Polypropylene injection molding processing technology


  PP is commonly referred to as polypropylene, bec […]


PP is commonly referred to as polypropylene, because of its good anti-breaking performance, it is also called pleated rubber. PP is a semi-transparent, semi-crystalline thermoplastic with high strength, good insulation, low water absorption, high heat forming temperature, low density, high crystallinity, etc. Modified fillers usually include glass fibers, mineral fillers, and thermoplastic rubber.  

The fluidity of PP for different purposes is quite different, and the PP flow rate generally used is between ABS and PC.  

1 Plastic processing  
Pure PP is translucent ivory white and can be dyed in various colors. For PP dyeing, only color masterbatch can be used on general injection molding machines. There are independent plasticizing elements to strengthen the mixing effect on the Ramada machine, and it can also be dyed with toner. Products used outdoors are generally filled with UV stabilizers and carbon black. The use ratio of recycled materials should not exceed 15%, otherwise it will cause strength drop and decomposition and discoloration. PP injection molded parts generally do not require special drying treatment before processing. 
2Injection molding machine selection   

There are no special requirements for the selection of injection molding machines. Because PP has high crystallinity. A computer injection molding machine with high injection pressure and multi-stage control is required. The clamping force is generally determined at 3800t/m2, and the injection volume is 20%-85%.
3Mold and gate design  
The mold temperature is 50-90°C, and the high mold temperature is used for higher size requirements. The core temperature is more than 5℃ lower than the cavity temperature, the runner diameter is 4-7mm, the needle gate length is 1-1.5mm, and the diameter can be as small as 0.7mm. The length of the edge gate is as short as possible, about 0.7mm, the depth is half of the wall thickness, and the width is twice the wall thickness, and it will gradually increase with the length of the melt flow in the cavity. The mold must have good air venting. The vent hole is 0.025mm-0.038mm deep and 1.5mm thick. To avoid shrinkage marks, use large and round nozzles and circular runners, and the thickness of the ribs should be small ( For example, 50-60% of the wall thickness). The thickness of products made of homopolymer PP should not exceed 3mm, otherwise there will be bubbles (thick wall products can only use copolymer PP). 
4 Melting temperature  
The melting point of PP is 160-175°C and the decomposition temperature is 350°C, but the temperature setting cannot exceed 275°C during injection processing. The temperature of the melting zone is preferably 240°C.  

5Injection speed  
In order to reduce internal stress and deformation, high-speed injection should be selected, but some grades of PP and molds are not suitable (bubbles and air lines in the human mantle). If the patterned surface appears with light and dark stripes diffused by the gate, low-speed injection and higher mold temperature are required.

6 Melt adhesive back pressure  
5bar melt adhesive back pressure can be used, and the back pressure of the toner material can be adjusted appropriately 
7Injection and pressure holding  
Use higher injection pressure (1500-1800bar) and holding pressure (about 80% of the injection pressure). Switch to holding pressure at about 95% of the full stroke and use a longer holding time.  

8Post-processing of products  
In order to prevent shrinkage and deformation caused by post-crystallization, products generally need to be soaked in hot water.


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