Plastic injection molding is a repeatable cycle in which resin is plasticized, injected into a mold, packed to compensate for shrinkage, cooled, and ejected. A stable process is not created by one temperature or pressure setting. It is developed by connecting part design, material data, mold design, machine capability, and measured trial results.
What are the four phases of injection molding?
| Phase | What happens | What the team controls |
|---|---|---|
| Fill | Melt flows through the sprue, runners, gates, and cavity until the mold is filled | Injection velocity, gate location, melt temperature, venting, and pressure |
| Pack or hold | Additional material is forced through the still-open gate to compensate for shrinkage | Pressure profile, hold time, change-over point, and gate-freeze behavior |
| Cooling | The part remains in the closed mold until it is stiff enough to eject | Cooling-channel layout, coolant temperature, mold temperature, and time |
| Mold open and eject | The mold opens and the ejection system removes the part and runner | Opening sequence, ejector support, release force, automation, and cycle timing |
Actual settings must follow the exact resin grade and mold. Autodesk describes the cycle as fill, pack, cooling, and mold-open stages, with cooling often the largest share of the cycle. See the Autodesk explanation of filling, packing, and cooling for the process physics behind these stages.
1. Filling: control the flow front
During filling, the screw moves forward and pushes the melt through the feed system. The melt cools as it contacts the mold wall and develops a frozen layer while the center continues to flow. Gate position, wall thickness, flow length, runner balance, and venting determine whether the cavity fills evenly or develops short shots, jetting, air traps, or weld lines.
Injection speed is usually profiled rather than held at one value. A slower start can reduce jetting near the gate; a faster middle section may fill a long thin wall before the front freezes; and a controlled end-of-fill speed can reduce pressure spikes and flash. The right profile depends on the resin and geometry.
2. Packing and holding: compensate for shrinkage
After the cavity is nearly full, the machine changes from velocity control to pressure control. Holding pressure pushes additional melt into the cavity while the gate remains open. The aim is enough packing to control volumetric shrinkage without overpacking the part, increasing stress, or forcing flash through a weak parting line.
Gate freeze is a useful process check: if hold pressure continues after the gate has solidified, it cannot add material to the cavity. A part-weight study or cavity-pressure measurement can help identify the useful hold-time window. Do not copy a hold pressure from another resin or mold without checking its response.
3. Cooling: control heat and dimensions
Cooling starts during filling but continues after packing while the part reaches an ejection condition. Thick sections, bosses, ribs, inserts, and deep cores store more heat and may remain soft after a thin wall looks ready. Uneven cooling creates differential shrinkage, warpage, sink marks, and dimensional drift.
Cooling-channel location and circuit balance should be reviewed during mold design. If a hot spot is caused by an inaccessible core, extending the cycle may help temporarily but does not replace a tool-design solution. Link cooling with mold making and DFM review.
4. Mold open and ejection
When the part is cool enough to release, the mold opens in the approved direction and the ejector system pushes the part from the core. Ejector pins should support the part where the ejection force can be accepted. Poor support, insufficient draft, vacuum effects, or premature ejection can cause white stress marks, deformation, sticking, or pin witness marks.
Which variables should be documented?
- Resin manufacturer, exact grade, color, filler, drying condition, and material lot.
- Melt and mold temperature targets, actual values, and allowable ranges.
- Injection velocity profile, pressure limit, change-over position, and screw recovery settings.
- Hold pressure profile, hold time, gate-freeze evidence, part weight, and cavity pressure when available.
- Cooling time, coolant inlet and outlet temperatures, circuit flow, and ejection temperature.
- Critical dimensions, part weight, appearance criteria, and inspection frequency.
From DFM to a stable production process
- Review the part CAD, drawing, resin, cosmetic zones, annual quantity, and machine constraints.
- Agree on parting line, draft, wall transitions, gates, runners, vents, cooling, and ejection.
- Run the first tool trial with documented settings and inspect fill, flash, sink, warp, dimensions, and appearance.
- Make the smallest justified tool or process change, record the result, and repeat until the acceptance criteria are met.
For production support, Cavity Mold can coordinate injection molding, tooling, and machining. Have a part under development? Send the STEP file, drawing, material, quantity, and target timing for a practical review.