Unfilled vs Glass-Filled PBT: A One-Page Decision Table for Design Reviews

Polybutylene terephthalate (PBT) is a semicrystalline polyester that enters design reviews as an unfilled molding resin or as a compounded short-glass-fiber product. The first process boundary is hydrolytic stability. At barrel temperatures between 240 °C and 270 °C, residual moisture attacks the ester linkage and reduces molecular weight. A production desiccant dryer is set to 120 °C for 3 h to 4 h; the return-air dew point is maintained at -30 °C or lower. The target moisture content is below 0.02 wt% per ISO 15512:2019. Glass-filled PBT is more sensitive because the melt temperature is shifted upward to 250 °C to 280 °C and hydrolytic chain scission accelerates. When ambient relative humidity exceeds 60 %, hopper residence times below 4 h create batch-to-batch variation that appears as gas splay, gate blush, and reduced tensile strength. For continuous production, dry-air conveying and hopper loading are specified to avoid re-absorption of moisture between the dryer and the feed throat.

On a reciprocating-screw injection molding machine with a general-purpose screw at L/D 20:1 to 25:1 and a compression ratio of 2.5:1 to 3.0:1, unfilled PBT is injected at 80 MPa to 120 MPa. Glass-reinforced PBT requires 100 MPa to 140 MPa. Mold temperature is part of the crystallization control loop. Unfilled PBT uses 40 °C to 80 °C; glass-filled PBT uses 60 °C to 100 °C. A mold surface variation above ±5 °C across the cavity can shift local crystallization and produce dimensionally unstable parts below 1.0 mm wall thickness. Injection speed should be moderate to high; for glass-filled PBT, fast injection reduces fiber orientation gradients but increases surface jetting and requires adequate cavity venting to avoid gas burns.

What changes when 30 wt% glass fiber is compounded into the PBT matrix?

Short glass fiber increases stiffness by load transfer from the polyester matrix to the fiber. In a 30 wt% short-fiber-reinforced grade, tensile modulus rises from 2,500 MPa to 2,800 MPa for unfilled PBT to 9,500 MPa to 11,000 MPa, measured under ISO 527-1/-2. Notched Izod impact under ISO 180/A rises from 3 kJ/m² to 5 kJ/m² to 6 kJ/m² to 10 kJ/m², but the improvement is sensitive to retained fiber length and fiber-matrix adhesion. The glass fiber also raises melt viscosity and reduces melt volume-flow rate; a grade that flows 15 cm³/10 min unfilled may drop to 8 cm³/10 min to 12 cm³/10 min when glass-filled at 250 °C and 2.16 kg under ISO 1133-1:2022. This reduction must be checked against minimum wall thickness and flow length before committing to a glass-filled grade.

Compounding a 30 wt% glass-filled PBT on a co-rotating twin-screw extruder with L/D 40:1 to 48:1 is usually performed with glass roving side-stuffed downstream of the melting section. Side-stuffing preserves fiber length and gives higher tensile strength than adding chopped glass at the main feed. The melt is vacuum-vented at -0.08 MPa or lower to remove residual moisture and low-molecular-weight esters. Pellet drying after compounding is still required; gaylord boxes and silo storage can introduce surface moisture that does not appear on a moisture certificate. Glass-fiber reinforcement also increases abrasion in the compounding screw, die plate, and pelletizer; segmented barrels and wear-resistant screw elements are specified for extended production runs.

Across multi-gated connectors and relay bases, knit-line regions are the governing structural weakness. Fibers orient parallel to flow and do not bridge the knit line. A 30 wt% glass-filled PBT may retain only 50 % to 70 % of the bulk tensile strength at the knit line, depending on melt temperature, mold temperature, and gate location. Unfilled PBT has a more isotropic knit-line response but its low notched Izod impact of 3 kJ/m² to 5 kJ/m² under ISO 180/A makes snap-fit arms brittle unless radiused and gated for flow. Fiber orientation solvers in injection-molding simulation software use fourth-order orientation tensors; their predictions for bow and twist should be calibrated against a physical shrinkage fixture on the first mold trial. Published data for a specific design configuration is limited, so design reviews should not substitute generic ISO bar data for end-use structural performance.

When the design cannot tolerate anisotropic shrinkage, unfilled PBT remains the reference

Unfilled PBT exhibits mold shrinkage of 1.5 % to 2.5 % in both flow and transverse directions under ISO 294-4. Glass fiber reduces flow-direction shrinkage to 0.2 % to 0.6 % but transverse shrinkage remains 0.4 % to 1.0 %. This anisotropy creates bow and twist in flat parts and bearing journals, especially when a single edge gate aligns fiber orientation along the long axis. Dimensional tolerance reviews should compare parallel-to-transverse shrinkage ratio, not a single shrinkage value. Unfilled PBT is the reference for low-stress electrical housings because its comparative tracking index under IEC 60112 is typically 600 V, placing it in IEC 60664-1 material group I. Glass-filled grades often fall to 250 V to 450 V because glass surfaces and flame-retardant packages create partial discharge and tracking paths. If a glass-filled connector must meet a CTI of 600 V, the specific grade must be verified; many halogen-free flame-retardant glass-filled PBT grades do not meet that threshold.

Thermal and Hydrolytic Boundaries in Continuous Service

Continuous service temperature in air is not identical to heat deflection temperature. Unfilled PBT has an HDT-A of 50 °C to 60 °C under 1.8 MPa per ISO 75-1/-2; a 30 wt% glass-filled grade reaches 200 °C to 215 °C. That does not make the glass-filled grade hydrolytically stable. Both compositions are subject to hydrolytic degradation in hot water and saturated steam; continuous exposure above 60 °C in aqueous media is generally outside the unfilled PBT design envelope. Glass-filled PBT should not be used for hot-water pressure vessels without a hydrolysis testing program under ISO 22088 or equivalent. Relative thermal index values under UL 746B are often 120 °C to 130 °C for unfilled PBT and 130 °C to 140 °C for glass-filled PBT, but these values are thickness- and grade-dependent. A design review must use the UL yellow card for the exact formulation, not a generic polymer class rating.

Chemical screening must address both fiber and matrix. PBT is resistant to aliphatic hydrocarbons, mineral oils, gasoline, and many chlorinated solvents at room temperature, but it is attacked by hot alkaline solutions, concentrated acids, and strong oxidizing media. Fuel and oil exposure should be tested under ISO 175 followed by tensile testing per ISO 527-1/-2; property retention varies with temperature and fluid composition. Avoid amine-based additives and alkaline processing aids because they can accelerate ester hydrolysis and cause surface chalking. For engine bay components, the design review should require immersion data at the upper operating temperature, not at room temperature.

Electrical testing of PBT connectors is performed at 23 °C and 50 % RH after conditioning per ISO 291. Unfilled PBT dielectric strength is 20 kV/mm to 25 kV/mm under IEC 60243-1; glass-filled PBT may be 18 kV/mm to 23 kV/mm. The reduction is modest, but internal air gaps between fiber and matrix can create partial discharge sites at high field strengths. This is relevant to electric vehicle connectors operating above 400 V; design reviews should include partial discharge testing under IEC 60664-1 or IEC 60270, particularly for glass-filled grades.

On production floors, glass-filled PBT accelerates wear of screws, barrels, check rings, nozzle tips, and mold gates. Bimetallic or nitrided barrels and hardened screw surfaces are specified for glass-reinforced polyester molding. A worn check ring causes cushion loss and short shots, even when shot size and transfer position remain unchanged. Unfilled PBT does not produce the same abrasive wear, but retained moisture and excessive barrel residence time are common root causes of brittleness and yellowing. Barrel residence times above 10 min at 270 °C degrade the melt; production schedulers should use appropriate shot-weight-to-barrel-capacity ratios and purge when starting and stopping.

The comparative table below is a one-page decision table for design reviews. It uses typical producer datasheet values for unfilled and 30 wt% glass-fiber PBT; values are not specification limits. Replace them with lot-specific data under ISO 10350-1 and the supplier’s UL yellow card.

The table differentiates design reviews by failure mode. A stiffness-limited bracket or structural housing moves to glass-filled PBT; the tensile modulus increases by approximately 3.5×. A dimensionally critical connector housing with constant wall stock may remain unfilled if the load is below 30 MPa and service temperature is below 80 °C. A gear or sliding wear component is generally specified in glass-filled PBT because fiber reinforcement reduces wear against steel and acetal counterfaces, but the counterface roughness should be below 0.2 µm Ra and lubrication must be validated. Published data for specific wear rates is grade-dependent; pin-on-disc tests under ASTM G99 should be requested from the resin producer.

Regulatory screening under RoHS Directive 2011/65/EU and REACH is grade-dependent. Brominated flame retardants in some PBT grades can impose permitting, recycling, and disposal constraints; halogen-free grades are available but may show lower CTI and different mold shrinkage. UL 94 V-0 flammability is a property of the exact grade and wall thickness; it cannot be assumed from the polymer family. Food-contact status requires FDA 21 CFR 177.1660 or equivalent; not every glass-filled PBT is compliant because fiber sizing, colorants, and processing aids are part of the formulation. The design review should require the supplier’s certificate of compliance and a full material data sheet with ISO 10350-1 data before tooling is released.