How to boost first-pass transfer efficiency in powder coating

Figure 1
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.

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 specific areas.

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 addressed later) affecting material transfer are consistent for good coverage. If the particles are 0 to 25 microns, you could have a difficult time both charging 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 are:

• 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, and pipe.

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.

Figure 2
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.

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 grounded object.

Several factors affect powder delivery. The powder hose diameter from the source to the gun, the 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 optimal aerodynamics.)

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 efficiency difficult.

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 hard-to-reach areas.

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 powder.

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 booth designs.

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).

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.