I found this interesting article about the feed rates and cooling of plastic.
This is good information that will help both the sheet press and extrusion machine development.
Like most materials, plastics shrink as the temperature of the plastic decreases, but they usually shrink a lot more than other materials. Plastics shrink at one rate when they are in the solid (frozen) state, but they shrink much more when they are still soft or in the molten state. The problem for the profile extruder is controlling this shrinkage when cooling the hot plastic, coming out of the extruder, all the way down to room temperature. Let’s take the simplest example of a flat sheet where one side cools faster than the other. When still soft both sides are shrinking at the same rate. Even if one side is cooling faster and shrinking faster the other side is still pliable enough to come along with the other shrinking side. However, once one side cools past the crystalline temperature or its glass transition temperature, two things happen. First, that material stiffens and is no longer pliable enough to follow the other side and the rate of shrinkage goes down significantly. It is as if the stiffened side is no longer shrinking while the other pliable side continues to shrink. Therefore, as the pliable side continues to shrink it is pulling on the stiffened side and causing a bow in the direction of the side that cooled last. In this example, and in other simple profiles, the part will bow in the direction of the material that cooled last. In more complex profiles the parts may twist, distort, or warp in all types of fashions depending on which sections of the part cooled last. We’ll cover more on this later.