1. Other screw and extruder types

Degassing extruders (two-stage screw)

When volatile products can be released during the melting phase, which can be detrimental to the quality of the extrudate, it is customary to use a two-stage extruder, with intermediate degassing (figure 1 ). The polymer is melted in the first stage, which dictates the flow rate. This first stage is a conventional screw, generally with three zones. At the end of this stage, a severe restriction is imposed to create pressure and eliminate any trace of infondus. The second stage then starts with a very wide air gap, where the polymer from the first stage cannot occupy all the space. This partial filling allows degassing through a hole drilled in the sheath and fitted with a vacuum pump. The role of the second stage...