Can you use a glass mold when casting clear polypropylene resin or will it bond with the glass?

Answer 1

You do not want to use a glass mold when casting resin.

Resin is a two-part compound that, when mixed in proper proportions, becomes completely hard when dry. When casting resin, you want to use a flexible mold, such as a silicone mold, to cast it.

Once the resin has hardened, you can flex a silicone or rubber mold, stretching and bending it to allow air between the mold and the shape, which allows the mold to release the resin form. The mold will revert back to its original state after the form has been released.

A glass mold has no flexibility. If you cast resin in a glass mold, the resin will harden exactly the size of the cavity in your glass mold. While it will not bond to the glass, the result is the same. Once the resin is hard, there will be no way for air to come between the resin form and the glass mold. The resin will essentially be vacuum-sealed in place. If you have a more complex form, such as a rose shape, the resin will be stuck in place permanently. A more simple shape, such as a square, might release over time. If you set the mold upside-down, a combination of gravity, and the effects of temperature and humidity on the glass and resin, may release the resin after many weeks or months.

If you have already cast your resin into the glass mold, your only option will be to wait to see if the cast will eventually release, or carefully break the glass mold from around the resin.