In high-voltage battery pack design, dimensional stability is assessed not only at moulded part approval but after moisture conditioning because components such as busbar insulators, cell contacting system carriers, high-voltage connector housings and interlock connectors must preserve creepage and clearance geometry under condensing humidity, coolant vapour and thermal cycling. Polybutylene terephthalate (PBT) is selected for these parts because its equilibrium moisture uptake remains low relative to aliphatic polyamides; unfilled PBT absorbs approximately 0.2 % water by mass at 23 °C and 50 % RH when conditioned in accordance with ISO 62:2008, and 30 % glass-fibre-reinforced PBT often exhibits 24 h water absorption below 0.1 % per ASTM D570-98(2018). The corresponding linear hygroscopic expansion is anisotropic in glass-fibre compounds because the fibres constrain expansion along the orientation direction while allowing greater movement transverse to flow. Dimensional measurements on injection-moulded plaques after 1,000 h at 85 °C and 85 % RH in a chamber conforming to IEC 60068-2-78:2012 show that length change, thickness change and warpage are not equivalent; skin-core fibre orientation, gate location and residual stress determine whether a component bows, twists or opens a snap-fit gap. Published data for this specific configuration is limited, but manufacturer datasheets for hydrolysis-resistant PBT grades confirm that unfilled PBT can reach 0.5 % moisture in liquid water at 80 °C, while filled grades plateau below 0.4 %. Because high-voltage creepage distances are specified in millimetres under IEC 60664-1:2020, a linear expansion of 0.05 % on a 200 mm part produces a 0.1 mm length change, which is significant when terminal alignment tolerances are held at ±0.15 mm.

What Limits the Hydrolysis Resistance of PBT Busbar Insulators at 85 °C and 85 % RH?

PBT busbar insulators in 800 V systems must maintain creepage distances under IEC 60664-1:2020; dimensional loss due to warpage can reduce clearance by more than 0.2 mm while the part remains electrically functional. Hydrolysis is autocatalytic: water attacks ester linkages, increases carboxylic acid end-group concentration, and accelerates molecular weight reduction. Viscosity number measured in phenol/tetrachloroethane solvent per ISO 1628-5:1998 falls from initial values near 105 mL/g to 120 mL/g to below 80 mL/g in unmodified PBT within 500 h of 85 °C and 85 % RH exposure, depending on sample thickness and surface-to-volume ratio. This degradation reduces tensile strength measured on ISO 527-2:2012 type 1A specimens from initial values of 55 MPa to 60 MPa for unfilled PBT and 130 MPa to 160 MPa for 30 % glass-fibre PBT by more than 30 %. Hydrolysis-resistant grades retard acid generation by end-group capping or carbodiimide stabilisation, and published datasheets report tensile strength retention above 70 % after 1,000 h at 85 °C and 85 % RH. However, the same additive chemistry can reduce the allowable melt-processing window: melt temperatures above 265 °C cause yellowing, viscosity shifts and reduced stabiliser efficiency. On injection-moulding lines producing high-voltage busbar insulators with 1,000 kN to 2,500 kN clamp force, melt temperature must be controlled at 250 °C to 265 °C, and residence time in the barrel limited to less than 45 s. Dimensional stability in humid conditions is therefore coupled to thermal history: a part moulded at 270 °C may exhibit acceptable initial dimensions but lower hydrolytic stability and greater warp after 85 °C and 85 % RH exposure because the molecular weight distribution has already been shifted. Moulders use ISO 1133-1:2022 melt volume-flow rate testing on dried granulate and after each production shift to detect viscosity loss; an increase in melt volume-flow rate of more than 20 % relative to virgin resin indicates that barrel residence, moisture or shear heating has degraded the melt and dimensional consistency should be revalidated. Creepage distance measurements on parts after hydrolysis are performed with the test methods of IEC 60112:2020 for comparative tracking index, but dimensional stability under pollution degree 2 conditions requires retention of creepage geometry rather than surface resistivity alone.

When glass-fibre-reinforced PBT is injection-moulded into cell contacting system carriers with nominal wall thickness of 1.5 mm to 2.0 mm, the solidification process creates a skin-core fibre orientation that controls subsequent hygroscopic expansion. Mould shrinkage measured per ISO 294-4:2018 on 60 mm × 60 mm plaques is typically 0.2 % to 0.4 % in the flow direction and 0.8 % to 1.2 % transverse to flow; the same asymmetry appears after moisture conditioning, with transverse hygroscopic expansion of 0.05 % to 0.15 % and flow-direction expansion below 0.03 % at saturation. Production trials on 220 mm × 160 mm busbar carriers moulded on a 1,500 kN electric injection moulding machine with a 45 mm screw and single edge gate showed free-corner warpage increased by 0.3 mm to 0.8 mm after 500 h at 70 °C and 95 % RH; moving the gate to a central film gate reduced measured warpage on a coordinate measuring machine by approximately half. Batch-to-batch variation in glass fibre length distribution of more than 5 % shifts melt viscosity and fibre orientation enough to alter mould shrinkage by 0.02 % to 0.05 %, which is above the tolerance of interlock connector slots. Moulders therefore use cavity pressure sensors and switchover position control rather than injection time alone, because dimensional stability in humid exposure begins with reproducible orientation and residual stress.

Creep Modulus and Compression Set in Humid Contact Zones

Humid exposure introduces a viscoelastic relaxation component that is not captured by instantaneous shrinkage measurements. Flexural modulus of 30 % glass-fibre PBT dry at 23 °C is commonly 8,000 MPa to 10,000 MPa per ISO 178:2019; after equilibrium moisture conditioning at 85 °C and 85 % RH, the same property falls to 4,000 MPa to 6,000 MPa, depending on fibre length and coupling. Tensile creep testing per ISO 899-1:2017 at 85 °C and 85 % RH shows creep modulus at 1,000 h of 2,500 MPa to 3,500 MPa for 30 % glass-fibre PBT, which permits progressive deformation in bolted busbar joints. A M6 threaded insert in a PBT boss tightened to 8 N·m can lose 20 % to 40 % of preload after 1,000 h humid ageing, allowing contact resistance variation in high-voltage busbar interfaces above 10 %. Dimensional stability therefore includes the reduction in compression set and creep compliance rather than only linear expansion. Components with snap-fit interlock connectors and tab pockets are designed with interference of 0.20 mm to 0.35 mm; after 1,000 h at 85 °C and 85 % RH, creep strain can consume 0.05 mm to 0.15 mm of that interference, reducing retention force below acceptable limits. Published data for this specific configuration is limited, but dynamic mechanical analysis on conditioned specimens shows a loss in storage modulus at 85 °C of more than 40 % relative to dry 23 °C values, which accelerates time-dependent deformation under constant load.

Before melt processing, PBT compounds require desiccant drying to below 0.02 % residual moisture, because hydrolysis in the barrel is autocatalytic and cannot be reversed after moulding. Desiccant dryers set at 120 °C for 4 h to 6 h with a dew point of −30 °C or lower reduce moisture to the specified limit, measured by Karl Fischer titration per ISO 15512:2019. At ambient relative humidity above 60 %, open granulate containers should not be left uncovered for more than 30 min; hopper blankets and low-dew-point dry-air conveying are required to prevent moisture regain. On compounding lines, a co-rotating intermeshing twin-screw extruder with an L/D ratio of 40:1 and segmented screw elements is standard; atmospheric and vacuum vents maintain melt devolatilisation at −0.08 MPa, and melt temperature must be controlled between 250 °C and 265 °C, a window of ±5 °C, because below 250 °C fibre wetting is incomplete and above 265 °C thermal degradation reduces hydrolytic stability. Injection moulding of high-voltage battery components uses electric machines with clamp forces of 1,000 kN to 2,500 kN; mould temperature is held at 80 °C to 120 °C to achieve sufficient crystallinity, because mould temperatures below 70 °C reduce crystalline order and increase post-mould shrinkage and moisture uptake. Residence time in the barrel should not exceed 45 s; hot-runner systems with dead zones and long sprue bushings cause viscosity loss, detectable as a melt volume-flow rate increase of more than 20 % per ISO 1133-1:2022 at 250 °C and 2.16 kg. These processing limits are not generic; they are threshold values observed on production-scale equipment and are especially critical for hydrolysis-resistant PBT formulations, whose stabiliser packages can be deactivated by a single overheating event.

When 30 % Glass-Fibre PBT Substitutes for Polyamide 66 in High-Voltage Interlock Connectors

The substitution of polyamide 66 with 30 % glass-fibre PBT in high-voltage interlock connectors changes the dominant dimensional failure mode from reversible moisture expansion to hydrolytic embrittlement plus creep. Polyamide 66 conditioned at 23 °C and 50 % RH reaches equilibrium moisture uptake of 2.0 % to 2.5 % per ISO 62:2008, while PBT reaches approximately 0.2 %; on a 200 mm connector body, polyamide 66 can expand 0.6 mm linearly, whereas PBT expansion remains below 0.10 mm. This difference is sufficient to violate creepage distance minima under IEC 60664-1:2020 for 800 V systems if terminal cavities shift. However, PBT has lower ductility than polyamide 66, and unmodified PBT exposed to 85 °C and 85 % RH loses tensile strength faster than polyamide 66 because ester hydrolysis dominates over polyamide hydrolysis. Hydrolysis-resistant PBT grades with acid-end-group control are therefore mandatory for humid high-voltage connectors, but their processing window narrows: melt temperature must not exceed 265 °C, and combinations with certain halogen-free flame retardants or amine-containing masterbatches can deactivate the stabiliser system and accelerate hydrolytic chain scission. PBT also dries at higher temperature than polyamide 66; a single desiccant drying schedule of 80 °C designed for polyamide 66 will not reduce PBT moisture below 0.02 % and will produce surface splay and molecular weight loss. Qualification testing for substitution requires parallel comparison of 30 % glass-fibre PBT and polyamide 66 in 85 °C and 85 % RH ageing at thicknesses of 1.0 mm, 2.0 mm and 4.0 mm, with tensile strength retention measured per ISO 527-2:2012 type 1A and dimensional scans after each 250 h interval. Published data for this specific configuration is limited, but the lower moisture expansion of PBT is well established in polymer data collections.

Supply-chain qualification for PBT in humid high-voltage battery components integrates dimensional stability, electrical safety and thermal endurance rather than treating them as separate approvals. Qualification test order follows drying verification, moulding process capability, dry-as-moulded dimensional inspection, moisture conditioning at 85 °C and 85 % RH, electrical clearance measurement and then mechanical strength verification. The test matrix below identifies the measurement methods and acceptance limits used on production-scale injection moulding and testing lines.