Introduction

Polyoxymethylene (POM), commonly known as acetal or Delrin, is a high-performance thermoplastic used in precision parts requiring high stiffness, low friction, and excellent dimensional stability. In manufacturing, ultrasonic plastic welding is a preferred joining method for POM due to its efficiency, speed, and ability to create strong bonds without adhesives. Understanding the molecular behavior of POM during ultrasonic welding is critical to optimising the process and ensuring the durability of the final product.

Molecular Changes During Ultrasonic Welding

1. Heat Generation and Polymer Softening

Ultrasonic welding relies on high-frequency mechanical vibrations to generate heat at the interface of two joining materials. POM, being a semi-crystalline polymer, experiences localized heating due to intermolecular friction and viscoelastic deformation. This heat causes the polymer chains in the amorphous regions to become more mobile, allowing them to entangle and fuse.

2. Melting and Crystallisation Effects

Unlike amorphous polymers, which soften gradually, POM has a distinct melting point (~175°C). During ultrasonic welding, the material quickly transitions from solid to molten state. The molten POM then undergoes recrystallisation upon cooling, which significantly affects the strength of the weld. Faster cooling rates can lead to a more brittle weld due to high crystallinity, while slower cooling allows for better stress relaxation and improved toughness.

3. Chain Alignment and Molecular Orientation

The rapid heating and cooling cycles influence the molecular orientation of POM chains. If plastic welding parameters such as amplitude and pressure are not optimised, excessive molecular alignment can occur, leading to anisotropic mechanical properties in the welded joint. Proper process control helps in maintaining a balanced microstructure that enhances bond strength.

4. Thermal Degradation and Oxidation Risks

POM is sensitive to thermal degradation at high temperatures, releasing formaldehyde when overheated. Prolonged exposure to temperatures beyond its decomposition point (~230°C) can lead to weakened welds and potential failure. Ensuring proper energy input and ventilation during welding minimises these risks.

Factors Influencing Weld Strength

1. Energy Input and Amplitude

Higher amplitude settings increase heat generation but may also cause excessive material degradation. Optimising amplitude and weld time is crucial for achieving a strong and uniform bond.

2. Pressure and Clamping Force

Insufficient pressure can lead to weak joints due to incomplete fusion, while excessive pressure may squeeze out too much molten material, reducing bond area. A balanced pressure setting ensures proper molecular interdiffusion.

3. Surface Preparation

Contaminants or oxidation on the POM surface can hinder molecular bonding. Cleaning and surface treatments, such as plasma activation, can enhance weldability by improving wetting and intermolecular adhesion.

4. Cooling Rate Control

Managing cooling rates through fixture design and environmental conditions can help achieve optimal crystallinity and mechanical performance in the weld zone.

Conclusion

Understanding the molecular behavior of POM during ultrasonic welding is essential for optimising process parameters and ensuring durable joints. By controlling factors like heat generation, crystallisation, and pressure, manufacturers can achieve strong, reliable welds with minimal defects. As ultrasonic welding continues to evolve, further research into molecular-level interactions will help enhance the quality and efficiency of POM-based assemblies.