§1What a plastic is
Plastics are polymers — long chain molecules of repeating units, mostly built on carbon. That molecular structure gives them their whole character: light, formable and inert, but soft and temperature-sensitive.
Because the chains are held together far more weakly than the atoms in a metal crystal, plastics are about a hundred times less stiff than steel, far weaker, and they soften or expand markedly with modest heat. In exchange they are a fraction of the weight, immune to the corrosion that plagues metals, cheap to mould into complex shapes in one shot, and often self-lubricating or electrically insulating. The engineering task is never to pretend a plastic is a metal, but to exploit what it does uniquely well while designing around its three main limits — low stiffness, low strength and high thermal movement.
Contents§2Thermoplastics and thermosets
Plastics split into two fundamental kinds by how they respond to heat — a division that decides how they are processed and whether they can be recycled.
Thermoplastics soften every time they are heated and harden again on cooling, reversibly, like wax — so they can be injection-moulded at speed, welded, and remelted or recycled. Most common plastics (polyethylene, nylon, PVC, acrylic) are thermoplastics. Thermosets undergo a one-way chemical cure into a rigid, cross-linked network that cannot be remelted — heat them again and they char rather than soften, like a boiled egg. Epoxies, phenolics and the matrix of most fibre composites are thermosets: harder, more heat- and creep-resistant, but unmeltable and unrecyclable. The distinction is the first question to ask of any plastic, because it fixes both the manufacturing route and the service temperature behaviour.
Contents§3Low density and the float test
Plastics are light — most between 0.9 and 1.4 times the density of water — so a quick tank of water sorts them into those that float and those that sink.
Against water at 1000 kg/m³: polypropylene (905) and polyethylene (950) float — the only common plastics that do, a genuine identification test. Nylon (1140), acrylic (1180), PVC (1400) and PTFE (2200) all sink. Even the densest common plastic, PTFE, is only about a quarter the density of steel, so a plastic part is a quarter to a seventh the weight of the same shape in steel. That lightness, more than any strength, is often the reason a plastic is chosen — and the float test is a field trick for telling the polyolefins from the rest.
§4Low stiffness
The property to respect most: plastics are roughly two orders of magnitude less stiff than steel, so a plastic part deflects far more under the same load.
Where steel has a modulus near 200 GPa, engineering plastics sit at 1–4 GPa — nylon at about 3 GPa is some 67 times less stiff than steel. A plastic beam or bracket therefore bends far more for the same load and section, which the designer answers with generous sections, ribs, and geometry that carries load in ways slender metal parts need not. Plastics also creep — they slowly deform under a steady load even at room temperature, so a plastic part under permanent stress keeps moving over months. The rule is to design plastics by stiffness and creep, not strength: they usually deflect too much long before they break.
Contents§5High thermal expansion
Plastics expand with heat far more than metals — commonly five to fifteen times as much — which dominates the design of any plastic part fitted to metal.
Steel’s expansion coefficient is about 11.7 µm/m/°C; nylon’s is roughly 90, polypropylene’s 150, polyethylene’s 200 — so polyethylene expands about 17 times as much as steel for the same temperature change (the hero). Over a 1 m span and a 50 °C swing, steel moves 0.6 mm but polyethylene moves 10 mm. This mismatch is why a plastic part clamped or pinned rigidly to metal will buckle, split or pull loose as temperature changes, and why plastic housings, panels and bushes are designed with slots, clearances and floating mounts that let them move. Thermal expansion, not strength, is often the property that sizes a plastic assembly.
§6The common plastics
A handful of families cover most mechanical use, each with a defining strength.
Nylon (PA)
Tough, wear-resistant, self-lubricating; gears, bearings, bushes. Absorbs moisture, which swells it.
Acetal (POM)
Stiff, low-friction, dimensionally stable; precision gears and mechanisms — the engineer’s plastic.
PTFE
The lowest friction of any solid, chemically inert, wide temperature range; seals, slides, non-stick — but soft and creeps.
Polyethylene / PP
Cheap, tough, chemical-proof, the floaters; tanks, pipe, containers. UHMWPE for wear.
Acrylic / PC
Clear; acrylic rigid and glossy, polycarbonate nearly unbreakable — glazing, guards, lenses.
PVC
Rigid for pipe and profile, or plasticised soft for hose and insulation; cheap and flame-retardant.
§7Quick reference
The working core of the page on one card rack.
Two kinds
thermoplastic (remelts)
thermoset (cures once)
Density
0.9–1.4 × water
PP, PE float; rest sink
Stiffness
1–4 GPa (~100× < steel)
design for deflection + creep
Expansion
5–15 × steel
allow it to move
Workhorses
nylon · acetal · PTFE
PE/PP · acrylic/PC · PVC