Cleaning Process – Rinsing – Preventing Carry-Over

A dedicated reader has requested that the blog address rinsing in more detail.  Rinsing ranks high on the list of process steps that are often not given their due attention in the specification of an overall cleaning process.  Many specifications just say “rinse” and nothing more.  In fact, rinsing is a secondary cleaning step intended to remove residuals left on parts from the primary cleaning step.  It is important, however, that one not make the mistake of counting on the rinse as a primary cleaning step.  If parts enter the rinse with the original contaminants still in place, it is the cleaning step, not the rinsing step that requires attention.  This blog is the first in a series that will address rinsing as an important part of the overall cleaning process.

One of the easiest ways to assure successful rinsing is to prevent cleaning liquid and contaminants from reaching the rinse in the first place.  By the time parts reach the rinse station, all contaminants that were initially on them should have been lifted and suspended by the cleaning chemistry used for the primary cleaning step.  In fact, most of the removed contaminants should stay in the cleaning station.  Some of the cleaning liquid, along with suspended contaminants removed in the primary cleaning step will, inevitably, be transferred to the rinse when parts exit the cleaning step.  This residue is called “carry-over.”  The first step in assuring good rinsing is to minimize carry-over from the cleaning station to the rinsing station.  Less carry-over means less burden on the rinsing process and a greater chance of thorough rinsing.

Some sources of carry-over are obvious.  Parts with geometries that include concave shapes and blind holes are good examples of obvious sources of carry-over.  Sometimes the solution to minimizing carry-over is as simple as orienting the part in such a way that the various part features will drain through the effects of gravity alone.  If the part has multiple features that prevent any single orientation from providing adequate draining, part rotation may be an answer.

Capillary spaces (narrow nooks and crannies) are also major contributors to carry-over and are often not recognized.  Heat exchangers, for example, with their high surface area to mass ratio, are notorious for carry-over.  Weighing a part before and after cleaning (before rinsing) will often reveal it’s potential to retain and carry liquid from the cleaning step to the rinsing step. Alternative techniques to minimize carry-over can be evaluated in this way as well.  Sometimes liquid retaining capillaries will drain if given sufficient time but, in many cases, the retained liquid just stays there until it evaporates of its own accord.  Adding time also brings the risk of  “dry-down.”  Dry-down occurs when the volatile compenent(s) of the cleaning liquid (usually water) evaporate leaving just the less volatile and solid components behind.  Once dry-down occurs, the likelihood of being able to successfully rinse the part is reduced drastically.  In fact, there are many cases in which even total re-cleaning won’t successfully recover parts once the cleaning chemistry has dried on them due to the resulting oxidation and/or other chemical reactions.

An air blow-off will often help remove liquid held in capillary spaces but the use of compressed air can be both costly and messy and requires line of sight access to the capillary spaces.  It may also cool the part (due to evaporation) which often makes rinsing more difficult in the long run.

Finally, a spray pre-rinse in air using either fresh water or water reclaimed from the following rinse(s) can, in essence, dilute or replace the contaminated cleaning liquid from the cleaning step with less contaminated water thereby reducing the burden on the rinse.  A pre-rinse also reduces the risk of dry-down as it keeps the part wet and reduces the concentration of contaminants.  A spray pre-rinse can be positioned either above the cleaning station or in a separate station between the cleaning and rinsing stations.

In summary, the first step to successful rinsing is to minimize the burden on the rinse by reducing the amount of contamination on the parts as they enter the rinse.

–  FJF  –