Thermoplastic
PBT (polybutylene terephthalate)
The connector plastic. Stable, insulating, and it barely takes up water.
Short answer: PBT is a semi-crystalline polyester with a density of 1.31 g/cm³ unfilled (1.53 GF30) and water absorption of only 0.08 %, which is why it keeps its electrical insulation in humid service. Unfilled it is a modest material (60 °C heat deflection); almost every structural or connector part uses a glass-filled grade, which raises tensile strength to 135 MPa and heat deflection to 210 °C.
Published values for unfilled grades at 23 °C unless stated. Confirm against the supplier datasheet for the grade you specify.
What it is
PBT is a semi-crystalline polyester chosen mainly for electrical parts. It holds its dielectric strength in humid conditions because it absorbs almost no water, and glass-filled grades survive reflow soldering temperatures.
Compared with nylon it is dimensionally stable out of the box, which matters when a connector has to hold pin positions to a tenth of a millimetre.
Where it is used
- Electrical connectors, housings, and switch bodies
- Automotive sensors and ignition components
- Appliance components exposed to heat and moisture
- Motor insulation and bobbins
- Under-bonnet electronics enclosures
Processing notes
Dry 3–4 hours at 120 °C. PBT hydrolyses readily.
Melt 240–265 °C, mold 60–100 °C. It crystallises fast, so fill quickly and gate near thick sections.
Fast crystallisation also means short cycles, which is part of why it is popular for high-volume connectors.
PBT data sheet: unfilled against GF30
The two columns most buyers choose between, using the same published values as the properties panel above. A data sheet for a named grade from the resin supplier is still the reference for the drawing.
| PBT, unfilled | PBT GF30 (30 % glass) | |
|---|---|---|
| Density | 1.31 g/cm³ | 1.53 g/cm³ |
| Tensile strength | 55 MPa | 135 MPa |
| Flexural modulus | 2.5 GPa | 9.0 GPa |
| Heat deflection (1.8 MPa) | 60 °C | 210 °C |
| Mold shrinkage | 1.5 to 2.0 % | Lower, and different along and across the flow because the glass fibres orient |
| Typical parts | Insulating parts with modest loads, housings, bobbins | Connectors, terminal blocks, sensor housings, under-bonnet parts |
PBT electrical behaviour
PBT is chosen for parts that carry current because its insulation does not drift when the air is wet: at 0.08 % water absorption it takes up less than half the water acetal does and a fraction of what nylon does, so pin spacing and dielectric strength stay where the drawing put them. Flame-retardant grades rated UL94 V-0, including halogen-free options, cover most connector and enclosure requirements.
For surface-mount connectors that pass through a reflow oven, LCP and PPS are common choices because both hold dimension at 260 °C better than PBT does. Glass-filled PBT is the usual answer for through-hole connectors, terminal blocks and sensor bodies. More on how these materials are chosen for connector work is on the electronics injection molding page.
How we mold PBT
Trumould molds PBT and glass-filled PBT in-house on its own presses. A few things we check on every PBT job:
- The resin is dried before molding, because PBT hydrolyses if it is processed wet, which weakens the molded part.
- The tool is cut for the grade: unfilled PBT shrinks 1.5 to 2.0 %, and a glass-filled grade shrinks differently along and across the flow, so the steel is sized for the resin named on the drawing. Tolerance planning by material is covered in the injection molding tolerances guide.
- UL94 V-0 grades run on request, with the resin and its certificate supplied by the buyer or sourced against a named grade.
Available through
Design notes
Unfilled PBT warps on flat panels because crystallisation shrinkage is high. Glass fill and correct gating fix it.
Design creep allowance into any snap fit that stays deflected in service.
Weld lines are weak in glass-filled grades. Place gates so they land outside the load path.
Grades and variants
- Unfilled, for insulating parts with modest loads
- GF15–GF30, the standard connector and structural grades
- Flame retardant V-0, halogen free options available
- Impact modified with elastomer phase
- Laser-markable grades for part identification
Specifying this material?
Send your CAD and target volumes. We come back with a price, a lead time, and any material or design notes that would reduce either.