Polybutylene Terephthalate Grade Map: Every Slot from Unfilled to GF50

Polybutylene terephthalate (PBT) is a semicrystalline aromatic polyester produced by the melt polycondensation of 1,4-butanediol and dimethyl terephthalate or terephthalic acid. The commercial grade map is structured primarily by glass fiber mass fraction, with standard slots at 0 wt%, 10 wt%, 15 wt%, 20 wt%, 30 wt%, 40 wt%, and 50 wt%. Densities measured under ISO 1183 move from 1.31 g/cm³ in unfilled resin to approximately 1.73 g/cm³ at 50 wt% glass. The crystalline melting point is approximately 223 °C and the glass transition is near 50 °C, but practical service temperature is governed by heat deflection temperature and fiber content. This grade map is not a continuous linear interpolation: each step changes mold shrinkage, weld-line strength, melt viscosity, and tool wear in ways that must be separated during part design.

For unfilled medium-viscosity PBT, tensile strength measured under ISO 527-2 falls between 50 MPa and 60 MPa, and flexural modulus under ISO 178 is typically 2.3 GPa to 2.7 GPa. Notched Izod impact at 23 °C using ISO 180/1A is generally 3 kJ/m² to 5 kJ/m², while heat deflection temperature at 1.8 MPa under ISO 75-2 remains between 50 °C and 60 °C. Mold shrinkage measured under ISO 294-4 is 1.5% to 2.0%. Unfilled PBT is characterized by fast crystallization and high flow length in thin sections, but it requires effective drying and is not suitable for continuous hot-water service above 60 °C because the ester linkage undergoes hydrolytic chain scission. On injection molding machines with standard general-purpose screws, the melt temperature window is commonly 240 °C to 260 °C, and mold temperature is held at 60 °C to 100 °C. ISO 1133-1:2022 melt volume-flow rate values for unfilled grades at 250 °C/2.16 kg typically range from 10 cm³/10 min to 30 cm³/10 min. The low heat deflection temperature of unfilled PBT limits its use in temperature-loaded sections; glass reinforcement is therefore the primary grade-axis variable.

How Glass Fiber Content Shifts Heat Deflection and Mold Shrinkage

The primary effect of short glass fiber is not simply tensile strengthening. It is the combined increase in heat deflection temperature and reduction in mold shrinkage. The following table represents the typical property ladder compiled from publicly available supplier datasheets using the cited methods.

The data show two nonlinear transitions. Between 10 wt% and 20 wt%, heat deflection temperature rises rapidly because the glass network begins to resist creep under load. From 30 wt% to 50 wt%, the tensile strength gain decelerates while flexural modulus continues to rise, indicating that the matrix-to-fiber stress transfer becomes the limiting factor rather than fiber content alone. When tested under ASTM D638 instead of ISO 527-2, absolute values may differ by 5% to 10% because of specimen thickness and extensometer type, but the grade-to-grade trend remains unchanged. Mold shrinkage becomes anisotropic: flow-direction shrinkage is typically 0.10% to 0.30% at 50 wt%, while transverse shrinkage can be 0.40% to 0.60%. This anisotropy must be compensated by gate placement and mold cooling layout. Unfilled and low-glass grades show less orientation sensitivity but higher absolute shrinkage.

When glass fiber loading is increased from 10 wt% to 20 wt%, the practical processing window narrows due to higher melt viscosity and increased shear heating. Tensile strength under ISO 527-2 rises from roughly 75 MPa at 10 wt% to 120 MPa at 20 wt%, but elongation at break drops below 5%. On production-scale injection molding machines with 20:1 L/D injection screws, the melt temperature must be raised toward 260 °C to prevent short shots; however, residence time above 5 min at that temperature can initiate thermal degradation. The main field failure mode in this intermediate slot is not strength loss but distortion: differential cooling between flow and transverse directions produces bowing in flat housings and connector headers. Tooling with conformal cooling or balanced multicavity runners is used to reduce warpage. Weld-line strength also declines as glass content increases; at 20 wt% glass, weld-line tensile strength may be 55% to 65% of the base tensile value depending on flow-front angle. Published data for this specific configuration is limited to supplier mold-filling studies and should be verified with an in-mold instrumented trial.

Rheological Boundaries at 30% Glass Fiber Reinforcement

At 30 wt% chopped-glass loading, PBT compound exhibits the most widely used balance of stiffness, heat resistance, and processability, but it also reaches a distinct rheological boundary. Melt volume-flow rate under ISO 1133-1:2022 at 250 °C/2.16 kg is typically 40% to 60% lower than the unfilled base resin, although absolute values vary by supplier. Injection pressure setpoints on hydraulic machines commonly rise to 100 MPa to 140 MPa for thin-wall sections below 1.0 mm. The compound is typically produced on a twin-screw extruder with 40:1 L/D, using side feeding of glass roving in the downstream zone to minimize fiber breakage; fiber length after compounding is generally retained in the 250 µm to 500 µm range. In injection molding, screw and barrel wear becomes measurable on standard nitrided screws after sustained glass-filled service, and common field failures include groove wear at the screw root and leakage across the non-return valve. Gate size should be at least 50% of wall thickness, and cold-slag wells must be oversized because glass-filled PBT generates a high frequency of unmelted particles at the flow front. Electrical connectors and sensor housings molded from 30 wt% glass PBT exhibit tensile strength under ISO 527-2 of 130 MPa to 145 MPa, flexural modulus under ISO 178 of 9.0 GPa to 11.0 GPa, and heat deflection temperature under ISO 75-2 at 1.8 MPa of 200 °C to 210 °C.

At 40 wt% and 50 wt% chopped-glass loadings, flexural modulus under ISO 178 exceeds 12.0 GPa and can reach 17.0 GPa, while tensile strength under ISO 527-2 plateaus between 155 MPa and 175 MPa. The linear coefficient of thermal expansion measured by ISO 11359-2 falls from roughly 8×10⁻⁵ K⁻¹ for unfilled PBT to 2.5×10⁻⁵ K⁻¹ to 3.5×10⁻⁵ K⁻¹ at 50 wt%. Dimensional stability is therefore improved, but impact performance becomes more orientation-dependent. Notched Izod impact under ISO 180/1A may remain at 8 kJ/m² to 11 kJ/m² in the flow direction, while transverse and weld-line regions can be significantly lower. The main process conflict is abrasion: glass-rich PBT causes progressive wear in standard hardened tool steels, so gate inserts, core pins, and ejector sleeves are specified in powdered-metal or through-hardened grades. Clamp force requirements must be calculated from high cavity pressure rather than from filled-material flowability alone. Mold temperatures above 80 °C are typical to maintain surface gloss and minimize fiber read-through. At these loadings, molded parts are used in structural brackets, pump housings, and automotive headlamp frames, but long-term alkaline or strong-acid exposure above 60 °C is incompatible with the polyester matrix.

Creep Modulus and Fatigue Life Do Not Scale Linearly with Glass Content

At 80 °C under a constant tensile load of 20 MPa, unfilled PBT creeps significantly because the test temperature is above its heat deflection temperature. At 30 wt% glass, the same load produces lower creep strain, but creep modulus measured under ISO 899-2 is not proportional to glass content. The transition from matrix-dominated creep to fiber-dominated creep occurs between 20 wt% and 30 wt%; below this interval, creep resistance improves slowly, whereas above it, further improvements depend on fiber orientation and fiber length distribution. Fatigue behavior under ISO 13003 or supplier-specific flexural fatigue protocols shows that weld-line regions at GF50 can fail earlier than the base resin because the weld line is essentially a glass-depleted plane. Thus, the grade map cannot be used as a simple fatigue-life ladder. In components subjected to cyclic loading, GF30 may outperform GF50 if the weld line is located in the stress path. This process conflict must be resolved by moving the weld line through gate repositioning rather than by selecting a higher glass content.

Electrical properties across the grade map are influenced by glass sizing and additive package, not only by glass mass fraction. Volume resistivity under IEC 62631-3-1 is generally above 10¹⁴ Ω·cm for unfilled PBT, but glass-filled grades may drop to 10¹³ Ω·cm depending on sizing and moisture. Dielectric strength measured under IEC 60243-1 at 1.6 mm is commonly 20 kV/mm to 25 kV/mm for unfilled and can fall to 15 kV/mm to 20 kV/mm at high glass content. Comparative tracking index under IEC 60112 for 30 wt% glass PBT is usually in the 250 V to 300 V range, which is lower than unfilled PBT. Therefore, high-voltage creepage requirements must be evaluated at the specific grade slot, not at a generic PBT level. This is especially relevant in power electronics where GF30 and GF40 are used for coil formers and terminal blocks near live parts.

When Flame-Retardant, Hydrolysis-Resistant, or Impact-Modified Grades Share the Same Glass Fiber Axis

Commercial PBT grade maps are not limited to neat glass-filled resins. Flame-retardant grades using brominated additives and antimony trioxide synergist occupy 15 wt%, 25 wt%, and 30 wt% glass slots and are rated UL 94 V-0 down to 0.8 mm. The same glass loading with an impact-modification package raises notched Izod impact under ISO 180/1A from about 7 kJ/m² to 12 kJ/m² or higher but reduces tensile strength by 10% to 20% relative to the unmodified glass-filled compound. Hydrolysis-resistant automotive grades use reduced carboxylic acid end-group concentration and are validated in hot-air and pressure-humidity aging per OEM specifications, not by a single generic ISO method. Food-contact PBT formulated with 30 wt% glass can comply with FDA 21 CFR 177.1660 and EU 10/2011 migration limits for specific food simulants, but the supplier must certify each glass-sizing chemistry. Flame-retardant, high-glass combinations can exhibit comparative tracking index values below 250 V under IEC 60112, limiting their use in unshielded high-voltage creepage paths. When a specification calls for GF50 plus UL 94 V-0 at 0.4 mm, published data for this specific configuration is limited and molders must not assume that a GF30 V-0 grade can be simply extended by increasing glass content.

Predrying in a desiccant dryer with a dew point of −40 °C is mandatory across the entire grade map, not just at high glass content. Residual moisture measured under ISO 15512 should be below 0.02% before melt processing. For glass-filled PBT, the melt temperature window is typically 250 °C to 270 °C; mold temperature is 80 °C to 100 °C for dimensional stability. Holding pressure amplitude depends on glass content: higher glass fractions require longer hold times to compensate for reduced compressibility and faster solidification. Processors running GF50 on hot-runner molds must maintain hot-runner manifold temperatures no more than 10 °C above melt setpoint to avoid fiber-bed separation and free polyester exudation at the gate. The main incompatibility across the grade map is moisture-driven hydrolytic degradation at melt temperature, not ambient humidity alone. A moist pellet may appear dry, but residual moisture hydrolyzes ester bonds within the first 2 min at melt temperature, reducing molecular weight and producing surface splay. Therefore, grade selection cannot be separated from drying and residence-time control.