This article breaks down the key injection molding parameters for commonly used plastics—covering melt temperature, mold temperature, injection pressure, and cooling time—with practical insights for mold designers and process engineers.

In daily mold shop practice, the difference between a stable production run and a scrapped batch often comes down to how well the process parameters match the polymer’s thermal and rheological behavior. For unfilled general-purpose grades like ABS and HIPS, a melt temperature of 210–250°C and a mold temperature of 40–80°C are typical, with injection pressure in the 60–100 MPa range. However, when moving to semi-crystalline materials such as PA66 or POM, the rules shift: PA66 requires melt temperatures around 270–300°C, mold temperatures of 80–120°C to ensure proper crystallinity, and a holding pressure that compensates for shrinkage—often 50–70% of injection pressure. Ignoring these numbers leads to warpage, sink marks, or short shots, especially in thin-wall geometries.

For engineering resins like PC and PBT, the window is narrower. PC needs a melt temperature of 280–320°C, a mold temperature of 80–120°C, and low shear rates to avoid stress cracking—so a larger gate and slower injection speed are mandatory. PBT, on the other hand, runs at 240–270°C melt, with mold temperatures of 60–100°C, but it is highly sensitive to moisture: even 0.02% water content causes hydrolysis and brittle parts. In our shop, we always pre-dry PBT at 120°C for 4 hours, and we verify the dew point of the dryer before every run. Cooling time is another parameter often underestimated—for a 3 mm wall in POM, a mold temperature of 90°C and coolant at 20°C yields a cooling time of roughly 20–25 seconds, but raising the mold to 110°C can cut cycle time by 15% without sacrificing dimensional stability.

What matters in real production is not memorizing a single set of numbers, but understanding how these parameters interact with the mold design—gate size, runner balance, and cooling channel layout. For instance, a hot runner system with a valve gate allows a lower injection pressure for PC, reducing residual stress, while a cold runner for PA66 requires a larger sprue to prevent premature freeze-off. Always start with the resin supplier’s data sheet, then fine-tune using short-shot trials and cavity pressure monitoring. For more detailed parameter tables and mold-specific troubleshooting tips, visit MoldWorld at www.moldw.com—a practical resource for sourcing molds and comparing process know-how from real production floors.