Tooling for Casting

From: MR ALBIN A HASTBACKA (KHVD07A@prodigy.com)
Date: Tue Sep 17 1996 - 00:00:00 EEST


-- [ From: Al Hastbacka * EMC.Ver #2.5.1 ] --

A feature article in JobShop Technology discussed Prototyping the Tool,
Not the Part. With permission from JobShop Technology, I have taken the
liberty to enclose this paper because of it's general interest to the RP
community.

 Prototype the Tool Not the Part

Courtesy of Job Shop Technology Magazine
Midwest Edition, September 1996

        American Industrial Casting, Inc. (AIC), a manufacturer of precision
investment cast components, has for a long time offered customers free
engineering assistance in the early stages of product development. This
has helped customers reduce time to market by avoiding changes to
tooling that might have added to the normal eight to ten weeks required
to produce new tools and samples from a new design. It also enabled AIC
to enhance its services to customers by bringing the customers' design
for manufacture and assembly practices into its own operation.

        To further increase its service to customers, AIC added a variety of
new capabilities. Included among these were the transfer of designs by e
-mail, fax, and modem; use of powerful software to view CAD files; and
the timely production of rapid prototype castings from customers' three-
dimensional CAD models.
        
        Since 1992, AIC has produced acceptable castings from the patterns
built by several of the leading rapid prototyping processes offered by
rapid prototyping design service bureaus.

        However, AIC was not satisfied with the overall speed, quality, and
cost of providing sample investment cast parts for many accounts. The
company felt that most of the rapid prototyping processes could not meet
the resolution and surface finish requirements of the intricate, fine-
featured castings for which AIC was noted. They also found that some of
the pattern materials from those processes can react in detrimental ways
with the materials used in the solid mold and shell mold investment
casting processes. This made it difficult, risky, and expensive to
achieve the high quality required by investment casting customers.

Doing It In-house
        Early in 1994, AIC studied the various rapid prototyping systems and
decided to purchase a low cost, desk-top, rapid prototyping system
called the ModelMaker System from Sanders Prototype Inc. They found that
this system would allow them to build patterns with the dimensional
accuracies and surface finishes their customers demanded.

        The Sanders ModelMaker System (Model MM-6B) uses a combination of
plotter/miller/inkjet printing technology to produce precise intricate
prototypes of a thermoplastic material that serves as the wax pattern in
investment casting. The system uses a dual printhead to deposit a
sequence of thin layers, or slices, of the pattern being built. One
printhead deposits a thermoplastic material to produce the pattern; the
other deposits a wax material to support the pattern's overhangs and
cavities during the build.

        The finished pattern is immersed in a solvent bath to dissolve away the
support wax, leaving the pattern ready for investment casting without
the need for further manual post-processing steps.
        
        AIC soon found that its ability to produce exact investment-ready
patterns with the new system was a major time-saving advantage. There
were no scaffolding support structures to be snapped off or broken away.
Thus, there is no manual touch-up or repair needed, as sometimes
required on models made by other types of rapid prototyping processes.

        The company discovered several other advantages of this three-
dimensional printing process. Dimensional tolerances were maintained by
precision X-Y plotter technology to within 0.001 inches. In the Z-axis,
dimensions were maintained by a horizontal milling technoique that
accurately maintains slice layer thickness from 0.0005 to 0.005 inch to
construct and cast extremely finely detailed parts with features such as
4-40 NF male and female screw threads, or cutting edges.

        Another use for the finest 0.0005 inch layer is building any pattern
where smooth, curved surfaces are required, such as jet engine turbine
blades, medical cutters and piercers, fine jewelry filigree, and the
like. More intermediate (0.0015 inch) and coarse settings (0.005 inch)
are used to produce tooling grade patterns and concept models,
respectively.

        Since AIC started rapid prototyping in-house, they have modeled and
shipped numerous parts as cast components in a week or two. They usually
use the customer's three-dimensional CAD designs supplied as .STL files.
The process has been ideal for producing a single unit, or a few small
patterns, for intricate cast components.

        The constructed parts are precise, with tolerances of 1 to 1.5 times
normal, dimensional resolution of 0.002 inch, and surface finishes as
normal production quality at 80 to 100 micro-inches, RMS. The process is
capable of producing parts of up to six inches on a side. However, the
build time for large patterns with thinly sliced layer thickness becomes
longer and more expensive.

Temporary Mold Prototypes
        
        When customers have needed a dozen or more larger parts to be
prototyped economically or quickly, AIC has occasionally used temporary
elastomeric molds made by quick processes, with moderate success. The
approach is to rapid prototype a single appropriately-scaled model of a
part. The prototyped part is investment cast as a beryllium /copper
alloy master part, which is used to make any one of three kinds of
temporary molds of rubber, RTV, or epoxy.

        Rubber molds can be made in about one day for under $100, with
tolerances of +/- 1/16 inch, but with considerable part distortion in
the process. On the other hand, RTV molds can be built in about 2 to 3
weeks for typically $500 to $1000, with tolerances of +/- 1/32 inch. RTV
molds exhibit less dimensional distortion than do rubber molds.
        
        For best results and minimum distortion, hard epoxy molds, sometimes
even with core elements, can be produced in 3 to 5 weeks for $1500 to
$2500. They have tolerances of +/- 0.005 inch or better. The lives of
such molds are somewhat limited, and yield experience of waxes for
investment casting from these molds is about 50 pieces for rubber, 100
pieces for RTV, and 200 pieces for epoxy.

        To overcome time, cost and accuracy constraints in making molds from a
beryllium/copper part master, AIC established a single step process to
directly prototype the part's mold instead of the part itself.

In this process, negative images of the part's surface are modeled.
These negative images become molds to create wax patterns of the part
for investment casting.

        As a test, a sample part of moderate complexity was chosen from those
in normal production at AIC. Virtual Concepts Design Inc., of North
Attleboro, MA, was asked to design mold components, starting from an .
STL file for the final part. Parting lines were created at agreed upon
places, and three-dimensional drawings were made by working into
negative space from the surfaces of the part. After mold components were
designed, a separate .STL file was produced for each.

        Each model component was scaled, oriented advantageously, placed beside
one another, and combined into a single .STL file for the best build
profile. Then the patterns for all the components were rapid prototyped
in a single overnight session using a low melting point polymer that
behaves like wax..

        On the third day, using the standard solid mold processing technique,
the rapid prototypes were investment cast in beryllium copper alloy,
which perfectly reproduced every detail of the prototype models. These
cast components underwent the regular processes of knockout, grind,
finish, inspection, and solution heat treatment by the fifth day.

        The final mold components were put together and used to hand injection-
mold several wax patterns of the final part by the seventh day. Those
wax patterns of the final parts were investment cast in solid molds on
the eighth day. Solution heat treating and layout inspection only took
one day more.

        AIC's lessons-learned from these rapid prototyping ventures have helped
to add value to their services by decreasing the company's response time
. Computerized rapid prototyping technologies and processes have become
an integral part of the company's operations.

        For more about American Industrial Casting, e-mail: info@aicasting.com.

        About the author– Tom Richards is a hands-on metallurgist and technical
account manager at American Industrial Casting. The company,
headquartered in Cranston, RI, precision investment casts components for
the aerospace, electronics, medical, instrumentation, and transportation
industries nationwide..



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