Machinery's Handbook, 31st Edition
1502 Shell Molding 5) The assembly is placed in an oven for a short period of time to complete curing of the resin. 6) The shell mold hardens around the pattern. 7) The mold is removed from the pattern using built-in ejector pins. 8) The second half of the shell mold is made, using the same steps. 9) Once complete, the two half shells are bonded or clamped together to form the mold. Support is provided by sand or metal shot in the flask when pouring the molten metal. 10) After casting and cooling, the part is cleaned and trimmed.
Fig. 9. Shell Molding Process, also called Dump-Box Technique: a) Pattern Heated and Clamped over a Box; b) Box Inverted; c) Box Repositioned; d) Half Shell Stripped from the Pattern; e) Shell Mold, Supported by Metal Shot in Flask, Ready for Poured Metal; f) Finished Casting Expendable-Pattern Casting Sometimes referred to as a lost foam process, expendable-pattern casting involves using a unique mold and pattern that must be produced for every casting. The pattern used in this process is made from polystyrene foam, which is 95 percent air bubbles; the material evaporates when liquid metal is poured on it. The pattern is made by molding. Polystyrene beads and pentane are put inside an alumi- num mold and heated. The material expands to fill the mold, taking the shape of the cavity. Then the pattern is removed from the mold and used for the casting processes as follows: 1) The pattern is dipped in a slurry of water and clay (or other refractory grains). It is dried to form a hard shell around the pattern. 2) The shell-covered pattern is placed in a container with sand for support. 3) Liquid metal is poured into a hole at the top. 4) The foam evaporates as the metal fills the shell. 5) Upon cooling and solidification, the part is removed by breaking the shell.
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