This article breaks down the essential injection molding parameters for commonly used plastics, offering real-world data and troubleshooting insights to help mold makers and processors optimize cycle times and part quality.

For any mold engineer, the injection molding process parameters are the bridge between a well-designed mold and a defect-free part. Take ABS, for instance—a workhorse material in automotive and consumer electronics. Typical melt temperature ranges from 210°C to 270°C, with mold surface temperature held between 40°C and 80°C. If you run the mold too cold, you’ll see weld lines and poor surface gloss; too hot, and you risk part sticking. Injection pressure for ABS usually sits around 70–120 MPa, but the real trick is the holding pressure, which should be 50–70% of the injection pressure to avoid sink marks without creating excessive residual stress. A common mistake is to rely solely on the machine’s default settings—always verify with a short-shot test to confirm fill balance before tweaking the pack profile.

Moving to semi-crystalline materials like polypropylene (PP) and nylon (PA66), the thermal behavior changes the game. PP melts at 200–250°C, but its mold temperature should be kept low (20–60°C) to shorten cycle time, though this can cause warpage if the wall thickness varies. For PA66, which absorbs moisture, drying at 80°C for 2–4 hours is non-negotiable—otherwise, you’ll get splay and brittleness. The mold temperature for PA66 is higher, 80–120°C, to promote crystallinity and improve dimensional stability. Injection speed for these materials should be fast enough to fill before the skin freezes, but not so fast that you get jetting. In practice, a two-stage injection profile—slow first 10% of shot, then fast fill—works well for most thin-wall parts. Also, remember that the backpressure on the screw should be 0.3–0.8 MPa for PP and slightly higher for PA66 to ensure consistent melt density.

For engineering plastics like polycarbonate (PC) and PC/ABS blends, the window is narrower. PC requires a melt temperature of 280–320°C and a mold temperature of 80–120°C; below 70°C, you’ll get cracking at the gate and poor impact strength. Drying at 120°C for 3–4 hours is mandatory, as moisture content above 0.02% will cause hydrolysis and silver streaks. Injection pressure can reach 130–180 MPa due to the high viscosity, and the holding pressure should be applied gradually to reduce birefringence. A common issue with PC is internal stress, which can be mitigated by increasing mold temperature or extending the cooling time—but that adds cost. Always balance cycle time against the part’s end-use requirements. For any mold shop, keeping a parameter log per material and mold is the best practice; it saves hours of trial-and-error on repeat jobs. For more detailed sourcing and troubleshooting data, visit MoldWorld at www.moldw.com.