REDUCING TOOLING LEAD TIMES

176 FUTURE DEVELOPMENTS IN DIE CASTING product engineers and designers the ability to evaluate designs quickly. These same rapid prototyping techniques can be applied to quickly manufacture die casting cavity inserts for use in high integrity die casting processes. 12–14 As the accuracy of CADCAM technology advances, carbon electrodes are being machined with greater precision for use with plunge electrodischarge machining EDM. Many die-making companies are also using prehardened tool steel inserts manufac- tured using EDM technology. As carbon electrode technology ad- vances, tool shops may be able to produce die cavities with a single electrode, eliminating the iterative EDM process commonly used today. Stereo prototyping methods are also being developed for direct production of die cavity tooling. Instead of using liquid resin, laser sintering with metal powder has been used in the development of this technology. Experimentation is underway utilizing several dif- ferent nontraditional metal powders, including polymer-coated zir- conium diboride, bronze–nickel mixtures, and 316 stainless steel. Casting cavity inserts are also being produced using an indirect form of rapid prototyping. Wax patterns of a casting cavity can be produced using polymer stereo lithography technology. These patterns can then be investment cast, producing die cast tooling inserts in H13 tool steel. However, the investment casting method has several problems. Large metal masses such as a casting cavity inserts are difficult to cast, resulting in porosity and distortion. To overcome these problems, experimentation is underway to test hollow casting cavity inserts investment cast with a uniform thick- ness.

12.7 LOST-CORE TECHNOLOGIES

A major limitation of high integrity die casting processes is the ability to produce complex internal geometries. Simple coring is regularly performed, but high integrity castings cannot be pro- duced with undercuts. However, most casting processes that do not utilize reusable molds regularly produce complex internal ge- ometries, as illustrated in Figure 12.3. With such casting pro- cesses, the core is lost. In the cases of resin-bonded sand, the heat

12.8 CONTROLLED POROSITY

177 Figure 12.3 Hollow a aluminum automotive suspension arm and b resin- bonded sand core. Courtesy of Teksid. of the metal causes the resin to combust and the core can be removed from the casting as loose sand. Die casting component producers have experimented with lost- core processes for several years. Finding a material is difficult. Several conflicting requirements must be met. The core material must withstand erosion during metal injection. The material must remain dimensionally stable under high pressures. After solidifi- cation, the core material must be easily removed. Trials have been conducted using salt core materials and low melting point alloys. Although these materials survive the metal injection, removal of the cores is time consuming and costly. Resin-bonded sand was abandoned years ago because sand cores could not stand up to the high metal injection velocities encountered in die casting. However, this technology is being re- examined for use with squeeze casting. Since metal velocities are significantly less for squeeze casting, sand cores may survive. Ex- perimentation is currently underway to prove this potential solu- tion.

12.8 CONTROLLED POROSITY

Porosity is a defect commonly found in die cast components. However, if the porosity does not affect fit or function, one cannot call it a defect.