How injection molding makes plastic parts

An injection molding machine melts plastic pellets in a heated barrel with a turning screw, then pushes the screw forward like a plunger to force the melt into a closed steel mold. The plastic cools into the shape of the cavity, the mold opens, and the part is ejected. A powerful clamp must hold the mold shut against the melt pressure, and that force grows with the part's projected area.

Controls

Labels

Animation speed

Projected part area

The part's shadow on the mold parting plane.

Cavity pressure

Cooling time

Step through it

Pellets inPlastic pellets drop from the hopper into the back of a heated barrel.
Melt and meterThe turning screw drags pellets forward. Friction and heater bands melt them, and a measured shot of melt gathers in front of the screw.
Inject and packThe screw stops turning and rams forward like a plunger, forcing melt into the mold at high pressure, then holds pressure while the part shrinks as it cools.
Clamp forceMelt pressure pushes the mold halves apart over the whole projected area. Double the area and the clamp must double too.
Cool and ejectCooling is usually the longest step. When the part is stiff, the clamp opens and ejector pins push it out.

Sizing the clamp

Clamp force is the pressure in the cavity times the projected area of everything the melt fills, including runners. Pressure in bar times area in square centimeters times 10 gives newtons; dividing by 9,807 gives metric tonnes of force.

Cavity pressure depends on the plastic, the wall thickness and the flow length. Thin, long-flow parts need far more pressure than thick, short ones, which is why molders often add a safety margin of 10 percent or more.

Try it: raise the projected area to 400 cm² at 800 bar. The clamp needed passes 320 tonnes, a large machine. Then cut cooling from 15 to 5 seconds and watch shots per hour jump from 171 to 327.

Cycle time

The cycle readout adds about 6 seconds for injection, packing, opening, ejection and closing to your cooling time, a simplified teaching estimate. Cooling time grows roughly with the square of wall thickness, so thin walls are the biggest lever for faster molding. The animation runs three times faster than real time.

Sources

2.008 Injection Molding (lecture)Massachusetts Institute of Technology
A Review on Injection Molding: Conformal Cooling Channels, Modelling, Surrogate Models and Multi-Objective OptimizationPolymers (MDPI, open access)

How to cite this model

Avid Solutions International. (2026, November 11). How injection molding makes plastic parts [Interactive 3D model]. Avid Field Lab. https://avidsolutionsintl.com/explore/injection-molding

Last updated November 11, 2026

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