How plastics and composites are made
Injection molding, extrusion, layup, autoclaves, filament winding, 3D printing — the ways raw polymer becomes a part.
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.
Two routes: melt-and-freeze vs shape-and-cure
A thermoplastic is melted, shaped, then frozen — and you can undo it by reheating. A thermoset is shaped soft and then cured, a one-way chemical reaction: the shape is set before the chemistry finishes, and it will never melt back.
Check yourself
1. Why is almost all thermoplastic manufacturing melt–shape–freeze?
2. How does making a thermoset part differ from a thermoplastic one?
3. Why do engineers say the process is part of the material?
Read next
Thermodynamics and fluids in plain language — and why curing an epoxy part is secretly a chemistry-of-heat problem.
The two ways monomers join into chains — chain-growth and step-growth — the mechanisms behind them, and the families of plastics, resins, and fibers they make.
A steel bar is equally strong whichever way you pull it. A carbon-fibre part can be ten times stiffer along the fibres than across them. What anisotropy is, why composites have it, and why it's a feature, not a bug.