Plastics

§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.

Example 1 — which plastics float

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.

Contents

§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.

Example 2 — plastic against steel over temperature

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.

Contents

§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.

Contents

§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

Contents

Handbook application: from concept to controlled practice

Purpose. This expanded section turns the original page into a practical handbook. It preserves the supplied material and adds a repeatable way to apply, check and review Plastics. It does not replace a contract, legislation, a controlled standard, competent engineering judgement or specialist advice.

The operating aim is to carry the subject from function and assumptions through design evidence, verification and controlled release. Read the original explanation first, then use the workflow and checks below to convert knowledge into evidence.

Apply Plastics by beginning with the duty, not the component or software command. Convert the key ideas—plastics, thermoplastics, thermosets, density, float—into measurable requirements and interfaces. Record operating and non-operating environments, duty cycle, expected life, loads, energy sources, human interaction and reasonably foreseeable abnormal conditions. When a value is not a project requirement or verified supplier datum, identify it as an assumption or illustrative value.

Create a calculation and evidence trail that another competent person can audit. Every input should carry a source, unit, revision and uncertainty or tolerance where relevant. Every model should state its boundary conditions and limitations. Keep nominal capacity separate from design capacity, and keep verification margin separate from an arbitrary safety factor. If a code or standard governs the work, confirm the applicable edition and contractual status rather than copying a number from a secondary summary.

Design for manufacture, assembly, inspection, operation and maintenance at the same time. A technically valid geometry can still fail because it cannot be fixtured, measured, cleaned, guarded, reached or replaced. Review process capability, datum or reference strategy, tolerance accumulation, access, error-proofing and changeover. Where people interact with plant, apply the hierarchy of controls and consult those who will operate, clean, maintain and recover the equipment.

Plan verification before release. Define the characteristic, method, equipment, sample or test condition, acceptance criterion, record and responsible person. Validation then asks a different question: whether the resulting system is effective and suitable in the intended use context. A passed drawing check or analysis does not by itself validate usability, maintainability or production performance.

Step-by-step operating method

Define the duty. Capture the required function, interfaces, operating environment, life, loads and unacceptable outcomes.
Establish the model. Identify governing principles, units, material or process data, assumptions and uncertainty.
Develop alternatives. Compare feasible concepts against performance, manufacturability, safety, maintainability and cost.
Verify the design. Use analysis, test, inspection or demonstration with acceptance criteria defined before execution.
Release and learn. Baseline the design, control changes, retain evidence and feed operating results into the next revision.

Illustrative design review record

Illustrative values only. Build a one-page record with the required function, input sources, assumptions, governing load or process condition, failure consequences, selected concept, verification method and acceptance criterion. Mark every numerical input as project requirement, verified supplier data, measured value, calculation output or assumption. Review the weakest evidence first. If an assumption can change safety, compliance, interchangeability or capacity, it must be resolved before release rather than buried in a calculation note.

Evidence class	Question	Release expectation
Requirement	What must the design do and under which conditions?	Approved and traceable
Input	Where did the load, property, tolerance or process limit come from?	Source, unit and revision recorded
Analysis	Which model and assumptions connect input to result?	Checkable calculation or simulation
Verification	How will conformity be demonstrated?	Method and acceptance criterion agreed
Validation	Will the solution work for intended users and conditions?	Representative use evidence

Common failure modes and recovery actions

1. Watch for

Starting detailed design before interfaces and operating limits are agreed.

Recovery: Return to the governing definition or requirement and restate the decision in one sentence.

2. Watch for

Using catalogue or typical values as though they were certified project inputs.

Recovery: Separate evidence from assumption, assign an owner and set a date for validation.

3. Watch for

Checking nominal performance while ignoring tolerances, degradation and foreseeable misuse.

Recovery: Run a small counterexample, boundary test, pilot or independent check before proceeding.

4. Watch for

Confusing verification of requirements with validation of user need.

Recovery: Record the consequence, decision and rationale, then update the controlled baseline.

5. Watch for

Releasing drawings or procedures without configuration, inspection and change controls.

Recovery: Escalate when the issue affects safety, compliance, acceptance, material value or an agreed tolerance.