Reducing Curl in Multilayer Blown Film Part II: Application of Predictive Modeling to a Barrier Cereal Linear Film, 2001 Polymers Laminations & Coatings Conference Proceedings

Barry A. Morris--Multilayer blown films often curl, particularly if the layers are not distributed symmetrically. A quantitative model is developed for predicting curl based on continuum mechanics: curl is the result of differential shrinkage between layers during quenching and is moderated by the stiffness and thickness of the layers. The difficulty in using such an approach is estimating differential shrinkage. Pressure-Volume-Temperature (PVT) data give good qualitative information on differential shrinkage, but they are generated under experimental conditions that differ greatly from commercial blown film processes. To correct the PVT data, a semi-empirical approach is utilized. The model is run "backwards" to compute the differential shrinkage in 2-layer structures where the curl has been measured. From this, PVT correction factors are obtained to predict the curl of multilayer structures.

The model is applied to a (HDPE - tie - EVOH - tie - sealant) cereal liner structure. A sensitivity analysis shows that increasing the thickness of the HDPE layer, reducing the shrinkage of the HDPE and reducing the thickness and stiffness of the EVOH layer can reduce curl. Experiments on a five-layer blown film line confirm the model predictions: a standard cereal liner structure had severe curl, yet by using the model as a guide, we were able to make essentially flat film.