Process characteristics of PP

　PP has good fluidity at the melting temperature and good molding performance. PP has two characteristics in processing:

　　 First: The viscosity of PP melt decreases significantly with the increase of shear rate (less affected by temperature);

　　 Second: The molecular orientation is high and the shrinkage rate is large. The processing temperature of PP is 220~275℃. It is better not to exceed 275℃. It has good thermal stability (decomposition temperature is 310℃), but it stays in the barrel for a long time at high temperature (270-300℃) There is a possibility of degradation. Since the viscosity of PP decreases significantly with the increase of shear speed, increasing the injection pressure and injection speed will improve its fluidity, improve shrinkage deformation and depression. For mold temperature (40~80℃), it is recommended to use 50℃.

　　 The degree of crystallization is mainly determined by the mold temperature, and should be controlled within the range of 30-50℃. PP melt can pass through a very narrow mold gap and appear a sharp edge. In the melting process of PP, it needs to absorb a large amount of heat of fusion (the specific heat is larger), and the product is hot after being out of the mold.

　　PP material does not need to be dried during processing, PP shrinkage and crystallinity are lower than PE. The injection speed is usually high-speed injection molding to minimize the internal pressure. If there are defects on the surface of the product, then low speed injection molding at a higher temperature should be used. Injection pressure: can be as large as 1800bar.

　　 runner and gate: For cold runner, the typical runner diameter range is 4~7mm. It is recommended to use a round injection port and flow channel. All types of gates can be used. The typical gate diameter range is 1~1.5mm, but gates as small as 0.7mm can also be used. For edge gates, the minimum gate depth should be half of the wall thickness; the minimum gate width should be at least twice the wall thickness, PP material can use hot runner system.

　　PP has always been the main raw material for the manufacture of fibers, because of its coloring ability, wear resistance, chemical resistance and favorable economic conditions.