They simply soften as they heat up. These materials (PC, PVC) can be shear heat sensitive so feedscrew meter depths are usually greater to reduce the shear rates in this area. Compression rates are also lower to avoid high shear.
Crystalline or semi-crystalline materials must go through a heat of fusion to relax the crystalline regions before they will melt. (Ice for example must melt entirely before the temperature will rise). These materials (Nylon, PP, PE) require shear heat energy to facilitate melting. Feedscrew meter depths should be shallower to increase the shear and contact with the barrel. Compression rate is also higher to facilitate shear. If these resins contain glass fiber filers then the compression rate will decrease depending on loading to avoid breaking the glass fibers.