Last updated: 2026-09-01 · 4 min read

What Does PBT Bring to a Molded Part — and When Does PET Make More Sense?

PBT (polybutylene terephthalate) is the semi-crystalline polyester you pick when the part needs dimensional stability at 120–150 °C and will sit in contact with oils or weak solvents. PET wins when the part has to go hotter, or when stiffness matters more than process forgiveness — PBT’s wider processing window is its main reason for being chosen over PET.

Cavity — The hollow space inside the mold that forms the part shape. Multi-cavity molds produce multiple parts per shot. Cavity count determines production output but also mold cost and complexity. Each cavity must be balanced for uniform fill. See also related guide.

Key Facts

  • Melt range 230–260 °C; mold temperature 60–90 °C for unfilled, 80–110 °C for glass-filled grades.
  • Unfilled PBT shrinks 1.5–2.0 %; glass-filled grades hold 0.3–0.7 % with directional bias.
  • Continuous service temperature approximately 120–140 °C; short peaks higher are documented for GF30.

    • Cavity — The hollow space inside the mold that forms the part shape. Multi-cavity molds produce multiple parts per shot. Cavity count determines production output but also mold cost and complexity. Each cavity must be balanced for uniform fill. The related guide covers adjacent topics.

    Why PBT Gets Chosen Instead of Other Engineering Resins

    PBT sits in the sweet spot between commodity resins and expensive polyaryletherketones. It processes easily on standard presses without the capital equipment that PEEK demands, and it survives conditions that would dissolve ABS or deform PA66 at elevated temperature. Typical applications include automotive connectors, small appliance housings, and electrical switch bodies — parts that are not glamorous but have to work for 15 years in warm, oily environments.

    The glass-filled grades (GF15, GF30) are where PBT becomes structurally interesting. A PBT GF30 bracket has modulus approaching that of die-cast zinc at a fraction of the weight, and the melt viscosity is low enough to fill thin walls that PC or PPO would refuse to flow into. The trade-off is notch sensitivity and hydrolysis — PBT parts that sit in hot, wet environments lose tensile strength over time, which is why dishwasher and under-hood applications usually specify a hydrolysis-stabilized grade.

    Grade Filler Typical Use Shrinkage
    Unfilled PBT none switch housings, small appliance 1.5–2.0 %
    PBT GF15 15 % glass fiber connector bodies, sensor housings 0.5–1.0 %
    PBT GF30 30 % glass fiber structural brackets, automotive under-hood 0.3–0.7 %
    PBT flame-retardant halogen-free FR electrical enclosures, terminal blocks 1.0–1.5 %
    PBT/PC blend impact modifier automotive exterior trim, low-temp toughness 0.8–1.2 %

    What the Mold Maker Has to Watch For

  • Dry the resin below 0.02 % moisture before every start — PBT hydrolyzes rapidly at melt temperature if moisture is present, and the part will fail tensile tests weeks after molding.
  • Use hardened steel for the cavity if production exceeds 50,000 shots in glass-filled grades; the fibers wear P20 noticeably after a few hundred thousand cycles.
  • Gate dimensions matter more than for most resins — PBT freezes quickly in thin sections, so gates smaller than the supplier minimum produce short shots and voids at the flow front.
  • Vent every predicted weld line; PBT weld strength is typically 60–75 % of bulk and needs aggressive venting to even that.
  • Ejector pin placement must avoid the cosmetic side — PBT marks easily and the 1.5–2 % shrinkage of unfilled grades creates enough grip on the core that a poorly placed pin will dimple the wall.

    • One failure mode worth flagging: a customer orders a PBT GF30 bracket, then later switches to a flame-retardant grade for an electrical application.
    The shrinkage difference (0.5 % vs 1.3 %) is enough to throw the cavity out of tolerance. The fix is to freeze the grade on the drawing before steel is cut.

    Frequently Asked Questions

    Should I use PBT or PET for a hot automotive part?

    Above 130 °C continuous, PET GF30 holds its modulus better. Below 120 °C, PBT processes faster and gives a more forgiving cooling window. Most automotive connectors run PBT; most engine-bay structural brackets run PET.

    Can PBT be overmolded onto metal inserts?

    Yes, but the insert should be preheated to within 40 °C of the mold temperature. Cold inserts produce amorphous skin at the interface that fails pull tests, and PBT’s hydrolysis sensitivity makes the interface a weak point in humid service.

    How do I know if a PBT part has hydrolyzed in service?

    A DSC trace of a hydrolyzed part shows reduced crystallinity and a lower melt peak compared to a fresh sample. Tensile testing will also show reduced elongation at break — that is usually the first field symptom.

    Key Takeaways

    1. PBT is the forgiving mid-range polyester — easier to process than PEEK, more durable in oil than ABS or PA66.
    2. Glass-filled grades deliver structural stiffness at a fraction of the weight of die-cast metal.
    3. Hydrolysis is the main failure mode in wet service; stabilized grades exist for dishwasher and under-hood applications.

    If the part runs below 140 °C and needs to survive in warm, oily conditions for years, PBT GF30 is usually the right call.
    If it runs hotter or needs the absolute stiffness ceiling, PET GF30 or a step up to PPS is worth the process difficulty — a good DFM review should do that comparison before steel is ordered.

    Sources
    Celanese — PBT product finder
    Prospector — plastics database
    MatWeb — material property data

    Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet