Screw Flight Diameter Along the Screw

I received this question as a follow-up question regarding a different blog, and decided it warranted it’s own entry.

Does the screw have the same diameter in the front, middle and back screw flight diameter?

My Response
The typical injection molding screw has the same outer diameter along the entire length of the screw. When measuring for wear, the highest amount of shear, and wear, typically occurs in the transition and metering zone.
I have seen some cases where a company may use variying clearances in the metering zone for mixing, or adjust the clearance in the feed zone to accomodate specific feed issues.
Some manufacturers, especially those for micro-molding, will occasionally use a tapered barrel to facilitate material conveyance in the feed zone.
Additional Thoughts
Keep in mind… if you are using a barrier screw, you will have two melt channels in the transition zone with one flight having more clearance than the other. 

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