Analysis of the injection molding process of polypropylene (PP)

PP is a translucent, semi-crystalline thermoplastic with high strength, good insulation, low water absorption, high heat forming temperature, low density, and high crystallinity. Modified fillers usually include glass fiber, mineral filler, thermoplastic rubber, etc.

The fluidity of PP varies greatly from application to application, and the general flow rate of PP used is between ABS and PC.

1. Handling of plastic.
Pure PP is translucent ivory white and can be dyed into various colors. the dyeing of PP in a general injection molding machine can only use color masterbatch. There is an independent plasticizing element on the WADA machine that strengthens the blending effect, and it can also be dyed with color powder. For products used outdoors, UV stabilizers and carbon black filling are generally used. The proportion of recycled material used should not exceed 15%, otherwise, it will cause a decrease in strength and decomposition, and discoloration. PP injection molding process generally does not require special drying treatment before.

2. Injection molding machine selection
There is no special requirement for the choice of an injection molding machine. Because PP has high crystallinity. Need to use high injection pressure and multi-stage control of computerized injection molding machine. The clamping force is generally determined by 3800t/m2 and the injection volume is 20%-85%.

3. Mold and gate design
Mold temperature 50-90℃, high mold temperature for high size requirement. The core temperature is more than 5℃ lower than the cavity temperature, the diameter of the runner is 4-7mm, the length of needle-shaped gate is 1-1.5mm, the diameter can be as small as 0.7mm, the shorter the length of wedge-shaped gate is, the better, about 0.7mm, the depth is half of the wall thickness, the width is twice of the wall thickness, and the length of melt flow in the cavity increases with each time. The mold must have good exhaust, the exhaust hole is 0.025mm-0.038mm deep and 1.5mm thick, to avoid shrinkage marks, we should use a large and round injection port and round runner, the thickness of reinforcement should be small (for example, 50-60% of the wall thickness). For products made of homopolymer PP, the thickness cannot exceed 3mm, otherwise, there will be bubbles (thick-walled products can only use copolymer PP).

4. Melting temperature
The melting point of PP is 160-175℃, and the decomposition temperature is 350℃, but the temperature setting cannot exceed 275℃ during injection processing. The melting section temperature should be 240℃.

5. Injection speed
In order to reduce internal stress and deformation, the high-speed injection should be selected, but some grades of PP and molds are not applicable (human mantle now bubbles, airlines). If the surface with pattern is engraved with bright and dark stripes spread by the gate, low-speed injection and higher mold temperature should be used.

6. Melt glue back pressure
The backpressure of melt glue can be 5bar, and the backpressure of color powder can be adjusted higher.

7. Injection and pressure-holding
Use high injection pressure (1500-1800bar) and holding pressure (about 80% of injection pressure). About 95% of the full stroke, turn to pressure-holding and use a longer pressure-holding time.

8. Post-processing of the product
In order to prevent shrinkage and deformation from post-crystallization, the product should be treated by hot water soaking.