Bowing Phenomenon in Double Bubble Tubular Film Processing for PA 6

This report is to discuss the bowing phenomenon and the sag elimination tension for biaxial oriented PA 6 film process, which consists of double bubble tubular film process and the thermosetting tentering.

The bowing phenomenon does not occur in the heat set of the triple bubble tubular process. However, there is a serious problem of the bubble stability when the sufficient thermosetting by the tubular process is carried out, so the heat set by the tenter process is required. The bowing phenomenon arises when the annealing is carried out by the tenter method. The bowing distortion becomes large when the annealing temperature is high and film relaxation ratio is large.

It is possible to reduce the bowing distortion when thermosetting is carried out in a two-step tenter process. The shrinkage stress of the stretched film was relaxed in the heat treatment during the second step whose temperature is higher than the first one. A two-step heat treatment was able to decrease the bowing distortion compared with the one-step heat treatment.

When the bowing phenomenon occurred, shrinkage percentage in hot water changes along the cross-direction of a film. The humidity expansion coefficient in the film which increases as the increasing shrinkage percentage in hot water is related to the molecular orientation. The sag also increased with increasing the bowing distortion. The bowing distortion and the sag elimination of tension have proportional connection.

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

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.