Originally Posted by
alastairduncan
Bob is correct but there may be a way to work around the problem of the shell strength. Thicken the shell with a meterial that will harden so that you have the cross sectional thickness you require, you mention clay but you could use something like milliput, car body filler or epoxy glue with some stiffening agent in it. Once it has hardened and it has been cleaned up, pack some of the Delft clay into the concave side and place this on a flat surface. Overfill it slightly and press it down onto the flat surface so that it is stable. Place the mould plate over the top and pack the casting sand over the top. When this is done turn up the other way and use a cocktail stick to loosen the sand in the shell and remove with a soft brush. Use the cocktail stick to ensure that your joint will be in the correct place and clean any residue away with the soft brush. Add the other mould plate, use the talc as a separator and pack the other side of the mould with the Delft clay. Split the mould and cut your sprues in the usual way. Be prepared to do this more than once ;-). I've not tried this but it should work provided the sand stays in the concave side of the shell. You may have to may have to place a flat surface over the upturned shell and turn both up the right way together so that the sand does not fall out.
I hope this helps.
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