In the diecasting process, metal is ladled into shot sleeves and forced under high pressure into the permanent mold or die. Te patterns are produced in dies via
injection molding. For the most part, the patterns are made of wax; however, some patterns are made of plastic or polystyrene. Because the tooling cost for individual wax patterns is high, invest- ment casting normally is used when high volumes are required. When cores are required, they are made of soluble wax or ceramic materials. Te ceramic shell is built around a
pattern/gating assembly by repeatedly dipping the “tree” into a thin refractory slurry. After dipping, a refractory ag- gregate, such as silica, zircon or alumi- num silicate sand, is rained over the wet slurry coating. After each dipping and stuccoing is completed, the assembly is allowed to thoroughly dry before the next coating is applied. Tus, a shell is built up around the assembly. Te required thickness of this shell depends on the size of the castings and tempera- ture of the metal to be poured. After the ceramic shell is complete, the entire as- sembly is placed into an autoclave oven to melt and remove most of the wax. Te majority of investment castings
weigh less than 5 lbs., but larger castings in the 10-30-lb. range are becoming more common. Castings weighing up to 800 lbs. have been poured in this process. Some of the advantages of investment casting include: • excellent surface finishes; • tight dimensional tolerances; • machining elimination or reduction;
• ability to produce titanium castings, as well as the other superalloys.
Ceramic Molding—Generally, these
processes employ a mixture of graded refractory fillers that are blended to a slurry consistency. Various refractory materials can be used as filler material. Te slurry then is poured over a pattern that has been placed in a container. First, a gel is formed in a pattern
2014 CASTING SOURCE DIRECTORY METAL CASTING DESIGN & PURCHASING 11
and stripped from the mold. The mold then is heated to a high temperature until rigid. After the mold cools, molten metal is poured into it, with or without preheating. Te ceramic molding processes have
proven effective with smaller castings in short- and medium-volume production runs. At the same time, the processes of- fer castings with excellent surface finish and good dimensional tolerances. Plaster Molding—Plaster molding is used to produce castings of the lower melting temperature metals, such as aluminum alloys. In the process, a slurry
containing calcium sulfate, sometimes called gypsum, is poured into a flask that contains the pattern. After the slurry has set, the pattern and flask are removed, and the drying cycle to remove the moisture from the mold begins. After the mold has cooled, the
cores and mold are assembled. After assembly, most molds are preheated before pouring. Because these molds have poor permeability in many cases, vacuum-assistance or pressure is re- quired during pouring. Plaster molding is well-suited to short run and prototype work.
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