Engineering plastics cover a tight band of semi-crystalline and amorphous resins — PA66, POM, PC, PBT, PPS and PEEK — sitting between commodity polyolefins and high-performance fluoropolymers, with densities of 1.1–1.45 g/cm³ against structural steel's 7.85 g/cm³ and tensile strength commonly in the 50–100 MPa window for unfilled grades [S6].
That weight-to-strength gap is the headline reason injection-moulded engineering plastics keep displacing cast iron and aluminium in housings, gears and pump bodies — but the spec engineer who treats them as "metal replacement" without checking continuous service temperature, moisture uptake and chemical resistance is signing up for field failures, not savings.
Where Engineering Plastics Win: Density, Corrosion and Net-Shape Output
Specific tensile strength (strength ÷ density) for glass-filled PA66 reaches roughly 70 kNm/kg, several multiples of mild steel at 50–63 kNm/kg, which is why automotive intake manifolds, pedal carriers and timing-chain tensioners migrated to engineering thermoplastics in the 2000s and have not migrated back [S6].
Net-shape injection moulding holds tolerances of ±0.05 mm on small parts and removes the secondary machining, stamping and welding that drive labour cost on metal components — a single 8-cavity mould typically replaces a CNC cell and three press operations for a connector housing or gear blank. Additives such as non-volatile plasticisers, glass or carbon fibre, PTFE and UV stabilisers are hot-mixed or dry-blended with the base resin to push heat-deflection temperature, wear life and outdoor durability into the operating window [S6].
For readers new to the resin family, the engineering plastic reference page covers the full taxonomy from PA through PEEK and the standard filler systems used to tune each property.
The Hard Ceiling: Heat, UV and Moisture
Continuous service temperature is where the deal breaks: POM and PC top out near 100–120 °C, PA66 around 150 °C with heat stabiliser, PPS near 200–220 °C and PEEK at 250–260 °C — a flat order of magnitude below the 550 °C continuous rating of carbon steel and nowhere near the 1000 °C-plus range of nickel super-alloys covered in the nickel alloy spec map. [S3]
PA66 absorbs 2.5–3.0% water by weight at 50% RH, swelling 0.2–0.3% and dropping tensile modulus by roughly 40%, which is why moulded PA66 gears need to be specified as "conditioned" or "dry-as-moulded" and not run through the same tolerance budget as POM or PC. POM's formaldehdye-emission tendency, PC's notch sensitivity to aromatic solvents, and PPS's processing window of 300–340 °C with mandatory corrosion-resistant tooling all push the realistic cap on engineering-plastic service well below the headline datasheet numbers [S6].
Main Resin Families Compared on Five Decision Criteria
Selection almost always runs through the same five filters: mechanical load, continuous heat, chemical exposure, dimensional stability, and cost per kg — and the table below is how a spec engineer lines them up against each other for unfilled or glass-filled general-purpose grades [S6].
PA66 (unfilled, dry): tensile 80 MPa, continuous 150 °C, weak acids/organics fair, moisture uptake 2.5–3.0% — cheapest of the five, used for housings and under-bonnet connectors. POM (homopolymer): tensile 70 MPa, continuous 100 °C, hydrocarbons excellent, low moisture — the go-to for precision gears and bearings. PC (unfilled): tensile 65 MPa, continuous 120 °C, weak acids fair, notch sensitive — chosen for transparent guards, instrument covers and the pressure transmitter windows that must survive impact.
PBT (30% GF): tensile 130 MPa, continuous 150 °C, hydrocarbons and fuels excellent — dominates automotive sensor bodies and connector blocks. PEEK (unfilled): tensile 100 MPa, continuous 250–260 °C, almost universal chemical resistance, very low moisture — the only unfilled engineering plastic rated for sustained hot oil, steam and aerospace interior applications. The 10× cost gap between PA66 at roughly USD 2–3/kg and PEEK at USD 80–120/kg in 2025 spot data is the reason PEEK is specified as a material of last resort, not first choice [S6].
Failure Modes and Field Limits the Datasheet Skips
Creep is the silent killer: PA66 in a constant 30 MPa load at 80 °C will deform to its end-of-life strain in weeks, and design codes such as those in ISO 899-1 are built around this rather than the one-shot tensile number printed in bold on page 1. Creep modulus, not tensile strength, is the working design value for any load-bearing injection-moulded bracket.
UV and weathering are the second non-obvious limit: most engineering plastics fail in 1–3 years of direct Florida or Arizona exposure unless a UV-stabilised or carbon-black grade is specified, which is why black-tinted housings dominate the outdoor industrial market and why the plastic pallet sector has shifted almost entirely to UV-stabilised HDPE for yard storage. For plumbing and process-pipe service, the answer is usually a steel-plastic composite pipe, not a plain engineering-plastic extrusion, because of the same creep and UV problem at the operating pressure boundary.
When to Specify Engineering Plastic — and When to Walk Away
Use engineering plastic when the part is net-shape injection-mouldable, weight saving pays for the resin premium, operating temperature stays inside the 100–260 °C band, and the chemical environment is mapped to the resin's resistance table — think pump impellers in benign chemicals, sensor bodies, gear trains, electrical housings and the housings that surround flow meter electronics. [S2]
Walk away when the spec needs sustained service above 260 °C, exposure to free halogens or concentrated hot acids, structural safety-critical loading without creep data, or a 25-year outdoor UV life — that is the domain of nickel super-alloys covered in [Nickel Alloy Advantages and Disadvantages: A 2026 Spec Engineer's Working Reference](/news/nickel-alloy-advantages-and-disadvantages-a-spec-engineer-s-working.html), and no engineering plastic currently bridges the gap. As a working rule: if the part spends its life indoors, under 150 °C and away from strong solvents, engineering plastic will almost always win on cost-per-part versus stamped or cast metal; outside that envelope, the lifetime cost curve flips fast.