Ultrasonic cleaning

Ultrasonic cleaners are efficient at removing a low level of dirt, oil and grease.

The film is transported through a tank of slightly heated solvent. Ultrasonic transducers are fitted to the outside of the tank and pulse sound waves at 20 KHz through the tank.

In air and most solids there is a continuous transition as the wave passes through. In most liquids there is continuous transition as long as the amplitude of the sound is relatively low.

As the amplitude increases the negative pressure in the area of rarefaction (that follows immediately behind the wavefront) becomes so great that the liquid can fracture or 'tear’. This phenomenon is known as 'cavitation’. Cavitation results in 'bubbles’ developing in the liquid. As successive waves pass through the liquid the bubbles oscillate and eventually grow to an unstable size and then collapse and finally implode violently (Fig 12.6).

Fig 12.6 Bubble formation due to cavitation

The implosion causes shock waves to be radiated through the liquid. It is the shock from the continuous imploding bubbles that moves the solvent around the tank. The constant movement provides a continuous supply of fresh solvent to attack the dirt.

Like all mechanised film transport equipment frequent maintenance is required to ensure that the films are not damaged during use.

Dirt adhering to rollers and incorrectly positioned jets or air knives can scratch the film. If the machine is fitted with air knives to remove the solvent as the film emerges from the cleaning tank these must be correctly aligned to ensure that the solvent is removed cleanly from the film surface and there are no streaks.

Regular maintenance also includes recovery of the cleaning solvent and replacement of the filters. Water separators and pH adjustment systems also need to be regularly checked.

The transport mechanism is relatively uncontrolled in terms of tension and the films are taken up at a very high tension. Prior to return to storage films must be rewound to a lower tension.