Ultrasonic welding: How I discovered the truth

The Challenge

How an ultrasonic weld initiates in glassy amorphous polymers such as polystyrene or PMMA, was a real puzzle to me for a long time. The materials should not heat up under the direct action of the ultrasound because they are just cyclically elastically compressed at 20 kHz. So I decided to see if there was any impact between the parts during the very early stages of the weld, or if they stayed together and were just compressed. I knew from high speed filming that the melt initiated within the first 10 milli seconds and so I needed a technique which could produce evidence of impact or not over a very short timescale.

The Switch

Using a silver conducting paint, I printed two tracks, one on each of the components to be welded. I connected wires to the tracks and assembled the parts so that the track on one, touched the track on the other. A battery and a large resistor completed the very simple circuit so that with the parts pushed together before welding, current flowed, and this could be seen on an oscilloscope connected across the resistor. Now if the parts separated at any time during the weld, this would be seen on the oscilloscope.

The Result

Asking the right question invariable delivers a good result and this one was rewarded at the first attempt, with a very clear indication that the impact between the parts accompanied the initial melting at the energy director.

Further Evidence

Adjusting the timebase on the oscilloscope and repeating the procedure on the next weld produced even more clear evidence of impact. The interval between switching events matched the 20 kHz excitation produced by the welder until the switch was destroyed by the melting polymer in the weld.

Outcome

A world first demonstration of the role of impact at the earliest stages of an ultrasonic weld in the glassy polymer PMMA leading to a model for energy dissipation based on plastic deformation at the tip of the energy director.

Reference

Wise R J: ‘Initiation of ultrasonic welding in amorphous thermoplastic polymers’, Conference on Joining Plastics, Dusseldorf, November 2010, DVS