Polybutylene Terephthalate (PBT) vs POM in Gears and Precision Parts: Choosing on Friction, Moisture and Cost

Polybutylene terephthalate (PBT) is a semi-crystalline aromatic polyester obtained by polycondensation of dimethyl terephthalate or terephthalic acid with 1,4-butanediol. The polymer’s melt temperature is near 223 °C, glass transition temperature is 45–60 °C, and density is 1.30–1.32 g/cm³ by ISO 1183-1. Unfilled injection-molding grades deliver tensile modulus between 2500 MPa and 2800 MPa under ISO 527-1/-2 and notched Izod impact values of 3–5 kJ/m² under ISO 180/A. PBT is used in gear trains, actuator housings, sensors, and fuel-system components because it combines rapid crystallization with low equilibrium moisture uptake compared with polyamide 66. The technical comparison “PBT vs POM in Gears and Precision Parts: Choosing on Friction, Moisture and Cost” arises when design loads are moderate, backlash tolerances remain below 0.05 mm, and ambient humidity varies widely during service.

Polyoxymethylene (POM) is an acetal resin with higher crystallinity and density near 1.41 g/cm³. Typical POM copolymer tensile modulus is 2800–3200 MPa, and notched Izod impact is 6–8 kJ/m², making it tougher than unfilled PBT at room temperature. However, the property gap narrows in thin-wall precision parts because PBT flows readily at high shear rates and can fill multi-cavity tools with reduced short-shot pressure. Neither polymer is a drop-in replacement for the other; the correct choice depends on tooth loading, lubrication regime, moisture exposure, and the cost of maintaining tolerance at production scale.

Why Does Moisture Uptake Shift Backlash in Unfilled PBT and POM Gear Pairs?

At 23 °C/50 % RH, unfilled PBT absorbs approximately 0.2 % moisture by ISO 62, and water immersion produces 0.4–0.5 % saturation. POM copolymer absorbs 0.20–0.25 % at 50 % RH and 0.7–0.9 % after saturation under the same method. The dimensional consequence in a precision gear is smaller for PBT in watery environments than for POM, but PBT has a separate hydrolytic degradation route above 60 °C continuous water exposure. Hydrolysis cleaves ester linkages, reducing intrinsic viscosity and tooth-bending fatigue retention; POM is not hydrolytically degraded by neutral water in the same manner but is attacked by acids and certain chlorinated compounds. In an automotive window-lift gear subjected to condensation and 80 °C soak cycles, PBT may require glass-fiber reinforcement or a hydrolysis-stabilized grade; otherwise the root fillet can lose tensile strength below the design limit after 1000 h. Published data for this specific configuration is limited, but OEM validation commonly uses retained impact strength per ISO 180/A and dimensional checks on a coordinate measuring machine after environmental cycling.

In dry-running or boundary-lubricated tooth contacts, the polymer transfers a molecular film to the steel counterface, and the coefficient of friction measured by ASTM G99 pin-on-disc against ground steel is generally reported as 0.25–0.35 for POM copolymer and 0.30–0.40 for unfilled PBT. Wear factor under ASTM D3702 thrust-washer tribology typically favors POM homopolymer in unlubricated steel-polymer contact because acetal forms a smoother transfer film and lower interfacial shear strength. PBT gear pairs often exhibit lower structure-borne noise at tooth-mesh frequencies above 1500 Hz because the polyester glass transition is closer to the gear surface temperature, increasing material damping. Internal lubrication shifts the ranking: PBT with 15 % PTFE or aramid fiber can lower dynamic friction to 0.20–0.28, while POM with 20 % PTFE may reach 0.18–0.25. High-shear dispersion of PTFE in PBT on a co-rotating twin-screw extruder with L/D 44:1 is required to prevent agglomerates that produce tooth surface pitting. Gear rig tests on an open center-distance gearbox with 450 mm center distance, 0.05 mm backlash, and laser displacement sensors separate transmission error from material damping; published data for this exact configuration is limited.

Thermal and Tribological Boundaries for Unfilled and Internally Lubricated Grades

Under ISO 75-1 at 1.8 MPa, unfilled PBT heat deflection temperature is 50–60 °C, while POM copolymer reaches 95–110 °C. This means unfilled PBT is not suitable for high-load gear meshes that exceed 60 °C bulk temperature unless the part is glass-fiber reinforced; a 30 % glass-fiber PBT grade can raise HDT to 200–220 °C but will increase wear on unfilled polymer mating gears. POM’s higher heat deflection under load supports higher contact pressure at moderate temperature, but acetal should not be processed above 230 °C because thermal depolymerization releases formaldehyde. Injection molding settings differ: PBT requires melt temperature 230–250 °C and mold temperature 40–80 °C, while POM requires melt temperature 180–220 °C and mold temperature 60–100 °C for consistent crystallization. Spiral-flow testing at 80 MPa injection pressure and 2 mm channel depth shows PBT can fill thin gear webs at lower viscosity at shear rates above 1000 s⁻¹, but POM provides more stable post-filling crystallization with less jetting.

Cost comparisons between PBT and POM are determined less by resin price per kilogram than by drying energy, cycle time, scrap rate, and tolerance capability. PBT must be predried with a desiccant dryer to below 0.02 % moisture at 120 °C for 4 h to prevent hydrolytic molecular weight loss during melting; POM with low surface moisture can often be processed after drying at 80 °C for 2–3 h. On an all-electric injection molding machine with 100-ton clamp force and a 16-cavity gear tool, PBT often consumes more heating energy because of the higher melt temperature, but its faster crystallization permits comparable or shorter cooling time than POM. Unfilled PBT mold shrinkage under ISO 294-4 ranges from 1.5 % to 2.0 % in the flow direction, whereas POM copolymer typically shows 1.8–2.5 %, giving PBT a tolerance advantage for fine-module gear teeth. If tooth-to-tooth composite error must remain below 0.02 mm, mold temperature uniformity of ±3 °C across the cavity is the controlling cost factor because nonuniform crystallization in POM produces greater post-mold ovality. For a 2.5 g gear, an 18 s cycle, and 2 % scrap, the cost per usable part is driven more by cavity balance, drying reliability, and regrind policy than by list resin price alone.

When Regrind Ratios and Drying Histories Govern Process Capability

Hot-runner tools processing PBT can sustain regrind ratios up to 30 % only if the regrind is dried to below 0.05 % moisture and melt viscosity retention is verified by melt volume-flow rate under ISO 1133-1:2022 at 250 °C/2.16 kg. POM regrind is more sensitive to thermal history; reprocessing above 230 °C increases formaldehyde generation and reduces Charpy impact strength under ISO 179-1/1eA. POM gear production often limits regrind to 20 % with vented barrels and local exhaust extractors. PBT’s hydrolysis risk during multiple heat cycles justifies intrinsic viscosity testing per ISO 1628-1 after every lot that exceeds 1000 ppm moisture. These process boundaries affect piece cost because regrind reuse lowers material cost per shot but increases incoming quality checks. A moisture content of 0.15 % in PBT can reduce melt viscosity by about one-third and produce splay marks at the tooth root under 70 MPa holding pressure. In a twin-screw compounding plant, additive distribution is measured by ash content under ISO 3451-1 to confirm that PTFE or glass filler concentration remains within ±1 % of nominal.

PBT offers resistance to automotive fuels, lubricating oils, and many solvents under ISO 175, but continuous exposure to ethylene glycol antifreeze at 120 °C can cause environmental stress cracking. POM is resistant to neutral hydrocarbons but should not be exposed to strong acids, chlorine, or hot water above 60 °C without grade-specific stabilizers. In gearboxes using chlorinated paraffin lubricants, PBT is often preferred because acetal can degrade rapidly; in fuel-adjacent mechanisms, PBT’s aromatic polyester structure provides lower permeation than POM. For food-contact precision parts, both material classes require grade-specific compliance; European Union migration limits for plastics in food contact are specified in Regulation (EU) No 10/2011, and verification is additive-dependent. Sharp internal corners with radii below 0.5 mm increase notch sensitivity in unfilled PBT under impact loading, so gear tooth root fillets should be at least 0.3 mm or an impact-modified PBT grade should be specified. POM homopolymer offers higher fatigue endurance in low-strain clock-gear applications, while PBT is selected when dimensional stability in humid air, lower shrink tolerance, or hydrocarbon resistance governs the system design.