Step by Step Injection Moulding Process

Injection moulding is a high-volume manufacturing technique to produce identical plastic components by injecting molten polymer into a precision mould cavity. A full production cycle consists of 6 sequential core stages.

1. Clamping & Mold Locking

The two halves of the mould (fixed half and moving half) close and are securely locked by the machine’s clamping unit with sufficient tonnage. This huge clamping force keeps the mould tightly shut to resist the high pressure from molten plastic during injection and prevent flash on finished parts.

Meanwhile, plastic pellets are fed from the hopper into the heated barrel. The reciprocating screw rotates to transport, compress and melt pellets through friction heat and barrel heater bands, forming homogeneous molten plastic. This melting process is called plasticization. The screw retracts backward and accumulates an exact volume of melt (called a “shot”) ready for injection.

2. Injection (Filling)

The screw stops rotating and moves forward rapidly like a plunger. Under high pressure, molten plastic flows through the nozzle, sprue, runner system and gate, filling the entire mould cavity. The filling phase normally takes only 0.5–4 seconds. Injection speed and pressure must be precisely controlled to avoid defects such as short shots, jetting or burn marks.

3. Packing & Holding Pressure

Once the cavity is nearly full, the machine switches from velocity control to pressure control. The screw maintains a constant holding pressure and pushes extra melt into the cavity to compensate for volumetric shrinkage as plastic cools down. Holding pressure continues until the gate freezes solid. This step eliminates sink marks, voids and ensures stable dimensional accuracy of the final plastic part.

4. Cooling & Solidification

Cooling is the longest phase, occupying 50%–80% of the whole cycle. Coolant (water or oil) circulates inside mould cooling channels and extracts heat from the hot plastic. The molten polymer gradually cools and hardens into the shape of the cavity. The part must become rigid enough to resist deformation before ejection. The cooling time depends on plastic material and part wall thickness. During cooling, the screw rotates again to plasticize material for the next cycle.

5. Mold Opening

After cooling completes, the clamping force releases. The moving mould half retracts and separates slowly along the parting line. For products with undercut features, side slides or lifters retract first to release the locked plastic part before full mould opening.

6. Ejection

Ejector pins, sleeves or stripper plates mounted on the moving mould half push the solidified plastic part off the mould core. The finished part falls onto a conveyor or is picked up by a robot arm. Sprue and runner scraps are trimmed in post-processing. The mould closes again automatically, and a new injection cycle starts.

7. Post-processing (Optional)

After ejection, secondary operations may be carried out: trimming sprues/runners, surface polishing, painting, assembly, ultrasonic welding and quality inspection.

Key Summary

Full cycle sequence: Clamping & Plasticization → Injection → Packing & Holding → Cooling → Mold Opening → Ejection → Repeat Common materials: ABS, PP, PC, PA, POM, PEEK and other thermoplastics Typical applications: automotive housings, electronic enclosures, medical parts, consumer goods.