Understanding the Injection Molding Process for Polypropylene (PP)

Polypropylene (PP) is one of the most widely used thermoplastics in injection molding. Understanding its material properties, processing characteristics, and injection molding conditions is essential for producing high‑quality parts.

Material Properties

PP is a white, waxy plastic that closely resembles polyethylene (PE) in appearance but is even lighter, with a density of only 0.90–0.91 g/cm³. It offers better strength, stiffness, hardness, and heat resistance than low‑density polyethylene (LDPE), and can be used continuously at temperatures around 100°C. PP also has excellent electrical properties and high‑frequency insulation, which are not affected by humidity. However, it becomes brittle at low temperatures, has poor wear resistance, and tends to age easily.

Processing Characteristics

① PP is a non‑polar, crystalline plastic. Its maximum crystallization rate occurs between 120–130°C, with a crystallinity of over 50%. It has low moisture absorption (about 0.03–0.04%) and generally does not require drying. PP is prone to melt fracture and can decompose after prolonged contact with hot metal surfaces.

② PP has good flowability, with a melt flow rate (MFR) of 2–9 g/10 min. Melt viscosity decreases as injection pressure increases or temperature rises. However, PP has a high shrinkage rate (1.0–2.5%) and exhibits anisotropic shrinkage, making it susceptible to voids, sink marks, and warpage.

③ PP has a higher melting and decomposition temperature than polyethylene, indicating good thermal stability. Its melting range is 165–170°C, and decomposition occurs above 350°C. The molding temperature window is relatively wide, typically between 205–315°C.

④ PP cools quickly. The runner and cooling systems should be designed to dissipate heat slowly, and molding temperatures must be carefully controlled. Low melt temperatures increase directional orientation, especially under low‑temperature, high‑pressure conditions.

⑤ PP has low oxidation resistance. An appropriate amount of antioxidant should be added before plasticization.

⑥ Wall thickness should be uniform and kept between 1–4 mm. Avoid material starvation and sharp corners to prevent stress concentration.

Injection Molding Conditions

① Melt temperature should be controlled between 200–250°C, with a maximum of 280°C. Thicker parts may require lower melt temperatures, but if the temperature is too low, molecular orientation increases, making the part more prone to warpage.

② Injection pressure typically ranges from 70–120 MPa. Higher pressure reduces melt viscosity, improves flow, eases molding, and also results in slightly lower shrinkage.

③ Packing (holding) pressure must be maintained long enough to compensate for material shrinkage during solidification. Beyond that, the injection and holding time should be minimized to reduce post‑molding shrinkage.

④ Mold temperature should be kept between 70–90°C. Higher mold temperatures improve crystallinity, reduce internal stress, and enhance both strength and appearance. Lower mold temperatures cause faster cooling and solidification, resulting in lower crystallinity, lower part density, higher internal stress, and poorer surface quality due to greater shrinkage.

⑤ Pay attention to post‑ejection shrinkage. Parts should be stored on a sizing fixture. For parts with tight dimensional requirements, heat treatment (annealing) is recommended.