The problem
A Tier-1 supplier molded a 32-position connector housing for an underhood application in PA66 with 30% glass fiber — the industry default for decades. Three field issues accumulated: terminal position drift after humidity cycling, rejected lots whenever molding happened without full drying, and slow cycles caused by PA66's moisture conditioning requirements. The connector sealing interface holds ±0.05 mm tolerances, and PA66's equilibrium moisture absorption (~2.5% at 50% RH) was moving dimensions seasonally.
Why PA66 struggled here
- Moisture absorption changes dimensions up to 0.4% between dry-as-molded and conditioned states — unacceptable for multi-pin sealing geometry.
- PA66 parts must be conditioned before measurement and assembly, adding logistics and scrap risk.
- Underhood exposure to coolant glycol and road salts slowly attacks PA66 over years; hydrolysis risk grows with temperature.
- Longer cycle time: PA66 crystallizes slower and demands strict drying (4 h at 80–90°C) with narrow re-exposure windows.
The evaluation
The team compared PA66-GF30 against PBT-GF30 on the properties that actually governed the part: dimensional stability after humidity exposure, chemical resistance to coolant and salt spray, weld-line strength at the multi-gate housing, and crystallization speed for cycle time.
| Criterion | PA66-GF30 | PBT-GF30 |
|---|---|---|
| Moisture uptake @ 50% RH | ~2.5% | ~0.4% |
| Dimensional change (moisture) | up to 0.4% | < 0.05% |
| Coolant / glycol resistance | Moderate, long-term risk | Excellent |
| Weld-line strength retention | Good | Lower — gated design review needed |
| Cycle time (crystallization) | Slower | ~15% faster |
| Material cost | Baseline | +10–15% |
The decision
PBT-GF30 won because the governing requirement was dimensional stability, not peak strength. Two engineering actions closed the trade-offs: gate locations were re-optimized to keep weld lines away from the sealing ribs (recovering margin on weld-line strength), and the slightly higher material cost was offset by cycle-time savings and eliminating the conditioning step entirely.
Outcome
- Terminal position variation after 85°C/85% RH cycling dropped below measurement noise.
- Cycle time reduced ~15%; drying window became far more forgiving.
- Lot rejections tied to moisture conditioning eliminated.
- Total landed cost per connector neutral-to-positive despite higher resin price.
The takeaway: PA66 remains the right answer where toughness and heat dominate, but for precision connector geometry in wet, chemically exposed environments, low-moisture semi-crystalline resins like PBT frequently win the total-cost argument. If your connector is fighting humidity, ask us to benchmark PBT and PA grades against your tolerance stack.
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