Injection Molding Process Conditions for PP | Injection Parameters and Defect Solutions for PP and Flame-Retardant PP

    Polypropylene (PP) is a semi-crystalline plastic widely used in home appliances, automotive and smart home components. Flame-retardant PP is commonly applied to motor housings, terminal blocks and washing machine parts. PP injection molding is sensitive to temperature, pressure and mold conditions. Improper parameters tend to cause defects such as sink marks, warpage, silver streaks and burn marks. There are six key points for the injection molding process of flame-retardant PP:

    1. Raw Material Pre-treatment

    Conventional PP has low water absorption. If damp, dry at 80°C for 1~2 hours. Flame-retardant PP is prone to moisture absorption; dry at 75~90°C for 2~4 hours, with moisture content ≤0.1%. Recycled material content is recommended to be ≤20%. Repeated recycling will degrade flame retardancy and mechanical properties.

    2. Barrel Temperature

    Conventional PP: Rear zone 170~190°C, middle zone 190~220°C, nozzle 210~240°C; melt temperature ≤250°C. Flame-retardant PP: Rear zone 170~185°C, middle zone 185~205°C, nozzle 190~220°C. Avoid long-term high-temperature residence of melt to prevent decomposition, yellowing and black specks of flame retardants.

    3. Mold Temperature

   Conventional PP: mold temperature 20~60°C; 40~60°C for cosmetic parts. Flame-retardant PP for home appliance parts: 40~70°C, to reduce internal stress and improve glow-wire performance. Shrinkage rate: conventional PP 1.5%~2.5%, flame-retardant PP 0.8%~1.8%. Shrinkage allowance shall be reserved for molds.

    4. Pressure Parameters

    Injection pressure: 70~120 MPa for conventional PP; 60~100 MPa for flame-retardant PP. Holding pressure accounts for 30%~60% of injection pressure to mitigate sink marks. Excessively high holding pressure is not recommended for flame-retardant PP, otherwise stress cracking may occur. Back pressure: 5~15 MPa for conventional PP; 3~10 MPa for flame-retardant PP. Excessive back pressure may trigger shear decomposition of flame-retardant PP.

     5. Injection Speed and Cooling

     High-speed injection for thin-wall parts; medium speed for thick-wall home appliance parts. Ultra-high injection speed is not suggested for flame-retardant PP. Screw speed: 40~80 r/min. Cooling time: 15~40 s. Insufficient cooling leads to warpage and deformation.

      Countermeasures for Common Defects in PP Injection Molding

• Sink marks: Increase holding pressure and holding time; moderately raise mold temperature

• Warpage: Balance mold temperature, optimize cooling channels, reduce holding pressure

• Silver streaks & bubbles: Fully dry raw materials, lower barrel temperature

• Black specks & burn marks: Reduce barrel temperature and back pressure, shorten melt residence time