Last updated: 2026-09-01 · 4 min read

PC vs PBT for Electrical Housings: Which Resin Fits the Application?

Polycarbonate (PC) and polybutylene terephthalate (PBT) are both used for electrical housings, connector bodies, and switch components — but they solve different problems. PC wins on impact strength and transparency; PBT wins on dimensional stability, chemical resistance, and cycle time. The selection depends on which failure mode the housing must prevent: cracking from drop impact (choose PC) or warping from solder-reflow temperatures and long-term chemical exposure (choose PBT).

Cycle time — The total time for one injection molding cycle: injection + packing + cooling + mold open/close + ejection. Typical range: 15–60 seconds. Cycle time determines production rate and cost per part — reducing cycle by 5 seconds saves 10–15% annual production cost. See also related guide.

Key Facts

  • PC has notched Izod impact strength of 600–850 J/m; PBT is 50–80 J/m unfilled and 80–120 J/m glass-filled. For housings that face drop impact, PC is the default.
  • PBT has a heat deflection temperature (HDT) of 200–220 °C at 1.8 MPa (glass-filled); unfilled PC is 130–140 °C. PBT survives lead-free solder reflow (260 °C peak) when glass-filled; PC does not.
  • PBT molding shrinkage is 1.5–2.0 % (unfilled) and 0.3–0.5 % (GF30); PC shrinkage is 0.5–0.7 %. PBT GF30 holds tighter dimensional tolerances in multi-cavity production.

    • Cycle time — The total time for one injection molding cycle: injection + packing + cooling + mold open/close + ejection. Typical range: 15–60 seconds. Cycle time determines production rate and cost per part — reducing cycle by 5 seconds saves 10–15% annual production cost. The related guide covers adjacent topics.

    Where Each Resin Wins

    Requirement PC PBT (GF30)
    Drop impact resistance 600–850 J/m — survives repeated drops 80–120 J/m — brittle at thin walls
    Solder reflow survival HDT 130–140 °C — deforms at 260 °C HDT 200–220 °C — survives reflow
    Chemical resistance (cleaning agents) attacked by ketones, esters, some alcohols resists most solvents, oils, dilute acids
    Dimensional stability (multi-cavity) shrinkage 0.5–0.7 % shrinkage 0.3–0.5 % (GF30)
    Cycle time longer cooling (amorphous, high Tg) faster cooling (semi-crystalline)
    Transparency (indicator windows) optically clear opaque

    What the Mold Design Changes Between PC and PBT

  • PC requires a mold temperature of 80–120 °C to prevent stress cracking and flow marks. PBT molds at 40–80 °C. The higher mold temperature for PC means longer cooling time and higher energy cost per cycle — typically 20–30 % longer cooling compared to PBT at equivalent wall thickness.
  • PBT is semi-crystalline and shrinks more than PC in unfilled form, but glass-filled PBT (GF30) shrinks less and more uniformly. The mold must be sized for the specific shrinkage rate of the exact grade — a PBT GF30 cavity is dimensioned differently from a PC cavity for the same nominal part.
  • PC is hygroscopic and must be dried to below 0.02 % moisture before molding. Inadequate drying causes hydrolytic degradation, which reduces molecular weight and produces brittle parts. PBT also absorbs moisture but is less sensitive — drying to 0.05 % is adequate for most grades.
  • Gate design differs: PC needs a larger gate (minimum 0.8 mm for edge gates) because of its higher melt viscosity. PBT flows more easily and can fill through smaller gates (0.5 mm pin-point is common). The gate vestige on PC is larger and may require secondary trimming for tight-tolerance housings.

    • When PC/PBT Blends Are the Answer

    PC/PBT blends (typically 40/60 or 60/40 ratios) combine the impact strength of PC with the chemical resistance and processability of PBT. The blend is amorphous-dominant and retains some transparency, with impact strength of 300–500 J/m and chemical resistance better than pure PC. The trade-off is that the blend’s heat deflection temperature (150–170 °C) falls between the two base resins — it does not survive solder reflow at 260 °C but handles wave soldering and hand-soldering temperatures without deformation.

    PC/PBT blends are common in automotive electrical housings (sensor connectors, fuse-box covers) where the housing faces both impact risk and moderate thermal exposure. The mold design follows PBT rules for shrinkage but PC rules for drying — the blend is hygroscopic and must be dried to 0.02 % moisture.

    Frequently Asked Questions

    Does PBT outgas during molding, and does it affect the mold?

    PBT releases tetrahydrofuran (THF) as a minor decomposition product at processing temperatures above 270 °C. THF is not corrosive to tool steel but can deposit on the cavity surface as a white film over thousands of shots. Regular cleaning every 10,000–20,000 shots removes the deposit. Vent slots should be sized at 0.02–0.03 mm depth for PBT to allow gas escape without flash.

    Can I switch from PC to PBT in an existing mold?

    Not without re-cutting the cavity. The shrinkage rates are different (PC 0.5–0.7 % vs PBT GF30 0.3–0.5 %), and the gate sizes are different. A cavity dimensioned for PC produces an oversized PBT part. The mold steel and cooling design are compatible, but the cavity dimensions must match the resin shrinkage.

    Which resin is better for snap-fit features?

    PC is significantly better for snap-fits because of its high elongation at break (100–150 % for unfilled PC vs 3–5 % for PBT GF30). Snap-fit arms in PBT GF30 must be designed with very low strain (below 1.5 %) to avoid brittle fracture during assembly. If the housing uses snap-fits for closure, PC or a PC/PBT blend is the safer choice.

    Key Takeaways

    1. PC is selected for impact resistance and transparency; PBT is selected for dimensional stability, chemical resistance, and thermal endurance. The failure mode that must be prevented determines the resin.
    2. PBT GF30 survives lead-free solder reflow (260 °C peak); PC does not. Any housing that passes through SMT assembly must use PBT or a high-temperature alternative.
    3. The mold is resin-specific — shrinkage rates, gate sizes, and mold temperatures differ between PC and PBT. Switching resins in an existing mold requires a cavity re-cut.

    The selection between PC and PBT is not a material-data-sheet exercise — it is a failure-mode exercise. The question is not “which resin has better properties” but “which failure mode is the housing most likely to encounter, and which resin prevents it.” A housing that faces drop impact and no solder reflow is PC. A housing that faces solder reflow and chemical exposure is PBT.
    A housing that faces both is a blend — or a redesign to separate the impact zone from the thermal zone.

    Sources
    Covestro — PC technical data
    BASF — PBT Ultradur data
    MatWeb — material property comparison

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