Using them only in the melting section provides for greater design freedom and negates the impact these devices can have on output and melt temperature.
It’s not unusual for polymer to exit the extruder with a uniform melt temperature, only to be fouled up by downstream flow pipes and the like. Here’s how to avoid this.
Processors ask screw designers this question a lot. But neither output nor melt temperature can be predicted without details on the polymer and head pressure.
The compression ratio of a screw does not provide enough detail on how it will perform. Screw design is a balancing act that takes many variables into account.