There are several reasons parts can be stuck into the tool and hard to remove.

1. Insufficient or the wrong type of release agent was used

Use the type of release agent recommended on the packaging.

2. The mould is old and rigid

Silicone moulds have a lifetime of 20 to 30 casts. Over that time the surface of the tool will be attacked by the hardener and will become brittle and hard.

You can see if a tool is old and brittle just by looking at it; if the otherwise transparent silicone turns opaque and brown in colour it is nearing the end of its life.

A new silicone tool may be required

3. Undercuts are too big

If overhangs and undercuts are too large, cut the tool to give loose pieces that enable the part to removed without having to stretch the mould too much.

4. Insert nuts have not been removed

Check that any screwed connections or insert nuts have been removed and try again.