Dynamic pressure analysis as a tool for determination of sharkskin instability by extrusion of molten polymers

Increasing requirements which extruded polymeric products should accomplish are imposed to quantitatively characterize the factors which have a considerable effect on extrusion stability. Investigation has been realized by means of a rheological measurement device including a single screw extruder, as well as a rheological die, signal acquisition and analysis system. Due to a specially designed measurement track and modern signal processing technique, the separation of cyclic instabilities from received signals (representing the course of melt pressure) and the definition of the influence on the extrusion process were feasible. This paper demonstrates an example of the application of two new methods which facilitate the detection of sharkskin instability, irrespective of visual observation of the extrudate. Autocorrelation function and frequency analysis based on a fast Fourier transformation (FFT) were used as numeric tools applied to register and control the extrusion process. The flow of molten polymer was studied under various processing conditions, giving an opportunity to split high frequency pressure fluctuations by means of the autocorrelation function and fast Fourier transform, and to create, in the future, a catalogue of data which can be used to define particular distortions in technological investigations.