By Frank Mohar
Editor’s Note: This article is adapted from Frank Mohar, “Variables That Affect First-Pass Transfer Efficiency in a Powder Coating Operation,” presented at
FABTECH ®, Nov. 9-12, 2015, Chicago.
Lean manufacturing principles are not limited to fabricating activities on the shop floor. They apply to powder coating as well.
Of course, the goal of any lean manufacturer is to
eliminate waste, and that should be the goal of any
shop running a powder coating line. It’s best to get
as much powder to cling to the metal part on the
first pass (see Figure 1), so that the part can go immediately to the curing oven, not to another staging
area where someone has to spend time touching up
Several variables affect first-pass transfer efficiency in a powder coating operation. By focusing
on these variables, fabricators who act as their own
powder coater can achieve more complete coverage of parts and reduce the amount of rework.
1. Powder Size
The particles that make up powder coatings come
in a variety of sizes. Powder particles that are either
too small or too large don’t have the greatest track
record for transfer efficiency.
For example, particles from 25 to 75 microns
boast a transfer efficiency of up to about 70 percent
or higher (depending on the application), meaning
that particles of this size have a very good chance
of attaching to the part, if other variables (to be ad-
dressed later) affecting material transfer are consis-
tent for good coverage. If the particles are 0 to 25
microns, you could have a difficult time both charg-
ing and fluidizing the powder. If the particles are 75
microns or bigger, they might be too large to hold a
charge and can cause a rough finish.
So what is good transfer efficiency for a powder
coating operation? Guidelines for transfer efficiency
• 10 to 40 percent for items such as wire goods,
bicycle frames, and small hardware.
• 30 to 60 percent for larger items such as wheels,
transformers, and light fixtures.
• 50 to 80 percent (or possibly even a higher percentage) for items with large surface areas, such as
flat panels, file cabinets, appliance panels, oil filters,
Many variables affect these ranges for transfer efficiency, obviously, but these percentages are useful
as basic benchmarks.
2. Electric Force
To be effective, a powder coating system needs the
right force—voltage and microamps—to work.
The most common powder coating guns rely on
corona charging. The powder particles acquire a
charge while traveling from the gun to the part. The
discharge—or the corona of powder material—acts
as an electric field that carries the particles to the
grounded metal part destined to be finished.
The most effective corona guns provide a high-voltage power supply of up to 100 kV or higher. The
guns also have an electrode optimally placed inside
the nozzle to control the working amperage.
Other factors, however, affect corona charging.
For instance, too much distance between the gun
and the part will not allow the charged particles to
be pulled to the substrate, meaning that the particles won’t have the velocity necessary to adhere to
the grounded parts. The electrode-nozzle configuration influences field strength as well.
In addition, the relationship between the gun’s
voltage and current can have a great effect on the
powder delivery to the part. But keep in mind that
more of either is not always better.
The charge strength actually can make the job
easier in some cases. If you can powder-coat a large,
flat surface by staying away from the edges, the
charge will paint the edges for you as the focus is
placed on the main body of the part.
Excessive voltage, however, can complicate the
coating of recessed areas. The high voltage enhances the powder particles’ deposition around the
edges of a recess.
A strong electric field also pushes free ions toward
the edges, resulting in a rapid charge accumulation
and back ionization development (in which charged
particles accumulate in one area and prevent even
distribution of the powder). The back ionization,
which can be identified by the presence of craters
on the surface of powder coatings, plays a dramatic
role in lowering the powder gun’s transfer efficiency.
Voltage and microamps react to gun-to-target distance. If the gun is moved closer, the voltage drops,
and the current draw goes up because of the higher resistance between the gun tip and the earth-grounded part.
Powder coating gun technology can help to overcome some of these obstacles. A current limiter can
be used to hold the current below a predetermined
level, which prevents the conditions that lead to
back ionization. Another alternative is the use of a
corona gun with an ion collector, which draws excess free ions back to an alternative ground reference, delaying the onset of back ionization.
3. Aerodynamic Force
Again, more is not always better. If the airflow rate in
the powder coating booth is too high, you will find
that the transfer efficiency is reduced, meaning that
the powder will go to the filtration system rather
than on the substrate.
One of the best ways to control airflow is to focus
on the powder delivery system. Fortunately, technology has come a long way since the 1980s in helping powder coaters fine-tune powder delivery to the
Several factors affect powder delivery. The powder hose diameter from the source to the gun, the
HOW TO BOOST FIRST-PASS TRANSFER
EFFICIENCY IN POWDER COATING
Fabricators need to keep a close eye on several variables
Fabricators know the pain associated with rework.
Because of that, the goal of any fabricator that runs a
powder coating line should be to maximize first-pass
efficiency. Having to pull a fabrication, such as this
large weldment, out of the flow to perform touchups
interrupts the automated process and takes a worker
away from another task.
A fabricator that is able to put a solid wall of parts in
front of powder guns is likely to see a higher percentage of powder coating material adhere to the parts and
not wind up on the floor.