Finisher's Think Tank: Rinsing: An Important Part of the Cycle

Rinsing as a dynamic operation refers to the diluting and subsequent removal of surface films and contaminants. It is a dynamic process of washing the surface of parts, conditioning them for the next treatment step, or to polish the final finish before drying. There may be more rinse tanks in a given cycle than process tanks. Yet, rinsing with its importance can be overlooked or undersold, to any finishing operation. 

Some critical rinsing factors to consider are: water purity, mechanical agitation, how much, for how long and how warm. It becomes especially challenging where water conservation becomes critical. 

The following points may apply to specific metal finishing operations, such as: plating, painting and powder coating, anodizing, colorizing, oxidizing and other specialties.

• Dial or select a dilution ratio of at least 1000:1, in order to provide a good start to remove soils off the surface efficiently. 
• Double rinsing is more beneficial than equal time spent to rinse in one tank.
• Quality rinsing improves as water temperature does, to a distinct limit. A suggestion is 75-85°F (24-29°C). In the coming months, some geographical areas will encounter the beginning of a chilly winter season. Incoming city or well water may enter the plant below 40°F (4.5°C). Cleaner drag-out and similar films from other processes are more difficult to remove in cold water. It is not uncommon to encounter plating deposit hazes, and adhesion problems that are related to chilly rinse waters. If practical, consider warming a stagnant rinse. 
• The temperature of parts being processed, such as cleaning, may detrimentally affect quality rinsing. An example is dry-on staining, where water quickly evaporates leaving dried on films of the cleaner. Proper range of the cleaner bath temperature, as long as it is effective to cleaning, should be confirmed. Additionally, do not exceed the cleaner concentration, to prevent this potential contribution to dry-on stains. If the problem gets past cleaning, then acid treatment will only tend to harden the film, making the overall problem worse.
• A rinse tank with little or no water inlet flow or plugged outlet will rapidly become contaminated. Sufficient turnover of rinse water can be confirmed by a simple visual check. Water discolorations, cloudiness, floating oil and grease residues, are some adverse observations to look for. A more sophisticated, but relatively simple rinse water control is the application of conductivity. Conductivity of water increases as the loading of dissolved salts does. At a level of these contaminants, the water conductivity will become critical. This problem is overcome by using a conductivity probe, preset to a conductivity range that automatically activates a water inlet, to refresh the water to a lower working conductivity. A conductivity activated rinse station minimizes water use while readily maintaining a preferred degree of quality rinsing.
• Decide if a single rinse or multiple rinses best meet the requirements between treatment and conditioning steps. A single dip in one rinse can be followed by multiple dips in the same rinse. Or, add agitation by air or eductor. Although air agitation is good, an eductor or pump can be more effective.
• Counter-flow rinsing (CFR) is a popular, effective method to improve quality rinsing, while minimizing water use. CFR can be two or more tanks in line, operating as one unit. For example, three rinse tanks follow a nickel plating tank. Rinse #3 (further removed) reconstitutes rinse #2 back flowing into it, which in turn reconstitutes rinse #1 in the same way. Rinse #1 is stagnant. As a concentrated drag-out, it can be added back to the plating tank, replacing evaporation loss. The bonus is returning plating solution back to the process tank. Rinse tanks #2 and #3 may provide a dilution ratio range of 30 to 40:1. 
• Spray rinsing can be very effective, much so to accommodate process lines having a premium on space. Mechanical energy of the spray can facilitate removal of films. Fog nozzles may be even better. Their effect is to dilute and wash films off the surface, while also conserving water use. Depressing a pedal as rack flight bars slowly rise out of the process tank can activate fog nozzles. The solution washes directly back into the tank.
• Keep the rinse tank water level at the proper volume depth. Neglected levels can result in top racked parts not getting immersed. Visually adjust the water level, or install an automatic valve to accurately maintain desired rinse water level.
• Gravity encourages solutions to flow downward. Racked parts should be, if possible, fixed on racks to encourage maximum draining and effect of rinsing. Barrel parts require drain holes that are not plugged and sufficient barrel rotations over an exiting rinse tank.
• Ion exchange can be very beneficial to rinsing. Rinse water can be purified in this way by removing dissolved salts. This water can be reused many times.
• Deionized water provides low conductivity / high resistivity water. This type of rinsing is very good to prevent spots and stains on parts in a final rinse before drying.