Problems that need to be paid attention to when the PVC protective film shrinks too much
Oct 27, 2022
Excessive shrinkage has two meanings:
1. The film shrinks too much during the stretching process and cannot meet the width required by the product;
2. The thermal shrinkage of the film is too large during the secondary lamination process, which exceeds the standard requirements and fails to meet the required width of the product.
Problem 1 can be explained by the malleability of plastics. PVC is an amorphous polymer. When subjected to a tensile force greater than the yield strength within a certain temperature range, plastic extension deformation occurs. During the deformation process, the polymer structural units (segments, macromolecules and crystallites) begin to stretch due to stretching. orientation.
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 ductility of the plastic is further improved, the stretching ratio can be increased, and even some polymers with poor ductility can be stretched. The deformation from room temperature to the vicinity of Tg (minimum processing temperature) is usually called "cold stretching". After the polymer is stress-hardened during the stretching process, the flow of polymer molecules will be restricted, thereby preventing the further increase of the stretching ratio. The film that has undergone "cold stretching" returns to its original shape after the stretching force is released, resulting in greater retraction.
In fact, the thermal insulation and air convection problems of the workshop should be considered when designing the stretch film workshop. Because the film loses heat very quickly during the stretching process, the thinner the film and the wider the stretching, the faster the heat dissipation. The author has measured that the surface temperature of the film drops from 175°C to 135°C within a short 3m from the stretching inlet to the stretching outlet, so the temperature in the production workshop is generally not lower than 20°C. In the design of the workshop, only exhaust windows are left at the top, and no windows are left at the bottom. In addition, in order to prevent cold stretching, a heat preservation chamber can be made at the transverse stretching position to ensure the ambient temperature of the stretched film. Appropriately reducing the speed ratio of each machine can also reduce the stretching of the film.
For problem 2, usually the lower the stretching temperature (above Tg), the larger the stretching ratio, the faster the stretching speed and the cooling speed, the higher the degree of orientation, and the polymer after stretching and orientation cooling will be reheated to When the temperature is above Tg, shrinkage will occur obviously, and the shrinkable film is made according to this principle. In order not to cause thermal shrinkage of the product, the product can be treated at the highest temperature for a certain period of time while maintaining stretching, and then cooled to room temperature, so that the product obtained in this way 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 warm-rolling wheel to slowly cool the stretched film, which can effectively reduce the stretched film. Heat shrinkage.
