I'm looking to make composite body tubes and I was wondering if anyone used pvc as a mold? Would a good release agent be enough to slide the tube off, or am I better just using cardboard and soaking it out after the cure.
Normal release agents won't work. There's no way you will be able to just 'slide the tube off'. what's needed is a release agent that will provide a decent gap between the product and the mandrel. Having said that even then the mandrel must be well covered with a non setting release so the 'gap material' can slide off.
I have been experimenting with molten candle wax. The idea is to melt some candle wax and brush it on the mandrel until a 5 to 10 thou thickness is built up. It would be better to spray it but difficult unless a hot spray gun can be used. Brushing leaves marks but they could possibly be smoothed out by using a hot air gun. If the wax is first applied with a brush the wax could then be re-melted to a smooth finish with the hot air gun.
For releasing it's simply a case of melting the wax. This can be done by sealing one end of the tube and pouring boiling water (or hot cooking oil) into the hollow mandrel. The wax melts and the tube releases. Hope that helps.