How plastics and composites are made

Thermoplastics: melt, shape, freeze

Because thermoplastics melt, almost all their manufacturing is melt–shape–freeze. Injection molding squirts molten plastic into a steel mold at enormous pressure and pops out a finished part every few seconds — LEGO bricks, bottle caps, phone shells. Extrusion pushes melt through a shaped hole continuously to make pipe, film, and filament. Thermoforming warms a sheet until rubbery and vacuum-sucks it over a mold — yogurt cups, blister packs.

3D printing (FDM) is a miniature extruder drawing the part line by line. It trades speed and strength for total geometric freedom — and its parts are weakest between the printed lines, a very composite-like directionality that surprises many first-time designers.

Thermosets and composites: shape first, then cook

Thermosets reverse the order: shape the soft, uncured material first, then trigger the chemistry that locks it solid. A boat hull is layers of glass cloth wetted with polyester resin in an open mold. Aerospace parts use prepreg — cloth pre-soaked with epoxy — stacked ply by ply (the layup defines the laminate!), bagged under vacuum, and cured under heat and pressure in an autoclave.

Filament winding wraps resin-wet fiber around a rotating mandrel for pressure vessels and pipes; pultrusion pulls fiber through a resin bath and a heated die to make continuous beams; resin transfer molding injects resin into dry cloth already sitting in a closed mold. In every one of these, the cure schedule — temperature versus time — decides the part's final quality.

Manufacturing is a property, not an afterthought

The same material makes different parts depending on process: fiber directions follow how the cloth was draped, voids form where air was trapped, weld lines form where two melt fronts met. Engineers say the process is part of the material. It is why the database on this site records test methods and conditions alongside every value — a number without its story is not data.