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These days, pretty much everyone's familiar with the traditional rapid-manufacturing techniques that have come to dominate the market: stereolithography, or SLA, and fused deposition modeling, or FDM. However, these technologies have been limited to plastics; those wishing to rapid prototype in metal have had some options - electron beam welding, chiefly - but the end result has not always been visually appealing, nor has the technology advanced to the point where parts can be constructed from more exotic metals.
Magnesium injection moulding (MAG) seeks to change all of that. MAG shapes parts out of a magnesium alloy in a semifluid state, but without using binders, resins, or sintering - which gives the technique an environmentally friendly face. Best of all, MAG produces net or near-net parts, which means that unlike parts produced via SLA or FDM, parts produced via MAG are oftentimes "ready to use," with little additional reinforcement work needed.
MAG is a thixomoulding process. "Thixomoulding" comes from thixotropy, which is a property of some non-Newtonian fluids to become less viscous when in shear stress. This is in contrast with rheopectic fluids, which show a time-dependent viscosity which increases with respect to shear stress. Thixomoulding can also produce parts with a lower minimum thickness of 0.025".
Companies such as Phillips Plastics of Hudson, Wis. claim that MAG combines injection moulding and die casting (a process in which the fabrication metal is poured into a shaped cavity while still in liquid form, and then allowed to solidify). MAG begins with chips of magnesium alloy - specifically, a combination of AZ91D and AM60B alloys - which are fed into a thixo machine's heated screw and barrel. The chips are thermally and mechanically processed into a semifluid and then injected into the cavity; vacuum assists in filling the moulds completely. This process allows MAG parts to be manufactured at 50º to 100ºF cooler than traditional die-cast parts. The magnesium semifluid behaves much like a thermoplastic at these temperatures, and exhibits a laminar flow. Part weights can range from 4lbs to 0.5lbs.
The end result are parts that are lighter and stronger than aluminium. MAG parts have low porosity, good surface finishes, excellent strength-to-weight ratios, and built-in EMI/RFI shielding. These parts also exhibit very low shrinkage rates, no air entrapment, and have a uniform microstructure.
MAG processes used to be difficult to manage, although now it has become a proven technology that has evolved to include new options such as overmoulding, plating finishes, and large parts of up to 200 square inches. In one application, MAG parts were used in the production of ITT Industries' Tactical Radio, said to be the smallest and lightest frequency-hopping, secure VHF device in the world. MAG allowed for the fabrication of the durable yet lightweight housing, which features a complex internal geometry and thin walls. Additionally, a proprietary elastomeric resin was developed to meet adhesion and durability requirements for the soft-touch overmould.
In another case, Niebel Engines of Edinburgh, Ind. used MAG to create a lightweight set of magnesium rocker-shaft covers for a racecar engine. The customer loved the smooth metal finish of MAG, which was available "hot off the press." Better still, the new cover weighed 5lbs less than the old design.
MAG may see yet more applications in the future, especially in industries such as aerospace where weight is critical and strength-to-weight ratio is paramount. Its ability to produce parts that are strong, lightweight, and naturally shielded against electromagnetic interference ensures that designers will be conjuring new ways to implement this technology for years to come.
http://www.machinedesign.com
http://en.wikipedia.org/wiki/Stereolithography
http://en.wikipedia.org/wiki/Fused_deposition_modeling
http://www.sciaky.com/
http://en.wikipedia.org/wiki/Thixotropy
http://en.wikipedia.org/wiki/Rheopecty
http://www.cast-it.com/thixo_chart.html
http://phillipsplastics.com/
http://www.itt.com
http://www.niebelengines.com/