Orthoses and splints for the orthopaedic and respectively medical sector are made of extruded plates. These are heated in ovens or on hot stages and then cooled off in molds. The shrinkage in the machine direction of the plate due to heat is vitally important. In particular the shrinkage due to heat of extruded PE plates of about 60 % can cause problems while thermo-forming. One part of this thesis was the development of a program, which computes the temperatures over the thickness of the plates starting from the nozzle till the end of the production line. For the verification and improvement of the calculation the surface temperatures of the extruded plates were measured with infrared thermometers. In the second part of this diploma thesis the production of PE-HD and PE-LD plates with a thickness of 3 and 4 mm was analyzed. Therefore the plates were produced with different settings of a solid sheet extrusion line and then the shrinkage was measured. The shrinkage due to heat was detected with a practical convection-oven test and with a standardized test (EN ISO 14632). Through these tests was determined that the temperature of the melt, the thickness of the plate, the temperature of the calendar stack, the nip of the tempering rolls and the bead in front of the first nip are the main influence factors for the shrinkage due to heat. Despite these results the shrinkage of the plates in the machine direction could not be changed considerably.