Material selection
POM vs PBT
Both are semi-crystalline engineering thermoplastics that hold a dimension well, and both are common alternatives to nylon where moisture movement is a problem. The difference that actually decides between them is whether the part's job is mechanical or electrical: one is chosen for how it slides and wears, the other for how it insulates when wet. The numbers below are the same published datasheet values used on each material's own page, set side by side.
| Acetal (POM) | PBT | |
|---|---|---|
| Density | 1.41 g/cm³ | 1.31 g/cm³ (1.53 GF30) |
| Tensile strength | 62–70 MPa | 55 MPa (135 GF30) |
| Flexural modulus | 2.6 GPa | 2.5 GPa (9.0 GF30) |
| Heat deflection (1.8 MPa) | 100 °C | 60 °C unfilled, 210 °C GF30 |
| Mold shrinkage | 1.8–2.5 % | 1.5–2.0 % unfilled |
| Water absorption | 0.2 % | 0.08 % |
| Relative cost | Medium | Medium |
Published values for unfilled grades at 23 °C unless stated, taken from each material's own datasheet page. Confirm against the supplier datasheet for the grade you specify.
What Acetal (POM) is
POM is the plastic to specify when a part has to hold a dimension and slide against something. It has the lowest coefficient of friction of the common engineering plastics, high stiffness at 100 °C heat deflection unfilled with no glass fill needed, and excellent fatigue and creep resistance, so it is the default for gears, bushings, cams, and snap-fit closures that get opened and closed thousands of times. The trade-off is that it is not a strong electrical insulator by design intent, has weak UV resistance without a stabilized grade, and bonds and paints poorly.
What PBT is
PBT is a semi-crystalline polyester chosen mainly for parts that carry current. It absorbs less than half the water POM does, so it holds its dielectric strength in humid conditions where POM's insulation would drift, and glass-filled grades survive reflow soldering temperatures that would distort an unfilled part. Unfilled PBT's stiffness and heat resistance both trail POM's, which is why almost every PBT connector or housing on the market is a glass-filled grade rather than the neat resin in the table above; add 30% glass and flexural modulus goes from 2.5 to 9.0 GPa and heat deflection from 60 to 210 °C. It also machines and slides worse than POM, so it is not the answer for a gear or a bearing surface.
POM against PBT
| Acetal (POM) | PBT | |
|---|---|---|
| Friction and wear | Lowest of the common engineering plastics, the default for gears and sliding contacts | Higher friction, not specified for wear surfaces |
| Electrical insulation in humid conditions | Adequate but not the focus; 0.2% water absorption lets dielectric properties drift | Purpose-built; 0.08% water absorption holds dielectric strength wet |
| Heat resistance, unfilled | 100 °C, no reinforcement needed | 60 °C unfilled; needs 30% glass fill to reach 210 °C for reflow soldering |
| Fatigue and repeated flexing | Excellent, the standard choice for snap-fits cycled thousands of times | Weaker, glass-filled grades are stiffer but more brittle under repeated flex |
| Dimensional stability | Very good, low shrinkage and low water absorption | Good unfilled, glass fill adds some anisotropic shrinkage |
| Relative cost | Medium | Medium, glass-filled grades cost more than unfilled |
Which one, for which part
- Gears, bushings, cams, rollers, and any part that slides against another surface is POM's case: it has the lowest friction of the common engineering plastics and holds up under repeated fatigue.
- Snap-fit closures cycled thousands of times favours POM for the same fatigue-resistance reason.
- Connectors, terminal blocks, and any part that carries current in a humid environment is PBT's case, almost always in a glass-filled grade: its low water absorption holds dielectric strength where POM's would drift.
- Surface-mount components that go through a reflow oven need glass-filled PBT specifically; unfilled PBT's 60 °C heat deflection will not survive that step, and POM is not rated as an electrical-grade insulator to begin with.
- A part that must hold a tight dimension without any glass reinforcement fits POM better; PBT's unfilled stiffness and heat resistance both trail it.
Our status
Trumould molds both POM and PBT in-house, including glass-filled PBT grades for connector and terminal-block work. Send the part, whether it carries current or slides against something, and target volume, and the quote states which material is recommended and why, not just a price for whichever one you asked about first.
Specifying this part?
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.