Plastics ship as pellets/powder and are melted before shaping. Also available as sheet, rod, tube.

Extrusion

Pellets fed into a heated barrel; a screw (feed → melt/transition → pumping sections) blends, melts (shear + heaters), and pressurizes the melt through a die; product cooled by air/water. Cheap tooling, long constant-section parts.

Sheet & film: flat die + cooled rolls; thin films/bags blown into a balloon (1.5–2.5×) for thickness, then drawn (strengthens by straining).

Injection Molding

Like hot-chamber die casting; barrel heated, but major heat is from friction (vs heaters in extrusion). Complex shapes, good accuracy (watch shrinkage). Cups, housings, knobs, toys. Cycle ~5–60 s (thermoplastics), minutes (thermosets). Molds: cold-runner 2-plate, cold-runner 3-plate, hot-runner (no runners). Insert molding embeds metal parts. Clamping force ~1–2 MN; machines $60k–$ 150k; dies $20k–$ 200k → needs high volume.

Reaction injection molding (RIM): two reactive fluids mixed and injected; chopped fibers optional; auto bumpers, fridge insulation.
Structural foam molding: solid skin + cellular core via a blowing agent (down to ~40% density); furniture, TV cabinets.

Other Molding

Blow molding: extrude a tube, clamp in a mold, blow with air → bottles, containers.
Rotational molding: powder in a thin-walled split mold rotated about two axes in an oven → large hollow parts (tanks, toys, footballs); low equipment cost, long cycles.
Thermoforming: heat a sheet, draw it over a mold by vacuum; needs high uniform elongation; cheap Al tooling; trays, panels.
Compression molding: charge/powder/resin in a heated mold, compacted by the upper half; flash/positive/semipositive types; dishes, handles, caps.

Processing Reinforced Plastics

Fibers are surface-treated for bonding. Partially cured forms:

Prepregs: aligned continuous fibers through a resin tank onto tape.
Sheet-molding compound (SMC): chopped fibers on resin paste, rolled.
Bulk-molding compound (BMC): billet form via extrusion.
Thick-molding compound (TMC): BMC (low cost) + SMC (strength).

Molding methods:

Vacuum-bag molding: prepregs in a mold, vacuum under a plastic bag for pressure/bonding.
Contact molding / hand lay-up: reinforcement wetted with resin by hand; low pressure → low dimensional quality.
Resin transfer molding (RTM): resin pumped into a fiber-filled mold.
Filament winding: resin + fibers wound on a rotating mandrel → very strong axisymmetric parts (pipes, tanks).
Pultrusion: prepreg pulled through a resin bath and die + microwave cure → rods, profiles, tubing; golf clubs, drive shafts.

Notes

Thermosets are hard to extrude (curing). Injection-molding composites is hard (fibers hinder flow).
Recycling reinforced plastics is difficult (separate fiber from thermoset matrix).
Blow-molded bottle lettering is raised (easier to machine into the mold).
Cycle time depends on material, part shape, and initial temperature.