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Rapid prototyping draws widening foundry interest.

It is said that when Napoleon ordered cannons for his armies, foundries understood that his orders were to be filled immediately. Delivery delays often were corrected by guillotine, and that fact made French foundry managers especially eager to fill royal artillery orders quickly. Their haste, however, resulted in some good cannons and some not so good. Frequently cannons burst when they were fired, resulting in the deaths of many cannoneers and shortening not a few foundry careers.

The unfortunate failures were mostly caused by faulty cannon design or flawed castings. Had they existed back then, a good quality control system would have hepped immensely, and proper cannon design and testing would have proved to be lifesavers. Rapid prototyping would have been an instant success.

Rapid Prototyping

The modern foundryman readily recognizes good casting design and product testing as necessary tools of his trade, but these take time. Thus, he is always alert to ways that can telescope the time between the design and molding of an acceptable casting. One new way to do just that is through rapid prototyping or stereolithography.

Rapid prototyping, or rapid modelling, is an extension of the patternmaker's traditional skills. It is a process used to create model parts directly from a computerized model without tooling or machining. It is a new concept that bears foundry investigation. It is important to note that none of the several rapid prototyping processes at this writing can produce metal parts directly. These processes create parts in plastic or wax materials. The parts created are nonfunctional, though this too is changing as the technology advances.

A leader in this recently developed technology is the process called stereolithography. Stereolithography (STL), simply put, is a method of transforming, in a matter of hours, computer-aided design (CAD) data (or 2-D engineering drawings converted to CAD by one of the new STL service bureaus) into solid models of virtually any geometry. Starting with the CAD model, STL uses proprietary software to horizontally slice the computerized representation of a part into very thin layers.

The computerized data, representing each slice or layer, is used to control a laser or ultraviolet (UV) beam of light that traces the shape of each layer, starting from the bottom of the part, onto the surface of a photosensitive resin. The resin changes from liquid to solid wherever the laser beam strikes it. The layers are drawn one on top of another until the object is complete.

With a solid representation of an engineering concept in hand quickly and relatively inexpensively, the customer and the foundry together can work out design imperfections, accuracy, casting potential and the ultimate functional capability of the part before committing to any production tooling.

Selective Laser


In addition to the photosensitive resin material used in STL, there are other processes available to make computer-generated prototype parts, one being a selective laser sintering (SLS) process that uses any material that softens under heat and then resolidifies. This includes a wide variety of powdered materials that sinter (soften and bond) such as polycarbonates, nylon, ABS plastic, investment casting waxes, ceramics, metals and advanced composites.

In the SLS process, a thin layer of heat-fusible powder is deposited into a container and heated to just below its melting point. An initial cross section of the object under fabrication is traced onto the layer of powder by a laser beam. The temperature of the powder impacted by the laser beam is raised to the point of "sintering" in a controlled atmosphere to chemically or metallurgically bond the particles and form a solid mass. As the process is repeated, each layer fuses to the underlying layer, and successive layers of powder are deposited and sintered until the object is complete.

Metal Castings

Although rapid prototyping processes vary considerably, usually they will be used in similar ways to speed the creation of prototype metal castings. In most of the methods discussed, the rapid prototyping process chosen will not have a major effect on the prototype casting produced.

Sand Casting

Vendors of rapid prototyping equipment have made vague statements about the use of rapid prototyping techniques to create patterns for sandcasting. Some have even hinted that the emergence of these technologies will speed the decline of the patternmaking industry. To date, however, rapid prototyping has had little impact in either the sandcasting or patternmaking industries. It is doubtful that rapid prototyping will ever replace patternmakers, but it is very likely the technique will become an important tool for patternmakers.

Loose Patterns

Although STL-produced parts require careful handling (they are not as durable as a wood or aluminum patterns), most of the materials available for rapid prototyping processes are durable and stiff enough to be used as prototype loose patterns to create several sandcast parts as shown in Fig. 1.

Rapid prototyping techniques provide an advantage to the patternmaker in that they allow him to work directly from the CAD model of the finished part. Starting with the CAD model, he can scale up the entire model to compensate for shrinkage. Machine stock and core prints can be added as necessary to the design. Draft can be checked and modified if required. Once the pattern design has been completed, the model can be conveyed to a rapid prototyping system to create the pattern. With little finishing, the pattern is complete.

For complex parts, stereolithography patterns can often be made faster and less expensively than a conventional pattern. Simple designs, however, typically are less expensive to make by conventional means. Patterns created by many of the rapid prototyping processes currently available can meet or exceed the accuracy of conventionally produced patterns.

To obtain more durability, copies of the pattern can be made in polyurethane or aluminum. Double Shrinkage can be incorporated easily into the stereolithography model to compensate for shrinkage if the durable patterns are to be from the rapid prototyping pattern.

In the same manner that rapid prototyping techniques can be used to make loose patterns, they can also be used as an advantage in the creation of matchplates. The CAD model of the finished part can be split along the parting line before scaling up, adding machine stock, core prints, etc. The split pattern can then be mounted on a parting line block and used to cast production matchplates.

Rapid prototyping techniques also be used to tie in the creation core tooling. Most CAD systems are capable of generating stereolithography models and will allow a CAD model of a cored area to be easily extracted from the model of the finished part, resulting in a rough model of the core print. Like the process for creating a loose pattern, the CAD model of the core print can quickly be scaled to compensate for shrinkage, and machining stock can be added to complete the design of the core. The core print then can be created from the finished CAD model by a rapid prototyping process and used as a pattern to make a corebox.

Diecasting Applications

Prototyping diecast parts has always been an expensive proposition. Parts can be machined from aluminum or zinc stock, but the machined prototype can be expensive and may not have the same mechanical properties as the diecast production part. Sandcast parts may come closer to the mechanical properties but may not have an acceptable surface finish. Single-cavity dies can be expensive to create and usually require long lead times.

Rapid prototyping techniques in combination with plaster casting, a process that has been around a long time, offers a fast and inexpensive way to create prototypes of diecast parts as illustrated in Fig. 2.

Plaster casting is similar to sandcasting except that the mold is made of plaster instead of sand. Wet plaster is poured into the flask on top of the pattern and allowed to set. Once set, the pattern is removed and the mold is dried. The mold can then be used to case most common diecasting materials. Once the casting has solidified, the plaster is broken to extract the part.

The surface finish is much better than sandcasting but not quite as good as a diecast part. There is more flash than would typically be obtained with a diecast part but that usually is not a significant drawback for prototype parts.

The primary advantage of using rapid prototyping techniques along with plaster casting to prototype diecast parts is time; cast prototypes can usually be obtained within three weeks. In addition, for small runs, the cost is often significantly less than either machining prototypes or creating a prototype die. Finally, the material propeties of prototypes made in this manner more closely approximate the die casting than a machined prototype.

Investment Casting

One area in which there has been much interest is in the use of rapid prototyping techniques to create investment cast parts. The benefits in creating the first few parts can be significant because tooling costs, especially for complex parts, can be very high. Lead times for tooling can be nearly as long as for the production of injection molds. The ability to create a functional casting for design verification before tooling is ordered is extremely attractive.

Several of the rapid prototyping systems can create patterns for use in investment casting. Two systems in particular, those produced by Stratasys, Inc. and by DTM Corporation, can create parts directly into investment casting wax. These parts can then be used by virtually any investment casting foundry to create metal parts.

Stereolithography, the process developed by 3-D Systems, Inc., has also been used directly to create investment casting patterns, but mold cracking and burnout have presented problems.

Most of the materials available for stereolithography tend to maintain strength at elevated temperatures. As a consequence, they tend to expand and crack thin shell molds. This can be overcome by using the older solid cast or flask casting technique that many investment casters are reluctant to use because of the difficulty in automating the process. Some of the newer stereolithography materials made specifically for investment casting applications contain a volatile component that evaporates when the part is heated, shrinking the part slightly so that it will not crack the mold. These patterns have been used successfully with thin shell molds.

All of the materials available for stereolithography are thermoset materials, which means they will not melt out of the cavity like wax. Consequently, the pattern must be burned out, typically for several days in an oven.

Another Approach

A process under development at the Massachusetts Institute of Technology takes a slightly different approach. Instead of making a pattern for investment casting, they have developed a process to make a ceramic shell. The process develops the shell in thin layers using inkjet technology to spray a binder on ceramic powder particles. The powder is added in thin layers and the process repeated for each layer. When the part is completed, the shel is removed from the tank of powder, any excess powder is poured out and the shell is fired to completely bind the ceramic powder. The shell is then ready for pouring.

There are two primary benefits from using a rapid prototyping technique to directly create patterns for investment casting.

First, because they create parts in thin layers, it is possible to construct geometries that cannot be molded or machined in a single piece. Thus, these techniques can be used to quickly create patterns that would be difficult to build by other means.

Second, because rapid prototyping techniques can create patterns without tooling, small numbers of castings can be created faster at less cost than machined molds. Rapid prototyping is, therefore, effectively used to create a few castings to test the design. However, it is not yet practical to use them to create patterns for even low-volume production. The cost of a pattern created by a rapid prototyping process is typically many times the cost of molding a was pattern. Also, the rate at which patterns can be produced by a rapid prototyping process is usually limited to a few per day, compared to the hundreds per day that can be molded.

Even though it may not be practical to directly create patterns for investment casting using rapid prototyping techniques, there are ways that rapid prototyping techniques can be used in combination with other techniques.

Rubber Pattern Molds

One successful technique is to use the rapid prototype as a pattern to create a silicon rubber mold in which wax patterns can be cast. The prototype is scaled to compensate both for wax and metal shrinkage. A parting line block (used to define the partine line in the mold, similar to a follow board in sandcasting) is created and used to make a silicon rubber mold. Molten wax can then be poured into the mold to cast wax patterns for investment casting.

Although the mold costs a few hundred dollars, the cost of the wax patterns is so much lower that this process can significantly reduce the cost of creating more than a few parts. The molds are typically good for 20 -- 40 pours before the silicon rubber is unusable.

Spray Metal Pattern Molds

When many parts are required, a spray metal tool can be created from the stereolithography part rather than a silicon rubber mold. In the spray metal process, an electric gun feeds two wires through an arc. The arc melts the wires into tiny droplets in the form of a spray. Compressed air drives the droplets onto the part where the metal collects and hardens to form the mold as in Fig. 3.

To create a mold for wax patterns, a rapid prototyping model of the part, scaled to compensate for wax and cast metal shrinkage is made. As in creating the silicon rubber mold, a parting line block is required to define the parting on the mold. Metal is slowly sprayed (about 0.002 in. per pass) onto the rapid prototyping pattern positioned in the parting line block until a metal layer approximately 0.080 in. thick is created.

The metal face is then backed with an aluminum-filled epoxy to provide a strong, rigid backing for the mold face. Once the epoxy has cured, the entire assembly is inverted, and the parting line block is removed, exposing the spray metal parting line and the pattern surface previously hidden by the parting line block.

The process is repeated to create the other half of the mold. When the second half is completed, the rapid prototyping pattern can be removed, leaving a cavity and core, which is the starting point for the mold. Such molds are typically gated at the parting line and can be used for several hundred to several thousand pours. Spray metal tools offer an attractive alternative of low-to-medium volume parts.

EPC Applications

Although there has been no investigation into the use of rapid prototyping parts in expendable pattern casting (EPC), rapid prototyping processes have been used to create tooling for expandable polystyrene patterns.

Tooling to create EPC patterns must withstand extreme conditions. After the polystyrene beads have been blown into the cavity, steam is introduced into the cavity through vents. The steam expands the beads and causes them to bond. Water is then sprayed on the back side of the mold surface to cool the pattern, allowing into to be easily extracted from the mold when opened.

In the last year and a half, several EPC molds have been made using rapid prototyping parts as patterns and the spray metal process to create the mold itself. The construction process is slightly different from an injection mold. The spray metal layer is approximately 0.375 in. thick rather than 0.080 in., and there is no epoxy backing. The spray metal component of the mold is made as an insert to be bolted on a fabricated aluminum steam chest. Spray metal EPC tools have been used to make several hundred patterns. Mold life is estimated to be 1000 pieces, but none have yet run to failure.

Accessing the Technology

Some larger foundries and pattern-makers have installed several varieties of rapid prototyping equipment in their operations. Equipment cost currently ranges from slightly less than $100,000 to nearly $400,000 depending on the manufacturer and size capability of the system. However, the equipment costs is only part of the cost of an installation. A CAD system, finishing equipment, specialized facilities and training are contributing factors that may increase the cost.

Service Bureaus

Even companies who could easily afford the cost of installing a system may be reluctant to invest in the first-generation equipment currently available. Future generations of equipment are likely to be substantially more productive, use better materials and obsolete current equipment. Some companies have chosen to wait until the direction of the technology has become more certain before investing in equipment.

A small industry has emerged to serve the rapid prototyping needs of companies who can benefit from the use of the techniques, but who have chosen not to purchase equipment. For them, working with a service bureau may be a cost-effective alternative.

Working with a service bureau can give a foundry access to rapid prototyping, providing them with the ability to add another service for their customers. Making castings better, faster and more accurately are three of the important benefits of this new technology.

Some Guidelines

To minimize the possibility of problems when working with service bureaus, here are some guidelines:


* Make sure that the bureau understands what the part is intended to accomplish. A part to be used as a direct pattern for investment casting will have a very different set of requirements from one to be used as a loose pattern for a sandcasting (surface finish, accuracy and applied scale factor).

* Make sure the design is complete. All of the rapid prototyping techniques require a complete definition of the geometry, including such details as fillet radii, definition of blended surfaces, etc. Additional time will be required to define the CAD model before the part can be built.

* All expectations concerning surface finish, accuracy and delivery should be in writing. Specifications will vary widely depending on the prototyping process used and the skill of the operator.

Service bureaus have a wide variety of capabilities. Each specializes in a specific type of rapid prototyping process, CAD system and application. To ensure that a specific bureau can meet your needs, some of the following information is useful.

What kind of process does the bureau use, and what materials does it use? There are a variety of both available, some more appropriate for some applications than others.

Determine the bureau's level of experience. Although manufacturers of the equipment will minimize its importance, the skill and experience of the operator is the single biggest determinant of the quality and accuracy of the parts created on most rapid prototyping systems.

Can the bureau accept your CAD data easily, or must it regenerate the data on another system? Service bureaus should be able to take your blueprints and generate CAD files adequate for rapid prototyping. If you have already generated a CAD model, however, it may be less expensive and faster to work with a service bureau that can handle your data directly.

What experience does the bureau have in your specific interest areas? If a bureau has never done a particular applications before, you may not want to risk a critical project with it.

Will the bureau supply customer references for similar work?

If the bureau subcontractors part of the process, how reliable are the subcontractors capabilities? Especially in the area of metalcasting, no service bureau provides everything in-house. It is important to establish that the bureau has a good relationship with the subcontractor and that the subcontractor is competent and reliable.
COPYRIGHT 1991 American Foundry Society, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1991, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

Article Details
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Author:Bex, Tom
Publication:Modern Casting
Date:Nov 1, 1991
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