PVC heat shrink film is how to shrink film?

PVC heat-shrinkable film is preheated and then stretched and cooled. Under Tf close to PVC resin, the resin cast sheet is stretched laterally and longitudinally, and then quenched, so that the molecular chain of the resin is frozen in the orientation state, forming a heat shrink film. From the raw material to the generation of heat shrink film is a process of entropy reduction, the thermodynamic state of heat shrink film is unstable, when the shrink film is heat treated, the molecular chain that has been oriented will quickly return to the random coil (curled state), resulting in contraction. However, the effect of longitudinal shrinkage and horizontal shrinkage of PVC heat shrinkable film is not the same, the longitudinal shrinkage rate is too large to make PVC film wrinkles, only the horizontal shrinkage rate is large, and the longitudinal shrinkage rate is small enough to maintain a relatively smooth film without wrinkles.