Polyoxymethylene (POM), commonly known as acetal or Delrin®, is an engineering thermoplastic widely used for precision components. Proper design of POM parts ensures optimal functionality, manufacturability, and cost-effectiveness. This guide covers the critical design specifications including wall thickness, draft angles, and tolerances for POM components.
1. Wall Thickness Recommendations for POM Components
Consistent wall thickness is crucial for POM parts to prevent defects and ensure dimensional stability:
- Standard wall thickness: 0.8mm to 3mm (0.031″ to 0.118″)
- Minimum wall thickness: 0.5mm (0.020″) for small parts
- Maximum wall thickness: 4mm (0.157″) without special considerations
- Ideal thickness range: 1.5-2.5mm (0.059″-0.098″) for most applications
Maintain uniform wall thickness throughout the part whenever possible. If variations are necessary, transition gradually (3:1 slope recommended) to avoid stress concentration and sink marks.
2. Draft Angle Specifications
Proper draft angles are essential for easy ejection from molds:
- Standard draft angle: 1° per side minimum
- Textured surfaces: Add 1° per 0.025mm (0.001″) of texture depth
- Deep ribs or tall features: 1.5° to 2° per side
- Critical dimensions: Minimum 0.5° with careful tool design
For internal features (holes, slots), increase draft angles by 0.5° compared to external surfaces. POM’s low coefficient of friction allows slightly smaller draft angles than many other plastics.
3. Tolerance Guidelines for POM Parts
POM offers good dimensional stability but requires realistic tolerance expectations:
Standard Tolerances (without post-machining):
| Feature Size | Tolerance |
|---|---|
| 0-25mm | ±0.1mm (±0.004″) |
| 25-100mm | ±0.15mm (±0.006″) |
| 100-200mm | ±0.2mm (±0.008″) |
Critical Considerations:
- Hole diameters: ±0.05mm (±0.002″) achievable for small holes
- Positional tolerances: ±0.1mm typical
- Angular tolerances: ±0.5° standard, ±0.25° possible with tool precision
- Allow 24-48 hours for complete dimensional stabilization after molding
4. Additional Design Considerations
For optimal POM component performance:
- Radii: Minimum 0.5mm radius on all corners to reduce stress
- Bosses: Keep height ≤ 2× diameter, use gussets for support
- Threads: Molded threads possible with ≥ 0.5mm pitch
- Shrinkage: Account for 1.8-2.5% shrinkage (varies by grade)
Conclusion
Following these POM design specifications for wall thickness, draft angles, and tolerances will result in components that are both functional and manufacturable. Always consult with your injection molding partner early in the design process, as specific POM grades (homopolymer vs. copolymer) and additives may require adjustments to these guidelines. Properly designed POM components offer exceptional mechanical properties, low friction, and long service life across countless industrial applications.
