Investment Casting LOM Patterns
From:
Albin Hastbacka (Sanders Design International, Inc.)
Date:
Wednesday, November 1, 1995
From: Albin Hastbacka (Sanders Design International, Inc.)
To: RP-ML
Date: Wednesday, November 1, 1995
Subject: Investment Casting LOM Patterns
A recent question was asked about investment casting with LOM patterns.
The answer to this question and the more general question of investment
casting from several different RP processes is provided below:
Sanders Prototype, Inc. Pine Valley Mill, PO Box 540, Wilton, NH, 03086 USA
Tel: 603-654-5100, Fax: 603-654-2616 http://www.sanders-prototype.com
Model MakerÂȘ Application Note #2 - Investment Casting Processes
A consolidated comparison of the various Rapid Prototyping processes
for investment casting is offered in this paper. The information was
obtained by discussions held with various investment casting firms
that have had experience with one or more of the reported processes.
Text and table information on Model Maker competitors supplied
courtesy Cercast Group, Montreal,Canada. Model-Maker data added by
Sanders Prototype, Inc.
InkJet Printing
Sanders Prototype, Inc. Model-MakerÂȘ System employs an InkJet Printing
technique to build a pattern using a thermoplastic material that is
very similar to investment casting wax. The "build" material can be
autoclaved out of the casting shell in the same way that investment
casting wax is removed. Based upon extensive use of the Model-Maker
System, the "build" material does not leave any residue in the
casting
shell. The method of building patterns with layers as thin as 0.0005
inches (12.7 microns) results in a surface pattern finish that is
superior to any of the other Rapid Prototyping systems on the market.
The overall accuracy and surface finish of the resulting cast metal
production tool will be superior to any of the other Rapid Prototyping
systems.
Stereolithography (SLA)
3D Systems, Inc. employs a laser based Stereolithography (SLA)
process that uses thermoset plastic resin materials that are
particularly not well suited for use as investment casting patterns.
Because
thermoset materials do not melt on heating, it is not possible to
remove SLA parts from investment casting shells using traditional
autoclave techniques. SLA patterns must be burned out of the casting
shells by subjecting them to high furnace temperatures for an extended
period of time, thereby leaving an unwanted ash that can contaminate
or mar the surface finish of the casting.
SLA materials also pose other problems for the investment, because of
their co-efficient of thermal expansion (CTE). Most of the SLA resins
have a CTE that is higher than the casting shells, which usually
results in the shell cracking. To solve the CTE problem, a process
called QuickCast was created. Using newer resins combined with a
lattice-like internal structure, QuickCast patterns are designed to
collapse in on themselves without cracking the casting shells.
QuickCast patterns require careful treatment. Breaks in the surface of
a QuickCast patterns will allow shell material to seep into the parts
when they are dipped into the ceramic slurry. This seepage will cause
surface flaws in the subsequent metal castings. Also, thin walled
patterns (less than 0.060 inches thick) are difficult to drain the
excess resin out before final curing. If resin remains trapped in the
QuickCast lattice, these walls will be solid instead of mostly hollow
and when heated during burnout, they will expand and crack a ceramic
casting shell.
Selective Laser Sintering (SLS)
DTM, Inc. employs a process called Selective Laser Sintering (SLS)
which uses the heat of a laser to fuse together particles of powdered
material. Two materials in current use by DTM are wax and
polycarbonate
which are suitable for creating investment casting patterns. These
materials are thermoplastics, which means they can be melted out of
the
invesment casting shells be using a conventional autoclave. DTM's wax
material is very similar to regular investment casting wax, making it
very compatible with traditional investment casting techniques.
Polycarbonate also can be melted out of casting shells by autoclaving.
The SLS surface finish is rougher than many other Rapid Prototyping
processes. The surface finish of investment cast parts can be improved
by coating the patterns with wax before making the casting shell. Even
with the wax coating, the rough surface finish of the SLS part is
usually transferred to the final metal part.
Laminated Object Manufacturing (LOM)
Helisys, Inc. employs a process called Laminated Object Manufacturing
(LOM) for Rapid Prototyping. The LOM process fabricates parts by
cutting and fusing together layers of coated paper. This type of
pattern must be burned out of an investment cast shell rather than
melted. These patterns do not expand as much as an SLA pattern, but
they are sensitive to moisture and will swell when exposed to steam.
For this reason, LOM parts should be sealed before use as investment
casting patterns.
LOM parts yield good surface finish on surfaces parallel to the build
plane, but layering lines perpendicular to the build plane will be
transferred to the cast metal parts. LOM parts also leave a
considerable amount of ash residue in casting shells after the
patterns have been burned out. This can cause problems with the finish
of the castings.
Fused Deposition Modeling (FDM)
Stratasys, Inc. employs a process called Fused Deposition Modeling
(FDM) which builds parts by extruding a bead of thermoplastic
material. One of the available materials is an investment casting wax.
The surface finish of FDM parts is generally better than the SLS
process, but the parts are quite porous and deliver only fair surface
finish on
the cast metal parts. The similarity of the Stratasys wax material to
conventional investment casting wax allows FDM parts to be processed
without modifying the traditional investment casting process.
Solid Ground Curing (SGC)
Cubital Corporation employs a Solid Ground Curing (SGC) for its
Solider System process. The Solider System produces solid plastic
models which behave similarly to solid non-QuickCast SLA parts.
Because of their relatively high coefficient of thermal expansion
(CTE), they are difficult to use as patterns for shell investment
casting. They can be used in the less accurate solid or flask casting
techniques.
Direct Shell Production Casting (DSPC)
Soligen Corporation employs Direct Shell Production Casting (DSPC) to
build a ceramic investment casting shell directly, rather than
fabricating parts for use as patterns. Castings can be formed by
pouring molten metal into these shells without any other treatment.
The DSPC system currently does not deliver very good surface finishes.
An additional drawback of the system is that it is nearly impossible
to inspect the inside of the casting shell before the molten metal is
poured.
Text courtesy of Cercast Group, Montreal, Canada. Model-Maker and
Soligen data
added by Sanders Prototype.
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