How can a factory make the same plastic shape thousands of times?

A plastic enclosure may have thin walls, ribs and little clips that fit another part. An injection-molding machine can form those features together by pushing softened plastic into a carefully shaped metal cavity.

For common thermoplastics, pellets enter a heated barrel. A turning screw mixes and melts them; heating and mechanical work both contribute. The screw then moves forward to drive a measured amount through a gate into the closed mold.

Pressure helps fill the cavity and compensate for some shrinkage while the material cools. The mold stays clamped because the pressurized plastic pushes its halves apart. Once the part is firm enough, the mold opens and ejector pins help release it.

The cavity is reusable, but the design has to permit escape. Slight wall slopes called draft can help a part release; uneven thickness can produce uneven cooling and distortion. A repeatable shape is therefore a joint achievement of geometry, material and timing.

The small gate mark and round ejector marks on a product can be clues to its history. A finished object sometimes carries evidence of the machine that made it.