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The shrinkage of the ziplock bag in the direction in which it is blocked is always smaller than the unobstructed direction, and the shrinkage near the insert portion is smaller than the shrinkage away from the insert portion. The shrinkage of a complex shaped article is less than the shrinkage of a simple shaped article. The flow direction of the ziplock bag is also significantly different from the contraction perpendicular to the flow direction, and generally the shrinkage rate in the flow direction is greater than the shrinkage rate in the direction perpendicular to the flow direction. The factors affecting the shrinkage rate of the ziplock bag are described in detail below.
The second thing to say is to increase the temperature of the zipper bag mold, which generally increases the shrinkage rate, especially for crystalline plastics. Extending the dwell time can reduce the shrinkage. However, once the gate has been closed, extending the dwell time will no longer affect the shrinkage.
Last but not least, increasing the pressure can lead to an increase in the density of the plastic ziplock bag and a reduction in shrinkage. Increasing the temperature of the material will cause the volume of the product to expand and the material pressed into the cavity to decrease, so that the shrinkage rate will increase; but the increase in the temperature of the material will reduce the viscosity, but it will facilitate the transfer of pressure into the cavity and shrinkage. The rate is reduced and the final shrinkage depends on the combined effect of the above two effects. Plastics whose viscosity is sensitive to temperature changes have a large effect on the latter effect, resulting in a decrease in shrinkage: plastics whose viscosity is not sensitive to temperature changes have a large influence on the former effect, resulting in an increase in shrinkage.
