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Today we have three questions from a reader asking about baskets for use in ultrasonic cleaning.
The first question is – – What is the minimum distance that is allowed between the bottom of the tank and the bottom of the cleaning basket before it affects the ultrasonics?
Answer – First, I am assuming that the ultrasonic transducers are located on the bottom of the ultrasonic cleaning tank. Proximity of the basket (provided that the entire bottom of the basket isn’t resting on the tank bottom) doesn’t have that much effect on the ultrasonics. A basket or rack placed directly on an ultrasonic transducer without some sort of standoff will cause undue wear on the tank bottom and shorten its life. In ideal cases, baskets or racks are suspended from the top of the tank to prevent any contact whatsoever with the ultrasonic transducer at the bottom of the tank. What is of concern, however, is that if parts are placed too close to the transducer there is a possibility that there may be cavitation damage to sensitive parts like brittle substrates and softer metals including aluminum and brass. This is because at the very bottom of the tank near the transducer, there is a highly intense band of ultrasonic energy. As a general rule, we try to avoid placing parts any closer than 3/4 inch from the bottom an ultrasonic cleaning tank for this reason. There are cases, however, when that highly intense zone is the “sweet spot” for some difficult applications. In these cases, the work is purposely placed close (but not touching) the transducer in a process commonly called “near field” cleaning.
Next Question – Is there an optimum location for the parts to be located in the cleaning tank for ultrasonic cleaning?
Answer – In a properly designed ultrasonic cleaning tank, the ultrasonic intensity will be relatively uniform throughout the volume of the tank. The exception may be near the tank walls where the intensity may be somewhat less. This is usually not a problem, however, as other constraints like positioning accuracy etc. preclude filling the entire volume of the cleaning tank with parts. If there is a concern, it would be tank overloading. Putting too many parts or parts too closely spaced can reduce the ultrasonic cleaning effect in even the optimal ultrasonic cleaning tank. The point at which a tank is overloaded with work depends on the shape, size, surface area and weight of the parts so it’s difficult to be very specific. An easy way to test for tank overloading is to reduce the load (fewer parts and larger spacing) and see if the cleaning improves.
And the last question – Is there a minimum and maximum mesh size allowed in baskets?
We know that anything between an ultrasonic transducer and the work being cleaned has a chance to disrupt the ultrasonic energy delivered to the parts. Unfortunately, there seems to be no clear relationship between mesh size and the reduction in ultrasonic effect. Interestingly, a thin sheet of stainless steel has almost no effect on the transmission of ultrasonics. There are a few things that we usually try to take into consideration in designing baskets – –
- Excess weight and surface area. Baskets or racks should be robust enough to support the parts being cleaned but should not be “massive.” The more open space around parts, the better the parts will clean. Remember, the basket or rack is just as much a cleaning load as the parts themselves.
- When using wire mesh, we have found than a mesh made of wire welded at the cross points seems to have less effect on the ultrasonic field than if the wire is not welded. It is assumed (not proven) that the vibration of the wire absorbs energy from the ultrasonic field. This effect seems especially evident when fine wire mesh is used.
- Soft and pliable materials like rubber and soft plastics are known to absorb ultrasonic energy and should be minimized in basket design. Today’s advanced ultrasonic cleaning systems and growing technology are making headway in minimizing the effect of materials like rubber and soft plastic on ultrasonic cleaning.
The surest way of determining if cleaning baskets or racks are having an adverse effect on cleaning is to eliminate the basket or rack altogether. For purposes of testing, suspend a single part in the cleaning tank using a piece of stainless steel wire. If the part does not clean to specifications when suspended and cleaned one part at a time, then the problem is not the basket or rack but some other facet of the ultrasonic cleaning process such as time, temperature, and chemistry.
I hope the above comments will help the reader who submitted the questions as well as others. If anyone would like to elaborate or challenge any of the above, your comments to the blog or to me in person at email@example.com are always appreciated.
– FJF –