Polypropylene (PP) processing technology
Polypropylene injection molding machine
PP is generally known as polypropylene, because of its good fracture resistance, also known as "100 fold rubber". PP is a kind of semi transparent and semi crystalline thermoplastic, which has the characteristics of high strength, good insulation, low water absorption, high thermoforming temperature, small density and high crystallinity. Modified fillers usually include glass fiber, mineral filler, thermoplastic rubber, etc.
The fluidity of PP with different uses is quite different. Generally, the flow rate of PP is between ABS and PC.
1. Treatment of plastics
Pure PP is translucent ivory white, which can be dyed into various colors. PP can only be dyed with masterbatch in general injection molding machine. It has independent plasticizing elements which can enhance mixing effect on the Huameida machine, and can also be dyed with toner. Products for outdoor use are generally filled with UV stabilizer and carbon black. The proportion of recycled material shall not exceed 15%, otherwise it will cause strength reduction and decomposition discoloration. PP generally does not need special drying treatment before injection molding.
2. Selection of injection molding machine
There are no special requirements for the selection of injection molding machine. 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 by 3800t / m2, and the injection volume is 20% - 85%.
3. Mold and gate design
The mold temperature is 50-90 ℃, and the high mold temperature is used for the high dimensional 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 shorter the edge gate is, the better it is. It is about 0.7mm. The depth is half of the wall thickness and the width is twice of the wall thickness. The die must have good exhaust performance. The exhaust hole is 0.025mm-0.038mm deep and 1.5mm thick. To avoid shrinkage marks, large and round injection port and circular flow channel should be used, and the thickness of stiffener should be small (for example, 50-60% of the wall thickness). The thickness of products made of homopolymer PP shall not exceed 3mm, otherwise there will be bubbles (only copolymerized PP can be used for thick wall products).
4. Glue temperature
The melting point of PP is 160-175 ℃, the decomposition temperature is 350 ℃, but the temperature setting cannot exceed 275 ℃. The melting temperature * is 240 ℃.
5. Injection speed
In order to reduce the internal stress and deformation, high-speed injection should be selected, but some grades of PP and mold are not suitable (bubble and air ripple appear in human mantle). If there are light and dark stripes on the patterned surface diffused by the gate, low speed injection and high mold temperature shall be used.
6. Glue back pressure
5 bar glue back pressure can be used, and the back pressure of toner can be adjusted appropriately.
7. Injection and pressure maintaining
High injection pressure (1500-1800bar) and holding pressure (about 80% of injection pressure) were used. About 95% of the whole stroke, the pressure is maintained for a long time.
8. Post treatment of products
In order to prevent shrinkage and deformation caused by post crystallization, products generally need to be soaked in hot water.