2. Tools

2.1 Extruders

The sheath blowing process uses relatively long single-screw extruders, i.e. with a length-to-diameter ratio of between 24 and 32. Screw diameter D largely determines throughput. The use of a grooved barrel with a length of around 4D under the hopper increases throughput. The presence of grooves increases the coefficient of friction between the pellets and the sheath, resulting in more efficient conveying towards the front of the screw and, consequently, higher throughput. Unlike the rest of the sheath, this small section is cooled to avoid clogging problems at the bottom of the hopper.