The first and most important stage in the spray washer is the cleaner stage. If the part is not completely cleaned while passing through this stage, the quality of the finished part will suffer. The cleaners are alkaline, and contain surfactants to lower surface tension. Spray pressure is important in effective cleaning. The highest pressure, without blowing the parts off the hanger is usually the most desirable. Cleaner solution concentration and temperature must be carefully controlled. Planned dump cycles must be implemented to assure that soils are not redeposited on the surface of the parts. Normal design dwell time in the cleaner stage is 90 seconds. If line speeds have been changed, the chemical supplier should be consulted for recommended changes in solution concentration and temperature. Between each stage is a drain zone. The drain zone should be long enough to keep solutions from being carried from one stage to the next, but short enough that the parts do not dry between stages.
The rinse after the cleaner is normally an overflowing clean water rinse. It is operated at the same temperature as the cleaner. The alkalinity of this stage is limited to no more than three percent of the cleaner concentration to limit the carry-over into the acid third stage.
Two generic types of phosphates are widely used in powder paint finishing systems, iron phosphate and zinc phosphate. From both the equipment and chemical standpoint, iron phosphates are more economical. They are easier to control and have fewer maintenance requirements.
A detergent iron phosphate product is available that requires only three stages. This system is used in some powder finishing systems, however, it is not recommended where a high quality end product is required. The ultimate performance of the final product, cost, and end use must be balanced to determine the phosphate to use.
Crystalline zinc phosphate and micro-crystalline zinc phosphate, when properly maintained, provide the best substrate, with respect to surface appearance, adhesion, detergent resistance, and corrosion resistance. However, the crystalline coatings exhibit poor flexibility, and generally require higher paint film thickness to achieve equal gloss or iron phosphated parts. No matter what generic type of phosphate is chosen, solution chemistry and temperature must be carefully controlled. Due to the fact that both iron and zinc phosphates form sludges in their normal operation, regular scheduled clean-out of all spray piping and tanks is required to assure adequate spray patterns.
The rinse after phosphate is an overflowing fresh water rinse, its concentration must be kept at no more that one percent of the phosphate tank concentration to avoid contamination of the passivation solution in Stage No 5.
The time and temperature of the dry-off oven is dependent upon the pretreatment and part shape. With iron phosphate, the requirement of changing the crystal structure is added. The desired crystal dehydration improves corrosion resistance. Usually, a metal temperature of 350°F for five minutes is sufficient to cause dehydration.