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PP Injection Molding Process in China

PP is also called polypropylene, because of its good anti-breaking performance, also known as "100 fold glue." PP is a translucent, semi-crystalline thermoplastic with these performances such as high strength, good insulation, low water absorption, low density and high crystallinity etc. The PP which has different uses has a large difference in fluidity. PP flow speed is between that of 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. Independent plasticizing elements with enhanced mixing on the injection molding machine can also be dyed with toner. Plastic 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 a strength to drop and discoloration. PP doesn’t require special drying before injection molding.

2. Mold and gate design
Mold temperature should be 50-90 ° C, for high dimensional requirements for high mold temperature. The core temperature is 5 °C lower than the cavity temperature, the runner diameter is 4-7 mm, the pin gate length is 1-1.5 mm, and the diameter can be as small as 0.7 mm. The shorter the length of the side 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 around the runner. The thickness of the ribs should be small (Such as 50-60% of the wall thickness). Plastic products made of PP shouldn’t exceed 3mm in thickness, otherwise, there will be bubbles (thick-walled products can only be copolymerized with PP).

3. Injection speed
In order to reduce internal stress and deformation, the high-speed injection should be selected, but some grades of PP and mold are not applicable (human air bubbles, gas lines). 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.

4. Melting temperature
PP has a melting point of 160-175 ° C and a decomposition temperature of 350 ° C, but the temperature setting during injection molding can’t exceed 275 ° C. The temperature of the melt section is preferably 240 °C.

5. The back pressure of the melt
The back pressure of the melt can use 5bar melting back pressure, and the back pressure of toner can be adjusted appropriately.

6. Selection of injection molding machine
There is no special requirement for the selection of the injection molding machine. Since PP has high crystallinity, it requires the computer injection molding machine with high injection pressure and multi-stage control. The clamping force is generally determined by 3800t/m2, and the injection volume is 20%-85%.

7. Injection and pressure holding
Using high injection pressure (1500-1800 bars) and holding pressure (about 80% of injection pressure). It is about to maintain pressure at 95% of the full stroke, with a longer holding time.

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