Polyoxymethylene (POM), also known as acetal, is a thermoplastic material widely used in precision engineering due to its high stiffness, low friction, and excellent dimensional stability. Ultrasonic welding is a popular method for joining POM components, but achieving consistent, high-quality welds requires careful material preparation. This guide outlines essential preparation tips to optimise your ultrasonic welding process for POM.
1. Ensure Proper Material Selection
Not all POM formulations behave the same during ultrasonic plastic welding. POM comes in two main types:
Homopolymer POM – Offers higher strength and stiffness but is more prone to degradation.
Copolymer POM – Provides better thermal stability and resistance to degradation, making it a preferred choice for ultrasonic welding.
If your application involves high mechanical stress, consider testing different grades of POM to determine which performs best under welding conditions.
2. Control Moisture Levels
POM is relatively resistant to moisture absorption, but even small amounts of moisture can cause welding inconsistencies. Trapped moisture can lead to:
Poor energy transfer
Bubble formation in the weld
Weak weld joints
How to Manage Moisture:
Store POM in a dry environment, ideally in sealed containers.
If the material has been exposed to humidity, dry it at 60°C for 2–4 hours before welding.
3. Ensure Clean, Contaminant-Free Surfaces
Surface contamination is one of the primary reasons for weak ultrasonic welds. Any dust, grease, or residues on the material can interfere with the welding process.
Cleaning Methods:
Wipe surfaces with isopropyl alcohol before welding.
Use compressed air to remove dust particles.
Avoid handling welding surfaces directly to prevent oil transfer from skin.
4. Optimise Part Design for Welding
The design of POM components significantly impacts the effectiveness of ultrasonic welding. Key factors include:
Joint Design
Selecting the correct joint geometry ensures even energy distribution and strong bonds. Common joint designs include:
Energy Director (ED) – A triangular ridge along the joint interface that helps concentrate ultrasonic energy for efficient melting.
Shear Joint – Recommended for airtight and watertight seals, as it promotes even material flow.
Step Joint – Suitable for aligning parts accurately, reducing flash and excessive material flow.
Wall Thickness
Maintain uniform wall thickness to prevent uneven heating and material deformation during plastic welding.
5. Check Material Compatibility
If welding POM to another plastic, ensure the materials are compatible. POM typically welds best to itself but may also bond with other thermoplastics under specific conditions.
Use a compatibility chart to verify which materials can be welded together or perform test welds to assess bond strength.
6. Precondition the Parts
If POM parts have been stored in fluctuating temperatures, precondition them to bring them to a stable room temperature before welding. Sudden temperature changes can lead to material shrinkage or expansion, affecting weld consistency.
Allow parts to sit in a controlled environment at 20–25°C for at least 1 hour before welding.
7. Align Parts Properly Before Welding
Misalignment can cause poor contact between surfaces, leading to weak welds. Before starting the welding process:
Ensure the parts fit together precisely in the fixture.
Use clamping tools or alignment jigs to maintain consistent positioning.
Avoid any gaps between parts that could result in uneven welding.
8. Inspect and Maintain Welding Equipment
Your ultrasonic welding machine must be in optimal condition to achieve consistent welds. Regular maintenance includes:
Sonotrode Inspection – Ensure the sonotrode is clean and undamaged.
Horn Calibration – Misaligned or worn horns can cause uneven energy distribution.
Fixture Maintenance – Check that holding fixtures are correctly adjusted for each batch.
9. Perform Test Welds
Before beginning full production, conduct test welds to assess weld quality. Look for:
Consistent weld strength – Check for mechanical integrity.
No voids or defects – Inspect the weld interface under magnification.
Appropriate energy settings – Fine-tune amplitude, pressure, and time settings if needed.
10. Document Welding Parameters
Keeping a record of successful welding parameters ensures repeatability in future production runs. Document settings such as:
Amplitude
Welding time
Clamp force
Temperature
Any environmental conditions
Having this data on hand makes troubleshooting easier if inconsistencies arise in later welds.
Final Thoughts
Successful POM ultrasonic welding starts with proper material preparation. By selecting the right POM grade, controlling moisture, ensuring clean surfaces, and optimising part design, you can achieve strong and reliable welds. Consistently monitoring your process and making necessary adjustments will further improve welding efficiency and product quality.
