injection molding
Injection molding is how the world makes billions of identical plastic parts: Lego bricks, bottle caps, phone cases, syringe bodies. You melt plastic and squirt it under pressure into a cold metal mold shaped like the part, let it cool solid, then open the mold and eject the piece, ready to repeat every few seconds. It is essentially a very fast, very precise metal-mold casting, but for polymer melts instead of molten metal.
Solid plastic pellets are fed into a heated barrel where a turning screw melts and mixes them, then the screw rams forward and injects the melt through a nozzle into the closed mold cavity. The melt fills the cavity, packs out under pressure to compensate for shrinkage as it cools, freezes against the cold steel, and is ejected. It works mainly for thermoplastics, which melt and resolidify reversibly; thermosets and rubber can be molded too but cure irreversibly in the hot mold. The whole cycle takes seconds, which is why cost per part is tiny once you own the mold.
The honest catch is that the flow leaves a fingerprint in the part. As the melt races into the cavity, the long polymer molecules line up along the flow direction and are frozen in that alignment, so the molded part is anisotropic and carries residual stress. Where two flow fronts meet around a hole they form a weld line that is a weakness, and thick sections cool unevenly and leave sink marks. And because the steel mold is expensive, injection molding only makes economic sense at high volume.
A polypropylene bottle cap is injection molded: pellets melt in the barrel, the screw injects the melt into a steel cavity, it packs and cools in a few seconds, and it ejects with molecules frozen aligned along the flow, leaving a faint weld line opposite the gate.
Fast fill freezes in molecular orientation and residual stress, so an injection-molded part is anisotropic by process.
Injection molding freezes in flow-induced molecular alignment and residual stress, and leaves weld lines where flow fronts meet; the high mold cost means it only pays off at high volume.