This article provides a practical overview of key injection molding parameters for commonly used plastics, with real-world data and process insights for mold engineers.

In daily mold shop operations, getting the process window right is often the difference between a stable production run and a scrap bin full of warped parts. For crystalline resins like PA66 and POM, melt temperature and mold temperature are the two most critical levers. PA66 typically runs at a melt temperature of 280–300°C, with mold temperature held between 80–120°C to promote proper crystallization and minimize post-mold shrinkage. POM, on the other hand, is more heat-sensitive—keep the melt at 190–210°C and mold at 80–100°C. Exceeding 220°C risks thermal degradation and formaldehyde gas release, which not only ruins part mechanical properties but also accelerates mold corrosion. For amorphous materials like ABS and PC, the focus shifts to flow length and residual stress. ABS processes well at 200–240°C melt, with mold temperature at 40–80°C, while PC requires a hotter barrel (280–320°C) and a mold surface of 80–120°C to avoid stress cracking. A common mistake is running PC with too low a mold temperature, which leads to high internal stress and poor chemical resistance.

Beyond temperature, injection speed and holding pressure directly affect part density and dimensional stability. For thin-wall parts in PP or HDPE, a fast injection speed (60–120 mm/s) is necessary to fill the cavity before the melt freezes. But for glass-filled PBT or PA, a two-stage profile—slow first 10–15% of stroke, then fast fill—prevents jetting and surface streaks. Holding pressure should be set at 60–80% of injection pressure, with a holding time calculated from gate freeze-off. As a rule of thumb, multiply the wall thickness (mm) by 2 seconds per millimeter for semi-crystalline resins; for amorphous resins, 1.5 s/mm is usually safe. Also, back pressure on the screw should be kept between 5–15 bar for most engineering plastics. Too low a back pressure gives poor melt homogeneity, especially with masterbatch colorants; too high causes excessive shear heating and screw wear, particularly with abrasive fillers like glass fibers.

Finally, remember that these numbers are starting points, not absolute rules. The actual optimal parameters depend on your specific mold design, gate geometry, and part wall thickness. Always run a short-shot series to verify fill behavior, and use a melt temperature probe at the nozzle to confirm the actual melt condition—not just the barrel setpoint. Also, monitor cycle time and part weight as quick indicators of process stability. For those sourcing molds or looking for a second opinion on process troubleshooting, visiting MoldWorld (www.moldw.com) is a solid resource for mold suppliers, tooling standards, and practical injection molding guides. Keep your data sheets handy, but trust your cavity pressure sensor more.