What was the root cause?
The film samples were analyzed and the “gels”/particles were determined to be “un-melts”; non-melted and incompletely mixed resin. The periodic variation suggested that the extruder be investigated.
Solid-bed break up in screw!
After analysing the extrusion screw, using the VEL™ Extruder module, we found that the compression section was too short. The following image shows a graph of the “Solid Bed Ratio”, along the screw, as predicted by the VEL™ Extruder module.
The above graph indicates that rate of compression was faster than the rate of melting which, often results in “solid-bed breakup”. The rapid increase of the extrusion screw root diameter would result in the solid bed of pellets getting temporarily “stuck” in the compression section where they would spin with the screw. The solid bed/particles would spin in the compression section and would slowly melt until they were sufficiently small to be released and continue down the screw channel. The process then repeats. This condition disrupts the melting process because the solid (un-melted) particles become blended into the melt pool and the pellets do not experience sufficient shear heating or mixing and melt slowly, due to conduction. Since they are not sufficiently blended, they re-appear as they re-crystallize when the film cools.