To crumble hash, it is best to use a metal grinder , also known as an aluminum grinder. This is because of the force required to properly crumble hash. A plastic grinder can easily break through this, so this is always not recommended. Depending on the hardness of the hash, in some cases it is possible to use a plastic grinder, but this is at your own risk. A metal grinder is strong enough for crumbling hash.