The in-line detection method of sharkskin melt flow instability during polyethylene extrusion based on pressure analysis

This paper is concerned with the issues of occurrence and detection of melt flow instabilities forming during polyethylene extrusion. A novel method of in-line detection and evaluation of sharkskin melt flow instability occurring during the extrusion of linear low density polyethylene (LLDPE) was developed. The application of the dynamic pressure analysis procedure with three different signal processing methods allows to obtain both quantitative and qualitative information about the phenomena which caused the distortion of the surface of the extrudate during the technological process. Investigations were carried out with two commercial LLDPE grades with different molecular weight distributions. In-line evaluation of the efficiency of the novel measuring system was preceded by off-line rheological experiments which allowed to obtain broad information about the influence of the macromolecular structure of selected polyethylenes on their properties in the molten state and their potential ability to create sharkskin instability. The preliminary multi-criteria assessment of the structure-rheological properties relationship allowed the verification of the sensitivity of the novel measuring system and the definition of proposals for the selection of appropriate signal processing methods for the different types of melt fracture instabilities formed on the extrudate surface.