POM (Acetal)
A strong, stable, low-friction engineering plastic
Typical values for the temper in the title
POM (polyoxymethylene, "acetal", trade name Delrin for the homopolymer) is one of the most common engineering plastics for mechanical parts. It is strong and stiff, absorbs very little water and is therefore dimensionally stable, with low friction and wear and good fatigue resistance.
It is used for gears, bearings and bushings, plastic springs and snap-fits, pump and valve parts, and precision machined parts. It is among the easiest plastics to machine.
Mechanical and physical properties
| Property | Value | Note |
|---|---|---|
| Tensile strength Rm | 70 MPa | |
| Elongation A | 25% | |
| Modulus of elasticity E | 3.1 GPa | |
| Density ρ | 1.41 g/cm³ | |
| Max continuous service temperature | 90°C | |
| Heat deflection temperature (HDT) | 110°C | |
| Machinability | 85% | relative rating in the site database |
Structure and family
| Feature | Value | Meaning |
|---|---|---|
| Type | semi-crystalline thermoplastic | strength and stability |
| Tm melting | about 165–175 °C (copolymer / homopolymer) | |
| Water absorption | low | dimensional stability |
Typical values per producer data sheets.
Grades and versions
In engineering polymers, the choice is between an unfilled grade, filled grades (glass or carbon fiber) and special grades (bearing, medical, food). Filler strongly changes strength, stiffness and expansion.
| Version | Properties | When to choose it |
|---|---|---|
| Homopolymer (POM-H) | stronger and stiffer | gears, loaded parts |
| Copolymer (POM-C) | more resistant to chemicals and hot water, less centerline porosity in thick rods | machined parts, food |
| PTFE-modified grades | lower friction | bearings |
Applications
- Gears
- Bearings and bushings
- Snap-fits and plastic springs
- Pump and valve parts
- Precision machined parts
Fabrication
Machining
Excellent: clean chips, stable dimensions. Among the best plastics.
Chemical resistance
Good to fuels and solvents. Strong acids and oxidizers attack it.
Thermal expansion
The expansion coefficient of an unfilled polymer is several times that of a metal. In a precision part or an assembly with metal, design clearances accordingly.
Compared with similar materials
| Material | Rm / Rp0.2 | Welding | When it is the better choice |
|---|---|---|---|
| PA66 (Nylon 66) | 70-85 MPa | Ultrasonic | PA66: more heat and impact resistant, absorbs water · material page |
| PBT | 55 MPa | Ultrasonic | PBT: stable, for electrical parts · material page |
| UHMWPE | 35 MPa | Not possible | UHMWPE: more abrasion resistant, weak and soft · material page |
Common mistakes
Using POM in a strong acid
It is attacked.
Overheating it beyond the processing temperature
POM decomposes and releases formaldehyde.
Using homopolymer thick rod for a pressure part
Thick homopolymer rods may have centerline porosity. Copolymer is better.
Equivalents, standards and supply
| ISO | POM |
|---|---|
| ASTM | D6778 |
| Standards | ASTM D6778 |
| Forms | plate · rod · pellets |
| Producers | DuPont Delrin · Celanese Hostaform · BASF Ultraform |
Learn more
Values are typical for an unfilled grade (unless stated otherwise) and for comparison only. In polymers, properties depend on grade, filler, temperature, moisture and load duration. For design, rely on the producer data sheet for the specific grade. Sources: ASTM classification standards (D8033, D5205, D5204, D6394, D4067 and others), producer data sheets, and the METAL HUB materials database.
Updated: October 31, 2026