Current economic pressures are putting new focus on injection molding, and in particular new processes that can even improve quality while cutting costs.
One idea to consider is gas-assist molding, especially when combined with overmolding. Gas assist is a process gaining speed right now. The basic idea is to introduce an inert gas, such as nitrogen, into the mold cavity after it partially fills with plastic. The gas presses the melt stream to the mold surface. It's a good way to eliminate sink marks that can be caused by shrinkage of thick sections. It's also a good way to improve surface finishes for cosmetic parts. And, of course, it's really effective at cutting resin costs because the gas can displace a substantial amount of plastic on large production runs. Because you need less force to clamp the press, you can move to a smaller press size, sometimes substantially smaller. Your production rates are hiked because cooling times are reduced.
It's estimated that at least half of the 100 largest injection molders in the U.S. now offer gas assist, or its newer cousin, water injection molding. Water is incompressible and can generate more pressure than gas. Water also has better heat exchange capability than gas, allowing parts to be cooled inside and out.
Three of the design winners at the recent awards competition held by the Alliance of Plastics Processors featured gas assist and are described in detail in the following report. One process even includes overmolding, in which a second injection barrel introduces a material that complements the properties of the core. Often, this is a thermoplastic elastomer that allows a firm, comfortable grip.
And injection molding isn't just about plastics.
Metal injection molding is growing at a 7-8 percent rate in the U.S. and close to 11 percent in Japan. It should be seriously considered whenever volumes start rising (at least 10,000 parts per year for most designs) for small parts with complex geometries. The savings grow if there are requirements for features, such as holes, that can be molded in. The MIM process produces high-density parts with mechanical properties that are generally superior to powder metallurgy and comparable to materials that are wrought.
Another thought is to use plastic injection molding instead of traditional metal processes. One significant example is plastic gears, which have received a bad rap in some design circles but for the wrong reasons. You can't apply rules of thumb used in designing metal gears to plastics. You really have to start from scratch when designing a plastic gear.
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 These orthodontic brackets are designed and molded by Oregon-based World Class Technology |
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