POM / Acetal Injection Moulding: Properties & Design Guide
POM (polyoxymethylene), also known as acetal, is the material of choice for components that need to move, slide, or snap together without lubrication. It combines high stiffness, low friction, and excellent dimensional stability—particularly in humid environments where Nylon would swell and lose tolerance. The typical application list reads like a catalogue of precision mechanical parts: gear trains, slide rails, snap clips, valve bodies, and pump impellers.
Mechanical and Thermal Properties
POM is highly crystalline, which gives it its characteristic rigidity and spring-like behaviour. That same crystallinity produces high mould shrinkage and a narrow processing window compared to amorphous materials like ABS.
| Property | POM Homopolymer | POM Copolymer | Test Standard |
|---|---|---|---|
| Tensile Strength | 65–70 MPa | 60–65 MPa | ISO 527 |
| Flexural Modulus | 2,700–3,200 MPa | 2,500–2,900 MPa | ISO 178 |
| Izod Impact (notched, 23°C) | 70–120 J/m | 60–110 J/m | ISO 180 |
| Heat Deflection Temp (1.8 MPa) | 100–110°C | 95–110°C | ISO 75 |
| Vicat Softening Point | 150–165°C | 155–175°C | ISO 306 |
| Density | 1.41–1.43 g/cm³ | 1.40–1.42 g/cm³ | ISO 1183 |
| Mould Shrinkage | 1.8–2.3% | 2.0–2.5% | ISO 294-4 |
| Friction Coefficient (vs steel) | 0.10–0.35 | 0.10–0.35 | ASTM D1894 |
Water absorption for both grades is only 0.2–0.3% (24 h). Parts won't swell or lose stiffness when the ambient humidity changes—a critical advantage over Nylon in anything that needs to hold a close fit.
Note that POM continues to crystallise slowly after ejection. Dimensional changes can continue for up to 24–48 hours at room temperature, which matters for very tight-tolerance parts measured immediately after moulding.
Common Applications
Precision Gears and Cams: Fatigue resistance under cyclic loading makes POM the first choice for plastic gears. It runs quietly and outlasts most alternatives in dry-running conditions.
Sliding Parts: Conveyor links, drawer slides, and zip sliders. The material's natural lubricity reduces wear without grease—important in food-contact machinery where lubricants are undesirable.
Snap-Fits and Clips: Buckles, automotive fasteners, and cable clips. POM's elastic recovery is excellent; it snaps back to shape after deflection, which is why it survives repeated assembly and disassembly where other plastics fatigue and crack.
Fluid Handling: Pump impellers, valve bodies, and showerheads. POM resists most fuels, oils, and weak acids, which explains its ubiquity in automotive fuel systems and domestic water fittings.
Processing and Design Considerations
POM needs discipline at the press. It has a narrow processing window and degrades quickly if left at temperature too long.
- Melt Temperature (190–230°C): Homopolymer grades tolerate up to 230°C; copolymers are safer kept to 210°C. Beyond these limits, formaldehyde gas and surface silvering result. Unlike PC or Nylon, POM gives very little warning before the degradation becomes visible.
- Mould Temperature (60–120°C): Higher mould temperatures reduce crystallisation stress and improve surface quality. Running too cold amplifies warping in flat parts.
- Ventilation: Formaldehyde risk is real. Moulding areas must be well-ventilated and barrel temperatures monitored precisely. Mould vents should be cleaned every 4–8 hours of production as white deposits of formaldehyde oligomers accumulate.
- Cooling uniformity: With ~2% shrinkage, any imbalance in cooling across a flat part will cause bow. Central gating and symmetrical cooling channels are important for panels and plates.
- Wall Thickness: Keep walls consistent. Ribs should not exceed 50% of the main wall thickness; the opposite side will show a sink mark if you go heavier.
- Draft Angles: Minimum 1.5°. POM's high shrinkage causes it to grip cores tightly as it cools—underestimating draft leads to drag marks even though the material is naturally slippery.
Material Grades
| Grade / Variant | Key Feature | Typical Use |
|---|---|---|
| Homopolymer | Highest stiffness | High-torque gears |
| Copolymer | Thermal and chemical stability | General mechanical parts |
| PTFE-Filled | Ultra-low friction | High-wear bearings |
| Glass-Reinforced | Structural stiffness | Pump housings |
| UV-Stabilised | Weather resistance | Outdoor clips, handles |
| Food-Grade | FDA/NSF compliant | Water fittings, food prep |
PTFE-filled grades can reduce the friction coefficient against steel to around 0.05—suitable for dry-running bearing surfaces where even POM's natural lubricity is not enough.
Advantages and Trade-offs
Why choose POM:
- Excellent wear and abrasion resistance without added lubrication.
- High fatigue endurance under cyclic stress.
- Dimensional stability in wet and humid environments.
- High stiffness without glass reinforcement (saves the abrasive wear on tooling).
What to watch for:
- High shrinkage: Holding ultra-tight tolerances on large parts is harder than with amorphous materials. Compensate in the tool design and allow stabilisation time.
- Bonding: Very difficult to adhesive-bond. Mechanical fasteners or ultrasonic welding are the standard joining methods.
- UV sensitivity: Standard grades yellow and embrittle in direct sunlight. Specify UV-stabilised grades for outdoor use.
- Chemical resistance limits: Strong acids (including bleach) and some oxidising agents attack POM. Check compatibility before specifying cleaning agents.
POM vs Nylon (PA6/66)
- Choose POM for parts that must stay the same size regardless of humidity, or for sliding wear applications.
- Choose Nylon for better impact resistance or if the part needs to survive temperatures above 110–120°C continuously.
Sustainability
POM carries resin code 7. Industrial regrind is common; up to 20% regrind can typically be reincorporated in non-critical components, provided the material has not been thermally degraded during the previous run.
Nordmould provides orders from 100 pieces with tooling starting at €3,000. Send us your STEP file for a free DFM review—we'll check wall thicknesses and help you choose the right POM grade for your application.