I received this follow-up question in a recent blog and decided to answer it in more detail using the blog...
MJ
Which kind of process parameters vary when the screw becomes worn?
My Response
Basically, the most common affects that arise from screw wear relate to the increased back-flow over the flights of the screw. This back-flow increases mixing, but also lowers material conveyance and increases residence time distribution.
As the screw wear increases, the number of revolutions necessary to recover the shot will also increase... thus resulting in more shear heating, a higher melt temperature, and possible polymer degradation.
Additional Thoughts
Since the melting and conveying properties of the screw become compromised, excessive screw wear can also cause a significant increase in the amount of energy which is consumed during screw recovery.
For a related discussion: Tolerance For Screw Wear
-Andy
Andy.
When screw becomes worn, screw recovery time will increase?
Milan
Milan,
Exactly... the wear will increase material back-flow which will reduce conveyance and eventually increase recovery time.
-Andy