This article breaks down the key injection molding parameters for widely used plastics, offering a practical reference for mold designers and process engineers to set up robust production cycles.

Getting the process window right from the start is what separates a smooth production run from a scrap-heavy shift. For most standard thermoplastics, the melt temperature and mold temperature are the two anchors that dictate everything else—from cycle time to surface finish. Take ABS, for instance: a melt range of 210–250°C paired with a mold temperature of 40–80°C gives a good balance of flow and gloss. Push the melt above 260°C and you risk decomposition, which shows up as silver streaks or burnt spots. On the tooling side, if you’re running a textured cavity or a deep rib, keep the mold closer to 80°C to ensure faithful replication. For PP, the melt sits around 200–230°C, and the mold can run cooler at 30–60°C, but don’t drop below 20°C on thin-wall parts or you’ll get flow marks from premature freezing. These numbers are not just suggestions—they are the baseline for calculating shrinkage, which for PP ranges from 1.5% to 2.5% depending on wall thickness and glass content.

Injection pressure and holding pressure are where most new process engineers make mistakes. For unfilled nylon (PA6), a typical injection pressure is 80–120 MPa, but the hold pressure should be around 50–70% of that—otherwise you’ll get sink marks near the gate or excessive warpage on flat panels. For PC, which is stiffer and more viscous, you’ll need 100–150 MPa injection pressure and a higher mold temperature of 80–120°C to avoid internal stress cracking. A common trick we use in the shop: set the packing time based on gate freeze-off, not just a fixed timer. For a 2mm wall section, gate freeze usually occurs at 4–6 seconds; running longer just wastes energy and can cause overpacking. Also, remember that screw back pressure for glass-filled materials should stay below 0.5 MPa to prevent fiber breakage, which directly impacts tensile strength. If you’re seeing short shots on a long, thin core, check the injection speed profile—start slow, then ramp up to 80% speed in the middle, and taper off before the final 10% of fill to avoid jetting.

Cooling time is the biggest lever on cycle time, and it’s often misjudged. For a 3mm thick ABS part, a mold temperature of 60°C and a coolant temperature of 20°C gives a cooling time of roughly 20–25 seconds. But if you drop the mold to 40°C, you might save 4–5 seconds per cycle—at the cost of poorer surface gloss and higher molded-in stress. For POM (acetal), cooling is faster, around 15–20 seconds for the same wall, but you need to watch the shrinkage anisotropy—it’s about 2% along flow and 1.5% across flow, so design your ejection with that in mind. Also, don’t forget to pre-dry hygroscopic resins like PA and PC—PA6 needs 2–4 hours at 80°C, and PC needs 4 hours at 120°C. Running them wet will cause hydrolysis, leading to brittle parts that fail at the gate. For more detailed process tables and mold sourcing tips, visit MoldWorld at www.moldw.com—it’s a solid resource for comparing tooling shops and getting real-world quotes.