Curling of films and laminations is due to unbalanced forces, which exist in the film. When a film curls, it can be impossible to use the film further because it will simply not lay flat, making printing, laminating, bagforming and heat-sealing very difficult, if not impossible to complete.
The theory of coextruded film and lamination curling has to come from the theory of composite structures because that is what they are. Composite materials, because they are made up of various material layers with oftentimes different thermal, environmental, and mechanical properties, will react in unexpected ways when forces, such as tensions, or compressions are applied. For instance, when bias-ply composites are subjected to tensile forces, they may exhibit an out-of-plane twisting. Understanding this feature of an oriented film or lamination could help a great deal in understanding curling, buckles and wrinkles in the transport sections of a laminator.
Tension control = flatter laminations
The forces, which will affect many coextruded films and laminations during use, are mainly due to frozen-in strains, temperature variations and the impact of humidity changes. Laminations are especially prone to frozen-in strains and stress relaxation. Unbalanced tensions during the lamination step are a primary source, and so the need to control tensions during lamination is paramount for good, flat laminated structures.
For films and laminations containing moisture-sensitive polymers, variations in humidity are important. It should be mentioned that at extrusion nylons, polyesters and other condensation polymers are dry, so as not to degrade during extrusion. However, once the film is extruded, these polymers will absorb moisture from their surroundings and will experience a dimensional change with water absorption. These changes in length will usually not be matched by the other polymers they are combined with and create a force which can curl or buckle the product. This is especially true for asymmetric structures.