As we all know, there is a certain degree of shrinkage in the stretch film, and there will be too much shrinkage in the production process. Here’s the reason why the shrinkage is too large: the stretch film is too large in the stretch process, which can’t meet the width requirements of the product; in the secondary composite process, the heat shrinkage is too large, which exceeds the standard requirements, which can’t meet the width requirements of the product.
The raw material of stretch film is amorphous polymer. In a certain temperature range, when the tensile force is greater than the yield strength, plastic tensile deformation occurs. In the process of deformation, due to tensile action, polymer structural units (chain segments, macromolecules and microcrystals) began to orient. With the increase of orientation degree, the interaction force between macromolecules increases, which leads to the increase of viscosity and hardening trend of polymer. The deformation tends to be stable and no longer develop. This phenomenon is called “stress hardening”.
With the appropriate increase of temperature, the elongation and the tensile ratio of the stretched film are further improved, and even some polymers with poor tensile properties can be stretched. The deformation near the very low working temperature at room temperature is usually called “cold drawing”. After stress hardening, the flow of polymer molecules will be limited, which prevents the further increase of tensile ratio. Cold stretching “the stretching film returns to its original state after the tension is increased, resulting in greater shrinkage.”.
In fact, in the design process, we should consider the heat preservation and air convection in the workshop, because the heat loss of the stretched film is very fast, and the heat dissipation of the stretched film is faster. “The surface temperature between the drawing mouth and the drawing mouth is reduced from 175 ℃ to 135 ℃, so the temperature in the production workshop should not be lower than 20 ℃. The purpose of the workshop design is to leave only exhaust windows at the top and no windows at the bottom. In addition, in order to avoid the phenomenon of cold drawing, the greenhouse can be made in the transverse stretching part to ensure the environmental temperature. By properly reducing the speed ratio of each machine, the tension of tensile film can also be reduced.
Shenzhen World Packing Industrial Limited
Top Stretch Wrap Film & Adhesive Tapes Manufacturer