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Rapid prototyping cuts time, costs to build finished parts.

Computer-controlled laser machine builds solid models for use as production patterns for sand, investment, plaster castings.

Computer-controlled rapid prototyping machine technology is a relatively new development that can reduce significantly the time and cost that long have burdened the foundry's role in responding to customers' needs in the new product development cycle.

The technology allows the immediate creation of complex, three-dimensional parts directly from the customer's computer image or computer-aided design (CAD) data. The parts are used as form, fit and function models for design verification to build patterns for secondary manufacturing processes and for foundry tooling development.

The rapid prototype (RP) process makes even the smallest foundry become a major player in the contest of helping customers move quickly from product concept to finished part. The proliferation of RP computer service bureaus and the affordability of setting up in-house RP operations adds a new competitive edge by letting foundries slash the time and cost of traditional methods of responding to the need for faster product turnaround.

The computerized automation of RP modeling easily replaces the traditional, hand-built pattern that relies on two dimensional (2-D) drawing information. It gives a foundry a far quicker, more accurate response for the verification of any part and substantially reduces the time required from the engineering design concept to a finished part.

Laminated Object Manufacturing

One accurate and fast method of producing RP patterns and secondary tooling uses a relatively new, proprietary laminated object manufacturing machine, or the LOM system.

The LOM machine uses a single laser beam and thin, roller fed sheet material to create solid objects having great complexity. The sheets, which are precoated with heat sensitive adhesives, are laminated one on top of the other to create a multilaminar structure. Once a layer is bonded, the C|O.sub.2~ laser cuts the outline of the specific cross section that represents one of the thin cross sections within the 3-D object. This process continues until all layers are cut and laminated, ultimately creating a solid model of the CAD part.

It is an automated process that uses a highly specialized machine consisting of a platform, a C|O.sub.2~ laser (optics), a heated roller, an x-y positioning device, a roll of polyethylene-coated (one-side) paper and a 486 PC computer.

Steps to fabricate sand and plaster cast metal prototypes are virtually identical, but the tooling selection is based on the quantity of parts required. The several case studies below cite our experience with LOM model fabrication and prototype tooling development to produce functional metal castings faster, more accurately and at less cost than conventional pattern building and prototyping methods.

The LOM machine operates as follows:

* A 486 PC computer first defines a 2-D cross section from a 3-D computer model. The cross section has a thickness equal to that of the material being used to construct the pattern (in our applications, a coated paper 0.0035 in. thick).

* The 2-D CAD data is used to guide an x-y optical positioning device that directs the laser beam onto the paper, cutting the paper around the periphery of the 2-D cross section only to the depth of one layer of paper. In areas around the periphery of the cross section, the laser cuts crosshatched squares to facilitate the separation of the model from the discarded material after the model is completed. The laser then cuts a boundary area around the outside of the crosshatched area to hold the area together as the model is being fabricated.

* The platform lowers at exact increments and the paper advances to a position where a new layer of paper can be bonded to the just-completed layer. The platform rises back into position and a heated roller moves across the upper layer of paper, pressing and melting the backcoated polyethylene of the upper layer. This heated ironing action causes the upper and lower layers of paper to bond to each other, resulting in a laminated structure.

* The height, or z-axis, of the current stack of paper layers is measured by the machine and fed back to the computer; the next 2-D cross section is then calculated from the 3-D computer model.

* The process is repeated until all 2-D cross sections, as defined by the 3-D CAD model, have been cut by the laser and bonded.

* The end product is a block of paper that has a model (pattern) encapsulated inside. The material surrounding the model, diced into small cubes as a result of the crosshatching, is separated easily from the model once the outside boundary has been broken away.

RP Tooling

Prototype tooling needs to be fabricated quickly and economically but be capable of producing parts acceptably close to final production specifications. Three widely used metalcasting processes for prototyping are sand, plaster and investment molding. Each has its own applications and advantages.

Sand Cast Process--The starting point for each of the RP casting processes is the creation of a 3-D CAD model (solid or surface), eliminating the need for 2-D detail drawings normally the basis for wood patterns. The use of the 3-D computer model instead of going the conventional 2-D route can reduce engineering concept-to-production time by as much as 5-8 weeks. The 3-D CAD computer models used to create a stereolithography (STL) file is formatted to replicate the CAD model into a LOM-usable format.

Mack uses LOM technology to fabricate "wood-like" paper models in place of traditional, hand-built wood patterns. A typical automotive-type model similar to the part shown in Fig. 3 can be LOM-built in as little as 4 hours for a simple part or take up to 3 days for a more complicated one. This compares with the 5-10 weeks necessary to build a pattern for the same types of parts using traditional pattern making methods.

The LOM model contains all necessary data to build foundry tooling, cope/drag patterns and core boxes.

Using all the design data, including a shrinkage allowance of a projected part, the special LOM slice software employs the STL computer file to create the individual 2-D computer sections that make up each laminated paper layer of the physical model. Parting surfaces can be incorporated in the 3-D computer design but Mack experience has found it more practical to build them conventionally.

After the LOM model is removed from the machine, it is sealed against moisture with a clear lacquer coat. Any radii or fillets not included in the 3-D computer model are easily added using wax. After the parting surface is in place, the steps to produce secondary tooling used to manufacture functional sand cast metal parts are as follow:

* build polyurethane molds from the LOM model;

* build polyurethane or epoxy pattern equipment (foundry tooling);

* produce expendable sand molds;

* cast the metal parts.

The last two steps are repeated for each casting required.

Polyurethane or epoxy cope and drag pattern equipment is built using wood or metal frames, and expendable sand molds are made ready for casting functional parts.

Polyurethane and epoxy secondary tooling used for sand casting make a durable pattern material sufficient to produce hundreds of castings from a single set of tooling to a tolerance range of 0.020-0.030 in.

Typical functional parts manufactured using the RP sand cast process include cylinder heads, manifolds (intake and exhaust), valve bodies, brackets and transmission housings.

In certain applications, the LOM model can be used as a loose pattern where sand molds are made directly from the LOM model and eliminating the first and second steps from the above noted tooling development process.

Plaster Cast Process--Plaster casts to manufacture simulated diecast prototypes use the RP process differently from sand casting in the type of tooling required. They utilize rubber patterns to produce expendable plaster molds that release easier from rubber patterns than from other materials. This reduces mold breakage and extends the LOM model life indefinitely.

Secondary tooling uses the LOM model just as for sand casting. A rubber, epoxy or polyurethane plastic mold is constructed from the LOM model and a rubber pattern is poured from which expendable plaster molds are made and metal parts are cast. The last two steps are repeated for each part manufactured, but any number of additional rubber patterns can be duplicated.

An average of 25-100 plaster molds can be made from the rubber pattern. The plaster cast process produces high quality simulated die-cast parts to tolerances in the range of +/-0.010-0.020 in.

Mack has used LOM techniques to produce functional frontend accessory drive brackets, electronic heat sinks, housings, bezels, front engine covers, oil pans, pump bodies, alternator housings and other parts where turnaround time is critical.

Investment Casting--LOM patterns for investment cast parts can be used directly or indirectly, depending on the number of parts desired. Traditionally, wax patterns are used for the ceramic shell mold process, and, if only a few parts are required, the cost to develop secondary tooling may be prohibitive. Now, however, highly accurate RP patterns are available as an alternative to wax patterns. Mack is using a LOM paper model in place of the wax pattern that has been dubbed the "lost paper" method of direct investment casting.

Steps for the "lost paper" investment cast process are as follows:

* apply a moisture-resistant sealant to the LOM model;

* develop an expendable ceramic shell mold using the LOM pattern and a slurry process to form a ceramic shell;

* place coated LOM pattern in autoclave to cure ceramic shell;

* burn out the LOM paper pattern in oven and remove ash from ceramic shell mold;

* cast metal into ceramic shell mold.

The LOM "lost paper" process is best used in the early product development stage when quantities are low and pattern changes are most likely. A new LOM model must be fabricated for each ceramic shell mold produced. As the number of LOM patterns increases, the cost may be great enough to justify building mold tooling that can produce multiple patterns from wax.

Three RP tooling options are available for indirect investment casting. Two use a LOM model to develop a mold from metal-filled epoxy or RTV silicone rubber, the third uses a LOM mold. All three, however, can produce multiple wax patterns. The metal-filled epoxy mold uses pressure-fed wax while the RTV silicone and LOM molds are gravity fed.

The steps in producing functional metal parts from RP indirect investment tooling are:

* fabricate a LOM mold or use a LOM model to develop an RTV silicone- or aluminum-framed, metal-filled epoxy mold;

* mold expendable wax pattern (repeated for each part);

* develop expendable ceramic shell mold using wax pattern;

* place in autoclave to cure ceramic shell and melt out wax pattern;

* place ceramic shell mold into oven to complete burnout of wax;

* cast metal into ceramic shell mold (repeated for each part).

The metal-filled epoxy mold is built with an aluminum frame and ejector pins for separating the mold halves and for ejecting the wax patterns from the mold. RP tooling can manufacture quickly at low cost highly accurate wax patterns for use in ceramic shell mold casting.

Depending on the complexity of the parts, hundreds of wax patterns can be produced in an epoxy mold of this type. Advantages of investment casting include the ability to cast certain types of features that would be difficult or impossible to machine, and to produce parts that reduce the amount of machining required on a casting.

Product development time can be shortened by weeks or months by incorporating RP methods into the prototype tooling process. Pattern equipment for sand, plaster and investment castings can be built directly from LOM models. Tooling selection and casting processes are based on the engineering objectives to be achieved with the prototype, the quantity of parts required and the desired tool life.

Functional metal parts can be made in a fraction of the time of traditional tool making methods while improving accuracy and replicating exact design intent from the 3-D CAD computer model into the finished cast parts.
COPYRIGHT 1993 American Foundry Society, Inc.
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Copyright 1993, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Title Annotation:Computers in the Foundry
Author:Warner, Merlin C.
Publication:Modern Casting
Date:Oct 1, 1993
Words:1992
Previous Article:A vision of computer-aided casting in the year 2000.
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