I received this question as a follow-up question from a related blog entry…
If an amorphous polymer requires approximately 8-10 diameters to melt the polymer, how much is needed for semi-crystalline polymers?
Generally speaking, amorphous polymers require a long transition zone to properly soften and melt the polymer. This provides a more consistent and homogeneous melt.
When processing semi-crystalline polymers, the polymer softens quickly once the polymer reaches it’s melting temperature. For this reason the transition zone is typically 5-7 times the diameter for semi-crystalline polymers.
Although the transition zone length is small, it is critical to ensure that a long fee zone is being used for semi-crystalline polymers. This allows the material to heat up using more energy from the heater bands to help melt the polymer with less shear. When processing these materials, it is very common to have a feed zone which is nearly half the overall length of the screw.