Polybutylene terephthalate (PBT) is a semi-crystalline engineering thermoplastic produced by the polycondensation of terephthalic acid or dimethyl terephthalate with 1,4-butanediol. Reading a Polybutylene Terephthalate Quote: What Is Included and What to Challenge begins with separating a generic grade call-out from a production-ready material specification. A technically complete quotation should state the base resin type, filler and reinforcement content in wt%, melt volume-flow rate per ISO 1133-1:2022, tensile and flexural data per ISO 527-2 and ISO 178, notched impact per ISO 179-1/1eA, heat deflection temperature per ISO 75-2, moulding shrinkage per ISO 294-4, flammability per UL 94, comparative tracking index per IEC 60112, and a residual moisture specification with drying instructions. A single test value without its method, specimen geometry, conditioning protocol, or the colour of the tested natural grade is not a data point; it is an open commercial item. The evaluation should assume that PBT responds to moisture, residence time, fibre orientation, mould temperature, and flame-retardant chemistry as coupled variables, not as individually adjustable line items.

Does the Melt Volume Rate Alone Define Process Stability?

Suppliers often state melt volume-flow rate (MVR) at 250°C under a 2.16 kg piston load. That single value is inadequate for thin-wall connector or sensor-housing geometries with wall thickness below 0.5 mm, because injection moulding fill occurs at shear rates from 10² s⁻¹ to 10⁴ s⁻¹ while MVR is a low-shear measurement. A quotation should therefore be challenged for capillary viscosity data at 1,000 s⁻¹ and 10,000 s⁻¹ at the proposed melt temperature, or for spiral-flow length determined on the same nozzle and cavity geometry. Unfilled PBT grades generally present MVR values of 10 cm³/10 min to 35 cm³/10 min under 250°C/2.16 kg, whereas 30 wt% glass-fibre-reinforced grades may report 5 cm³/10 min to 15 cm³/10 min. The reported value is meaningful only when barrel residence time is below 5 min and residual moisture is below 0.02%, because hydrolytic chain scission in PBT lowers apparent viscosity while permanently reducing molecular weight and mechanical strength. On production-scale injection moulding machines with hot-runner systems, a manifold temperature shift of 5°C can change thin-wall fill pressure by more than 0.5 MPa; quotations that do not specify a hot-runner nozzle temperature profile leave this interaction unresolved. The lot-to-lot MVR tolerance should also be challenged, preferably as a process capability statement or as ±2 cm³/10 min around nominal, because batch variation transfers directly to cushion position, holding-pressure drift, and gate-seal time.

Glass Fibre Loading, Tensile Properties, and What the Data Must State

Quotations that state only “PBT-GF” without nominal fibre content, fibre sizing, and compounding route cannot support tool design or mechanical simulation. For a 30 wt% glass-fibre-reinforced PBT, publicly available supplier datasheets typically report tensile stress at break from 125 MPa to 140 MPa per ISO 527-2, flexural modulus from 8.5 GPa to 10.0 GPa per ISO 178, notched Charpy impact from 7 kJ/m² to 11 kJ/m² per ISO 179-1/1eA, and heat deflection temperature from 200°C to 220°C at 1.8 MPa per ISO 75-2 method A. These ranges are not transferable across specimen geometry, conditioning history, or fibre length distribution. The quote should be challenged for fibre diameter, residual fibre length after compounding, and coupling-agent chemistry; without these details, tensile modulus can vary by more than 15% at identical nominal glass content. Compounding on a corotating twin-screw extruder with L/D 40:1 and side-fed glass fibre changes fibre attrition depending on screw speed, feed rate, and specific mechanical energy input. Quotations that omit pellet size, fibre source, or the compounding line should be questioned when parts are intended for dynamic or fatigue loads. Glass-filled PBT also exhibits anisotropic moulding shrinkage, so the quotation must include parallel and perpendicular shrinkage values per ISO 294-4, not a single averaged percentage.

When a Quote Omits the UL Yellow Card and Tracking Index Details

For electrical and electronic components, a flammability statement without a complete UL Yellow Card is not a production release datum. The quotation should identify the grade by manufacturer, colour, thickness, and the minimum thickness at which UL 94 V-0 applies, because a material may pass V-0 at 1.6 mm and fail at 0.8 mm. Comparative tracking index per IEC 60112 should be reported for the exact flame-retardant chemistry; brominated systems with antimony trioxide can produce CTI values below 250 V, while certain halogen-free phosphinate systems can exceed 600 V. A continuous-use temperature claim should be supported by a relative thermal index from UL 746B assigned for the specific thickness and colour, not a generic datasheet value. Halogen-free flame-retardant PBT may introduce mould deposit, plate-out, and volatiles that attack tool steel; the quotation should state whether cavity steel should be hardened to at least 52 HRC and whether venting clearances must be increased to control deposit accumulation. For appliance-housing applications, glow-wire ignition temperature per IEC 60695-2-13 and glow-wire flammability index per IEC 60695-2-12 may be required; a quote listing only UL 94 V-0 should be challenged for the missing glow-wire and ball-pressure data. Colour carriers and pigment concentrates can suppress flame retardancy, especially in halogen-free packages, so every coloured material in the quotation must be supported by its own UL Yellow Card or a documented equivalence statement.

Hydrolytic degradation remains a common cause of field failure when PBT is processed outside its moisture specification. As a condensation polyester, PBT undergoes chain scission in the presence of moisture at melt temperatures above 250°C, and the rate accelerates with temperature and residence time. A production quotation should therefore include a maximum residual moisture content, typically 0.02% by Karl Fischer titration per ISO 15512, and a drying specification of 120°C for 4 h with a desiccant dryer delivering a dew point of -30°C or lower. If these values are absent, the quote should be challenged before moulding trials, because wet resin can produce splay, lower molecular weight, and tensile stress loss of 10% to 30% depending on exposure time. Water absorption values per ISO 62 or ASTM D570 do not replace a melt-processing moisture limit; they describe long-term water uptake at service temperature rather than residual moisture after drying. Quotations for hot-runner systems with manifold temperatures above 260°C or predicted melt residence times longer than 5 min should demand melt viscosity retention data after drying. Packaging should be specified as moisture-barrier bags with desiccant, and the quotation should state whether vacuum-sealed foil packaging is used. Continuous service in hot water above 60°C remains outside the reliable range for unmodified PBT, regardless of drying quality.

A Shrinkage Value Alone Is Not a Tooling Specification

Moulding shrinkage in PBT is anisotropic and process-dependent. A single percentage on a quotation is insufficient for tool cutting unless the value is tied to plaque thickness, flow direction, mould temperature, injection speed, and holding-pressure profile. Post-mould shrinkage must also be separated from total moulding shrinkage because PBT continues to crystallise after ejection, particularly at service temperatures above 80°C. For 30 wt% glass-fibre-reinforced PBT, shrinkage parallel to flow commonly falls between 0.2% and 0.6%, while perpendicular shrinkage may range from 0.7% to 1.2%; unfilled PBT often ranges from 1.4% to 2.0%. These ranges reflect mould temperature, flow length, and cooling-rate effects on crystallinity. The quotation should be challenged for shrinkage data generated on edge-gated plaques per ISO 294-4 with the exact mould temperature and holding-pressure profile recorded. Without that, a tool cut to a single nominal shrinkage can produce parts outside tolerance, particularly in gear housings or connector bodies where bore-to-bore distance may be held within 0.05 mm. Coefficient of linear thermal expansion per ISO 11359-2 should also be requested for service spanning -40°C to 140°C, because PBT expansion behaviour changes after the glass transition and with fibre orientation.

A supplier’s certificate of analysis attached to a PBT quotation is not equivalent to a defined production window. The COA typically reports one batch’s MVR, moisture content, and selected tensile values from standard specimens; it does not state the permitted lot-to-lot variation, the compounding line, or the screw configuration used. For long-run moulding of thin-wall connectors, the quotation should be challenged for minimum and maximum MVR limits, preferably with process capability data, not a single value. Production of glass-fibre-reinforced PBT on a twin-screw extruder with L/D 40:1 can produce lot-to-lot fibre length differences that change notched impact by 2 kJ/m² even when nominal glass content remains 30 wt%. If the COA does not report fibre length or ash content, that gap should be identified before validation. Colour masterbatch addition can reduce tensile strength by 5% to 10%; therefore a quote for a coloured moulding compound must be based on the actual coloured material, not only the natural base resin. Requesting the COA for the production lot, not a laboratory reference lot, is essential.