The Ultrasonic Cleaning Process
An ultrasonic cleaning tank is simply a metal tank [stainless steel] that has pizeo ceramic transducers bonded to the bottom or side. These transducers have a unique property of changing size almost instantly when excited by an electrical signal. When excited the transducer increases in size and causes the tank bottom or side to move. This creates a compression wave in the liquid of the tank.
By using an electrical generator that puts out a high frequency signal [20 to 50 kHz] the transducer rapidly induces compression and rarefaction waves in the liquid. During the rarefaction cycle the liquid is torn apart. This creates a vacuum cavity within the liquid. These cavities will grow larger and smaller as the compression waves are continued. When the cavity reaches a certain size [based on the frequency and the wattage of the signal] the cavity can no longer retain its shape. The cavity collapses violently and creates a temperature of 5,000 degrees centigrade and a jet of plasma that impacts against whatever object is in the tank. There are millions of these bubbles created and collapsing every second in an ultrasonic tank.
Compression and rarefaction waves in an ultrasonic tank
It is these collapses that clean the part. The jet will explode the dirt or any other material off the surface of the part. By adding soap or other chemical to the water in an ultrasonic tank, you can increase the effectiveness of the cleaning operation. Heat also improves ultrasonic cleaning by eliminating entrapped air in the water and making the detergent more effective. The best temperature to clean with is 80% of the boiling temperature of the solution. You should always use a basket to hold the parts you are cleaning. Never put parts directly on the bottom of an ultrasonic cleaner.
Basic Types of Ultrasonic Cleaning Systems
- A single tank cleaning system:
A single tank cleaning system is most often used when only a small number of parts are cleaned. It is important to remember that you should use an industrial grade ultrasonic cleaning system, not one of the standard table top types. Most table top systems are very lightly constructed and have small transducers, with the generator, tank and heater in the same shell. These systems are used primarily for cleaning small parts, once or twice a day. They are not designed to provide high throughput or a high end clean part.
Industrial table top tanks always have the generator as a separate unit; this prevents damage by the heat generated in the ultrasonic tank and allows more room for larger transducers on the tank bottom. Single industrial table top tanks can be set up to clean parts as well as a multiple tank system at far less cost. Of course there is no free lunch. A single tank system requires a manual cleaning procedure with full operator attendance. Another restriction is that you are limited by the size of a table top to smaller loads, especially if you are using a multiple beaker cleaning system where 3 or 4 beakers are placed in a holder and filled with cleaning and rinsing solutions. The parts are manually transferred from one beaker to another. Single tank systems are ideal for low volume, lab type or prototype cleaning lines.
- Industrial single tank cleaning systems:
Large single tank floor mount ultrasonic cleaning tanks can be used for in line cleaning of parts in plating or industrial cleaning applications; there is no limit to the size of the tank.
- A Multi tank cleaning systems:
Multi tank cleaning system consist of an ultrasonic detergent tank, spray rinse tank, and an ultrasonic rinse tank, at the minimum. Additional stations such as multiple, detergent and spray rinse tanks, ultrasonic rinse tanks can be added to the basic system for quicker throughput and higher cleanliness levels. With multiple tanks you can process a lot of parts fast and still maintain a high level of cleanliness. Usually these systems have many automated features such as auto fill, drain, filtration, DI water heaters, etc. The exact configuration of the system can be worked out after discussing the cleaning requirement.
- Automation:
Automation in a cleaning system eliminates the inconsistency in process. By automating your cleaning system parts will always be cleaned in the same manner from lot to lot. Choosing an automation system is a decision that will be worked out between you and the manufacturer.