BATAVIA, OHIO — Although co- injection — molding that uses one core material and another skin material — has been around for some 20 years, many molders still know very little about it, according to speakers at a recent conference. The Oct. 16 Benchmarking Program on Coinjection Molding, sponsored by the Society of the Plastics Industry Inc.'s Midwest Regional Office, drew about 80 people to Cincinnati Milacron Inc.'s Plastics Machinery headquarters in Batavia.
Bemis Manufacturing Co. of Sheboygan Falls, Wis., got into coinjection molding to consume its own regrind.
``We know we have a lot of plastics we have to dispose of, and what a beautiful way to dispose of it, by burying it inside the part,'' said Susan Braun, coinjection manager of the custom injection molder.
In 21/2 years of coinjection, Bemis has been able to whittle down its regrind pile, using its three 725-ton Cincinnati Milacron coinjection machines. But co- injection is about more than getting rid of scrap. Possibilities are nearly endless, she said, including molding a soft skin over a hard core, using foam on the inside to make sound-deadening parts for lawn mowers or cars, or combining a structural core molded of glass-fiber composites with a high-quality outer surface.
A molder also can use the technology to cut costs for colorants and additives.
``We've even run black inside white,'' Braun said. ``Eventually we would like to become a net purchaser of regrind.''
Speakers at the SPI Midwest conference said that German press-maker Battenfeld GmbH obtained some of the first patents for coinjection molding. Since then, Battenfeld has built a few hundred coinjection presses, but the technology never really took off in the United States, where Battenfeld has sold about 20 of the machines, Braun said.
Contacted after the conference, Wolfgang Meyer, president of Battenfeld of America Inc. in West Warwick, R.I., agreed with that number. He said coinjection at first was marketed to replace structural foam in large computer housings. But when computers got smaller, companies were able to use straight injection molding and did not need coinjection, he said.
Meyer said interest in the technology has re-emerged, especially in the automotive industry.
Milacron introduced its 725-ton VL725 coinjection press at NPE '94. The machine, with two injection units linked to a patented manifold, boasts better control over thickness of the skin and core materials, Milacron said.
Incoe Inc. of Troy, Mich., manufactures the V-shaped manifold. It has a pin that directs resin from one injection, or the other, or both. John Blundy, Incoe's vice president of business development, called the manifold ``a little machine in itself'' that delivers exact control of the pin, heat profile and melt consistency. The two injection units slide back for purging or access to the manifold, which itself can be easily removed.
Tony Lutarewych, Milacron national sales manager for injection presses, said all the companies — Milacron, Bemis, Incoe and resin supplier GE Plastics — had to work together to develop the improved technology.
``The machinery manufacturer can't do that without knowing what the processor wants,'' he said.
Braun said the savings add up with coinjection. Bemis officials figure that on a standard part, coinjection saves the company 18 percent on a per-piece basis. With engineering resins, savings can reach 30 percent.
Addressing the materials perspective was Jim Fassett, senior technical development engineer at GE Plastics' Midwest Region in Lisle, Ill. The core and skin materials should have a similar processing temperature range and viscosity. If possible, the higher-viscosity of the two materials should be used for the inner core. The skin should be the material that shrinks more, so that no gaps are created when the part cools, he said.
Fassett gave the results of GE Plastics' testing of a full-size Bemis mold to make a lawn mower deck, using combinations of various engineering resins.
Fassett said GE Plastics also is studying applications with a flame-retardant skin over a core of nonflame-retardant resin.
Attendees toured Milacron's Batavia injection press assembly operation. They saw a Milacron coinjection machine running a prototype lawn mower deck.