powder pump, the design of the powder gun’s throat, and nozzle designs can
influence how powder is delivered. (Additionally, the gun’s ability to provide the
optimally charged powder particles arguably represents half of the formula for
success in coating the most difficult geometries; the other half of the formula is
Traditional venturi pumps, made between 1980 and 2008, used a considerable amount of compressed air to drive powder to the gun. That compressed air
provided a lot of velocity—almost too much—which made high rates of transfer
Newer generations of venturi pumps can feed more powder using less compressed air, while still using a combination of air supplies to ensure a smoother
delivery of powder. Atomizing air is used to maintain a correct delivery velocity
within the powder hose. A separate feed of flow air helps to regulate the amount
of powder fed to the gun; a vacuum is created through an injector, which pulls
the powder from the hopper and delivers it into the powder hose.
Recently dense-phase application technology was commercialized. Also
called high-density, low-volume (HDLV) systems, this powder coating delivery
method relies on digital control to find the optimal combination of airflow to
ensure repeatability and uniformity for repeat jobs.
Because HDLV systems use significantly less air to deliver the same amount
of powder as conventional venturi systems, they can run with smaller hoses.
This is beneficial to the powder coater because smaller-diameter hoses can be
cleaned out much more quickly than larger-diameter hoses. In fact, because
these systems can be cleaned automatically—powder is purged back to the
powder supply—powder lines can be flushed quickly, which helps speed up the
color change time, as well as reduce potential for cross-contamination.
Perhaps the most important aspect of dense-phase technology is that it delivers a denser cloud at a reduced velocity, more than double the powder density
of conventional venturi systems. The high charge density helps to overcome
aerodynamic forces that typically work against efficient powder application.
Manual powder coating using HDLV guns also will achieve better coverage of
How can you determine if your powder coating process is operating with excessive airflow rates? Can you see through the spray booth during operation? If
not, that’s a pretty good sign that you aren’t spraying just the right amount of
powder with the minimum amount of air necessary to do the job.
Looking for something more solid in terms of a benchmark? Here are some
guidelines for output rates for certain powder coating applications. Keep in
mind that other factors need to be taken into consideration as well:
• 30 lbs. per hour for simple parts (flat panels, file cabinets)
• 25 lbs. per hour for medium-sized or difficult-to-access parts (wheels, transformers, oil filters)
• 20 bs. per hour for complex parts (wire goods, bicycle frames, door hinges)
4. Powder Booth Design
The powder booth design can affect first-pass transfer efficiency as well. Stainless steel walls, for example, act as a strong magnet for powder, and certain
polycarbonate materials, while not as bad as stainless steel, also can attract
Booths made of proprietary engineered plastics have emerged in recent
years. Powder attraction tests have demonstrated that these new materials attract fewer powder particles than more traditional materials used in powder
It should be noted as well that the openings where parts enter and exit, slots
where powder guns are located, and the main extraction points all affect transfer efficiency. Experienced equipment suppliers can ensure that a booth is designed to deliver airflow that will maximize transfer efficiency, not work against
the manual powder coater or the automated powder guns.
5. Part Density/Racking
If you want more powder on the parts, you need to put more parts in front of the
gun. In this case, a solid wall of parts in front of the guns drastically improves
first-pass transfer efficiency (see Figure 2).
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If you have gaps in the part spacing, advanced controls can assist in boosting
transfer efficiency by identifying parts as they enter the booth and adjusting application parameters as the parts move into position to be coated. For instance,
as a panel moves into a booth, it can pass through a bank of electronic eyes or a
light curtain. The size of the part is identified, and the powder coating guns are
programmed to spray only when parts are present.
These same advanced controls also can adjust airflow rates, gun voltage, as
well as gun triggering parameters. If the automated guns have the ability to
move vertically, the controls can adjust their height placement as well.
As for the racks themselves, they need to be free from cured powder, especially where the part and hook make contact. Powder coating buildup on the
hooks can cause the parts to be less conductive, which makes them unable to
attract the electrostatically charged powder particles. As a result, the powder
ends up on the floor, not on the part.
6. Preventive Maintenance/People
This is arguably the biggest factor for metal fabricating operations relying on
manual application of powder coatings. The powder coating technicians need
to be engaged in the operation, spraying in a focused manner as parts are presented and staying on top of maintenance, which means cleaning equipment
and replacing items such as nozzles and venturis when needed.
Powder coating can be overlooked as a vital aspect of the manufacturing operation because it’s near the end of the production line, but that doesn’t mean it
should be. Fabricators have plenty of opportunity to root out waste in the powder coating operation just by focusing more closely on the material, equipment,
and people involved in the process.
Frank Mohar is a powder systems specialist, Nordson Corp., 28601 Clemens Road, Westlake, OH
44145, 440-892-1580, www.nordson.com.