??????Shrinking too much includes two meanings: the film shrinks too much during the stretching process and cannot reach the width required by the product; the film shrinks too much when heated during the secondary compounding process, exceeding the standard requirements and not meeting the product requirements width.
??????Can be explained by the malleability of plastics. PVC is an amorphous polymer. When it is subjected to a tensile force greater than the yield strength in the temperature range, plastic extension deformation occurs. During the deformation process, the polymer structural units (segments, macromolecules, and crystallites) begin to orientate due to stretching. . As the degree of orientation increases, the force between the macromolecules increases, causing the viscosity of the polymer to increase and the tendency to harden. The deformation also tends to be stable and no longer develops. This phenomenon is called 'stress hardening.' If the temperature is properly increased, the ductility of the plastic is further improved, the stretching ratio can be increased, and even some polymers with poor ductility can be stretched. Generally, the deformation from room temperature to around Tg (processing temperature) is called 'cold stretching'. After stress hardening of the polymer 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 state after the stretching force is released, resulting in greater retraction.
??????In fact, when designing a stretched film workshop, we must consider the heat preservation and air convection problems in the workshop, because the film loses heat very quickly during the stretching process. The thinner the film and the wider the stretch, the faster the heat dissipation. The author has measured that the surface temperature of the film from the stretch entrance to the stretch exit is only 3m from 175 ° C to 135 ° C, so the temperature in the production workshop should not be lower than 20 ° C. In the design of the workshop, only the exhaust windows are left above, and no windows are left below. In addition, in order not to cause cold stretching, an insulation chamber can be made in the transverse stretching section to ensure the ambient temperature of the stretched film. Properly reducing the speed ratio of each machine can also reduce the stretching of the film.
??????Generally, the lower the stretching temperature (above Tg), the greater the stretching ratio, the faster the stretching rate and cooling rate, the higher the degree of orientation, and then reheat the polymer after cooling by stretching orientation to a temperature above Tg At this time, retraction obviously occurs, and the shrink film is made according to this principle. In order not to cause heat shrinkage of the product, the product can be treated at a high temperature while being kept stretched, and then cooled to room temperature. The product thus obtained has good stability. This can add a heating device under the rear half of the stretching machine to maintain the stretching temperature; in addition, you can also pass warm water to the No. 2 tempering wheel to cool the stretched film slowly, which can effectively reduce the stretched film Heat shrinkability.