Polybutylene Terephthalate: What GF15, GF20, GF30 and GF50 Each Buy You

Polybutylene terephthalate is a fast-crystallizing semicrystalline polyester with a melting temperature near 225 °C and a glass transition temperature near 50 °C. Glass-fiber-reinforced PBT compounds are produced by melt blending PBT with chopped glass strands, typically 10–13 µm in diameter, in a co-rotating twin-screw extruder. The glass fraction is expressed as weight percentage: GF15, GF20, GF30, and GF50. Each loading changes stiffness, strength, dimensional stability, flow length, and processing demand. The property envelope is governed less by the polyester alone than by fiber length distribution, glass sizing chemistry, and the quality of fiber wet-out during compounding.

A Comparative Data Baseline for Four Glass Loadings

The values in Table 1 are consolidated from publicly available commercial datasheets and are typical for hydrolysis-stabilized PBT grades corresponding to ISO 7792-1 designation PBT, GF15, GF20, GF30, and GF50. They should not be read as absolute maxima; fiber diameter, sizing, and heat stabilizer packages can shift individual values by 10–20 %.

Glass fiber raises tensile modulus more rapidly than tensile strength because the fiber modulus of E-glass, approximately 72 GPa, dominates stiffness while strength remains limited by interfacial adhesion and fiber length. Heat deflection temperature rises steeply from unreinforced PBT but plateaus near 220 °C because the crystalline PBT matrix, not the glass, limits the upper thermal resistance. Mold shrinkage decreases with glass content, but the difference between flow and transverse shrinkage remains significant and becomes the dominant source of part warpage above 20 % glass. A GF30 pellet with heavily degraded fiber length can exhibit tensile strength closer to GF20 while retaining GF30 density; therefore compounding screw design and injection molding parameters are inseparable from the material specification.

When Moisture, Residence Time, and Fiber Attrition Govern the Processing Window

PBT hydrolyzes at melt temperature when residual moisture exceeds 0.02 wt%. Desiccant drying at 120 °C for 4 h or 130 °C for 3 h is required before compounding and molding when ambient relative humidity is above 60 %. A hopper-mounted desiccated-air supply with a dew point below -30 °C is standard on production lines. Melt temperature measured at the nozzle is maintained between 250 °C and 265 °C; excursions above 270 °C accelerate chain scission and reduce molecular weight, producing a measurable drop in tensile strength and an increase in melt flow rate. Melt residence time above 270 °C longer than 5 min can reduce intrinsic viscosity by more than 10 %, as measured by ISO 1628-5, even if the molded article appears visually acceptable.

Compounding on a 40:1 L/D co-rotating twin-screw extruder with glass side feeding downstream of the melting section minimizes fiber attrition. When glass is added at the main throat, screw torque rises and measured number-average fiber length after pelletizing can fall by 30–50 %, effectively reducing a GF30 compound toward GF15 stiffness. For GF50, side stuffing and reduced-shear mixing elements are required; excessive kneading blocks create fiber breakage and fines, which lower notched impact and increase melt viscosity variability. In injection-molded parts, a core–shell fiber orientation structure develops: fibers in the shell align with flow, while core fibers take transverse orientation. This differential orientation becomes more pronounced above 20 % glass and is the primary cause of out-of-plane warpage in flat connectors and housings.

Injection molding of GF30 and GF50 requires mold temperatures of 60–90 °C for crystallization. A differential scanning calorimetry cooling scan at 10 K/min typically shows a PBT crystallization peak near 190 °C, so low mold temperatures below 60 °C create an under-crystallized skin that reduces dimensional stability after thermal cycling. Published molding data for wall thicknesses below 1.0 mm often show injection pressures above 120 MPa for GF30 and above 150 MPa for GF50. Machine clamp force must be calculated from projected area at maximum cavity pressure, with a margin of 20–30 %. Regrind levels above 20 % are not recommended for load-bearing parts because repeated heat history shortens fiber length and lowers tensile modulus.

At 15 % glass fiber by mass, the compound retains enough flow to fill thin-walled, multi-cavity tooling while moving flexural modulus from approximately 2500 MPa for unfilled PBT to 5500–6500 MPa. This grade is specified for relay housings, door lock mechanisms, mirror housings, and sensor housings where creep resistance under low mechanical load and dimensional control at elevated temperature are required. Tensile strength measured by ISO 527-2 typically falls between 90 MPa and 105 MPa, with elongation at break near 3.5–5.0 %; the retained ductility reduces the risk of brittle snap-fit fracture compared with higher glass levels. Mold shrinkage by ISO 294-4 is lower than unfilled PBT but still anisotropic; flow-direction values of 0.4–0.9 % and transverse values of 0.7–1.1 % require a well-positioned gate to avoid visible sink and warpage. Processing benefits include lower screw torque, reduced gate wear, and shorter hold-pressure time. The limitation is heat deflection temperature near 200 °C under 1.8 MPa, which is insufficient for soldering-adjacent electrical hardware or hot oil exposure.

Between the minimally reinforced GF15 grade and the structural GF30 grade, the 20 % glass-filled compound is used when snap-fit retention, dimensional stability, and a measurable increase in flexural strength are needed without the higher tool wear of 30 % glass. Flexural modulus measured by ISO 178 generally falls between 7000 MPa and 8000 MPa, while tensile strength reaches 105–125 MPa. Notched Charpy impact at 23 °C remains in the 6–8 kJ/m² range, making it serviceable for automotive connector housings and fuse clips that are assembled by snap-fit. Flow-direction shrinkage of 0.3–0.7 % is sufficiently low to hold post-molded contact pin spacing in multi-cavity connectors, provided gate location is optimized with mold-filling simulation. Melt temperature can be held near 250 °C, which reduces degradation during high-cavitation molding. The 20 % loading is not specified for maximum structural stiffness; it is specified because it combines a more balanced fiber orientation state than 15 % glass with lower pressure drop than GF30 in tools with long flow paths.

What Justifies the 30% Glass Benchmark in Connector and Bobbin Molding?

At 30 % glass fiber, PBT reaches a stiffness plateau that electrical component specifications commonly treat as the default for heat-resistant connectors, relay bobbins, solenoid caps, and sensor bodies. Tensile strength by ISO 527-2 of 130–150 MPa and flexural modulus of 9000–11000 MPa support thin-wall designs that must survive heat spikes and vibration. Heat deflection temperature under 1.8 MPa is 205–215 °C, close to the practical ceiling for PBT because the polyester crystallites dominate the upper thermal limit. Flame-retarded GF30 compounds are available with UL 94 V-0 at 0.4 mm and often 0.75 mm, but the specific flame-retardant package changes tracking resistance; IEC 60112 comparative tracking index values can shift by 100 V or more depending on whether a brominated, phosphorus, or halogen-free system is selected. The CTI value must be verified for the exact grade rather than assumed for all GF30 PBT materials.

Mold filling of GF30 is more pressure-intensive than GF20. In multi-cavity connector tools with 0.4 mm wall sections, injection pressures of 100–140 MPa are regularly required, and mold temperatures below 80 °C can produce an under-crystallized skin that reduces dimensional stability after thermal cycling. Gate diameter should not be reduced below 1.0 mm unless the runner system is equipped with pressure transducers and the material has been characterized on a capillary rheometer at 250 °C. Shrinkage anisotropy is the main quality risk: flow-direction shrinkage of 0.2–0.5 % and transverse shrinkage of 0.5–0.8 % are sufficient to bow a long connector body if glass orientation is unidirectional. Mold-flow simulation with fiber orientation tensors is used to place weld lines away from pin arrays and snap fits.

In structural brackets, gear housings, fan impellers, and pump wear parts, 50 % glass-fiber reinforcement is selected for metal replacement and high-load creep resistance. Flexural modulus rises to 13000–16000 MPa and tensile strength to 150–175 MPa, but elongation at break drops below 2.5 % and the material behaves as a highly anisotropic solid. Heat deflection temperature under 1.8 MPa reaches 215–220 °C, yet this does not imply long-term thermal stability above the PBT melting point; continuous use at such temperatures is not supported. The chief benefit is increased stiffness-to-weight ratio and reduced creep under bolted compressive or short-beam flexural loads, provided the load path is not tensile at a weld line.

Processing GF50 imposes the hardest demands on the molding cell. Melt flow rate at 250 °C and 2.16 kg can fall to 3–8 g/10 min by ISO 1133-1, requiring high injection velocity, hold pressures up to 160–180 MPa, and mold temperatures of 90 °C or higher in thin sections to fill before freeze-off. The glass fibers produce accelerated gate and vent wear; mold inserts are typically hardened to at least 56 HRC, and gates are positioned in the thickest section to reduce jetting. Shrinkage is very low in the flow direction, 0.1–0.3 %, but transverse shrinkage of 0.4–0.7 % still creates warpage in flat parts longer than 150 mm; ribbed geometry or curvature is required to resist out-of-plane distortion. Published data for the specific fatigue performance of PBT GF50 in lubricated gear meshes is limited; gear applications therefore require prototype dynamometer testing rather than interpolation from static tensile data.