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Reducing Curl in Multilayer Blown Film: Experimental Results, Model Development, and Application to A Cereal Liner Film

Multilayer blown films often curl, particularly if the layers are not distributed symmetrically. Experimental results from simple two- and three-layer structures are described in an effort to understand the underlying mechanisms behind curl. A model from the literature based on “beam theory” was adapted to film applications. In this model, force and momentum balances are used to solve for curl as a function of each layer’s thickness, stiffness, and shrinkage during fabrication. 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 was utilized. The model was run “backwards” to compute the differential shrinkage in two-layer structures where the curl has been measured. From this, PVT correction factors are obtained to predict the curl of multilayer structures. The model was applied to a (HDPE-tie-EVOH-tie-sealant) cereal liner structure. A sensitivity analysis showed that curl can be reduced by: increasing the thickness of the high density polyethylene (HDPE) layer, reducing the shrinkage of the HDPE, and reducing the thickness and stiffness of the ethylene vinyl alcohol (EVOH) layer. Experiments on a five-layer blown film line confirmed 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.