A brief introduction to the injection molding process of polypropylene (PP)

PP is called polypropylene, because of its good anti-breaking performance, also known as "one hundred percent plastic." PP is a semi-transparent, semi-crystalline thermoplastic that has high strength, good insulation, low water absorption, high heat-to-shape temperature, low density, and high crystallinity. Modified fillers typically have glass fibers, mineral fillers, thermoplastic rubber, and the like.

PPs with different uses have different flowability, and the commonly used PP flow rate is between ABS and PC.

1, plastic processing.

Pure PP is translucent ivory white and can be dyed in various colors. PP dyeing can only use color masterbatch in general injection molding machine. On the Ramada machine, there are independent plasticizing elements that enhance mixing and can also be dyed with toner. Products used outdoors are generally filled with UV stabilizers and carbon black. The proportion of recycled materials should not exceed 15%, otherwise it will cause a decrease in strength and discoloration. PP injection molding process generally does not require special drying.

2, injection molding machine selection

There are no special requirements for the selection of the 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 can be 20%-85%.

3, mold and gate design

The mold temperature is 50-90°C, and the high mold temperature is required for the high dimension. The core temperature is 5°C lower than the cavity temperature, the flow channel 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 length of the edge gate is, the better is about 0.7mm, the depth is half of the wall thickness, the width is twice the wall thickness, and the length of the melt flow in the cavity increases. The mold must have good exhaustability, exhaust hole depth 0.025mm-0.038mm, thickness 1.5mm, to avoid shrinkage marks, it is necessary to use a large round nozzle and round flow channel, the thickness of the reinforcement ribs should be small ( For example, it is 50-60% of the wall thickness). Homopolymer PP products, thickness can not exceed 3mm, otherwise there will be bubbles (thick-walled products can only use copolymer PP).

4, melt temperature

The melting point of PP is 160-175°C and the decomposition temperature is 350°C, but the temperature setting cannot exceed 275°C during injection processing. The melting zone temperature is preferably 240°C.

5, injection speed

In order to reduce the internal stress and deformation, high-speed injection should be selected, but some grades of PP and molds are not applicable (bubbles and gas lines are present in humans). If the patterned surface appears bright and dark streaks diffused from the gate, low speed injection and high mold temperature are used.

6, melt back pressure

5 bar melt back pressure can be used, the back pressure of the color powder can be properly adjusted.

7, injection and maintenance pressure

Use higher injection pressures (1500-1800 bar) and holding pressure (about 80% of injection pressure). About 95% of the full trip, it will be transferred to hold pressure, with a longer dwell time.

8. Post-processing of products

In order to prevent shrinkage caused by post-crystallization, the products generally need to be soaked in hot water.