Introduction

Material Selection

1. Thermoplastic vs. Thermoset Plastics:

   • Thermoplastics: These materials soften and melt when heated, making them ideal for ultrasonic welding. Common thermoplastics used in ultrasonic welding include ABS, SAN, acrylic, polystyrene, nylon, polyethylene, and polypropylene.
   • Thermosets: These materials do not melt when heated, making them unsuitable for ultrasonic welding.

2. Material Compatibility:

   • Amorphous Polymers: These materials, such as ABS and polycarbonate, are easier to weld ultrasonically due to their gradual softening over a temperature range.
   • Semi-Crystalline Polymers: Materials like polyethylene and polypropylene have a more defined melting point and are more challenging to weld. They generally require higher energy inputs and specific joint designs.

3. Dissimilar Plastics:

   • When joining parts made from different plastics, consider the glass transition temperature (Tg), chemical compatibility, and melt flow index (MFI). For successful bonding, the Tg of the two materials should be within 40°F, and they should have similar chemical structures and MFI values.

Joint Geometry

1. Butt Joint with Energy Director:

   • This is the most common joint design used in ultrasonic welding. It features a small triangular ridge (energy director) molded into one of the mating surfaces. The energy director focuses the ultrasonic energy at its apex, causing the plastic to melt and bond the parts together.
   • For amorphous polymers, the energy director height should be between 0.008" and 0.025" (0.2 and 0.6 mm). For semi-crystalline polymers, a height between 0.015" and 0.020" (0.4 and 0.5 mm) with a 60° included angle is recommended.

2. Shear Joint:

   • Shear joints are used for semi-crystalline materials that transition quickly from solid to liquid. The joint is formed by pushing the sidewalls of opposing parts together, creating a strong bond.

3. Step Joint:

   • This joint design involves overlapping the edges of the parts to be welded. It provides a larger contact area, which can improve bond strength and reduce the risk of misalignment.

4. Tongue and Groove Joint:

   • This design features a protrusion (tongue) on one part that fits into a corresponding groove on the other part. It provides excellent alignment and can improve bond strength.

Design Considerations

1. Contact Area:

   • Ensure sufficient contact area to generate adequate heat and achieve a strong bond.

2. Horn and Tooling:

   • The horn, which transfers ultrasonic energy to the materials, must be designed to match the joint configuration. Proper tooling design is crucial for efficient energy transfer.

3. Preload and Pressure:

   • Adequate pressure ensures that the materials remain in contact during the plastic welding process, facilitating efficient energy transfer and a strong bond.

4. Joint Geometry:

   • Focus on creating a joint geometry that supports the welding process. This includes designing energy directors, shear joints, or other configurations that facilitate efficient energy transfer and melting.

Conclusion