Improving Mixing…

Screw speed is not the most consistent method of improving mixing within the barrel. Back Pressure and Melt Temperature are more consistent and reliable methods of mixing your polymer.

Question: I read your post on using a slow screw for long cycle time for processes like ours. My techs tell me they need to use a high screw speed to get the material properly mixed, is this true?
My Response: High rotational speeds will add shear to the material. Excessively high speeds will also cause heating due to excessive polymer chain breakage and degradation, especially if semi-crystalline polymers are being forced through the transition zone before they are ready to melt. Both of these results are not consistent & reliable, and will add variability to your process. Good mixing typically requires shear, temperature, or both. If you need more back flow & shear, then you should add back pressure to the process. If the material needs more heat to mix or melt properly, then you should increase the middle and front zones to increase the melt temperature. In any case, you should document the Melt Temperature (probe inserted into purge), Back Pressure (in plastic pressure), and Recovery Time (in seconds) when the mixing is adequate so your melting profile can be easily duplicated in the future.

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