6. Extrusion – calendering

6.1 Presentation

The differences compared to traditional calendering lie in the fact that the cylinders are cooled, that the material bulge upstream of the air gap is much smaller (between 2 and 3 times the air gap) and that the calender frame, often referred to as a "calandrette" for this reason, is much less rigid than in traditional calendering. A key issue with this "finishing calendering" is matching the calender's throughput to the extruder's throughput. This is, in principle, a delicate task:

• if the flow rate of the extruder and its flat die is higher than the flow rate of the calender, the bead will grow and cause the sheet to widen until it overflows on both sides of the cylinders;

• If, on the...