Engineering Plastics

What Engineering Plastics Are

An engineering plastic is a polymer specified to carry mechanical, thermal, or electrical duty in a functional part, not merely to enclose or decorate it. The boundary against commodity plastics (polyethylene, polypropylene, PVC, polystyrene) is functional rather than chemical. Commodity grades typically deliver tensile strength below 40 MPa and serve below 80 degrees Celsius. Engineering grades deliver roughly 60 to 110 MPa tensile strength, heat deflection temperatures from 90 to well over 250 degrees Celsius, and retain shape and stiffness under sustained load. In procurement terms, an engineering plastic is the material an engineer reaches for when the part must do a job a metal once did: a gear, a bushing, a pump impeller, a connector body, an insulator.

Structurally, engineering plastics are almost all thermoplastics: long-chain polymers that soften on heating and can be injection-molded, extruded into stock shapes, or machined. Their performance comes from three molecular features. First, chain rigidity from aromatic rings or polar backbones, which raises stiffness and temperature resistance. Second, crystallinity: semi-crystalline resins such as POM, PA, PBT, PPS, and PEEK pack into ordered regions that give a sharp melting point, good fatigue and wear resistance, and chemical resistance, while amorphous resins such as PC, PSU, and PEI offer transparency, toughness, and tighter dimensional tolerance. Third, interchain bonding, where the hydrogen bonds in polyamides give nylon its strength but also its moisture sensitivity.

The industrial history of engineering plastics is roughly a century old. DuPont commercialized nylon (PA66) in 1938, the first synthetic fiber and the first true engineering thermoplastic. Polyoxymethylene (acetal) followed as DuPont Delrin in 1960, prized for fatigue and wear. Bayer and General Electric independently commercialized polycarbonate around 1958 to 1960, giving industry a transparent, impact-resistant glazing and housing material. The super-engineering tier arrived later: polyphenylene sulfide (Ryton) in the 1970s, polyetherimide (Ultem) and the first commercial PEEK (Victrex) in the early 1980s. Each addition extended the upper temperature ceiling and the chemical-resistance envelope.

In application scale, engineering plastics span an enormous performance range that no single resin can cover. Continuous service temperature runs from about 80 degrees Celsius for the lowest general-purpose grades to 260 degrees Celsius for PEEK and PTFE. Tensile strength runs from roughly 50 MPa for unfilled POM to 240 MPa for carbon-fiber-reinforced PEEK. Cost spans two orders of magnitude, from a few US dollars per kilogram for nylon to well over one hundred US dollars per kilogram for PAI and aerospace PEEK grades. The essence of engineering selection is mapping a defined duty (load, temperature, chemistry, tolerance, regulatory) to the cheapest resin and grade that clears every requirement with margin.

Four engineering metrics dominate most selections: tensile strength and modulus (mechanical capacity), heat deflection and continuous service temperature (thermal ceiling), water or chemical absorption (dimensional stability and degradation), and the cost per finished part. A material that looks cheaper per kilogram can be the more expensive choice once moisture swelling, creep under load, or thermal softening forces oversizing, scrap, or early replacement. The chapters that follow decode each of these in turn.

Material Families and Classification

Engineering plastics are conventionally split into two performance tiers plus a separate fluoropolymer class. General-purpose engineering plastics (PA, POM, PC, PBT, PET, modified PPE) cover the majority of mechanical parts at moderate temperature. Super engineering plastics (PPS, PEI, PSU, PAI, PEEK) extend service into the 170 to 260 degrees Celsius range at a steep price premium. Fluoropolymers such as PTFE form a third group defined by chemical inertness and low friction rather than mechanical strength. Choosing the wrong tier is the most common and most expensive selection error: paying super-grade prices for a moderate duty, or specifying a general-purpose grade into a thermal or chemical environment that destroys it. The table below summarizes the families.

Polyamide (PA, nylon) is the volume workhorse. PA6 and PA66 are the dominant general-purpose grades, with PA66 offering a higher melting point (about 255 to 265 degrees Celsius) and better creep resistance, while PA6 melts near 220 degrees Celsius and is tougher and easier to process. Both deliver roughly 75 to 85 MPa tensile strength dry, with excellent fatigue and abrasion resistance, which is why nylon dominates gears and bearings. Their defining weakness is moisture absorption, 1.5 to 3 percent at equilibrium, which swells parts and lowers stiffness. Specialty grades PA46, PA11, PA12, and the semi-aromatic PPA (PA6T, PA9T) extend the temperature and dimensional envelope.

Polyoxymethylene (POM, acetal) exists as homopolymer (Delrin) and copolymer. It offers high stiffness, excellent fatigue and wear resistance, low and stable friction, and very low moisture absorption (about 0.2 percent), which makes it the default for precision gears, cams, and snap fits where nylon would swell. Tensile strength is roughly 60 to 70 MPa with a melting point near 175 degrees Celsius. POM's weaknesses are poor flame resistance, sensitivity to strong acids, and limited bondability.

Polycarbonate (PC) is the principal amorphous engineering plastic: transparent, with outstanding impact strength, tensile strength near 60 to 70 MPa, and good dimensional stability with very low moisture pickup. It is the material of safety glazing, light covers, and impact housings. PC is sensitive to certain solvents and to hydrolysis in hot water, and is often blended with ABS (PC/ABS) for cost and processing. PBT and PET (thermoplastic polyesters) are stiff, dimensionally stable, and electrically excellent, dominating connectors and electrical housings, usually in glass-filled grades.

In the super tier, PPS offers continuous service to about 220 degrees Celsius, near-universal chemical resistance below 200 degrees Celsius, and inherent flame retardance, making it a workhorse for under-hood automotive and chemical pump parts, almost always glass or mineral filled. PEI (Ultem) is an amber, transparent, inherently flame-retardant amorphous resin serving continuously near 170 degrees Celsius. PEEK sits at the top of the melt-processable tier with a 343 degrees Celsius melting point, 260 degrees Celsius continuous rating, V-0 flammability, and broad chemical and hydrolysis resistance, at a price 30 to 100 times that of nylon. PTFE, though chemically a fluoropolymer rather than a structural engineering plastic, belongs in any selection discussion for its unmatched 0.05 to 0.10 friction coefficient and chemical inertness.

Grades, Fillers, and Reinforcement

A resin name such as PA66 or PEEK identifies only the base polymer. The grade, set by molecular weight, additives, and fillers, determines the delivered properties and can change a single resin's strength, stiffness, temperature ceiling, and friction by a factor of two or more. Understanding the filler and additive notation is essential to reading a datasheet and to comparing quotes from different suppliers, because two parts labeled the same resin can behave entirely differently. The table below summarizes the main reinforcement and additive systems and their effect on the base resin.

Short glass-fiber reinforcement is the single most common modification. At a 30 percent loading (written GF30 or G30), tensile strength typically roughly doubles and flexural modulus roughly triples, while heat deflection temperature climbs sharply toward the resin's melting point. For PA66, unfilled tensile strength near 80 MPa rises to roughly 170 to 180 MPa at GF30, and HDT at 1.8 MPa jumps from near 70 degrees Celsius to over 240 degrees Celsius. The cost is anisotropy: fibers align with flow, so the part shrinks and is stronger along the flow direction than across it, complicating tight tolerances. The surface becomes rougher and more abrasive, which accelerates wear on mating metal parts and on cutting tools.

Carbon-fiber reinforcement delivers even higher stiffness at lower weight than glass, is electrically conductive (useful for static dissipation and EMI), and warps less, but costs substantially more and is available only in dark colors. Carbon-fiber PEEK is a standard aerospace and semiconductor structural grade, reaching tensile strengths well above 200 MPa. Internal lubricants (PTFE, graphite, molybdenum disulfide, or silicone oil) are compounded into POM, PA, and PEEK to lower the friction coefficient and wear rate for unlubricated bushings and gears, at the expense of some strength.

Mineral fillers such as glass beads and talc give isotropic (uniform) shrinkage and good dimensional stability with less strength gain than fiber, which suits flat, warp-sensitive parts. Flame retardants raise resins such as nylon and PBT to a UL 94 V-0 rating for electrical enclosures; halogen-free FR systems are increasingly specified for environmental compliance. Impact modifiers are elastomer phases added to boost toughness and cold-temperature impact, trading away stiffness and heat resistance. Reading a grade code (for example PA66-GF30 FR or POM-AF, an acetal with PTFE fibers) tells the engineer at a glance what was added and roughly how the part will behave.

Chemical and Environmental Resistance

For many parts the limiting factor is not load or temperature but the service environment: the chemistry of the fluid in contact, the humidity, the UV exposure, and whether the part is sterilized or cleaned with aggressive agents. A mechanically ideal resin can fail by swelling, stress-cracking, hydrolysis, or embrittlement long before its strength rating matters. Chemical compatibility must always be verified against the specific concentration, temperature, and exposure time, but the families have characteristic behaviors that guide first selection.

Moisture and hydrolysis separate the families sharply. Polyamides absorb 1.5 to 3 percent water, which plasticizes the polymer: stiffness and tensile strength fall, impact toughness rises, and the part swells up to 0.5 to 0.8 percent linear, enough to throw a precision gear out of tolerance. This is why nylon datasheets quote properties in two states, dry-as-molded (DAM) and conditioned at 50 percent relative humidity. POM, PBT, PC, and the super grades absorb far less (POM about 0.2 percent, PC about 0.15 percent) and are the dimensionally stable choices for humid or wet service. Polyesters (PBT, PET) and polycarbonate can hydrolyze in hot water or steam, so PPS or PEEK is specified for sustained steam contact.

Acid, base, and solvent resistance follows crystallinity and backbone chemistry. Semi-crystalline resins generally resist solvents better than amorphous ones: PC and PEI are attacked or stress-cracked by ketones, chlorinated solvents, and strong alkalis, while POM, PPS, and PEEK resist a far broader range. POM is notably vulnerable to strong acids. PTFE and the fluoropolymers are essentially inert to all common industrial chemicals, which is why they line valves and pumps in aggressive service even though their mechanical strength is low. PEEK resists most acids, bases, and hydrocarbons up to high temperature, with concentrated sulfuric acid as a notable exception.

UV, weathering, and thermal aging matter for outdoor and long-life parts. Unstabilized PC, PA, and POM yellow and embrittle under UV; carbon-black or UV-stabilized grades are specified for outdoor use. Long-term thermal aging is captured by the UL relative thermal index (RTI), a continuous rating distinct from the short-term heat deflection temperature. The table below is a quick-reference lookup for common environments and suitable resin families. It is for initial screening only; before committing, obtain the manufacturer chemical-resistance chart and verify the exact concentration, temperature, and stress state.

Key Specification Parameters

Reading a plastics datasheet is a core procurement skill. A single grade may list 20 to 40 single-point values under ISO 10350 or the CAMPUS format, but only a handful drive most selections: density, tensile strength and modulus, flexural modulus, impact strength, melting point or glass transition, heat deflection temperature, continuous service temperature, water absorption, and the UL 94 flammability rating. The key-specifications comparison below collects typical published values for the most common families, all for unfilled grades unless noted, so they can be read like for like. Values are nominal and vary by supplier and grade; always confirm against the specific datasheet.

Density sets the weight per part and, with cost per kilogram, the cost per part. Most engineering plastics fall between 1.1 and 1.4 g/cm³, roughly one-sixth the density of steel, which is much of their appeal. PTFE is the heavy outlier at about 2.2 g/cm³. Glass and mineral fillers raise density in proportion to loading.

Tensile strength and modulus (ISO 527 or ASTM D638) quantify load capacity and stiffness. Strength is the stress at yield or break in MPa; modulus is the stiffness in MPa or GPa. Note that values depend strongly on test speed, specimen geometry, temperature, and moisture state, so an ISO 527 number is not directly comparable to an ASTM D638 number from another supplier. Flexural modulus (ISO 178) often matters more than tensile for parts loaded in bending, and Izod or Charpy impact (ISO 180 / ISO 179 or ASTM D256) captures toughness, where notched values are far lower than unnotched and are the realistic design figure.

Thermal limits require two distinct numbers. The melting point (for semi-crystalline resins) or glass transition temperature Tg (the governing limit for amorphous resins such as PC and PEI) sets the processing and absolute ceiling. The heat deflection temperature (HDT), per ISO 75 or ASTM D648 at 0.45 or 1.8 MPa, is a short-term softening indicator, not a lifetime rating. The continuous service temperature, often from a UL RTI listing, is the temperature a part survives for 20,000-plus hours with no more than a 50 percent loss of a key property. HDT and continuous rating can differ by 100 degrees Celsius or more, so thermal duty must be sized against the continuous figure.

Water absorption (ISO 62 or ASTM D570) predicts dimensional stability and property loss in humid service, and is the single most important number for polyamide parts. The UL 94 flammability rating classifies burn behavior from HB (slowest, just self-extinguishing horizontal) through V-2, V-1, to V-0 (self-extinguishing vertical, no flaming drips), with 5VA the most stringent. PEEK, PEI, PPS, and PVDF are inherently V-0; nylon and POM need flame-retardant grades to reach V-0. For electrical enclosures the UL 94 rating is frequently a hard, non-negotiable requirement.

Selection Decision Factors

To turn the preceding chapters into a specific resin and grade, work the decision sequence below in order. Most selection mistakes come not from a single wrong answer but from deciding at the wrong level too early, for example fixing on a resin before the thermal and chemical envelope is defined. These eight steps double as a fixed RFQ template that suppliers can quote against.

1. Define the load and stiffness duty: Establish the worst-case stress, whether it is steady (creep matters, favor POM, PPS, PEEK) or cyclic (fatigue matters, favor PA, POM, PEEK), and the deflection budget. This sets the minimum tensile strength and flexural modulus and decides whether glass or carbon reinforcement is required.
2. Set the thermal envelope: Specify both the peak short-term temperature and the continuous service temperature, and size against the continuous (RTI) rating, not HDT. This step often eliminates whole tiers: above 150 degrees Celsius continuous, general-purpose grades drop out and the choice narrows to PPS, PEI, or PEEK.
3. Specify the chemical and moisture environment: List every fluid in contact with concentration and temperature, plus humidity, steam, UV, and cleaning or sterilization agents. Reject any resin that swells, stress-cracks, or hydrolyzes in that environment before considering its mechanical fit.
4. Fix the tolerance and stability requirement: For precision gears and bearings, prefer low-absorption, low-warp resins (POM, PBT, mineral-filled grades) and account for moisture swell and post-mold shrinkage. Tight tolerance often outweighs raw strength in the choice.
5. Resolve friction and wear: For unlubricated sliding contact, choose a low-friction base (POM, PTFE-filled, UHMW-PE) or an internally lubricated grade, and check the PV limit (pressure times velocity) and the hardness of the mating surface.
6. Apply regulatory and safety requirements: Confirm UL 94 flammability class for electrical parts, food-contact (FDA, EU 10/2011) or potable-water approval, medical (USP Class VI, ISO 10993), and low-outgassing (NASA, semiconductor) where applicable. These are pass or fail and frequently force a specific grade.
7. Choose the form and process: Decide between injection-molded parts (high volume, complex geometry) and machined stock shapes (rod, plate, tube for low volume or large parts). The same resin can differ in property between molded and extruded forms, so match the datasheet to the form.
8. Compare total cost of ownership: Weigh resin cost per kilogram against part weight, scrap rate, tolerance yield, expected service life, and replacement labor. A super grade that costs ten times the resin price can still be the cheaper lifetime choice when it eliminates failures, downtime, or qualification re-work.

One last dimension is commonly overlooked: supply security and serviceability. Confirm the grade is from a named, traceable resin producer (BASF, Celanese, Covestro, Solvay, Victrex, DuPont, SABIC, Ensinger, Mitsubishi Chemical, DSM among others), that a documented datasheet and CAMPUS entry exist, and that second-source or equivalent grades are available to avoid single-supplier risk. For machined parts, verify that stock shapes are held in the required dimensions and that the converter can certify the resin lot. These factors seem secondary at quote time but determine lead time and continuity over a multi-year production program.