PET(polyethylene terephthalate), preferably scrap material, is extruded by a twin screw extruder(4) while being degassed. Melt pressure is measured in front of and behind a filter screen(11) at the output end of the extruder. Dependent on the measured pressures the metering of PET material into the extruder and/or the extruder speed are varied. Molten plastic enters a spinning head(13) following the filter screen and emerge in the form of a band which is subsequently cooled and stretched. The PET material is fed to the extruder by a metering screw(3) which under feeds the twin extruder screws so that they are only 25-60%, preferably 30-50% full. Both screws rotate in the same direction. Degassing is effected by a pump. PET melt is fed to the spinning head(13) by a melt pump(12). Band emerging from the spinning head is cooled in a water bath.