Key takeaways
- PA66 is tougher and more forgiving of impact, but it absorbs moisture and changes dimension.
- PBT holds dimension and electrical properties more steadily in humid conditions.
- The tracking index and flame rating belong to the grade and wall thickness, not to the polymer name.
- Choose on the worst condition the housing will see, then confirm with a conditioned test sample.
What the material choice actually decides
PA66 and PBT are both semi-crystalline engineering thermoplastics used for breaker housings, terminal blocks, connectors, coil formers and equipment enclosures. On a data sheet at 23°C and 50% relative humidity they can look close enough to substitute. On a part that sees humidity, heat, a thin wall and a high tracking requirement, they behave differently enough to change the design.
The choice decides four things: how much the part moves after moulding, how it behaves when it is dropped or snapped, how well it resists tracking across a contaminated surface, and how forgiving the moulding process is. Cost and supply matter too, but they are secondary to a housing that fails its electrical test or will not fit its cover.
This comparison is about unfilled and glass-filled grades in general terms. A specific grade can move the answer, so the data sheet and a conditioned sample are part of the decision.
Moisture and dimensional stability
PA66 is hygroscopic. Unfilled grades can pick up around 1.5 to 2.5% moisture at 50% relative humidity, and more when immersed. The water plasticises the polymer and makes the part swell, so a dimension measured dry can grow after conditioning and a part that fits on the day of moulding may not fit a week later.
PBT absorbs far less water, typically 0.1 to 0.4% at 50% relative humidity for unfilled grades. The dimensional change is smaller and more predictable, which is why PBT is common for connectors and tight interfaces. Glass-filled grades reduce the movement further, but flow direction and gate position still matter.
The design response is different for each material. With PA66, specify the conditioned dimension, not the dry dimension, and state the conditioning condition on the drawing or in the inspection plan. With PBT, the risk is lower, but the part still shrinks during moulding and still warps if the wall is uneven.
Measure the part in the condition it will be used
A dry PA66 part and a conditioned PA66 part are not the same part. Agree the condition before the first article is measured.
Toughness, stiffness and temperature
PA66 has the edge in toughness and impact resistance, especially at low temperature and in unfilled grades. It also has good abrasion resistance, which suits hinges, latch features and parts that flex during assembly. The trade-off is lower stiffness than a glass-filled grade and a greater tendency to creep under a sustained load.
PBT is stiffer and has better creep resistance at moderate temperatures, but it is more notch sensitive. A sharp corner is more likely to crack in PBT than in PA66, so radius the corners and avoid abrupt section changes. Glass-filled PBT suits stiffness and dimensional stability.
Both polymers lose strength as temperature rises, and both are available in heat-stabilised grades. Check the continuous use temperature and the impact strength at the lowest service temperature. For a housing near a breaker or a busbar, the local temperature may be higher than the ambient figure on the drawing.
Tracking, insulation and flame rating
The comparative tracking index, or CTI, measures how well a surface resists forming a conductive path under contamination and moisture. It is not a single number for a polymer family. It depends on the grade, the filler, the colour and the surface condition. Some PA66 and PBT grades reach a high tracking index, and some do not. Ask for the CTI of the exact grade at the thickness and condition you will use.
Flame retardance works the same way. A UL 94 rating applies to a specific grade, colour and thickness. A V-0 rating at 1.6 mm does not automatically apply at 0.8 mm. If the housing has a thin wall, confirm the rating at that wall or choose a grade that is rated there.
Moisture changes electrical behaviour as well. PA66 that has absorbed water has lower surface resistivity and can track more easily than the dry data sheet suggests. PBT is less affected by humidity, which is one reason it is often chosen for high-voltage or high-tracking applications. Neither material removes the need for creepage and clearance distance; the material supports the design, it does not replace it.
| Requirement | PA66 | PBT | What it changes in the design |
|---|---|---|---|
| Moisture uptake | High, around 1.5 to 2.5% unfilled | Low, around 0.1 to 0.4% unfilled | PA66 needs conditioned dimensions and a moisture allowance |
| Dimensional stability | Moves with moisture and shrinkage | More stable, especially glass-filled | PBT suits tight cover and connector interfaces |
| Impact and toughness | Good, including at low temperature | Lower, more notch sensitive | PA66 suits latch and hinge features; PBT needs radii |
| Stiffness and creep | Lower unfilled, good with glass | Higher and more creep resistant | PBT suits sustained loads and stiff walls |
| Tracking index | Grade dependent, affected by moisture | Grade dependent, less affected by moisture | Confirm CTI for the exact grade and condition |
| Flame rating | Grade, colour and thickness dependent | Grade, colour and thickness dependent | Check the rating at the actual wall thickness |
| Processing window | Higher melt and mould temperature | Lower melt and mould temperature | PA66 needs drying and hot moulds; PBT also needs drying |
Indicative ranges for unfilled grades. A specific grade, colour and wall thickness can move any of these values, so confirm with the material supplier and a conditioned test sample.
Processing and tool design
Both materials need drying before moulding, but for different reasons. PA66 absorbs moisture quickly and must be dried to a low moisture level to avoid hydrolysis and splay. PBT is also moisture sensitive, and a wet pellet can hydrolyse during melting, which reduces molecular weight and embrittles the part. The dryer, the dew point and the residence time belong in the process sheet.
PA66 typically runs at a higher melt temperature and a hotter mould than PBT. A hot mould improves crystallinity and reduces the difference between flow and cross-flow shrinkage, but it adds cycle time. PBT runs cooler and often cycles faster, which can matter at high volume.
Shrinkage is higher for unfilled PA66 than for unfilled PBT, and glass-filled grades shrink less but warp more. The gate, wall section and cooling decide whether shrinkage becomes a dimension problem or a warp problem, so a flow study is worth more than a shrinkage table.
Cost, supply and the grade you can actually buy
PA66 and PBT move on different raw material chains, so their relative price is not fixed. PA66 is linked to adiponitrile and hexamethylene diamine, and its price has been volatile when supply is tight. PBT follows a different chain, so one year of prices can mislead over a five-year programme.
The more practical question is which grade is available in the colour, flame rating and filler content the part needs. A specialised flame-retardant grade may have a longer lead time or a higher minimum order than a standard grade. Ask for the grade certificate, the UL yellow card reference and the supplier's moisture and shrinkage data before the tool is cut.
Do not choose a polymer to save on the pellet price if it adds a conditioning step, a longer cycle or a higher reject rate. The part cost includes drying, cycle, scrap and inspection, not just the material.
A practical way to choose
Start with the worst condition. If the housing will sit in high humidity and must hold a tight interface, PBT has the advantage. If it will take an impact or flex during assembly, PA66 is more forgiving. If tracking is severe, compare the CTI of the candidate grades at the intended wall thickness and in the conditioned state.
Then check the process. Confirm that the moulder can dry the material to the specified level, hold the mould temperature and control the cycle. Ask for a conditioned sample and measure the critical dimensions after the agreed conditioning period. For a flame-rated part, confirm the rating at the wall thickness and colour, not just the grade name.
Finally, run a small design of experiments if the decision is close. Mold two or three candidate grades into the same tool, condition them together and compare dimensions, impact and tracking. That evidence is more useful than a data sheet argument, and it gives the programme a defensible record for the material choice.
Frequently asked questions
Is PBT better than PA66 for electrical housings?
Not in every case. PBT is more dimensionally stable and less affected by moisture, which suits tight interfaces and humid conditions. PA66 is tougher and more forgiving of impact, which suits latches and parts that flex. The right choice depends on the worst condition the housing will see.
Does PA66 change dimension after moulding?
Yes. PA66 absorbs moisture, and a moulded part can grow after conditioning. Specify the conditioned dimension and agree the conditioning condition before the first article is measured. Glass-filled grades move less, but flow direction and gate position still matter.
Can I use the UL 94 rating from the data sheet at any wall thickness?
No. A UL 94 rating applies to a specific grade, colour and thickness. A V-0 rating at 1.6 mm may not apply at 0.8 mm. Confirm the rating at the wall thickness you are using and keep the yellow card reference in the material record.
Do both materials need drying before moulding?
Yes. PA66 and PBT are both moisture sensitive. PA66 can hydrolyse and splay if it is too wet, and wet PBT can lose molecular weight during melting. The dryer, dew point and residence time should be specified in the process sheet and checked at the machine.
Sources and further reading
- ISO 62:2008, Plastics, determination of water absorption
- IEC 60112, Method for the determination of the proof and the comparative tracking indices of solid insulating materials
- UL 94, Standard for safety of flammability of plastic materials
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