Magnesium injection molding (MAG) shapes parts out of magnesium alloy in a semifluid state, without the need of binders, resins, or sintering. This gives the relatively new technique an environmentally friendly tag. MAG is a socalled “thixomolding” process — the name comes from the word “thixotropy,” the property of certain fluids to exhibit changes in viscosity depending on sheer stresses. MAG produces net or near-net parts.
Companies such as Phillips Plastics Corp. in Hudson, Wis. (phillipsplastics.com), that provide MAG say the technique combines plastic injection molding and die casting (a process in which liquid material is poured into a mold with a shaped cavity and then allowed to solidify). To start, chips of magnesium alloy are fed into a thixomolding machine’s heated screw and barrel. The press thermally and mechanically processes the alloy into a semifluid state and then injects it into the mold cavity. A vacuum assists in the filling of the molds.
MAG components are made at temperatures 50 to 100°F cooler than die-cast parts. At these temperatures, the magnesium flows almost like a thermoplastic, with a controlled, laminar flow. Part weights range from about 4 lb (a large electronics enclosure, for example) to under 0.5 lb (a housing for a tiny radio, for example). The magnesium is a combination of AZ91D and AM60B alloys.
MAG produces parts lighter and stronger than aluminum. MAG parts have low porosity, good surface finishes, excellent strength-to-weight ratios, and built-in EMI/RFI shielding. In addition, the parts have low shrinkage rates, no air entrapment, and a uniform microstructure.
Though the MAG process has been difficult to manage in the past, it is now an established, proven technology, says Phillips. In fact, the company says there are new options such as overmolding, special plating finishes, and large parts of up to 200-in. total projected area including gates, runners, and overflows.
In one application, the technique handled the production of White Plains, N.Y.-based ITT Industries’ (itt.com), Tactical Radio, said to be the smallest and lightest frequencyhopping, secure VHF device in the world. MAG fabricated the durable yet lightweight housing, which had a complex geometry and thin walls. In addition, a proprietary elastomeric resin was developed to meet adhesion and durability requirements for a soft-touch overmold.
In another case, Niebel Engines in Edinburgh, Ind., needed an innovative, lightweight, and strong set of magnesium rocker-shaft covers for a race-car engine. Here, production entailed adjusting a previous supplier’s poorly designed mold by modifying the feed system and changing the gate location. The customer loved the right-off-the-press smooth metal finish because it complimented the high-tech look of the engine. Better yet, the new cover set weighed 5 lb less than the old design.
Finally, supplier of lawn and garden equipment, The Toro Co. in Bloomington, Minn. (toro.com), also needed help with a single-cavity mold for an impeller on its leaf vacuum-blower-mulcher. The mold could not produce parts fast enough and the company saw a substantial order backlog in a short period of time. In this case, what was needed came in the form of a two-cavity mold. It easily got the company back on track and handled the high-volume production.
Edited by Leslie Gordon