Polybutylene Terephthalate Vs Pom Where The Polyester Beats The Acetal
The material comparison Polybutylene Terephthalate vs POM, where the polyester beats the acetal, is not settled by dry tensile strength alone. Selection of polybutylene terephthalate for injection-molded automotive sensor brackets and connector backshells is frequently driven by dimensional repeatability after exposure to 50 °C and 95 % relative humidity rather than by short-term mechanical values. On a 120-ton hydraulic injection molding machine using a 25 mm general-purpose screw with 22:1 L/D, unfilled PBT is processed with a barrel profile of 245 °C to 265 °C and mold temperature of 60 °C to 80 °C. Desiccant drying at 120 °C for 4 h to a residual moisture below 0.02 % is mandatory; higher moisture initiates hydrolytic molecular weight loss that appears as splay and inconsistent melt pressure. Under ISO 62, 24 h immersion at 23 °C gives water uptake of 0.08 % to 0.10 % for unfilled PBT and 0.20 % to 0.25 % for acetal copolymer. The difference of approximately 0.10 % to 0.15 % is sufficient to alter hole-to-hole pitch on a 100 mm connector footprint by a measurable amount after equilibrium moisture conditioning, exceeding the tolerance band for press-fit electrical terminals. PBT also exhibits lower density at 1.31 g/cm³ to 1.34 g/cm³ measured under ISO 1183-1, compared with 1.40 g/cm³ to 1.41 g/cm³ for acetal copolymer.
Glass-fiber-reinforced PBT compounds are compounded on a 32:1 L/D co-rotating twin-screw extruder with 20 wt% short glass fiber side-fed after the melt seal at 260 °C and screw speed of 300 rpm. The fiber attrition on that line reduces mean glass length from 450 µm to 210 µm, while tensile modulus rises from 2 500 MPa to 7 500 MPa under ISO 527-2. Weld-line tensile strength in the glass-reinforced grade typically drops to 45 % to 55 % of the unreinforced value, a limitation that governs gate placement in battery-module housings.
Why Does Moisture Absorption Create Different Dimensional Tolerance Drift Between the Polyester and the Acetal?
The comparative data in Table 1 are consolidated from unfilled injection-molding grades tested under standard specimen preparation. The moisture-uptake differential is not the only variable: PBT also exhibits lower mold shrinkage and lower coefficient of linear thermal expansion, which reduce the cumulative thermal-plus-moisture deviation. A 100 mm PBT housing subjected to a 30 K temperature rise from 23 °C to 53 °C expands 0.18 mm to 0.30 mm, whereas an equivalent acetal copolymer housing expands 0.27 mm to 0.33 mm; the additional moisture-induced growth of the acetal adds a larger second-order error.
| Property | Test method | PBT, unfilled | POM copolymer, unfilled |
|---|---|---|---|
| Density | ISO 1183-1 | 1.31–1.34 g/cm³ | 1.40–1.41 g/cm³ |
| Water absorption, 24 h, 23 °C | ISO 62 | 0.08–0.10 % | 0.20–0.25 % |
| Tensile stress at break | ISO 527-2 | 50–60 MPa | 55–65 MPa |
| Tensile modulus | ISO 527-2 | 2 500–2 800 MPa | 2 600–3 100 MPa |
| Flexural modulus | ISO 178 | 2 200–2 700 MPa | 2 300–2 900 MPa |
| Notched Izod impact strength, 23 °C | ISO 180/A | 4–6 kJ/m² | 6–8 kJ/m² |
| Heat deflection temperature, 1.82 MPa | ISO 75-2/A | 50–60 °C | 90–110 °C |
| Coefficient of linear thermal expansion, parallel | ISO 11359-2 | 60–100 × 10-6 K-1 | 90–110 × 10-6 K-1 |
| Mold shrinkage, parallel | ISO 294-4 | 0.5–1.5 % | 1.6–2.2 % |
After molding, the lower equilibrium moisture content of PBT reduces the swelling contribution in terminal blocks and sealed connectors. The test condition of IEC 60068-2-78, 40 °C and 93 % relative humidity for 56 days, is used to qualify dimensional stability for outdoor telemetry housings; PBT grades retain a larger fraction of their dry dielectric properties under this conditioning cycle than acetal copolymer because the polyester absorbs less water into the bulk matrix. Published data for specific PBT-POM dielectric comparisons under this exact profile is limited, but the lower saturation moisture content of the ester is consistently reported in resin manufacturer technical bulletins.
An all-electric molding cell with 80-ton clamp force and 18 mm screw diameter is used to mold two-piece PBT sensor housings with a laser-weld seam. The unfilled PBT grade is specified because its lower density reduces part mass by 7 % to 8 % for the same cavity volume. In through-transmission laser welding at 980 nm, unfilled PBT is routinely specified; its semicrystalline morphology scatters less near-infrared radiation than the higher crystallinity acetal. The weld seam is tensile-tested according to ISO 527-2; a PBT weld line in the referenced housing retains at least 70 % of the parent tensile strength when the laser power is set to 160 W and the scan speed is 30 mm/s. Acetal copolymer often requires surface activation to reach comparable weld quality, and published data for this specific configuration is limited.
Processing Boundaries That Establish Thermal Margins for PBT
PBT crystallizes with a peak crystallization temperature of 190 °C to 210 °C during cooling from the melt at 20 K/min under differential scanning calorimetry per ISO 11357-3; acetal copolymer crystallizes at 140 °C to 150 °C. The higher crystallization temperature of PBT permits stable filling at mold temperatures as low as 40 °C, though 60 °C to 80 °C is applied when maximum crystalline perfection and heat deflection resistance are required. Under ISO 1133-1, unfilled PBT with a melt volume-flow rate of 10 cm³/10 min to 20 cm³/10 min at 250 °C and 2.16 kg can fill thin-wall sections down to 0.8 mm, while acetal copolymer with comparable flow at 190 °C may require higher injection speeds to avoid premature freeze-off in 0.6 mm walls.
| Processing parameter | PBT, unfilled | POM copolymer, unfilled |
|---|---|---|
| Drying | Desiccant dryer 120 °C, 4–6 h, dew point −40 °C, residual moisture <0.02 % | If saturated: 80 °C, 2–3 h, residual moisture <0.1 % |
| Melt temperature | 240–270 °C | 180–220 °C |
| Mold temperature | 40–80 °C | 60–90 °C |
| Hot-runner gate type | Open or valve gate with 1.5–2.5 mm gate diameter | Valve gate preferred to reduce drool |
| Shutdown purge | Polycarbonate or polyolefin purge after long production stops | Polyolefin purge mandatory to displace acetal before heater shutdown |
Production-floor failure modes differ. Inadequately dried PBT produces silver streaks on visible surfaces and a measurable loss in melt viscosity; on a 16-cavity hot-runner tool, screw recovery time can drop from 3.2 s to 2.7 s when moisture-induced degradation lowers molecular weight. Acetal copolymer does not hydrolyze, but thermal degradation in a blocked hot-runner gate releases formaldehyde and can cause black specks and mold plate corrosion. PBT therefore requires desiccant drying infrastructure, while POM requires aggressive purge protocols and vented barrel zones when residence times are long. The PBT processing window also provides a wider margin between the upper melt setting of 270 °C and the start of thermal-oxidative degradation above 280 °C; acetal copolymer has degradation onset at 230 °C to 240 °C, leaving a narrower margin over the upper melt setting of 220 °C.
When PBT is molded at the lower end of the mold-temperature range at 40 °C, the cooling rate suppresses crystallite perfection. Post-mold shrinkage under ISO 294-4 can then increase by 0.2 % to 0.5 % after 48 h at 80 °C. Molding at 60 °C to 80 °C stabilizes the crystalline fraction and reduces subsequent dimension changes. In acetal copolymer, the same post-mold annealing effect is smaller because its mold shrinkage is already higher and its crystallization window is lower.
When Continuous Service Temperature and Dry Sliding Loads Reverse the PBT Advantage
Although unfilled PBT has a heat deflection temperature at 1.82 MPa of only 50 °C to 60 °C, glass-fiber reinforcement raises the PBT value to 195 °C to 205 °C under ISO 75-2/A. Acetal copolymer retains a higher unfilled heat deflection temperature of 90 °C to 110 °C, but its reinforced grades do not match the upper end of glass-filled PBT in every application. In dry sliding gear mechanisms, however, acetal copolymer retains the tribological advantage. Unfilled PBT exhibits a coefficient of friction against steel in the range 0.25–0.45, while acetal copolymer typically runs at 0.15–0.35, depending on velocity and surface finish. Wear factor measurements under ASTM G133 show that unfilled PBT is not suitable for continuous dry sliding contact without PTFE or silicone internal lubricants; published data for specific gear geometries is limited.
Fatigue endurance is another boundary. POM homopolymer has higher tensile fatigue strength and creep resistance at room temperature; PBT is selected in snap-fit arms only when the strain is below the yield point and when the loading frequency is below 5 Hz. Under ISO 527-2 cyclic loading, unfilled PBT shows earlier crack initiation at a given stress amplitude than acetal copolymer. POM therefore remains the default for gear wheels, cams, and rollers in copiers and automotive window-lift mechanisms, while PBT is specified for housings, sensor bodies, fuse boxes, and connector shells where dimensional stability in humid air is the controlling requirement.
Exposure to automotive coolant at 85 °C in a 50:50 glycol/water mixture reveals the polyester’s boundary condition. PBT is susceptible to hydrolysis of ester linkages when residual moisture and temperature remain high; hydrolysis-stabilized PBT grades are required for continuous immersion above 60 °C. In contrast, acetal copolymer offers hydrolysis resistance but is attacked by strong aqueous acids and oxidizing environments. PBT is therefore specified for battery-cell carrier components needing resistance to weak acids, motor oil, and brake fluid, while POM is retained for sliding door rollers and gear mechanisms where dry friction and fatigue dominate. The selection is made against material-specific chemical compatibility tabulations from resin producers; published data for specific field-aged configurations in combined acid and glycol service is limited.
