Key Points of PP Injection Molding Parameter Settings - China Plastic Mold

Polypropylene (PP) is one of the most widely used crystalline plastics in the injection molding industry. It features excellent fluidity, low density, and strong molding adaptability. However, due to its high shrinkage rate and fast crystallization characteristics, PP products are prone to sink marks, warpage, weld lines, and dimensional instability if the process parameters are not properly controlled. Combined with mature manufacturing experience from China plastic mold factories, this article systematically summarizes the standard parameter setting principles and practical adjustment methods for PP injection molding.

1. Barrel Temperature Setting Rules

PP has a conventional melting temperature range of 180℃ to 240℃, and the specific temperature should be adjusted according to product wall thickness and material modification status. For general homopolymer PP, the rear barrel temperature is set at 180℃–190℃, the middle section at 190℃–210℃, and the nozzle temperature at 200℃–220℃. Copolymer PP and glass fiber reinforced PP require a slight temperature increase of 10℃–20℃ to ensure full plasticization.

Long-term temperature above 250℃ will cause molecular degradation, resulting in brittle products and silver streaks. If the temperature is too low, the melt will be uneven, causing material patterns and cold material marks. In actual production, the nozzle temperature is appropriately increased to prevent cold material from entering the cavity. For short-term shutdown, the holding temperature is controlled at 160℃–180℃ to avoid material aging.

2. Mold Temperature Control Standards

Mold temperature directly affects the crystallization degree and dimensional stability of PP products. Based on China plastic mold mass production experience, the mold temperature of ordinary PP is stably controlled at 30℃–50℃. Low mold temperature can speed up cooling, shorten the molding cycle, and improve product toughness, but it easily produces internal stress and later deformation.

When the mold temperature rises to 50℃–70℃, the material crystallization is more sufficient, the surface gloss is improved, and sink marks are effectively reduced. For glass fiber modified PP, the mold temperature is maintained at 40℃–60℃. The cooling water channels must be evenly arranged to avoid local temperature differences, which is the key to controlling product warpage and distortion.

3. Injection Pressure and Holding Pressure Matching Technology

PP has good fluidity, so the basic injection pressure is set between 60MPa and 100MPa. For thin-wall products and long-flow structural parts, the injection pressure can be increased to 90MPa–130MPa to ensure complete filling. Thick-wall products are not suitable for high-pressure one-time filling to prevent excessive internal stress.

Holding pressure is the core parameter to eliminate sink marks and stabilize dimensions. The conventional holding pressure is 50%–80% of the injection pressure. Thick-wall parts need extended holding time to supplement the shrinkage gap during cooling. Reasonable pressure switching can avoid over-holding defects such as sticking mold and stress cracking. Complex products adopt multi-stage injection pressure to ensure stable filling quality.

4. Injection Speed Adjustment Strategy

The injection speed is adjusted according to product appearance and structural requirements. Products with high surface requirements and obvious weld line positions adopt medium and low-speed injection to reduce melt turbulence, air entrapment and weld line marks. Thin and long products need high-speed filling to prevent premature cooling and material shortage.

In standardized China plastic mold production, the common method is low-speed passing through the gate and medium-speed filling in the later stage. Full high-speed injection will cause air trapping and burning, while long-term low-speed filling will lead to insufficient weld strength and easy cracking of stressed parts.

5. Screw Speed and Back Pressure Parameters

The conventional screw speed of PP molding is 40–80 r/min, which ensures uniform plasticization of raw materials. Excessively high speed will generate excessive shear heat, resulting in virtual high melt temperature and reduced product toughness. Too low speed will reduce plasticization efficiency and prolong the production cycle.

The recommended back pressure is 0.3–0.8MPa. Appropriate back pressure can homogenize the melt, discharge tiny bubbles, and improve product surface quality. For glass fiber reinforced PP, low back pressure is adopted to avoid shear damage to glass fiber structure and strength reduction.

6. Cooling Time and Internal Stress Control

PP releases a large amount of heat during crystallization, so sufficient cooling time is required for stable molding. The cooling time of ordinary thin-wall products is 8–20 seconds, while products with wall thickness over 3mm need extended cooling time. Insufficient cooling will cause secondary crystallization after demolding, resulting in shrinkage deformation and size deviation.

Controlling reasonable demolding temperature can effectively avoid top white and product distortion. For products requiring assembly and long-term use, proper stress relief treatment can reduce later warpage risk.

7. Drying, Venting and Auxiliary Process Management

Pure PP has extremely low water absorption and does not require drying under normal environment. Modified PP with filler, flame retardant or recycled materials needs drying treatment at 80℃ for 2–4 hours to eliminate silver streaks and bubbles.

The vent groove depth of PP mold is controlled at 0.015–0.025mm to ensure smooth exhaust while avoiding flash. When recycled materials are mixed in production, the temperature and back pressure parameters should be fine-tuned according to fluidity changes to maintain stable molding quality.

Conclusion

PP injection molding quality depends on reasonable temperature, pressure, speed and cooling matching. Combining mature China plastic mold practical experience, stable product quality can be obtained by standardizing barrel temperature 180℃–240℃, mold temperature 30℃–70℃, and scientific pressure and speed adjustment. Targeted parameter optimization for sink marks, warpage, weld lines and bubbles can effectively improve molding yield and stabilize mass production efficiency.