Injection molding method of polypropylene (PP)
PP, generally called polypropylene, has another name "hundred-times folding rubber" due to its good performance of break-resistance. PP is a kind of semitransparent and semi-crystal thermoplastic plastic, which has high strength, good insulation, low bibulous rate, high shaping temperature, low density and high crystallinity. Modified filler normally includes fiberglass and thermoplastic rubber, etc.
The fluidity of PP with different functions has great difference. Common fluidity rate of ppranges from ABS to PC.
1. Disposal of plastic
Pure PP is semitransparent ivory and can be dyed into any color. PP’s dyeing can only use masterbatch in ordinary injection molding machine. Productions used in outdoor are always filled with UV stabilizer and conductex. The rate of reclaimed materials in use should be no more than 15%, or the strength will decrease and it will decompound and change color. Generally, PP does not need drying process before Injection molding processing.
2. Selection of Injection molding machine
There is no special requirement on the selection of injection molding machine. Due to high crystalline of PP, only Computer injection molding machine of high injection pressure and multistratum controlled can be used. Generally the clamping force is 3800t/m2, the injection amount ranges from 20%-85%.
3. Design of mold and gate
Mold temperature ranges from 50-90 Celsius degree. High mold temperature is needed for products with high requirement on size. Mold core temperature is 5 Celsius degree lower than mold cavity temperature. Diameter of runner is 4-7mm, and the length of needle gate is 1-1.5mm, and the minimum diameter length could be 0.7mm. For polygon gate length, the smaller the better, about 0.7mm. The depth is half of the wall thickness, the width is twice of the wall thickness and increases with the length of melt flow in mold cavity. The mold should exhaust well. The exhaust vent depth is 0.025mm-0.038mm, the thickness should be 1.5mm.Big and round pouring spot and round runner are needed to avoid shrinkage. The stiffener thickness should be thin (50-60% of the wall thickness). Thickness of products produced by homopolymer PP should be no more than 3mm, or it will blister(thick wall products only use copolymerization PP).
4. Melt temperature
Melting point of PP is 160-175 Celsius degree, decomposition point is 350 Celsius degree. But the temperature should be no more than 275 Celsius degree in injection processing. The better temperature in melting section should be 240 Celsius degree.
5. Injection speed
High speed injection is needed to reduce internal stress and distortion, which is not suitable for certain PP and molds (the mantle will blister and crinkle). If alternately dark and bright stripe which is spread from gate appears on the surface with pattern, it needs low speed injection and high mold temperature.
6. Melt packing
5bar melt packing is needed and the toner material can be adjusted higher moderately.
7. Injection andpressure maintaining
High injection pressure (1500-1800bar) and packing pressure (nearly 80% of injection pressure) are needed. The time of turning into pressure maintaining should be 95% of total excursion and long packing time is needed.
8. Post-processing of product
Productsare needed to be immersed in hot water to avoid shrinkage distortion caused by the final crystallization.