Can I reuse cured silicone in a new mould

Yes. Old moulds and casting leftovers can be cut or ground into chunks and cast into freshly mixed silicone rubber. The chunks fill volume, the fresh rubber forms the matrix that binds everything together. On larger block moulds this saves a considerable share of your new material.

The method is as follows. Use clean material only, chunks carrying residues of release agent, silicone oil, wax, plaster or resin will not bond. Cut or grind to pieces of roughly five to fifteen millimetres and make sure they are bone dry and free of grease. First pour a pure face coat of five to ten millimetres against the model, so the chunk material never touches the casting surface. Then build up in layers, scattering the chunks into the still liquid rubber and pouring fresh silicone over them so everything is fully enclosed.

Stay below roughly thirty to fifty percent chunk material by volume. More means too little matrix, and the mould falls apart.

There are two important limitations. Do not mix material of different curing types. Chunks from a tin cured condensation mould, or chunks carrying residues of release agent, sulphur bearing clay or latex, can cause inhibition in platinum cured addition silicone, leaving the fresh rubber uncured around those chunks. Always run a small test cast if you are unsure. Tear strength also drops, because tears propagate easily along the interfaces. Use the technique for simple block moulds, not for moulds with thin or fragile sections, and not for applications with requirements around food contact, skin contact or dimensional accuracy.

If you want to save material without compromising quality, use a filler core instead. Place a solid block of casting foam or closed cell plastic in the heart of the mould, leaving at least fifteen to twenty millimetres of silicone rubber all around. You do not pay for the volume the core occupies, while the mould stays solid and homogeneous around the model.

Read more: cure inhibition in silicone