injection technology of PP material
PP is commonly referred to as polypropylene because of its good fracture resistance. PP is a translucent, semi-crystalline thermoplastic with high strength, good insulation, low water absorption, high heat distortion temperature, low density and high crystallinity. The modified filler usually has glass fibers, mineral fillers, thermoplastic rubber, and the like.
The PP of different uses has a large difference in fluidity, and the PP flow rate generally used is between ABS and PC.
1. Plastic processing
Pure PP is a translucent ivory white that can be dyed in a variety of colors. The dyeing of PP can only be used with color masterbatch on general injection molding machines. On some machines, there are separate plasticizing elements that enhance the mixing action, and can also be dyed with toner. Products for outdoor use are typically filled with UV stabilizers and carbon black. The proportion of recycled materials should not exceed 15%, otherwise it will cause strength drop and discoloration. PP does not require special drying before injection molding.
2. Injection molding machine selection
There are no special requirements for the selection of injection molding machines. Since 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 ° C, and the high mold temperature is required for the size. The core temperature is 5 °C lower than the cavity temperature, the runner diameter is 4-7 mm, the needle gate length is 1-1.5 mm, and the diameter can be as small as 0.7 mm. The shorter the length of the edge gate, the better, about 0.7 mm, the depth is half of the wall thickness, the width is twice the wall thickness, and the thickness of the melt flow in the cavity increases.
The mold must have good exhaustibility. The vent hole depth is 0.025mm-0.038mm and the thickness is 1.5mm. To avoid shrinkage marks, it is necessary to use large and round nozzles and circular flow passages. The thickness of the ribs should be small ( For example, 50-60% of the wall thickness). Products made of homopolymerized PP should not exceed 3mm in thickness, otherwise there will be bubbles (thick-walled products can only be copolymerized with PP).
4. Melt temperature
The melting point of PP is 160-175 ° C, and the decomposition temperature is 350 ° C, but the temperature setting during injection processing cannot exceed 275 ° C, and the temperature of the melting section is preferably 240 ° C.
5. Injection speed
In order to reduce internal stress and deformation, high-speed injection should be selected, but some grades of PP and mold are not suitable (bubbles, gas lines appear). If the surface of the pattern has a light and dark streak spread by the gate, a low speed injection and a higher mold temperature are used.
6. Melt back pressure
The back pressure of the 5 bar melt can be used, and the back pressure of the toner can be appropriately adjusted.
7. Injection and pressure retention
Use a higher injection pressure (1500-1800 bar) and a holding pressure (about 80% of the injection pressure). It is about to maintain pressure at 95% of the full stroke, with a longer holding time.
8. Post-treatment of the product
In order to prevent shrinkage deformation caused by post-crystallization, the product generally needs to be immersed in hot water.
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