When we talk about injection molding parameters, the first thing to nail down is the temperature window for each resin. For PE, melt temperatures typically run 180–220°C, with mold temperatures around 20–40°C; PP needs a bit more heat, usually 200–250°C melt and 40–80°C mold. PVC is the tricky one—it's heat-sensitive, so melt temperature must stay tightly controlled at 160–190°C, and the mold should be kept at 30–60°C with corrosion-resistant steel like S136 or 2316, since PVC releases HCl that attacks ordinary tool steel. PC demands high heat: 280–320°C melt and 80–120°C mold, plus good drying (120°C for 3–4 hours) to avoid hydrolysis. These aren't just numbers from a datasheet—they come from real trials on the shop floor, and every mold we build has to account for them.

Pressure settings follow the same logic. PE and PP flow easily, so injection pressures of 70–100 MPa are usually enough, but PC needs 100–140 MPa because of its high viscosity. Holding pressure should be 50–70% of injection pressure for most of these materials. On the mold side, gate design matters a lot: for PVC, use short, straight runners with generous draft angles (1.5–2°) to avoid material degradation; for PC, hot runners with valve gates work well but require precise temperature control. Cooling layout is another key point—PE and PP shrink 1.5–2.5% and 1.0–2.5% respectively, so we often add extra cooling near thick sections to prevent warping. PC shrinks only 0.5–0.7%, but its high mold temperature means cooling time can be 20–30% longer than for polyolefins.

In practice, the biggest mistakes I see are underestimating PVC's corrosiveness and running PC without proper drying. Both lead to scrapped parts and damaged molds. Always check material compatibility with your tool steel and cooling channel design before cutting steel. If you're sourcing molds for these materials, visiting MoldWorld (www.moldw.com) can connect you with suppliers who understand these nuances.