A previous blog discussed the ramifications of contaminant loading on cleaning. This time let’s look at the long term effects on cleaning chemistry. In lab testing, the ability of chemistry to withstand extended use is often not challenged. Chemistry is prepared and used for short term cleaning trials to verify the cleaning process. In long term use, however, chemistry inevitably looses its ability to clean as more and more parts are processed. This may be due to one or more reasons. In many cases, components of the chemistry are consumed by the cleaning process. Surfactants, for example, are consumed as more and more oil is emulsified. Once the chemistry is saturated, it loses its ability to emulsify more oil with the result that oil remains on parts after cleaning. In other cases, chemistry may expend its alkalinity, acidity or other properties which are essential to effective cleaning.
Although adding more chemistry to maintain cleaning for a longer is a common practice, this tact has its limits. Unfortunately, determining if a deficiency in cleaning is due to contaminant loading as discussed earlier or due to consumption of chemistry is very difficult. If a cleaning solution consists solely of water and dissolved chemistry, there are several ways to determine concentration. Specific gravity, refractive index (refractometer), and pH (titration) all give a good indication of concentration in a freshly mixed batch of chemistry. As soon as other components are added (contaminants removed from parts), however, these methods become less and less reliable. This is not to say that monitoring chemical concentration using any of these methods is not valuable, but just that it must be done with the realization that there are limits beyond which the numbers cease to be a reliable indicator of the ability of the chemistry to clean.
The question of how often to replace chemistry is a tough one. Clearly, nobody wants to spend more on chemistry than necessary, but, at the same time, parts must be adequately cleaned. There may be means in some cases to extrapolate data from cleaning trials to anticipate chemistry life. In my experience, although this can be a good indicator, it is far from infallible. I prefer to use some real-life numbers.
The best answer to how often to change chemistry in my opinion (as much as I know that nobody wants to hear this) is trial and error. In the startup of a cleaning system, the cleanliness of parts should be closely monitored. Once a degradation of cleaning approaching unacceptability is detected, it is time to change the chemistry. In most cases, this interval can be expressed in the number of parts being cleaned given that the parts are similar or cleaned in the same relative mix or as a period of time, again given that production is relatively constant.
I find that most facilities change out cleaning solutions on a regular basis for convenience and to minimize the risk of cleaning failures. Since there is some down time associated with emptying, cleaning and re-charging tanks, this work is usually relegated to an off-shift operation such as an evening or weekend. Although this practice may not squeeze every bit of useful life out of the chemistry, it is often more efficient than waiting for cleaning failures to indicate the need for a chemistry change.
– FJF –