Know-How: Four Process Parameters To Clean Parts

Posted on Friday, February 20, 2015

Cleaning parts using aqueous chemistries is a lot like washing dishes in a sink by hand. The conditions of soil loading and the ways to overcome these using four key process parameters are the same for both washing dishes or using a multi-stage pretreatment washer. For example, a particularly heavily soiled plate that has a high amount of grease, dried food, and stains may be a difficult to get clean as the most heavily soiled part produced in any manufacturing process. Keeping this analogy in mind, let’s look at the four key process parameters we can use to overcome these stubborn soils on both dirty dishes and dirty parts for powder coating. Of course, you should always consult with your chemical supplier before making any changes to your pretreatment process.

1. Chemical Concentration: Just like you may be inclined to add more dishwashing detergent when faced with cleaning a particularly dirty plate, adding more cleaning chemical to a wash stage, or combination wash/phosphate stage, is often done to attain a cleaner part for powder coating. There are limits to how high the concentration can be raised, as your chemical supplier will attest to.

2. Temperature: Everyone who ever had problems with getting stubborn grease off a dirty plate knows that increasing the temperature of the dishwater is an effective solution. The same goes for chemical pretreatment systems, where increasing the tank temperature is an effective way to remove heavy soils (grease & oil) from parts, especially paraffin-based oils.

3. Pressure: Just like using more “elbow grease” to clean a particularly dirty plate when washing dishes, adding more nozzle pressure can be very effective at removing particularly heavy soils from parts to be powder coated. The higher nozzle pressure will result in higher mechanical force to break the bond of the soil to the part.

4. Time: If everything stays the same (i.e., concentration, temperature, and pressure), then the only way you can clean that dirty dish is to spend more time washing it. The same applies to pretreatment systems, as well. If you cannot change chemical concentration, tank temperature, or nozzle pressure, slowing your conveyor speed to get through a batch of unusually soiled parts may be the easiest fix you can implement. However, this solution is least effective when the “unusual” part soil conditions become the “normal” part soil condition, as the line speed change will affect productivity of the line.

In this case, you need to try one, or more, of the other solutions to overcome the problem.

These four process parameters are important to keep in mind when specifying or selecting a new piece of equipment, as well. Having some headroom in these parameters is normally easy to accommodate when a pretreatment system is being designed and the added costs are often inconsequential. Some general target goals for these parameters are 180°F maximum tank temperature, 25 to 30 psi maximum nozzle pressure, and at least 60 to 120 seconds dwell time for active stages and 30 to 45 seconds dwell time for rinse stages. These parameters are conservative enough that the chemical supplier will be able to handle most any soil condition without having to use very high chemical concentrations to attain the desired part cleanliness.

Nick Liberto, P.E., is president of Powder Coating Consultants, division of Ninan Inc., an independent technical consulting firm in Bridgeport, Conn. He can be reached at pcc@powder coat.com .