Films that are stretched in the solid state, i.e.: above Tg but below Tm, will have a wide range of recoverable strain based on the stretch ratios employed as well as the annealing they are subjected to. This is true for double-bubble as well as MD-only oriented thin films.
Biaxially oriented films derived from cast sheet are generally MD-oriented first, and then TD-oriented in a tenter frame . Annealing in this context means reheating the stretched film under restraint but with some relaxation of tensions or dimensions to lessen the recoverable strains (limit shrinkage) while maintaining film dimensions. Free, unrestrained shrinkage results in thickening of the film with a change in MD and TD dimensions.
To anneal or not to anneal
Double-bubble films are not annealed during manufacture and are often times used for shrink films for wrapping items for point-of-sale and will have a wide range of shrinkage levels and shrinkage force controlled by the polymer and the stretch ratios.
Tenter films are generally annealed in the last step of production and predominantly in the TD direction. This is done by reducing the width of the tenter chain, while the film is held by the clips (called toe) Because the clips are generally on a fixed pitch, the film is prevented from any MD relaxation. Tenter films will generally have a low TD and higher MD, or unbalanced shrinkage due to the relaxation. Often times at lower temperatures, the TD dimension will increase as the MD dimension decreases, but as temperature rises both TD and MD dimensions decrease.
For bubble films, annealing can be performed on a tenter frame as well as by a third inflation bubble where the film is inflated and annealed under MD tension and internal air pressure to “heat-set” the film and reduce shrinkage. However, this approach cannot match the overall output levels of tenter-based films. There are tenters, such as the pantographic and LISIM®, which can allow an MD relaxation to give more balanced, or lower MD/TD shrinkage, but they are not common for packaging films.
MD-oriented only films are generally stretched in the MD through a gap where the film is sped up from a slow roll to a fast roll. MD shrinkage can be controlled by hot-roll annealing.
All of the films described above will have various patterns and levels of MD and TD shrinkage. Hopefully, this recitation will enable you to better understand the potential shrinkage you will experience with each sort of film process encountered.