Calendering of PVC: Defects in calendered PVC films and sheets

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Abstract
Major types of defects appearing in calendered films and sheets are described. The conditions for the appearance of air bubbles and mattness were studied for the particular case of calendering crystal-clear, rigid, toughened PVC. A correlation was found between the presence of air bubbles in the film and a distribution of air inclusions in the bank. The existence of air inclusions is also related to a critical pressure, which has been calculated with the aid of a theoretical model (pseudoplastic fluid). The variation of mattness with various parameters of the calendering process, such as speed, friction, temperature, and lubrication, was measured on a laboratory calender. The influence of the calender size (laboratory and industrial) on the occurrence of mattness was also investigated. The calculated critical parameters to be considered, in order to define the onset of mattness, appear to be a shear stress or a maximum pressure. A comparison of model calculations and experimental observations allows the definition of appropriate boundary conditions for calendering a given PVC compound as a function of calender size.

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