coated
investments 30 are sifted by refractory grains to provide a
plurality of layers. The investments 30 are heated while attached
to runners 60, and investment 30 is melted out, forming a mold. An
air and a water supply are coupled to central sprue 62 in order to
supply the interior of the mold with the air and water in order to
remove carbon and other trash, purifying the interior of the molds.
The molds are heated, and hot metal fills the molds. An accurate
casting is achieved by gravity, pressure/vacuum or centrifugal
force. The mold material is broken away from the castings and the
gate stubs which correspond to gate 16 are removed, for example by
grinding, from the castings.
[0027] The investment is computer and machine generated, which
significantly reduces the manual labor and the cycle time for the
fabrication of the casting and provides a high degree of part
accuracy. The hollow investment 30 is stable, while enabling
consistent burn-out and low ash content. The shell and labyrinth
also enables the use of a variety of polymer formulations which may
present other benefits such as cost, green strength and safety.
[0028] This process generates casting or mold patterns with highly
accurate internal and external details in comparison with other wax
coating techniques. This is a result of the CAD defined polymer
exterior and hollow investment. This new process provides patterns
which are compatible with industry accepted casting processes and
equipment.
[0029] Although the present invention and its advantages have been
described in detail by way of a preferred embodiment, it is to be
understood that this is for example only and that various changes,
substitutions and alterations can be made without departing from
the spirit and scope of the invention as defined by the appended
claims.
* * * * *