Appliance Design - February 2008 - (Page 48) JOINING Think Zinc rather than joining a magnet and pre-manufactured steel shaft with a zinc alloy bond, the shaft is eliminated from this assembly. It is formed as it is cast directly to the magnet during the joining process. Injected metal process joins small parts instantly. M by les agnew Les Agnew is manager, engineering and development, FisherTech, Div. of FisherCast Global Corp., Peterborough, Ontario. 48 applianceDESIGN February 2008 olten zinc alloy is not commonly considered as an instant curing “glue,” nor as an alternative to instant-cure adhesives for bonding small components. But it has been used for more than 60 years in FisherTech’s Injected Metal Assembly (IMA) process to join components in much the same way as adhesives. While zinc alloy can’t replace adhesives in all applications, it can do so in many assemblies without the problems of peeling and thermal degradation inherent with conventional adhesives, and without the need for special surface preparation. The process of using zinc alloy as a bonding agent resembles that of using injection molding around inserts. Zinc alloy can join a wide range of materials such as metals, plastics, ceramics, glass, paper, engineered synthetics, textile fibers, and elastomers. The method of joining the materials is such that part-to-part consistency is ensured even over long production runs. The IMA process has much in common with adhesive bonding, such as excellent stress distribution, joining of dissimilar materials, and joining materials of differ- ing thickness. But the molten alloy bond requires little or no surface preparation of the materials. The bond also performs well in harsh environments where only specialty adhesives and a few injection-molding resins could maintain their integrity. In the IMA process, the components are automatically positioned in a precision fixturing tool. In less than 20 milliseconds, the molten alloy is injected into a die cavity between the components being bonded. The alloy solidifies in less than a second, and the completed assembly is ejected from the tool ready for use. The alloy has a predictable 0.7 percent shrinkage that is compensated for in the tool design. While the injected molten alloy exhibits temperatures up to 815 DegF (435 DegC), the solidification speed prevents the materials from thermally degrading. Any heat distortion stresses are extremely brief, as the solidification behavior of the zinc alloy mitigates them within seconds. In the case of plastics, zinc’s extremely high thermal diffusivity (up to 100 times higher than that of plastic), lets solidification complete before the thermal influence zone of the heat-sensitive www.applianceDESIGN.com http://www.appliancedesign.com
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