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 .