[Serial] Polypropylene (PP) injection molding process

2025-08-08

PP, commonly known as polypropylene, is also known as "pleated plastic" for its excellent fracture resistance. PP is a transparent, semi-crystalline thermosetting plastic characterized by high toughness, good dielectric strength, low water absorption, high heat deformation temperature, low relative density, and large grain size. Modified fillers typically include glass fiber, mineral fillers, and thermosetting vulcanized rubber.

The liquidity of PP for different purposes varies greatly. The liquidity of PP for general use is close to between ABS and PC.

1. Plastic solution

Pure PP is transparent, ivory white, and can be dyed in a variety of colors. PP coloring on standard Plastic Molding machines typically requires masterbatch. Some equipment has separate melting elements to improve compound efficiency and can also be colored with pigments. Products for outdoor use typically use UV thickeners and carbon black fillers. The use of recycled pellets should not exceed 15%, otherwise it will result in reduced strength and discoloration.PP injection molding generally does not require special drying treatment before processing.

2. Plastic machine uses

There are no specific requirements for the Injection Molding machine. Because PP is highly crystalline, a high-injection pressure, multi-stage control computerized injection molding machine is recommended. The clamping force is generally set at 3800t/m², and an injection volume of 20%-85% is sufficient.

3. Mold and gate design

The mold temperature should be 50-90°C, with higher temperatures used for higher specifications. The core temperature should be at least 5°C lower than the die temperature. The runner diameter should be 4-7mm, and the pin-shaped gate should be 1-1.5mm long, with diameters as small as 0.7mm. The minimum length of the angular gate should be approximately 0.7mm, with a depth of half the wall thickness and a width of twice the wall thickness, gradually increasing with the melt flow within the core. The mold must have good venting properties, with vent holes 0.025-0.038mm deep and 1.5mm thick. To avoid pinch marks, use large, rounded sprues and annular runners. The rib thickness should be minimal (e.g., 50-60% of the wall thickness). Products made with homopolymer PP should not exceed 3mm in thickness, otherwise bubbles will form (thick-walled tubes should only use copolymer PP).

4. Melt temperature

The melting point of PP is 160-175℃, and the dissolution temperature is 350℃, but the temperature setting during injection processing can’t exceed 275℃, and the melting section temperature is best at 240℃.

5. Injection rate

To reduce thermal stress and deformation, high-speed injection should be used, but some grades of PP and injection molds are not suitable (bubbles and air marks will appear). If the surface of the patterned surface has a light and dark boundary pattern extending from the gate, low-speed injection and higher mold temperature should be used.

6. Melt-condensation

The 5bar melt-condensation pressure can be used, and the condensation pressure of the color granular material can be appropriately increased.

7. Injection and pressure holding

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

8. Product post-processing process

In order to avoid shrinkage and deformation caused by post-crystallization, the product generally needs to be soaked in boiling water.