Creating a Model
Q: I need to make a prototype that will look as much like the final production part as possible. This part will be a metal casting in production. Where can I get a high-quality prototype made?
A: First, you have to decide whether you want your casting made from aluminum, zinc alloy, iron or brass. You also have to decide how precise and smooth you want the surface finish to be. If you were making an office stapler, for example, the base on the better quality staplers is made by die casting a zinc alloy. Its surface is smooth and requires no further treatment before painting. On the other hand, if you were making a fancy aluminum lawn chair, die casting would be prohibitively expensive due to the mold size and a somewhat rougher surface can be tolerated. In this case, sand casting would be the economic and cosmetic choice.
High-volume metal parts, especially small parts, are most often die cast because the process is fast and not labor intensive. Die casting only works well with zinc or aluminum allows.
Iron, steel and brass, especially for medium to large parts, can be cast by the age-old foundry method of a wooden pattern and a sand mold. Another form of casting, known as investment casting, is an option for smaller parts or parts requiring a better finish and greater accuracy than the sand-casting method.
Now, to answer your question: For medium to large-sized parts that can be sand cast, there's often no need for a prototyping process. A pattern is needed for either one piece or a thousand, and once you have the pattern, recyclable foundry sand is your mold. So essentially the prototype and production processes may be one and the same. However, stereolithography can be used to create a plastic pattern rather than wood. Wood is much more durable and should be your first choice if changes aren't anticipated and volume production is imminent. Sand casting can be used for almost any kind of metal.
For parts that will be die cast in volume production, two main prototyping processes are available. Investment casting (the lost wax process) will produce excellent imitations of die-cast parts. Investment casting requires a master pattern, which is usually made by stereolithography. A silicone rubber mold is made from the pattern, and then wax replicas of the master pattern are cast. The wax replicas are coated with plaster, the wax melted out, and molten metal is poured in under a vacuum to eliminate trapped air. The plaster shell is destroyed when removing the part, but the process of investing is fairly quick and even robotic in many facilities. The tooling typically will cost about $2,000, depending on complexity and size. But this is a lot less expensive than a $35,000 die casting mold.
Large die cast parts are often made by the plaster mold process. This process is similar to investment casting in that the mold is destroyed with each part made. But a "hard" master pattern is used to make the two-piece plaster mold rather than melting out the wax.
Investment casters can be found in the Thomas Register of American Manufacturers. Die casters sometimes offer plaster mold prototyping in their volume-production facilities. Ask those that don't offer it where to go for this service-many work with outside sources. Stereolithography and sand casting can also be found in the Thomas Register.
Jack Lander is a prototyper for inventors. Prior to starting his own business, he worked for several years as a corporate manufacturing engineer and later, as a mechanical design engineer, acquiring 13 product patents. You can contact Jack at (203) 792-1377 or visit his Web site, The Inventor's Bookstore, at www.inventorhelp.com.
The opinions expressed in this column are those of the author, not of Entrepreneur.com. All answers are intended to be general in nature, without regard to specific geographical areas or circumstances, and should only be relied upon after consulting an appropriate expert, such as an attorney or accountant.
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