You should be able to, but it might stick. I have had a real problem getting my soap out of molds like the Milky Way soap molds. If you rub some mineral oil or vaseline, a light coating, in the mold it should really help. You might also want to put them in the freezer before you unmold, for an hour or two. Silicone is the easiest to unmold. I wait about 24 hours to unmold. What type of plastic mold is it? Is it for soapmaking?