PBT Injection Moulding: Engineering Plastic Properties Guide
PBT (polybutylene terephthalate) is a semi-crystalline engineering thermoplastic in the polyester family, closely related to PET but formulated for injection moulding. Unreinforced PBT offers good electrical insulation, low moisture absorption relative to nylon, and resistance to oils and fuels. Glass reinforcement is what unlocks PBT's full potential: 30% glass fibre raises heat deflection temperature from a modest 50°C to over 200°C, making GF30 PBT the dominant material for automotive electrical connectors, relay and sensor housings, and SMT-compatible PCB components.
What are the mechanical and thermal properties of PBT?
PBT's semi-crystalline structure gives it a sharp melting point and predictable performance up to that melt. The unfilled resin has modest heat deflection temperature at high load. Glass reinforcement is the standard lever for structural applications.
| Property | Unfilled PBT | 30% GF PBT | Test Standard |
|---|---|---|---|
| Tensile Strength | 50–60 MPa | 120–160 MPa | ISO 527 |
| Flexural Modulus | 2,400–2,700 MPa | 8,000–10,000 MPa | ISO 178 |
| Izod Impact (notched) | 40–60 J/m | 60–90 J/m | ISO 180 |
| Heat Deflection Temp (1.80 MPa) | 50–60°C | 200–210°C | ISO 75 |
| Melting Point | 220–225°C | 220–225°C | DSC |
| Density | 1.30–1.32 g/cm³ | 1.50–1.55 g/cm³ | ISO 1183 |
| Mould Shrinkage | 1.5–2.5% | 0.3–0.8% | ISO 294-4 |
| Water Absorption (24 h) | 0.08–0.10% | 0.05–0.08% | ISO 62 |
| Dielectric Strength | 15–20 kV/mm | 15–20 kV/mm | IEC 60243 |
The jump in flexural modulus from approximately 2,500 to 9,000+ MPa between unfilled and 30% GF grades illustrates why glass reinforcement is standard for structural PBT applications. The 24-hour water absorption of 0.08–0.10% is the PBT property that most clearly differentiates it from nylon — PA6 and PA66 absorb 2–9% at equilibrium, which causes swelling and dimensional shift in humid or submerged service.
Where is PBT injection moulding used?
PBT dominates the electrical and automotive connector market because of its combination of dielectric performance, low moisture uptake, and — in glass-filled grades — heat resistance that nothing in the commodity thermoplastic range can match.
Automotive electrical connectors and housings: ECU connectors, harness connector bodies, relay sockets, fuse box components, and sensor housings. GF30 PBT with UL 94 V-0 flame retardancy is among the most widely specified materials for automotive connectors.
Consumer electronics connectors: USB connector bodies, HDMI housings, RJ45 socket frames, and PCB-mounted connector housings. GF30 PBT's HDT of approximately 205°C at 1.80 MPa, combined with its semi-crystalline structure, allows it to tolerate the brief peak excursion to 260°C in lead-free SMT reflow without permanent distortion — a critical requirement for surface-mount PCB components.
Electric motor components: Brush holders, commutator rings, motor end caps, and small motor housings. Dimensional stability under combined thermal and electrical load is the decisive requirement here.
Industrial sensors and meters: Body housings for temperature sensors, pressure sensors, flow meters, and proximity switches. Resistance to oils, hydraulic fluids, and mild cleaning agents makes PBT a practical choice for process-industry field instruments.
Appliance components: Dishwasher spray arms, washing machine internals, iron housings, and pump covers. Unfilled PBT serves here when surface finish and chemical resistance matter more than structural stiffness.
Lighting components: LED holder bodies, track-lighting connector frames, and luminaire bodies that must tolerate sustained proximity to heat sources.
What are the moulding characteristics of PBT?
PBT crystallises fast and has a sharp melt point. That combination rewards correct processing — short cycle times and dimensionally stable parts — but punishes incorrect drying or temperature management more severely than most engineering resins.
Melt temperature: 240–270°C. The processing window between the crystalline melt (220°C) and onset of thermal degradation (above 280°C) is narrower than for ABS or PC/ABS. Temperature uniformity across the barrel matters.
Mould temperature: 60–80°C. PBT needs a warm mould to complete crystallisation. Cold moulds produce amorphous skin layers that are dimensionally unstable and more susceptible to solvent attack — a particular problem on precision connector housings.
Injection pressure: 60–150 MPa. GF grades require higher pressures due to increased melt viscosity. Short shots are more common in GF-PBT than in unfilled grades; gate sizing and runner balance must account for this.
Drying: 3–4 hours at 120°C in a dehumidifying dryer. PBT hydrolyses at melt temperature in the presence of moisture, permanently reducing molecular weight and producing brittle parts. This is more damaging than moisture-related defects in ABS or SAN and makes pre-drying genuinely non-negotiable.
Shrinkage: 1.5–2.5% for unfilled PBT; 0.3–0.8% for GF30. Anisotropy between flow and transverse directions is significant in unfilled grades — a critical factor in tool design for connectors with precision pin spacing.
Draft angles: Minimum 1° on standard surfaces; 1.5–2° for GF grades where fibre orientation at the surface increases ejection friction. Textured surfaces require 2–3°.
Sink and warp: Unfilled PBT is prone to warp due to high and anisotropic shrinkage. Glass reinforcement dramatically reduces both. For precision connector housings, symmetric gating and balanced runner systems are standard practice.
Which PBT grades and variants should you consider?
| Grade / Variant | Key Feature | Typical Application |
|---|---|---|
| Unfilled PBT | Good surface finish, flexibility, chemical resistance | Appliance parts, decorative housings |
| PBT GF10 | Moderate stiffness increase, better aesthetics than GF30 | Enclosures where surface quality matters |
| PBT GF30 | High stiffness, HDT 200–210°C, dimensional stability | Connectors, SMT components, motor parts |
| PBT GF30 + FR (V-0) | Flame retardant, halogen-free options available | Electrical enclosures, switchgear |
| PBT GF30 + mineral fill | Reduced anisotropy vs GF alone | Precision connector bodies |
| Toughened PBT | Elastomer-modified for improved impact | Industrial connector covers, snap-fit housings |
| Hydrolysis-stabilised PBT | Improved hot-wet stability | Under-bonnet automotive, outdoor weathering |
Hydrolysis-stabilised PBT grades are specifically formulated for applications involving prolonged exposure to hot water, steam, or high-humidity heat cycling — conditions that degrade standard PBT molecular weight over time.
Advantages and limitations of PBT
Advantages:
- Excellent dielectric properties stable across humidity and temperature range
- Very low moisture absorption relative to nylon — parts hold pin spacing and dimensional tolerances in humid service
- Fast crystallisation means short cycle times on well-designed tools
- Good chemical resistance to oils, fuels, hydraulic fluids, and dilute acids
- Wide range of GF and FR grades covering automotive, electrical, and SMT-compatible applications
Limitations:
- Unfilled PBT has low HDT at 1.80 MPa (50–60°C); glass reinforcement is required for virtually all structural applications
- High shrinkage anisotropy in unfilled grades demands careful tool compensation
- Susceptible to hydrolytic degradation under prolonged hot-wet exposure in standard grades; hydrolysis-stabilised grades required for those environments
- Notch-sensitive; internal corners must be radiused to prevent stress-concentration cracking
- Attacked by strong alkalis and hot concentrated acids
- Brittle in unfilled form at sub-zero temperatures
When to choose PBT over alternatives
PBT vs nylon (PA6/PA66): Use PBT when dimensional stability in humid conditions is the priority — connector pin spacing, sensor body tolerances, any application where moisture-induced swelling has caused field failures. Nylon is preferred for higher impact resistance, better sliding wear, and lower unfilled-grade cost.
PBT vs POM: Use PBT when electrical insulation or flame retardancy is needed alongside stiffness. POM is the better choice for precision sliding contact, low friction, and fatigue-resistant snap-fits where dielectric properties are irrelevant.
PBT vs PET: Both are polyesters; PBT crystallises faster and is the standard injection moulding choice. PET is used for moulding only in specific nucleated grades and is more commonly processed by ISBM for packaging.
PBT vs LCP: LCP is preferred when HDT above 270°C, walls below 0.5 mm, or extreme flow-length-to-wall ratios are required. LCP costs significantly more. PBT is the appropriate default for standard connector and sensor housing applications.
Nordmould regularly recommends glass-filled PBT to customers converting from nylon in connector or sensor housings where humidity-driven dimensional variation has caused field failures.
Is PBT recyclable?
PBT is a thermoplastic polyester (resin code 1 group relates to PET; PBT is coded 7 — Other) and is technically recyclable. Industrial recycling of PBT regrind is practised by large-volume connector manufacturers, where clean production scrap is re-blended with virgin resin. The hydrolysis sensitivity of PBT limits regrind content — excessive moisture during reprocessing causes molecular weight loss. Glass-filled grades can accumulate fibre length reduction through reprocessing, reducing impact performance of recycled content parts.
Frequently asked questions
What makes PBT suitable for electrical connectors?
PBT has excellent dielectric strength, low moisture absorption compared to nylon, and dimensional stability that doesn't change with humidity. These properties make it the dominant material for automotive and industrial electrical connectors, where consistent pin spacing and electrical isolation must be maintained across temperature cycles.
What is the shrinkage rate of PBT in injection moulding?
Unfilled PBT shrinks 1.5–2.5%, which is high and anisotropic — flow-direction shrinkage differs from transverse shrinkage. Glass-filled PBT (30% GF) reduces shrinkage to 0.3–0.8% and substantially improves isotropy. Nordmould designs tool compensation based on the specific grade and part geometry.
Does PBT absorb moisture like nylon?
PBT absorbs significantly less moisture than nylon (PA6 or PA66). Equilibrium moisture absorption for PBT is around 0.3–0.5% versus 2–9% for nylons. This makes PBT dimensionally stable in humid environments and reduces the urgency of pre-drying compared to nylon, though drying is still recommended before moulding.
What is the heat resistance of PBT?
Unfilled PBT has a heat deflection temperature of 50–60°C at 1.80 MPa — modest, but adequate for non-structural applications. Glass-filled PBT (30% GF) raises HDT to approximately 205°C at 1.80 MPa. This is below the 260°C peak of lead-free SMT reflow, but PBT GF30's semi-crystalline structure and the brief duration of the peak exposure allow it to pass through reflow without permanent distortion in most PCB-mount connector designs. Grade-specific SMT suitability should be confirmed at DFM.
Can PBT be used in automotive engine-bay applications?
Glass-filled PBT is widely used in automotive engine-bay and underbonnet applications: connector housings, sensor bodies, relay bases, and small motor housings. It meets the combined requirements of heat resistance, dimensional stability, and chemical resistance to oils and coolants that these environments demand.
How does PBT compare to POM for precision mechanical parts?
POM (acetal) is preferred for sliding contact, precision gears, and parts requiring low friction and high fatigue life. PBT is preferred when electrical isolation, heat resistance, or chemical resistance to hydrocarbons and oils matters more than tribological performance. Both are semi-crystalline with high stiffness.
What wall thickness is recommended for PBT injection moulding?
A wall thickness of 1.5–3.5 mm is standard for PBT. Walls below 1.0 mm risk incomplete fill, particularly in glass-filled grades with higher viscosity. Walls above 4.5 mm increase sink mark risk and extend cycle times. Nordmould reviews wall thickness during DFM.
What is the minimum order quantity for PBT parts at Nordmould?
Nordmould's minimum order is 100 pieces across all materials including PBT and glass-filled PBT variants. Tooling starts from €3,000, with aluminium bridge tooling available for low-to-mid volumes.
Send your STEP file to Nordmould for a free DFM review and written quote — typically returned within one business day.