Oct 14 2004
The "X-Melt" expansion injection molding process developed by Engel Austria GmbH opens up a whole range of new possibilities for manufacturing ultra-thin-walled components. It enables cost-effective production of wall thicknesses significantly less than 0.5 millimeters.
Bayer MaterialScience AG has developed a customized blend of polycarbonate and acrylonitrile-butadiene-styrene copolymer (PC/ABS) especially for this process. The qualities of the flame-retardant Bayblend® grade are demonstrated in battery covers for cellphones. Engel currently manufactures this component with a wall thickness less than 0.18 millimeter. "Up till now, this type of PC/ABS cover couldn't be made so thin," says Hans-Joerg Dahmen, an expert for polycarbonates in Business Development at Bayer MaterialScience. He sees considerable potential for applications in cellphone components, for example battery covers and casings and cellphone casings. The material could also have many uses in the production of micro-precision components.
In the X-Melt expansion injection molding process, the screw exerts pressure up to 3,000 bar on the plastic melt in the cavity in front of it, compressing the melt by around 10 percent of its original volume. A special shut-off system then opens, allowing the melt to expand instantaneously into the mold. The cavity fills in a few hundredth of a second with melt front speeds of up to five meters per second. "The mold can therefore be filled quickly and evenly without the melt setting too quickly," explains Hans-Joerg Dahmen. Injection times are extremely short, typically ranging from 10 to 50 milliseconds depending on the size and wall thickness of the component. This results in cycle times of less than five seconds.
The Bayblend® grade has the perfect qualities for expansion injection molding. It is highly compressible, a fundamental requirement for use with this technology. Its processing stability means it is also able to withstand the long periods in the heated screw and its flowability allows the production of thin walls. A further strength is its high degree of mold surface reproduction, ensuring that even very fine, small script is replicated exactly.
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