The reasons for the excessive shrinkage of the stretch film

Do you know that stretch film has a certain degree of shrinkage, and there will be excessive shrinkage during the production process? What is the reason for this?
There are two reasons for the excessive shrinkage of stretch film:

stretch film

1. The stretch film shrinks too much during the stretching process and cannot reach the width required by the product;



2. The stretched film shrinks too much during the secondary compounding process, which exceeds the standard requirements and fails to meet the width required by the product.



The raw material of the stretch film is an amorphous polymer. Plastic extension deformation occurs when subjected to a tensile force more significant than the yield strength within a specific temperature range. During the deformation process, the polymer structural units (segments, macromolecules, and crystallites) Extend and begin to orient. As the degree of orientation increases, the force between macromolecules increases, causing the viscosity of the polymer to increase and the tendency to harden and the deformation tends to be stable and no longer develop. This phenomenon is called "stress hardening."
Appropriately increasing the temperature, the flexibility of the stretched film is further improved, the stretching ratio can be increased, and even some polymers with poor flexibility can be stretched. The deformation from room temperature to the vicinity of Tg (minimum processing temperature) is usually called "cold stretching." After the polymer undergoes stress hardening during the stretching process, the flow of polymer molecules will be restricted, thereby preventing the further increase of the stretching ratio. The stretched film that has undergone "cold stretching" will return to its original state after the stretching force is released, resulting in more significant retraction.



Generally, the lower the stretching temperature (above Tg), the larger the stretching ratio, the faster the stretching speed and the cooling speed, and the higher the degree of orientation. After stretching and orientation cooling, the polymer is reheated to a temperature above Tg. The stretch film is made according to this principle when shrinkage occurs. In order not to cause thermal shrinkage of the product, the product can be treated at the highest temperature for a specific time while maintaining the stretching and then cooled to room temperature so that the obtained product has good stability. This can add a heating device under the back half of the stretching machine to maintain the stretching temperature; in addition, warm water can be passed to the No. 2 tempering wheel to slowly cool the stretched film, which can effectively reduce the stretched film—heat shrinkage.