Last updated: 2026-09-05 · 5 min read

PBT Injection Molding — When Does This Polyester Beat Nylon for Electrical Connectors and Automotive Under-Hood Parts?

Polybutylene terephthalate (PBT) is a semi-crystalline polyester with a melting point of 225°C, moisture absorption of only 0.09% at equilibrium (vs. 2.5% for PA66) and a resin cost of $3–5/kg — roughly half the price of PA66. It is the default material for electrical connectors (CTI >250V, arc resistance 130s), automotive sensor housings and appliance components where dimensional stability in humid environments matters more than impact strength.

Key Facts

  • PBT absorbs 30× less moisture than PA66 (0.09% vs. 2.5% at 23°C/50% RH). This means PBT parts do not swell, soften or lose dimensional accuracy in humid environments — a connector molded at ±0.05mm in the factory stays at ±0.05mm after six months in a tropical climate. PA66 parts, by contrast, absorb moisture and shift dimensions by 0.5–1.5% — enough to break a press-fit connection.
  • PBT has excellent electrical insulation properties: dielectric strength 25 kV/mm, CTI (Comparative Tracking Index) >250V for unfilled grades and >600V for mineral-filled grades. These values make PBT the default material for electrical connectors rated for 250V and above — PA66’s CTI drops to 175–225V in humid conditions because the absorbed moisture conducts current on the surface.
  • PBT processes at lower temperatures than nylon (melt 240–270°C vs. 270–290°C for PA66) and does not require drying as strict (120°C for 2–3 hours vs. 80°C for 4 hours for PA66). The mold temperature is 60–80°C — standard cooled mold, no heating required. Cycle time is 5–10% shorter than PA66 because the crystallization is faster at lower mold temperatures. See the PA6 vs PA66 comparison for when nylon is the better choice.

How Does PBT Compare to PA66 and PPS for Electrical and Automotive Applications?

Parameter PBT GF30 PA66 GF30 PPS GF40
Melt temperature 225°C 260°C 285°C
Processing melt 240–270°C 270–290°C 310–350°C
Mold temperature 60–80°C 60–80°C 130–160°C
Moisture absorption 0.09% 2.5% 0.02%
CTI (unfilled) >250V 175–225V >250V
Resin cost $3–5/kg $4–7/kg $10–18/kg

CTI (Comparative Tracking Index) — the maximum voltage at which a plastic surface resists electrical leakage when contaminated with conductive fluid. Higher CTI = safer for high-voltage connectors. PBT unfilled: >250V; mineral-filled: >600V. PA66 in humid conditions drops to 175–225V because absorbed moisture conducts on the surface. See the CTI deep-dive for how CTI drives material selection across connector standards.

PBT wins on moisture resistance and cost vs. PA66. PPS wins on chemical resistance and continuous use temperature vs. PBT. For electrical connectors in humid environments at ambient temperature, PBT is the standard choice. For under-hood parts exposed to coolant or brake fluid, PPS is required.

What Three Design Rules Govern PBT Parts for Electrical Connectors?

  1. Wall thickness: 0.8–2.5mm for connectors, uniform within 0.3mm transitions. PBT crystallizes quickly at 60–80°C mold temperature — thin walls (below 0.6mm) freeze before the cavity is full, requiring injection pressures above 120 MPa. Thick walls (above 3.0mm) develop internal voids because the core crystallizes before the skin seals. For connector housings with varying wall thickness, the transition must be gradual (3:1 ratio) to prevent warpage from differential shrinkage.
  2. Gate design: pin-point gates for connectors, fan gates for housings. Connector pins require small pin-point gates (0.5–0.8mm diameter) that leave minimal vestige on the contact surface. Fan gates (width 3–5mm, depth equal to wall) are used for connector housings where the gate vestige is on a non-cosmetic surface. Submarine gates (tunnel gates) are used when the gate must be automatically trimmed during ejection — the gate breaks off at the parting line as the part ejects. The gate type guide covers the full decision tree for each gate geometry.
  3. Glass-filled grades for connector stiffness — 30% glass is the standard. Unfilled PBT has a flexural modulus of 2.5 GPa — too flexible for connector housings that must withstand insertion forces of 30–80N without deflecting. Glass-filled PBT (30% glass) reaches 9–12 GPa modulus — stiff enough for thin-wall connectors (1.0–1.5mm wall) without adding weight. The glass fiber also reduces shrinkage from 0.002 to 0.0005 mm/mm, critical for connector pitch accuracy (0.4–1.0mm pin spacing).

PBT: semi-crystalline polyester, Tm 225°C, moisture 0.09% (30× less than PA66), CTI >250V. Molding: melt 240–270°C, mold 60–80°C (standard cooled), dry 120°C/2–3h. Resin cost $3–5/kg (half PA66). GF30 standard for connectors: modulus 9–12 GPa, shrinkage 0.0005 mm/mm.

  • Moisture at equilibrium confirmed — PBT: 0.09% (no swelling in humid environments); PA66: 2.5% (press-fit connections at risk).
  • CTI matches connector rating — unfilled PBT >250V for standard connectors; mineral-filled >600V for high-voltage applications.
  • GF30 for stiffness — modulus 9–12 GPa; connector housings must withstand 30–80N insertion force without deflection.
  • Mold temp 60–80°C — standard cooled mold; below 50°C causes under-crystallization and post-mold cracking.
  • Stress-crack-resistant grade for brass pins — specify “hydrolysis stabilized” PBT when mating with brass or bronze press-fit pins.

Frequently Asked Questions

Why does PBT sometimes crack during assembly with press-fit pins?

Two causes: residual stress from molding or stress cracking from the pin material. If the mold temperature is too low (below 50°C), the PBT does not crystallize fully and retains frozen-in stress that releases when the press-fit pin creates a hoop stress concentration. The fix: raise the mold temperature to 60–80°C to ensure full crystallization before ejection.

The second cause is stress cracking from brass or bronze pins — certain brass alloys (with high zinc content) catalyze hydrolysis of the PBT ester bond in the presence of moisture. The fix: use nickel-plated pins or specify a stress-crack-resistant PBT grade (typically labeled “hydrolysis stabilized”).

Can PBT be welded with PA66 or other resins?

PBT can be ultrasonic welded to itself (20 kHz, energy director 0.3–0.5mm) and to PC using compatible processing parameters. PBT cannot be welded to PA66 — the two resins have different melting points (225°C vs. 260°C) and different crystallization rates, so the weld interface is weak and fails under load. For multi-material assemblies, mechanical fasteners or adhesive bonding are preferred over welding.

Solvent bonding of PBT is limited — the resin resists most solvents (which is why it is used for chemical-resistant housings). THF (tetrahydrofuran) can soften PBT for bonding, but the joint strength is typically 30–50% of the base material strength.

What is the difference between PBT and PET?

PBT and PET are both polyesters, but PBT has a shorter polymer chain (4-carbon butylene glycol vs. 2-carbon ethylene glycol in PET). This makes PBT crystallize faster (better for injection molding cycle time), flow more easily into thin walls and process at lower temperatures (240–270°C vs. 270–290°C for PET). PET is used primarily for bottles and fibers; PBT is the engineering-grade polyester for molded parts.

PET has slightly higher tensile strength and better barrier properties (for gas and moisture), but its slower crystallization rate makes it less suitable for injection molding — the cycle time is 15–25% longer than PBT for equivalent part geometry.

Key Takeaways

  • PBT: Tm 225°C, moisture 0.09% (30× less than PA66), CTI >250V.
  • Molding: melt 240–270°C, mold 60–80°C, standard cooled — no heating needed.
  • GF30 for connectors: modulus 9–12 GPa, shrinkage 0.0005 mm/mm.
  • Cost $3–5/kg: half PA66, one-third PPS. Default for electrical connectors.

Send the connector drawing with the pitch, insertion force and environmental requirements — the quotation evaluates whether PBT GF30 meets the stiffness and CTI requirements, or whether PA66 or PPS is needed for higher temperature or chemical exposure conditions.

Sources: DuPont — Crastin PBT Resin Technical Data Sheets and Connector Design Guidelines · BASF — Ultradur PBT Product Range and Electrical Application Notes · Plastics Technology — PBT Injection Molding Parameters and Troubleshooting · Society of Plastics Engineers (SPE) — Polyester Resin Processing Guidelines · MatWeb — PBT (Polybutylene Terephthalate) Material Property Database

— Arthur, Tooling Engineer at aCatMould