338 Section 4 Nonferrous Metallurgy Plunger injects liquid metal Gooseneck Die Die locked in place during metal injection Liquid metal fills plunger cylinder Liquid metal Goodheart-Willcox Publisher Figure 15-18. With hot-chamber die casting, a vertical plunger forces liquid metal into a “gooseneck” and thus into a closed steel die, without exposure to air. The liquid metal solidifies rapidly and takes on the exact shape and surface texture of the die cavity. There is no possibility of oxide, or dross, getting into the cast part. Die cast parts are near net shape as they exit the molding machine. Often the parts may be used without further surface finishing. Die casting dies must be reworked occasionally to maintain the dimensions of the molded parts, due to erosion from the liquid metal. Steel alloys for die casting molds are selected to minimize heat checking, which is the formation of surface cracks due to repeated thermal cycling. If the melt temperature is too low or the ram velocity is too low, then liquid metal will not get all the way into the part—a situation called a short shot. Such a part is remelted. When the ram pressure is too high or the dies are worn excessively, then metal is forced between the die halves, creating a thin flash of metal on the part. Flash is usually removed by manual grinding. Whether the molding machine is controlled automatically or manually, molding operators and technicians must pay close attention to die molding process conditions to assure consistently high yields and sound parts. Improving Cast Properties by Hot Isostatic Pressing (HIP) Pores form in most nonferrous alloys during casting at atmospheric pressure, due to the shrinkage from liquid to solid and from gases coming out of solution as the liquid solidifies. In a process called hot isostatic pressing (HIP), the casting is compressed in an isostatic chamber at high pressure and temperature, so that pores in the metal are closed and healed. The term isostatic means that pressure is applied equally from all directions. Using HIP, the yield and tensile strength, elongation, and fatigue strength of the parts increase. Aerospace and critical-to-safety castings often require HIP processing. Copyright Goodheart-Willcox Co., Inc.