The Affect Of Screw Diameter On Shear Rate

One of our customers brought up this question…
Jim M.
How does shear rate vary for the same fill rate going from a small screw to a large screw?  The volumetric flow rate being the same of course, but is there a difference in shear rate caused by the barrel / screw size differences?
My Response
With respect to shear during first stage injection, virtually all the shear occurs in the nozzle, runner, gates, and mold cavity. The diameter of the barrel, in comparison, is inconsequential.
If you used the same rotational speed (RPM) with a larger diameter screw, then you would create more shear during recovery since the circumferential speed would increase. For this reason… you should try to match the recovery times to help balance the shear caused during screw recovery.
Additional Thoughts
What is usually overlooked is the ID and length of the nozzle which may change from machine to machine as well as the resulting melt temperature due to recovery… These oversights often make people think it’s the barrel that is causing the difference.

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