Q. can you explain Shell molds?
These molds are prepared by pouring a mixture of sand and thermosetting resin over the heated surface of a metallic pattern, which results into the formation of a thin and rigid layer or shell of uniform thickness around the pattern, which, when separated from the pattern surface, forms one part of the shell mold and two such parts are joined together to form the complete shell mold.
The process of making shell mold, a brass or aluminum pattern is clamped on a steel plate which also carries projections to produce runners, risers, etc. in the mold cast on this plate. The assembly of pattern is heated to 1750C to 3700C and later sprayed with a silicon release agent to help easy removal of mold shell from the pattern. A molding mixture comprising fine silica sand and 3 to 10 % synthetic resin is prepared and dumped over the hot pattern and plate assembly held in a box and is allowed to remain in contact with hot pattern assembly for about 30 seconds. This results in the softening of resin and formation of a shell or coating of uniform thickness around the pattern assembly. Latter the assembly of pattern with the formed shell is cured by heating to a temperature. Example of shell mold cast products include: cylinder heads, connecting rods, gear housings and other mechanical parts.