Shell moulding is also similar to sand moulding except that a mixture of sand and 3-6% resin holds the grains together. Shell moulding also uses sand with a much smaller grain than green-sand. Set-up and production of shell mould patterns takes weeks, after which an output of 5-50 pieces/hr-mould is attainable. Aluminium and magnesium products average about 13.5 kg as a normal limit, but it is possible to cast items in the 45-90 kg range. Shell mould walling varies from 3-10 mm thick, depending on the forming time of the resin.
Shell moulding is used for small parts that require high precision. Some examples include gear housings, cylinder heads and connecting rods.
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It is also used to make high-precision moulding cores. This process makes it so complex parts can be cast with less labor. |
There are a dozen different stages in shell mould processing that include:
| 1 |
Initially preparing a metal-matched plate |
| 2 |
Mixing resin and sand |
| 3 |
Heating pattern, usually to between 505-550 K |
| 4 |
Inverting the pattern (the sand is at one end of a box and the pattern at |
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the other, and the box is inverted for a time determined by the desired |
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thickness of the mill) |
| 5 |
Curing shell and baking it |
| 6 |
Removing investment |
| 7 |
Inserting cores |
| 8 |
Repeating for other half |
| 9 |
Assembling mould |
| 10 |
Pouring mould |
| 11 |
Removing casting |
| 12 |
Cleaning and trimming. |
The sand-resin mix can be recycled by burning off the resin at high temperatures. |
