In The BIG Four, I talked about the four important variables in cleaning. These are time, temperature, chemistry and agitation. In that blog, I mentioned that a shortcoming in one variable can, to a point, be overcome by changing another of the variables. For example, increasing temperature or changing chemistry may reduce the time required for ultrasonic cleaning. It is important, however, to understand that the interactions of these four variables are not absolute and, in many cases, non-predictable. Also, most processes have constraints on one or more variable. In one case, for example, cleaning may need to be accomplished with a chemical that has a pH of 7 ± 1. In another, the total cleaning time might need to be under 2 minutes to meet the required production. In still another, the temperature might need to be under 100°F to prevent damage to the part. Finding the right combination of variables to produce the desired result while not violating the constraints put on the process often requires some degree of testing and ingenuity.
In ultrasonic cleaning involving soluble contaminants, one must think of ultrasonics as a means of agitation which enhances the cleaning process. Ultrasonics, by itself, is seldom the only mechanism required to produce a desired cleaning result any more than is temperature. Simply applying ultrasonic energy to or heating a part to a given temperature alone is not likely to make it clean (although there are exceptions). In ultrasonic cleaning as practiced in removing soluble contaminants (as opposed to particles), the primary mechanism is usually the interaction of the cleaning chemistry with the contaminant. The blog Ultrasonic Cleaning – Soluble Contaminants provides more detail on this.
Typical Ultrasonic Cleaning Times –
In my experience, typical, appropriate ultrasonic cleaning times range from a few seconds to several minutes. Times above several minutes may be simple “overkill.” Although less time may still produce the desired result, the relationship between cleanliness and ultrasonic time has not been adequately investigated to show this. Somewhere along the way it was determined that 20 minutes, for example, was “enough,” and shorter times were never investigated.
When times as determined by applications testing exceed a few minutes it usually indicates one of two things – –
- There is a shortcoming in one or more of the other of the “big four” process variables. A change in chemistry or temperature may reduce the required ultrasonic time significantly.
- Ultrasonic cleaning alone is not the appropriate process for this particular cleaning application. In some cases, the addition of a pre-cleaning step prior to ultrasonics may be the answer. In others, other cleaning options such as spray washing may be more effective and appropriate.
Why Does Ultrasonic Time Matter? –
First and foremost, if cleaning can be accomplished in a shorter time, higher production rates can be achieved. This is important especially in cases where cleaning is a process bottleneck. If production rate is not an issue, the same work can be done using a smaller, less expensive ultrasonic cleaning system which uses less resources (water, electricity, manpower, etc.).
The life of an ultrasonic cleaning system is directly related to its use. Operating an ultrasonic system longer than necessary during each cleaning cycle may shorten its life.
Excessive exposure to ultrasonic cavitation and implosion may cause part damage. Limiting ultrasonic cleaning times on materials including aluminum, brass and many other soft metals which are subject to cavitation damage is imperative.
Ultrasonic cleaning times in excess of several minutes should trigger an investigation into the rational or background that lead to using an ultrasonic time that may be inappropriate. Simple applications testing may justify reducing the time or indicate process changes which will shorten and/or enhance cleaning.
– FJF –