Injection molding of PP material

Polypropylene (PP) is a thermoplastic “addition polymer” made from the combination of propylene monomers. It is used in a variety of applications to include packaging for consumer products, plastic parts for various industries including the automotive industry, special devices like living hinges, and textiles.

1. Treatment of plastics.

Pure PP is translucent ivory white and can be dyed in various colors. For PP dyeing, only color masterbatch can be used on general injection molding machines. Products used outdoors are generally filled with UV stabilizers and carbon black. The use ratio of recycled materials should not exceed 15%, otherwise it will cause strength drop and decomposition and discoloration.

2. Selection of injection molding machine

Because PP has high crystallinity. A computer injection molding machine with higher injection pressure and multi-stage control is required. The clamping force is generally determined at 3800t/m2, and the injection volume is 20%-85%.

 3. Mold and gate design

The mold temperature is 50-90℃, and the high mold temperature is used for the higher size requirements. The core temperature is more than 5℃ lower than the cavity temperature, the runner diameter is 4-7mm, the needle gate length is 1-1.5mm, and the diameter can be as small as 0.7mm. The length of the edge gate is as short as possible, about 0.7mm, the depth is half of the wall thickness, and the width is twice the wall thickness, and it will gradually increase with the length of the melt flow in the cavity. The mold must have good venting. The vent hole is 0.025mm-0.038mm deep and 1.5mm thick. To avoid shrinkage marks, use a large and round nozzle and a circular runner, and the thickness of the ribs should be small. The thickness of products made of homopolymer PP cannot exceed 3mm, otherwise there will be bubbles.

4. Melting 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 temperature of the melting zone 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 molds are not suitable. If the patterned surface appears with light and dark stripes diffused by the gate, low-speed injection and higher mold temperature should be used.

6. Melt adhesive back pressure

5bar melt adhesive back pressure can be used, and the back pressure of the toner material can be adjusted appropriately.

7. Injection and pressure keeping

Use higher injection pressure (1500-1800bar) and holding pressure (about 80% of the injection pressure). Switch to holding pressure at about 95% of the full stroke, and use a longer holding time.

8. Post-processing of products

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

Post time: Feb-25-2022


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