If its not too late.. use ACETAL all the way.
I know its more expensive, but it machines so much cleaner and nicer than HDPE, and is also more rigid, while having the same "anti stick" property.
Ive worked a fair bit with both materials as and when I move to this method Ill be using acetal for sure.
directplasticsonline are an awesome supplier.
http://www.directplasticsonline.co.uk/Acetal_Sheet.html
AH