Help with using a plastic mould and plaster of Paris ?

Coating the mold with talcum powder should do the trick!

11 Plaster of Paris tips | Top Tips - MontMarte

https://www.montmarte.net/.../11-plaster-of-paris-tips

here is more info

https://www.ehow.com/how_5682162_keep-plaster-paris-sticking-mold.html#:~:text=Coat%20the%20mold%20with%20talcum,moisture%20from%20the%20mold%20itself.

A couple of choices, petroleum jelly, mold releasing spray, talcum powder.

Spray the mold’s surface with a mold release agent purchased at most art supply stores or hobby shops. The mold release agent will form a layer between the plaster of paris and the mold that prevents the plaster from sticking to the mold’s surface. Only a light layer of release agent will be required.

Spray cooking oil like Pam

coat it in oil - like a spray cooking oil.

I would rub in some olive oil really well.

Try spraying it with non stick spray or you could line it with foil

Hi Georgia, Plaster of Paris doesn't normally stick to plastic, but you can use a mold a release agent (available at most art supply stores) or use vegetable oil, mineral oil, cooking spray or petroleum jelly. Just be careful with the oils, they do tend to stain the Plaster of Paris.

I would go with a mold releasing agent since they're made for the job. Cooking spray might leave a sticky residue or stain behind.