The Prediction of Bowing Distortion of Film after Transverse Stretching with Consideration of Heated Air Flow in a Tenter

Abstract

Abstract The bowing distortion of polypropylene (PP) film during transverse direction (TD) stretching with a tenter was newly predicted with consideration for heated air flow. The distribution of the heat transfer coefficient on film was calculated by the results of air flow analysis in the tenter in order to obtain a more realistic heat transfer condition than in previous works based on an impinging jet model and a wall jet model. Bowing distortion after stretching was calculated by applying the distribution of heat transfer coefficient obtained from air flow analysis under the assumption that the deformation of film obeys an elastic-plastic rule. Another bowing distortion was also calculated by applying an impinging jet model and a wall jet model in order to evaluate the applicability of the distribution of heat transfer coefficient obtained from air flow analysis. The authors experimentally measured the bowing distortion and the film thickness after stretching using a pilot plant. It has been shown that the calculated results based on the distribution of heat transfer coefficient obtained from air flow analysis are closer to the experimental results than previous works based on an impinging jet model and a wall jet model, particularly at faster line speed. It is revealed that the distribution of heat transfer coefficient has a strong influence on the temperature of film, and the difference of the predicted temperature causes a difference in the thickness change in film and longitudinal stress. The authors concluded that the difference in thickness change in film and longitudinal stress brought about the difference in bowing distortion.