Engineering Plastics in Machine Design: PA, POM, PEEK, and UHMWPE Compared

Engineering plastics are no longer just a cost-cutting substitute for metal — in the right application, PA, POM, PEEK, and UHMWPE outperform metals on tribological, weight, and chemical resistance dimensions that matter most.

The challenge in specifying engineering plastics is that their behavior is far more sensitive to temperature, absorbed moisture, and sustained load than metals. A POM bearing that works perfectly at 23°C and 50% relative humidity can fail by creep at 80°C, and a nylon gear that machines beautifully on Monday can be 0.3 mm oversize by Friday after absorbing ambient moisture. This guide covers the engineering plastics most commonly used in mechanical components, with emphasis on the practical design considerations that are not always obvious from a datasheet.

Nylon (Polyamide, PA): Versatile but Hygroscopic

Nylon is the most widely used engineering plastic for structural and tribological components. The two most common grades are PA6 (polycaprolactam, nylon 6) and PA66 (polyhexamethylene adipamide, nylon 6/6). PA66 has slightly higher stiffness and thermal stability (Tm ≈ 265°C vs 220°C for PA6); PA6 has better impact resistance and is somewhat easier to process. Both are available in a variety of filler systems — glass fiber reinforced (PA66-GF30: 30% glass fiber, UTS ~180 MPa), MoS₂ lubricated, and heat-stabilized grades.

The critical design issue with nylon is moisture absorption. Both PA6 and PA66 absorb significant moisture from the environment — PA66 absorbs approximately 2.5% water at 50% RH and up to 8% when saturated. Moisture acts as a plasticizer: tensile modulus drops by 30–50%, yield strength drops proportionally, and dimensions change. A PA66 component sized at 20°C and 50% RH will grow approximately 0.3–0.5% in the short direction when exposed to humid service. Design rules:

• Always design for conditioned (moist) dimensions when tight fits are required

• Specify machining in conditioned state (soak parts in water for 24–72 hours before finish machining for precision) or calculate the swelling offset

• Do not use PA6/PA66 for parts requiring dimensional stability in humid environments; consider PA12 (lower moisture absorption, ~1.5%) or POM instead

Nylon has excellent tribological properties — low coefficient of friction against steel (µ ≈ 0.2–0.4 dry, 0.1–0.2 lubricated) and good abrasion resistance. It is widely used for gears, bushings, cam followers, slide pads, and conveyor components. The PV limit (pressure × velocity, a measure of bearing capacity) for PA66 is approximately 0.10 MPa·m/s (dry). PA66 + MoS₂ or oil-impregnated grades extend this to 0.14 MPa·m/s.

POM (Acetal / Delrin): Precision’s Best Friend

Polyoxymethylene (POM), marketed as Delrin (DuPont homopolymer) or Celcon/Hostaform (copolymer), is the go-to material when precision dimensions, stiffness, and low friction must coexist. POM absorbs only approximately 0.2% moisture (compared to 2.5% for PA66), making its dimensions far more stable in varying humidity environments.

Mechanical properties (POM homopolymer, “Delrin 150”): UTS 69 MPa, tensile modulus 3.1 GPa, yield strength 67 MPa, elongation at break 25%, Rockwell M hardness 94. It machines exceptionally well — smooth surface finish, tight tolerances, and excellent thread-forming characteristics. POM is one of the few plastics where precision H7/h6 bore-shaft fits are routinely achievable.

Tribologically, POM is outstanding: coefficient of friction against steel ≈ 0.10–0.20 dry, low wear rate, good chemical resistance to hydrocarbons and solvents. It is the standard material for precision bushings, linear bearings, cam followers, small gears, conveyor flights, and any part where a lubrication-free running fit is needed. The PV limit for POM is approximately 0.10 MPa·m/s dry.

Limitations: POM is notch-sensitive — avoid sharp inside corners in high-stress designs (minimum r = 0.5 mm). It degrades in strong acids (pH < 4) and strong alkalis. The homopolymer (Delrin) has higher strength; the copolymer has better resistance to hydrolysis at elevated temperatures and in hot water service. For food contact applications, verify FDA compliance of the specific grade.

PEEK: High-Performance Engineering Plastic

Polyetheretherketone (PEEK) is the premium engineering plastic for demanding structural, thermal, and chemical applications. Its semi-crystalline structure provides a unique combination of properties that no other common plastic matches: continuous service temperature to 250°C, UTS 100 MPa (neat), tensile modulus 3.6 GPa, and resistance to virtually all organic solvents, hydraulic fluids, and most acids at ambient temperature.

Filled PEEK grades significantly expand performance: PEEK + 30% carbon fiber reaches UTS 200 MPa and modulus 14 GPa — approaching aluminum territory at one-fifth the density. PEEK + PTFE + graphite (bearing grade) provides excellent dry-running tribological performance with PV limits up to 0.06 MPa·m/s continuous (lower than POM but at much higher temperatures). PEEK bushings and washers are used in aerospace, semiconductor equipment, oil and gas downhole tools, and medical implants where sterilizability (autoclave compatible) is essential.

The principal limitation is cost: PEEK rod stock is approximately 50–100× the price of POM per kilogram. Specify PEEK only when the application genuinely requires high temperature, high chemical resistance, or high strength-to-weight in a plastic material. Don’t use PEEK at 80°C when POM or PA would work fine.

UHMWPE: The Wear Champion

Ultra-high molecular weight polyethylene (UHMWPE) has a molecular weight of 3–6 million g/mol, compared to 200,000–500,000 for standard HDPE. This extreme chain length creates an entangled network that cannot be melted and reprocessed conventionally — UHMWPE is ram-extruded or compression-molded into sheets and rods. The result is a material with the lowest coefficient of friction of any solid plastic (µ ≈ 0.05–0.10 against steel), outstanding abrasion resistance (10× better than nylon in standard abrasion tests), and excellent impact toughness even at cryogenic temperatures.

Applications: conveyor wear strips and chain guides, mine chutes and hoppers, marine dock fenders, food processing equipment wear components, orthopedic bearing surfaces (total joint replacements use highly crosslinked UHMWPE). Its weakness is relatively low stiffness (modulus ≈ 0.7 GPa) and poor machinability compared to POM — it tends to gum up cutting tools. Specify UHMWPE for high-wear, low-load sliding applications; use POM when precision dimensions and higher load capacity are required.

PTFE: The Sealing Specialist

Polytetrafluoroethylene (PTFE, Teflon) has the lowest coefficient of friction of any solid material (µ ≈ 0.02–0.10) and outstanding chemical resistance to virtually all chemicals except molten alkali metals and fluorinating agents. However, pure PTFE has very poor creep resistance and wear resistance — it deforms under sustained load (cold flow) and wears rapidly under sliding contact.

Filled PTFE composites address these weaknesses: PTFE + 25% glass fiber for wear resistance; PTFE + 40% bronze for high thermal conductivity (heat dissipation in bearings); PTFE + graphite for low-friction seals. PTFE is primarily used for seals, O-ring backup rings, guide rings in hydraulic cylinders, piston rings in pneumatic cylinders, and anti-friction liners (PTFE-woven fabric in spherical bearings). It is not a structural material — don’t use it for load-bearing brackets or precision-dimension bushings. For dry-running bearings, POM or filled PEEK are better choices.

Engineering Plastics Comparison Table

Material	UTS (MPa)	Modulus (GPa)	Max Cont. Temp (°C)	Moisture Abs. (%)	Friction vs Steel (dry)	Chemical Resistance	Relative Cost
PA6 (nylon 6)	75	2.8	80	2.5–3.5	0.2–0.4	Good (not strong acids)	Low
PA66 (nylon 66)	82	3.3	100	2.0–2.5	0.2–0.4	Good	Low
PA66-GF30	185	9.0	120	1.5	0.3–0.5	Good	Low–Medium
POM homopolymer	69	3.1	90	0.2	0.10–0.20	Good (not strong acids)	Low
PEEK (neat)	100	3.6	250	0.5	0.35–0.45	Excellent	Very High
PEEK-CF30	200	14	260	0.1	0.25–0.35	Excellent	Extremely High
UHMWPE	35	0.7	80	0.01	0.05–0.10	Excellent	Low
PTFE (neat)	25	0.5	260	0.01	0.02–0.10	Excellent	Medium
PET (Ertalyte)	80	3.0	100	0.3	0.20–0.35	Good	Medium
PPS (neat)	80	3.8	200	0.02	0.3–0.4	Excellent	High

Thermal Expansion: A Critical Design Factor

Engineering plastics have coefficient of thermal expansion (CTE) values 5–10× higher than steel. POM: 110 µm/m·°C; PA66: 80 µm/m·°C (dry), 120 µm/m·°C (moist); PEEK: 47 µm/m·°C — compared to steel at 11–13 µm/m·°C and aluminum at 23 µm/m·°C. This means a 100 mm POM shaft running 50°C hotter than assembly temperature will be approximately 0.55 mm longer and 0.055 mm larger in diameter than at assembly. Design clearances must account for this differential expansion when plastic components interface with metal housings.

Practical approach: calculate the worst-case clearance at maximum operating temperature (plastic at max expansion, metal housing at minimum size) and minimum clearance at cold start (plastic at minimum size, metal housing at maximum size). For tight fits, this analysis often reveals the need for larger radial clearances than would be used for metal-on-metal interfaces.

Creep: Long-Term Loading Behavior

Unlike metals, which creep only at elevated temperatures (above approximately 0.4 Tm), plastics creep at room temperature under sustained stress. Creep data (stress vs. strain vs. time at various temperatures) is available from material suppliers (Ensinger, Quadrant Engineering Plastics, Röchling) and should be used for any part under sustained compressive or bending load. As a rough guide, limit sustained stress to 20–30% of the short-term yield strength to keep creep strains below 1% over the expected service life. PEEK and PPS are significantly more creep-resistant than PA or POM at elevated temperatures.

Conclusion

The plastic selection process requires matching the material to the specific combination of temperature, moisture, chemical environment, load type (sustained vs. dynamic), precision requirement, and cost budget. POM is the first choice for precision bushings and dry-running bearings at moderate temperatures; PA for tougher, higher-load dynamic applications where moisture can be controlled; PEEK when temperature or chemical resistance exceeds what POM can handle; UHMWPE for high-wear, lower-precision wear surfaces; PTFE for sealing and anti-friction liners rather than structural bearings. Consult full material datasheets with creep and moisture correction data — not just room-temperature dry values — before finalizing a specification.