Like many materials, plastics have the characteristics of thermal expansion and cold contraction. Plastic self sealing bags will also encounter such problems in the production process. So what are the factors affecting the shrinkage of self sealing bags? How to solve this problem has become a common concern of the industry.

，同一制品中壁厚较大部位的收缩率总是大于壁厚较小部位。自封袋收缩受到阻碍方向的收缩率总比无阻碍方向要小，靠近嵌件部位的收缩要小于远离嵌件部位的收缩。形状复杂制品的收缩要小于形状简单制品的收缩。流动方向与垂直于流动方向的收缩也有明显差别，一般为流动方向收缩率大于垂直于流动方向的收缩率。

First of all, the shrinkage rate of the parts with larger wall thickness in the same product is always greater than that of the parts with smaller wall thickness. The shrinkage rate of the self sealing bag in the blocked direction is always smaller than that in the unimpeded direction, and the shrinkage near the insert is smaller than that far away from the insert. The shrinkage of products with complex shape is smaller than that of products with simple shape. There is also a significant difference between the shrinkage in the flow direction and that perpendicular to the flow direction. Generally, the shrinkage in the flow direction is greater than that perpendicular to the flow direction.

其次要说的是提高自封袋模具温度，一般会使收缩率大，特别是对于结晶型塑料。延长保压时间，可以使收缩率减小。但是一旦浇口已经封闭，再延长保压时间便不再会对收缩率有影响。

Secondly, it should be said that increasing the mold temperature of self sealing bag will generally make the shrinkage rate large, especially for crystalline plastics. The shrinkage can be reduced by prolonging the holding time. However, once the gate has been closed, extending the holding time will no longer have an impact on the shrinkage.

还有一点就是提高压力可以导致塑料自封袋密度增加，使收缩率减小。提高物料温度，会使制品体积膨胀而佼压入型腔的物料减少，使收缩率加大；但物料温度的升高会使粘度减小，却有利于向型腔内传递压力，又使收缩率减小，收缩率的大小取决于这上述两种效应的综合影响。一般粘度对温度变化敏感的塑料，后一种效应影响大，致使收缩率减小：粘度对温度变化不敏感的塑料，前一种效应影响大，导致收缩率加大。

Another point is that increasing the pressure can increase the density of plastic self sealing bags and reduce the shrinkage. Increasing the material temperature will expand the volume of the product, reduce the material pressed into the cavity and increase the shrinkage; However, the increase of material temperature will reduce the viscosity, which is conducive to the transmission of pressure to the cavity and reduce the shrinkage. The shrinkage depends on the combined influence of the above two effects. Generally, for plastics whose viscosity is sensitive to temperature change, the latter effect has a great impact, resulting in the reduction of shrinkage rate; for plastics whose viscosity is not sensitive to temperature change, the former effect has a great impact, resulting in the increase of shrinkage rate.

了解了影响自封袋收缩率的因素，我们就不难一一突破解决，为自封袋的质量再提一个台阶。

Knowing the factors affecting the shrinkage rate of self sealing bags, it is not difficult for us to break through and solve them one by one, so as to raise the quality of self sealing bags to a new level.