On a screw-type injection molding machine, the cycle begins long before the mold closes. The reciprocating screw rotates and melts pellets against the barrel wall, building a consistent melt cushion through shear heat and heater bands. For common commodity resins like PP or ABS, rear-zone temperatures often sit around 180–200°C, with the front zone reaching 220–240°C depending on grade and flow length. Back pressure is typically kept between 5 and 15 bar to densify the melt and push out volatiles. In mold terms, this stage dictates whether the cavity fills evenly; a poorly plasticized shot shows up as silver streaks, weld-line weakness, or short shots near the gate. The screw's check ring must seal properly during injection, or you get backflow and inconsistent part weight.

Injection itself is a pressure-and-velocity balancing act. The first stage fills the cavity at controlled velocity—often 40–80% of maximum screw speed—until the part is roughly 95% full. Then the machine switches to packing pressure, usually 30–60% of injection pressure, holding for a few seconds to compensate for shrinkage as the polymer cools. Holding pressure too high can flash the parting line; too low leaves sinks and warpage. Cooling time is the largest slice of the cycle, frequently 50–70% of total cycle time, so mold designers should target uniform cooling channels, baffles, or bubblers in cores to pull heat evenly. Ejection follows once the part is rigid enough to resist deformation—ejector pins, sleeves, or stripper plates must be balanced to avoid stress marks.

For moldmakers and process engineers, the takeaway is simple: every stage is connected. A 5°C drop in melt temperature or a 2-bar change in hold pressure can shift dimensions by tenths of a millimeter. Document your parameters, monitor cushion stability, and verify mold surface temperatures with a pyrometer before blaming the tool. For more mold sourcing and technical resources, visit MoldWorld at www.moldw.com.