Injection Molding Process — Interactive Simulator

A step-by-step animated cross-section of the six stages of the plastic injection molding cycle. See how clamping, injection, packing, cooling, mold opening, and ejection work together inside a real mold — with detailed anatomy of the barrel, screw, sprue, runner, gate, cavity, core, ejector pins, cooling channels, tie bars, and clamp unit.

About the injection molding process

Plastic injection molding is the most widely used manufacturing process for producing plastic parts in volume. From bottle caps and Lego bricks to automotive bumpers and medical syringes, almost every plastic part you touch was made by injection molding. The cycle is repeatable to within a fraction of a second and can produce identical parts at rates of hundreds of thousands per day from a single mold.

The six stages explained

1. Mold clamping

The hydraulic clamp cylinder pushes the movable platen against the fixed platen until the two mold halves meet along the parting line. Clamp force ranges from 30 tonnes for small parts to over 6000 tonnes for large automotive components. The clamp must exceed projected area × peak injection pressure, otherwise the mold flashes open during injection.

2. Injection (filling)

The reciprocating screw — now acting as a plunger — drives molten polymer at 180–300 °C through the nozzle, sprue, runner, and gate into the cavity. Injection pressure spikes to 70–200 MPa over 0.5–4 seconds. The mold parting line is vented at 0.02–0.05 mm depth so trapped air can escape; without venting, the air compresses and burns the polymer (the "diesel effect").

3. Packing and holding

At 95–98% full, the machine switches from velocity control to pressure control (the V→P transfer). Additional melt is forced in at 40–100 MPa to compensate for thermal shrinkage as the part cools. This continues only until the gate freezes off — afterwards no more polymer can enter regardless of pressure.

4. Cooling

Water or oil circulates through channels machined close to the cavity surface, conducting heat out of the part. Cooling typically takes 60–80% of the entire cycle time and is the single biggest lever for productivity. Conformal cooling channels — 3D-printed to follow the part geometry — can cut cycle time by 20–40%.

5. Mold opening

The clamp retracts. Because the polymer shrinks onto the B-side core features during cooling, the part stays gripped on the core and travels with the moving half, exposed in the open space between the two mold halves. Opening stroke is at least 2× part depth.

6. Ejection

The ejector plate advances, driving knockout pins through the core face against the back of the part. The part releases and falls by gravity onto a conveyor below. Pin placement leaves small witness marks (~0.05 mm), so pins are placed on non-cosmetic surfaces. Typical total cycle: 8–60 seconds.

Frequently asked questions