Surging
Vol.20 #2, July 1993
Single screws seldom have a perfectly matched feeding and pumping rate. If they do, its only at one screw speed and/or set of operating conditions. Consequently, it's necessary to have sufficient head pressure to assure that the metering section is filled for at least several turns or the output may be unstable.
This type of instability (or surging) is usually characterized by a steady, rhythmic surge that defies correction with operating conditions. It's very prevalent in two stage screws where the second stage has been designed for excessive capacity verses the first stage.
If you experience this kind of surge, addition of screens, restrictive breaker plates or other means of building head pressure will improve stability. Although you may not completely eliminate the instability with increased head pressure you will see a reduction in the instability or the surge is probably due to another cause.
See also:
- Extra breaker plates
- Head pressure
- Patience
- Screen packs 2
- Screw cooling
- Vent location
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Extrusion Hints