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Injection molding is a manufacturing process for producing parts by injecting molten material into a mold. The process is widely used for thermoplastics, and also for some thermosetting polymers and elastomers. In general, plastic material is melted in a heated barrel, then forced at high pressure into a closed mold where it fills the cavity and takes its shape as it cools and solidifies.

The mold is held closed by a clamping unit, and after the material has filled the cavity, pressure is maintained to pack the material and compensate for shrinkage. Once cooling is complete, the mold opens and the part is ejected, often with automation such as worms, grippers, or conveyors assisting the cycle. The cycle time depends on material, part geometry, and cooling efficiency.

Materials commonly used include thermoplastics such as polypropylene, ABS, polystyrene, polycarbonate, and nylon, with thermosets and elastomeric compounds used in niche applications. The process relies on an injection unit consisting of a heated barrel and screw that melts and then injects the material into a mold made of hardened steel or aluminum.

Key design considerations include mold layout with gates and runners, venting, cooling channel design, and wall thickness balance to minimize warpage and sink marks. Injection molding enables high-volume production with tight tolerances and good surface finishes, making it a mainstay in automotive, consumer electronics, packaging, and medical devices.

Variants and related techniques include insert molding, overmolding, gas-assisted molding, and multi-shot or two-shot injections, which expand part functionality and assembly options but require more complex tooling.