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INVESTMENT CASTING (LOST WAX) In the investment casting process, also known as the lost wax process,


wax is injected into an aluminum die to produce a pattern that is an exact replica of the part to be produced. Patterns are assembled in clusters around a coated sprue to create the casting tree, which is repeatedly dipped into an agitated vat of ceramic and allowed to dry. Typically, after a shell thickness of approximately 0.375 in. (9.525 mm) has been formed, the molds are dewaxed by fl ash fi ring at high heat [1,400F (760C)] or autoclaving (pressure and steam), and the wax is drained and recycled back into the process. The hollow ceramic shells then are preheated to 800-2,000F (427-1,093C), depending on the requirements of the alloy to be poured, and the molten metal is cast into the hot shell. The casting process is performed by either gravity-pour or vacuum (counter-


gravity) methods. After cooling, the ceramic is vibrated and blasted off the metal parts and discarded. The cast components are broken off the tree and run through cleaning/fi nishing processes, which are similar to those of other casting processes, such as cut-off, grinding, heat treatment, straightening and blasting.


WHY USE INVESTMENT CASTING? Diecasting and thixomolding require a higher tooling cost than invest-


ment casting. This makes a signifi cant difference in the economic analysis for low-volume production runs, such as prototyping. Pickholz noted that the cost per piece is higher with investment casting, so for high-volume markets such as automotive, fi nal production can be transitioned to other processes. Leonard touted the advantage of investment cast prototyping for its ability to mimic the fi nal output of more high-volume production methods. 


will degrade and fail if they are allowed to exceed their rated temperature. T e heat is dissipated through components designed into the cast part itself, gener- ally produced as A380 aluminum alloy extrusions or die castings. “T e issue is that diecasting is limited


in scope,” Pickholz said. “Our solution is to employ magnesium for three reasons: It is one-third lighter, it has similar thermal conductivity properties to ef- fectively remove heat from the LED and carry it to where you can dissipate it, and we can injection mold it via thixomold- ing for high volume production, which means we can do the same things with magnesium we can do with plastics.” As automotive manufacturers and


the transportation industry as a whole address rising fuel costs, lightweight parts are becoming increasingly critical. “When you’re talking about only


two-thirds the weight, it doesn’t sound like a whole lot, but when you put that in your hand and you feel that, it offers a weight savings like no other,” Leonard said. Pickholz noted the advantage of the


thin walls that can be achieved with mag- nesium. “You’re making [the part] with a low material content which, in addition to magnesium’s weight versus aluminum, makes the product extremely lightweight and very cost eff ective. You’re essentially ticking all the right boxes in the process while accomplishing what you need to do, which is to remove the heat from the lamp, from the LED proper.” T e shell system Aristo-Cast uses


produces a shell that is seldom over 0.1875 in. (4.76 mm) thick, versus the more conventional investment casting shell thickness of 0.375 in. (9.525 mm). “It enhances our ability to fi ll very thin sections, and it cuts material costs,” Ziemba said. Beyond weight savings, investment


casting enables part design without draft. “It allows the LED products to be as lightweight as possible, but it also allows the rendering of a suffi ciently smooth fi nish capable of yielding a class ‘A’ refl ec- tive inside light surface,” Leonard said. Aristo-Cast’s magnesium invest-


ment casting process has opened up a whole new industry for the company. “High-powered LEDs are the future of the lighting industry,” Leonard said.


32 | METAL CASTING DESIGN & PURCHASING | Sept/Oct 2012


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