A practical breakdown of how injection pressure and melt temperature interact with flow resistance and packing compensation, with real-world data for mold quoting and process troubleshooting.

When quoting or troubleshooting injection molds, the relationship between injection pressure and melt temperature is often oversimplified. In practice, the pressure required to fill a cavity is directly proportional to the flow length-to-wall thickness ratio (L/t) and the melt viscosity at the set temperature. For a typical ABS part with an L/t of 150, a fill pressure of 600–800 bar is common at a melt temperature of 230°C. But if the mold has poor venting or a cold slug well is undersized, that pressure can spike by 20–30% due to trapped air compression. Always calculate the pressure drop across the runner system first—a 2 mm diameter sub-runner can consume 40% of the available machine pressure before the melt even reaches the gate.

Temperature is not just a process setting; it is the primary lever for managing flow resistance. Raising the melt temperature by 10°C for a semi-crystalline polymer like POM can reduce viscosity by roughly 15%, which directly lowers the required injection pressure. However, this comes at a cost: higher melt temperature increases shrinkage and warpage risk, especially in glass-filled grades. For example, a 30% glass-filled nylon 66 molded at 290°C will show a 0.5–0.8% shrinkage difference between flow and cross-flow directions if the mold temperature is not held at 80–100°C. In practice, I set the mold temperature first to stabilize the cooling rate, then adjust melt temperature to hit the target fill pressure—never the other way around.

Packing compensation is where most quoting errors occur. The switchover point from fill to pack should be at 95–98% of the cavity volume, not at a fixed pressure value. After switchover, the holding pressure should be 50–70% of the peak fill pressure, but only if the gate is large enough to allow material flow during packing. A gate thickness less than 50% of the wall thickness will freeze off too early, leaving sink marks regardless of packing time. For a real-world example: a polycarbonate lens mold with a 0.8 mm gate and 3 mm wall required 850 bar fill pressure, but only 450 bar hold pressure for 4 seconds to achieve zero sink. The melt temperature was 310°C, and mold temperature was 120°C. These numbers are your baseline—always record them during first trials and compare them against the simulation. For more mold sourcing and process data, visit MoldWorld (www.moldw.com).