Nylon, acetal, UHMW-PE, HDPE or PTFE: choosing a plastic for washers, spacers and bushes

Plastic washers, spacers and bushes fail for predictable reasons: they swell, creep, soften with heat or wear out. The figures below come from the material producers' own datasheets. They are indicative values for comparison, not guaranteed minimums, and test methods differ between producers (ISO 62 and ASTM D570 for water absorption), so treat close numbers as similar.

Quick choice

• Wet, humid or outdoor, and the size must hold: acetal (POM-C).
• Sliding or abrasive wear, wet, moderate temperature: UHMW-PE.
• Lightly loaded, chemically exposed, low cost: HDPE.
• Heat or aggressive chemicals, light load: PTFE.
• Dry service, impact and toughness: nylon PA6; PA66 for a little more strength and heat resistance.

Nylon (PA6 and PA66)

Mitsubishi Chemical Group lists water absorption at saturation in water at 23 °C of 9% for Ertalon 6 SA (PA6) and 8% for Ertalon 66 SA (PA66). Its continuous service temperature in air is 70 °C for the PA6 grade and 80 °C for the PA66 grade, and tensile strength 80 MPa and 90 MPa, all for dry material. Ensinger describes extruded PA6 as tough and wear resistant, suitable for mechanical parts as long as dimensional tolerances are not critically tight. Use nylon where it stays dry, or where a fit can tolerate moisture uptake.

Acetal (POM-C)

Ensinger lists 0.18% water absorption after 24 hours and 0.80% at saturation (ASTM D570) for TECAFORM AH, with low moisture absorption, high dimensional stability and good machinability among its main features. That makes acetal the usual choice for spacers and bushes that must keep their size in damp conditions.

UHMW-PE

Röchling lists water absorption below 0.01% for Polystone M Slide (PE-UHMW, PE 1000), a dynamic friction coefficient of 0.07 to 0.10 and a maximum service temperature in air of 180 °F (about 82 °C). It is a sliding and wear material, not a stiff one: tensile strength at yield is listed at 3000 psi (about 21 MPa).

HDPE

Röchling lists 0.01% water absorption for Polystone G HD (PE-HD, PE 300), a long-term service range of −50 to 80 °C and good weldability and chemical resistance. It suits lightly loaded parts where cost matters more than wear life.

PTFE

Ensinger lists TECAFLON PTFE for long-term service up to 260 °C, with very good chemical resistance and sliding properties and 0.01% water absorption. Its compressive strength is low, 5 MPa at 1% strain, so PTFE is a poor choice for a washer under high bolt clamp load.

Allow for thermal expansion

Plastics expand far more than metals. Producer figures for linear expansion: PA6 90 µm/(m·K) between 23 and 60 °C (Mitsubishi Chemical Group), UHMW-PE 1.0 × 10⁻⁴ per K (Röchling), HDPE 150 to 230 × 10⁻⁶ per K (Röchling) and PTFE 13 × 10⁻⁵ per K between 23 and 100 °C (Ensinger). Leave clearance on long spacers and in bores.

What to send

1. A drawing or the dimensions, with tolerances.
2. The service conditions: wet or dry, temperature range, chemicals, sliding or static, and the load.
3. The material used now, if known, and any approval needed, such as food contact.
4. Quantity and destination.