THERMAL SHRINKAGE OF PLASTIC FILMS
During the manufacturing process, internal stresses are induced into the material and remain there until it is heated which has a tempering effect causing the material it to shrink.
The residual stress patterns are indicative of the direction in which the film is produced, and therefore, before test samples are cut from the material, the orientation must be recorded by marking the "X" and "Y" axis.
To accurately determine the percentage shrinkage that takes place, test samples are immersed in a temperature controlled bath so that a graph can be produced of percentage shrinkage versus temperature. From the graph, quality control sampling temperatures can be established so that the material can be acurrately and quickly monitored
A less accurate method to obtain a general guide of the degree of shrinkage is the hot plate method. The apparatus is extremely portable so it can be used throughout the plant. The hot plate is coated with silicon oil and the test sample is placed on top for a predetermined time.