A compound under consideration for re-qualification is unfilled polybutylene terephthalate with a melt volume-flow rate of 15 cm³/10 min at 250 °C and 2.16 kg per ISO 1133-1:2022, a tensile modulus of 2.5 GPa per ISO 527-2:2012, and a density of 1.31 g/cm³ per ISO 1183-1:2019. The review “Is PBT Still the Right Polyester? A Re-Selection Checklist Before You Re-Qualify” is not a datasheet comparison; it is an audit of whether the selected grade survives the actual conversion and service boundary. A re-selection checklist must begin with the semi-crystalline polyester’s melting point of 223 °C and crystallization temperature near 190 °C by differential scanning calorimetry per ISO 11357-3:2018, because these values define the thermal corridor between adequate flow and degradation. The checklist must also verify glass content, additive package, regrind fraction, and moisture state before accepting any mechanical property from the supplier’s certificate of analysis.

Why Re-Conditioning Data Is Not Transferable Across Sourcing Regions

PBT absorbs moisture at a low but re-qualification-relevant level. Immersion data per ISO 62:2008 typically show water absorption of 0.08 % after 24 h and saturation values near 0.4 %. A hydrolysis-stabilized grade from one polymerization site can carry a different carboxyl end-group concentration than a superficially identical grade from another site, even when the datasheet melt volume-flow rate is identical. Re-conditioning data from a plant at 80 % RH therefore does not transfer to a plant with winter indoor dew points below −10 °C. Pre-drying must be set to 120 °C for 4 h or until the residual moisture by Karl Fischer titration per ISO 15512:2019 is below 0.02 %. Hydrolysis resistance is not inferred from a single tensile value; it is measured by tensile testing according to ISO 527-2:2012 after exposure at 85 °C and 85 % RH for the time specified by the end-use specification. PBT without hydrolysis stabilization is generally unsuitable for continuous contact with hot water above 60 °C or for steam-sterilisation service.

The same re-qualification logic applies to additive migration. Flame-retardant systems, nucleating agents, and mold-release chemistries alter crystallization kinetics and surface behaviour. A re-selection checklist should require a lot-by-lot additive report, not a proprietary formulation description, and should compare the ash residue by ISO 3451-1:2019 against the approved reference. If the ash value drifts outside the approved control limit, the shipment is not re-qualified without further mechanical and flammability testing.

During the mold-filling phase, the melt-temperature window between 230 °C and 250 °C is not a recommendation; it is the boundary between short-shot risk at the lower end and thermal degradation above 260 °C. Injection molding lines with general-purpose screws of L/D 20:1 to 24:1 and compression ratios of 2.5:1 to 3.5:1 process PBT adequately, but start-up delays or size-change stops that leave melt in the barrel for more than 8 min at melt temperature produce discolouration and a measurable rise in melt volume-flow rate. The barrel-temperature profile should be flat-to-reverse, with the feed zone 10 °C to 20 °C lower than the metering zone to prevent early melting and screw slip. The mold temperature for unfilled PBT is held between 40 °C and 80 °C; for glass-reinforced PBT the temperature is raised to 80 °C to 120 °C to reduce free glass on the surface and improve knit-line strength. Packing pressure should be held between 60 MPa and 100 MPa to suppress sink marks. Re-qualification after tool transfer must include a melt-stability trial: a hold test at 250 °C for 20 min, followed by a melt volume-flow-rate measurement per ISO 1133-1:2022, compared with the virgin pellet baseline.

Regrind fraction in PBT must be controlled because dried regrind carries additional carboxyl end-group concentration and reduced hydrolytic stability. A re-qualification trial at 20 % regrind may be acceptable for a bracket with a wide mechanical safety factor; the same 20 % regrind level in a sealed relay housing may fail the 85 °C/85 % RH tensile retention requirement far earlier than virgin material. The checklist should set the maximum permissible regrind level as a numerical value tied to the end-use requirement, rather than a nominal “closed-loop regrind” permission.

Hot-Shear, Gate Vestige, and Mold Shrinkage Variability

Mold shrinkage measured per ISO 294-4:2018 for unfilled PBT typically falls between 1.5 % and 2.0 %. The same measurement on a 30 wt% glass-fibre grade can be anisotropic, ranging from 0.2 % to 0.4 % in the flow direction and 0.8 % to 1.2 % transverse to flow, depending on gate type, wall thickness, and fibre orientation. Post-molding shrinkage and warpage must be checked after annealing at 120 °C for 2 h or after the component’s upper service temperature excursion, because incomplete crystallization at low mold temperatures allows later dimensional change. Hot-shear degradation at the gate, where melt velocity is high, can produce gate vestige brittleness and a local loss of tensile elongation. Re-qualification of a tool move therefore requires tensile specimens cut from gate-adjacent and far-from-gate regions, not only from a generic tensile mold. The data are compared with ISO 527-2:2012 tensile bars and ISO 179-1:2010 Charpy bars to detect shear-induced molecular-weight loss.

Weld-line performance in glass-reinforced PBT is especially variable. A 30 wt% glass-fibre tensile bar without a weld line can show 130 MPa; a moulded part with a knit line may fail at less than 60 % of that value when the flow fronts meet after long flow lengths. Re-qualification after changing gate position or wall thickness includes Charpy impact bars with a central weld line per ISO 179-1:2010 or component burst testing, because the datasheet does not capture this defect region.

Electrical connector re-qualification exposes PBT to thermal aging, vibration, and leakage-current stress at narrow terminal pitch. For unfilled PBT, dielectric strength per IEC 60243-1:2013 is commonly reported at 20 kV/mm for a 1 mm specimen, volume resistivity per IEC 62631-3-1:2016 is near 1 × 10¹⁵ Ω·cm, and the comparative tracking index per IEC 60112:2003 can reach 600 V in unfilled, unreinforced grades. These values are not retained automatically when glass reinforcement, regrind, colour masterbatch, or flame retardants are added. The tracking index of a V-0 grade is often lower than that of an unfilled HB grade; a connector housing moulded from a flame-retardant PBT may require a CTI of at least 250 V or 400 V depending on the electrical spacing and pollution degree defined in the clearance and creepage standard. Flammability class must be read from the UL Yellow Card at the exact wall thickness used in production, not inherited from a nominal datasheet. A 0.75 mm V-0 listing does not qualify a 0.40 mm wall section.

Colour concentrates can reduce tracking resistance by introducing polar pigments or conductive carbon black. A black V-0 PBT connector housing should not be re-qualified by testing a natural analogue; the re-selection checklist demands the exact production colour masterbatch because the leakage-current path along the surface is influenced by pigment dispersion and additive bloom. The IEC 60112:2003 tracking test is run on the moulded colour at the minimum wall thickness, not on a generic plaque.

When Glass Reinforcement Changes the Re-Qualification Criteria

The addition of short glass fibres to PBT is not a drop-in substitution; it converts an isotropic unfilled polymer into an anisotropic composite. The re-selection checklist must change acceptance limits for modulus, impact, shrinkage, and flammability because glass reinforcement alters the combustion pathway and the mechanical failure mode. At 30 wt% glass fibre, tensile strength per ISO 527-2:2012 rises from 55 MPa to approximately 130 MPa, but tensile strain at break falls to a few percent. The gain in flexural modulus from 2.5 GPa to 9.0 GPa is accompanied by a change in notched Charpy impact from 5 kJ/m² to approximately 9 kJ/m² at 23 °C; the improvement is not uniform at low temperature and depends on fibre sizing. A glass-reinforced grade must be re-qualified for knit-line strength at the weld line, which is controlled not by the tensile datasheet but by the interaction of fibre orientation, melt temperature, mold temperature, and venting.

Glass content is not assumed; it is verified by ash residue per ISO 3451-1:2019 or thermogravimetric analysis per ISO 11358-1:2022. The re-qualification engineer should reject a shipment that meets tensile strength but contains less glass than the approved reference, because the compounder may have compensated with a brittle mineral filler that does not show up in the tensile number. Fibre length distribution after molding is also a process output: high back pressure, aggressive screw speed, or excessive regrind fraction reduces fibre length and lowers impact performance even when the pellet glass content is correct.

Immersion in hot engine coolant at 120 °C and contact with commercial brake fluid, alkaline cleaning solutions, or chlorinated solvents reveal the boundary between PBT’s solvent resistance and its hydrolysis sensitivity. Aliphatic hydrocarbons, mineral oils, and many automotive fuels at room temperature cause only limited property loss, but strong bases, hot acids, ketones, and chlorinated solvents attack the polyester. Underhood connector re-qualification per USCAR-2 class requirements typically demands chemical exposure followed by tensile or insulation-resistance verification, because a housing that survives thermal cycling at −40 °C to 125 °C may still crack after exposure to windshield-washer fluid or calcium chloride road salt. The checklist must state the exact test fluid, exposure time, and temperature; “chemical resistance” without a fluid list is not a re-qualification protocol.

For a gear or actuator component running in a dry enclosure at 80 °C, unfilled PBT may still be re-qualified if the load is below the creep modulus limit and the dimensional requirement tolerates post-molding shrinkage. The same part in a washing-machine pump exposed to warm detergent solution at pH 11 will fail the hydrolysis boundary before the mechanical check. That operational difference is the reason the re-selection checklist is structured around the combination of temperature, fluid, and electrical stress rather than around a single polymer name.