Bemis Manufacturing Co., Sheboygan Falls, Wis., a custom molder known for its expertise in coinjection, has found a way to reduce the cost of solid parts by using a well-known gas-assist molding technique—without the gas. For years, Bemis has used coinjection “sandwich” molding to embed a core layer of reprocessed scrap between skins of virgin material as a cost-saving measure. But Bemis had bumped up against limits on how much core material it could get into its parts because it could not distribute the core layer throughout the part. According to Gary Vande Berg, director of engineering for injection molding, in a paper he delivered at the Molding 2011 Conference in San Diego this spring, experiments showed that 10% to 15% more core material could be injected by using overflow wells such as are used to receive material displaced by injected gas in hollow-core molding. Instead, Bemis used overflow wells to receive skin material displaced by core material. This helped extend the core layer throughout more of the part.
This saves money in two ways: First, by incorporating more scrap material. Second, by enabling use of a lower-cost material in the virgin skin layer. That’s because the sandwich structure produces a “plywood effect,” according to Vande Berg, which can increase impact strength threefold over a homogenous structure. That avoids the need for a more costly high-impact material in the skins.
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