Single-thread three-zone screws with screw lengths of 24 to 35 D and a constant screw pitch have proven to be effective in extruding Desmopan® and Texin™.
Fig. 1 illustrates the structure and dimensions of this type of screw. The compression should be between 2 : 1 and 3 : 1.
Fig. 1: Extruder screw for processing Desmopan® and Texin™
Constant taper screws (PVC screws) can also be used.
The advantage of using barrier screws that are specially designed for processing TPU is that they permit a lower L/D ratio. Alternatively, throughput can be increased by selecting a screw of approximately the same length as a conventional screw. Fig. 2 shows the basic structure of a barrier screw that is suitable for extruding Desmopan® and Texin™.
Fig. 2: Barrier screw
Short-compression screws are not suitable for processing TPU.
Heating of the screw is not necessary.
Shearing and mixing sections are not needed for homogenizing Desmopan® and Texin™, although it may be an advantage to use mixing sections if particularly high color dispersion is needed, for example with thin film or coatings.
The shearing section (Fig. 3) and mixing section (Fig. 4) should be designed in such a way that the energy input is as low as possible. Dead spots must be avoided under all circumstances.
Fig. 3: Spiral shear section
Fig. 4: Toothed disk mixing section