Investment Castings (The Lost Wax Process)
Touted as the process that will get the manufacturer of complex pieces to meet near-net shape reproduction, investment casting eliminates certain machining, finishing and assembly steps. Holes, slots, cutouts and undercuts can be cast to size. Complicated internal configurations and exterior shapes such as thin walls, blind pockets, internal seats, fillets and radii can be achieved.
Dating back to 4000BC, investment casting is accomplished with an aluminum die comprised of upper and lower halves. A hole in the outside of the die accepts a nozzle for wax injection. Heated to 135-140F and pressurized to 300 psi, wax fills the cavities of the die. After cooling and removal from the die, the wax pattern is dipped into a ceramic slurry six to eight times to generate a 3/8" to 1/4" thick ceramic shell. A steam autoclave melts out the wax leaving the shell.
The shell is heated to 1500F-1800F and metal is poured into it. When the metal has solidified, the shell is broken away. Thousands of years ago a new original had to be created every time. Today, precise wax patterns are mass produced from aluminum molds and attached to "trees" for volume production. A tolerance guideline is 0.005 in./in.
Materials suited for casting include 300 and 400 stainless, plain carbon steel, Monel, tool steel, brass and bronze. According to one source the attitude is that if the metal can be melted it can be poured.
Precise, repeatable and predictable investment casting yields lower cost and higher quality for communications, food-process equipment, medical, dental, transportation, valve and fluid-handling applications.
Implementation of SPC, TQM and computerized processing of information are important enhancements of this precise operation. Freedom of design, alloy selection, cost savings, elimination of weldments and inventory reduction are important features.
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