POM — Polyoxymethylene (Acetal)
Polyoxymethylene (Acetal)
POM (polyacetal) is a semi-crystalline thermoplastic offering excellent slip, high fatigue resistance and remarkable dimensional stability. It is the reference for precision mechanical parts (gears, clips, guides), but its narrow thermal window demands rigorous process control.
- Mold temperature: 80-120°C
- Melt temperature: 190-220°C
- Shrinkage: 1.8-2.5%
- Density: 1.41-1.42
- Manufacturer grades: Delrin 500P (DuPont), Hostaform C9021 (Celanese), Ultraform N2320 (BASF)
Processing
POM is barely hygroscopic: light drying is optional. The thermal window is narrow (190 to 220 °C): never exceed 220 °C or extend residence time, otherwise degradation releases formaldehyde (gas, black specks, burn marks). Regulate the mold between 80 and 120 °C and compensate for its high shrinkage (1.8 to 2.5 %) with careful holding to limit sink marks and warpage.
Advantages
- Excellent slip and low coefficient of friction
- Very good fatigue and wear resistance
- Remarkable dimensional stability (low moisture uptake)
- Good resistance to hydrocarbons, solvents and flexural fatigue
Limitations
- Narrow thermal window: degradation and formaldehyde above 220 °C
- High shrinkage (1.8 to 2.5 %) prone to sink marks and warpage
- Low adhesion: bonding and painting difficult without treatment
- Sensitive to strong acids and to UV without stabilizer
Common injection molding defects
- Sink marks — High shrinkage: increase holding pressure and time, review rib thickness
- Black specks — Thermal degradation: lower melt temperature below 220 °C and purge the barrel
- Burn marks — Overheating or excessive residence time: lower temperature and purge regularly
Typical applications
Gears, bearings, springs, clips, precision parts