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.

，同一制品中壁厚較大部位的收縮率總是大于壁厚較小部位。自封袋收縮受到阻礙方向的收縮率總比無阻礙方向要小，靠近嵌件部位的收縮要小于遠離嵌件部位的收縮。形狀復雜制品的收縮要小于形狀簡單制品的收縮。流動方向與垂直于流動方向的收縮也有明顯差別，一般為流動方向收縮率大于垂直于流動方向的收縮率。

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.

其次要說的是提高自封袋模具溫度，一般會使收縮率大，特別是對于結(jié)晶型塑料。延長保壓時間，可以使收縮率減小。但是一旦澆口已經(jīng)封閉，再延長保壓時間便不再會對收縮率有影響。

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.

還有一點就是提高壓力可以導致塑料自封袋密度增加，使收縮率減小。提高物料溫度，會使制品體積膨脹而佼壓入型腔的物料減少，使收縮率加大；但物料溫度的升高會使粘度減小，卻有利于向型腔內(nèi)傳遞壓力，又使收縮率減小，收縮率的大小取決于這上述兩種效應的綜合影響。一般粘度對溫度變化敏感的塑料，后一種效應影響大，致使收縮率減?。赫扯葘囟茸兓幻舾械乃芰希耙环N效應影響大，導致收縮率加大。

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.

了解了影響自封袋收縮率的因素，我們就不難一一突破解決，為自封袋的質(zhì)量再提一個臺階。

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.