HPDE is also non-stick, but not as rigid as acetal.
This is my prefered UK plastics supplier and the product in question-
http://www.directplasticsonline.co.uk/Acetal_Sheet.html
Tip-
Finding the line and splitting a two part mold can sometimes be a pain, even with mold release, smear a very thin layer of vaseline along the top edge of the mold (not the bottom as it could cause leaking) to ensure you can find the line and pull the mold apart instantly with no hassel. Even lazier method- put a "tab" of sellotape on one corner of the mold before pouring the second half!