Extruded plastics are cooled in a variety of ways-by water, air, and chill rolls. Chill-roll cooling has the most easily definable cooling calculation, but I still see considerable ambiguity when it comes to the selection of roll stacks for sheet. For sheet extrusion it's common to have some geometric variation of a three- roll vertical stack, with possibly some additional rolls for very high-speed lines where the contact time is short.