With the continuous improvement of injection molding processing accuracy requirements, in addition to the correct selection of materials and the modification of the selected plastics, how to correctly select the injection molding processing conditions, reasonable analysis of the comprehensive shrinkage of products and the relationship between the parameters of the processing process, as well as the various causes of shrinkage and the impact of processing factors on it is a very important topic.

Shrinkage process and mechanism

Carefully observing the injection of PP samples under the combination of process conditions, it can be found that the shrinkage rate of the whole product is 1%-3%, and this shrinkage is composed of three parts, that is, the contraction before the solidification of the gate, the cooling contraction and the free contraction after stripping.

The product shrinks before the gate solidifies

The shrinkage of PP products before solidification at the gate largely depends on the compensation ability of the melt. As soon as the material enters the mold, because the mold temperature is low at this time, the melt temperature drops rapidly, and the density and viscosity of the melt are constantly increasing. The compensation capacity of the melt depends on the size of the holding pressure, and the entire mold filling time that continues to transfer the melt into the mold at that pressure. This process continues until the gate is solidified and closed. Therefore, it can be considered that the holding pressure and the holding time have a great relationship with the contraction of this stage, if the holding pressure is large and the holding time is long, the shrinkage rate of the product is relatively low.

The product shrinks during cooling

The whole stage from the beginning of solidification at the gate to the stripping stage is the cooling stage. At this stage, no other new melt enters the mold cavity, and the weight and density of PP products do not change. During the cooling process, the shrinkage of the product is related to the mold temperature and cooling rate and cooling time, the cooling speed is fast, the product has no time to relax, and the shrinkage rate may be small; If the mold temperature is high, the cooling rate is slow, and the molecules have sufficient relaxation time, the shrinkage rate may increase.

Free shrinkage of the product after stripping

PP products after demoulding need to stand at room temperature for a period of time, this is because the product after demoulding has a free shrinkage process. If the process conditions are not set properly, some deformation after natural stress release may occur during this period.

Therefore, the total shrinkage measured from the product is the sum of the above three parts. It is normal for plastic materials to shrink, but how to control and master the relationship between product shrinkage and process factors is a complicated research topic, and it must be analyzed from the process factors that affect the shrinkage rate of products.

Process factors affecting the size of shrinkage

Effect of cavity average pressure on shrinkage and stability

In the PP injection process, the melt is filled into the mold cavity at a specific temperature and pressure and continues until the gate is sealed, this section of pressure can also be called the sealing pressure. Sealing pressure plays a decisive role in product shrinkage, the higher the sealing pressure, the smaller the product shrinkage; If the process conditions and equipment at the time of injection can not ensure a stable sealing pressure, it will cause shrinkage fluctuations of the product, affecting the dimensional stability of the product.

The pressure of the mold cavity before sealing depends on the pressure holding pressure, and the pressure holding pressure can actually be regarded as the injection pressure at the stage of maintaining the pressure holding. If the freezing sealing time of the gate is certain, then the sealing pressure of the gate must be increased by increasing the pressure holding pressure. The influence of the pressure holding pressure on the sealing pressure of the gate actually reflects the influence on the shrinkage rate. The holding pressure is actually a direct reflection of the injection pressure.