The idea of transfer efficiency is a simple one: put all of the powder on the part. In practice, however, it is much more complex. It is not a matter of just putting powder on the part, but coating the part uniformly, and at the optimum film thickness needed to achieve certain functional or aesthetic properties (like corrosion resistance or opacity). It does no good to just pile up powder on one edge and leave exposed bare metal on another.

To make things more challenging, real-world production parts are not a continuous flat panel passing in front of the spray gun. They are usually a wide range of complex shapes with corners and edges. Perhaps they are wire goods with more air than metal. In reality, they have gaps, holes, and spaces between them on the rack. And to make matters worse, they are usually large enough that they must be coated using several guns in a booth where the air currents designed to contain powder within the booth push and pull powder particles around as they travel from the gun tip to the part. In short, navigating a course toward better transfer efficiency means having to contend with many hazards along the way (Fig. 1).

Most well-run powder coating operations work hard to optimize first-pass transfer efficiency. They know that getting powder on the part uniformly and at the optimum film assures better quality, lower scrap and rework, and saves a lot of money. There is much that a good operator can do to help transfer efficiency. Increasing rack density, proper hook design, proper gun placement, and adjustment of air, powder flow, and electrostatics will all help increase the proportion of powder that goes on the part.

But equipment manufacturers also play a major role in determining the potential transfer efficiency that a particular plant can hope to achieve. From proper airflow in the booth to gun design, the hidden engineering that goes into equipment design can have a dramatic effect on the final results.

A number of advanced “space-age” tools now allow powder equipment designers to move beyond where conventional design methods once ended. For instance, the same simulation software that allows aircraft designers and Formula One teams to minimize air resistance allows spray gun designers to make guns and pumps that are far more efficient.

Although a number of innovations in plastic and composite materials and airflow design have improved powder booth design and its influence on transfer efficiency— resulting in products like the SuperCube—this article focuses on advances in powder delivery and application equipment.

Three basic components make up the simple delivery system for most manual and automatic spray systems; the powder pump, spray gun, and the interconnecting hoses (Fig. 2).

Let’s review a few of the major design concepts that can improve transfer efficiency of this system of components and how they influence each other.

Powder is applied by mixing solid powder with air and propelling the fluidized powder through the electrostatic field of a high-voltage electrode (Fig. 3). The charged powder particles are electrostatically drawn to a grounded part.

The uniformity of the coating’s film is determined, in large part, by the uniformity of the spray pattern coming from the gun, the efficiency of powder particle charging, and the absence of other forces that distort or disturb the powder “cloud.” Booth designers have recently gone to great lengths to create “quiet” zones near the spray guns in order to minimize the interference of air in the booth on the spray pattern. Unfortunately, the air used to fluidize the powder itself has a demonstrable effect on film build uniformity and transfer efficiency.

In the simplest terms, fluidized powder shoots past the electrode toward the part (Fig. 4). In practice, the velocity of this powder has to be fast enough to deliver a certain volume of powder within a certain timeframe. Typical powder flow rates are 180–200 grams of powder leaving the gun per minute. On conventional systems, this commonly requires air pressures of about 90 psi.

But when the powder cloud strikes the part surface, the transport air must go somewhere. Some will cause the part to deflect slightly, or will blow nearby powder particles from the part (Fig. 5). But most of the air dissipates by flowing along the part surface or bouncing off the part.

This bouncing air creates a great deal of turbulence. Some air inevitably disturbs the uncured powder film, collides with powder in the powder cloud, and creates eddy currents that prohibit uniformity. The higher the airstream velocity, the more havoc it causes, and all of this turbulence reduces the system’s transfer efficiency.
Shown (Fig. 6) is a simulation of turbulent airflow among particles in a uniform particle stream that was computer-generated by scientists at MIT Labs. It is easy to see how complex the interactions are and how forces that disturb simple, laminar airflow are created even in a simple scenario.

Air is a necessary evil in the world of powder delivery. Without air we cannot achieve proper fluidization, and no powder transport would be possible. But transport air disturbs the gentle electrostatic deposition process required for a uniform film buildup.

Wagner engineers have successfully tackled the problem of reducing air velocity by reworking the design of the three fundamental components of the powder deliver system—namely the pump, hose, and gun.

The industry-standard pump, the Venturi pump (Fig. 7), has long provided a simple, reliable means of delivering powder. Feed air passes across an injector nozzle and creates a Venturi effect. A small vacuum draws powder from the hopper where transport, or dose air, helps carry the powder away.

In conventional Venturi pump designs, the majority of air is feed air, with a small proportion of dose air.

Wagner Industrial Systems has redesigned the conventional Venturi pump so that significantly less air is required to deliver the same amount of powder. This efficient delivery, or “ED” pump, uses 30–50% less total air (Fig. 8). Interestingly, the ED pump also uses the majority of air as transport or dose air and a smaller fraction of the total air consumption for feed air.

The new ED pump improves upon the best features of the Venturi pump. The pump’s simplicity of design, with no moving parts to break, make it extremely reliable and far more cost effective than pumps that use moving diaphragms or pistons. Powder can be an extremely abrasive material, so avoiding direct impingement of powder on delicate mechanical parts is desirable in proving a long life span with minimum maintenance. The new ED pump solves the traditional problem of air efficiency by utilizing new injector and cavity designs that require minimal feed air to achieve efficient powder delivery (Fig. 9).

Because the ED pump makes it possible to deliver more powder using less air, there is another benefit that comes from switching to the ED Venturi pump. Because of the increased air efficiency, the delivery hoses, the powder pipeline between the pump, and the gun can all be smaller in diameter.

Decreasing the diameter of the powder hose also means that the density of powder particles within the hose is much higher (Fig. 10). The illustration shows how switching from an 11-mm diameter powder hose to a 9-mm diameter hose decreases the volume of the hose by nearly 50% (because the volume is a function of the square of the radius).

By keeping the powder delivery rate constant (between 180–200 grams/minute) we see that the density of powder in the hose increases markedly.

The powder in the hose is now moving at a slower speed and at a higher density. Slower moving, denser powder moving past the spray gun electrode provides much more uniform charging—and softer, more uniformly charged powder results in better transfer efficiency.

Another innovation in powder transport hoses lies in the materials and construction of the hose itself. Wagner has patented a unique hose design that contains an integral, electrically conductive material along the entire length of the hose (Fig. 11). This conductive material is connected to the earth ground.

Normally, as powder flows through a plastic or rubber transport hose, the powder rubs against the hose walls and builds up a frictional, or triboelectric charge. This charge is not only a safety hazard because it can cause electric shock or arcing, but the charge imparted to some particles disrupts the uniform charging of the powder at the spray gun electrode.

The final leg of the application equipment triangle that has undergone improvement is the powder gun itself. The latest generation of Wagner Hi-Coat spray guns, the Model C4 gun, has been intentionally designed to accommodate the dense delivery of powder using less total air arriving to it.

The internal chambers and cavities of the gun have been designed to provide smooth, laminar flow of the powder past a newly designed nozzle body that uses a specially designed “powder wedge” placed in the powder/air stream (Fig. 12). The unique geometry and location of this wedge results in a powder cloud of exceptional uniformity with a wide range of popular spray gun nozzles.

This design provides for more uniform charging and lower velocities at the tip of the C4 gun. A softer, wider powder cloud provides even charge distribution and less bounce-back and air turbulence at the part surface. The result is a more uniform film buildup and better coating of difficult part geometries. Faraday cage effects are minimized, and the performance with metallic powders is greatly enhanced. The overall impact is a spray gun that coats better while at the same time being easier and more flexible for operators to use.

The photos in Figure 13 show the softer, more uniform powder cloud of the C4 gun (right) compared to a standard spray gun (left). Even the naked eye can see that the powder distribution is more uniform and less turbulent. The gun is less sensitive to nozzle orientation, part distance, and other characteristics that make painting or setup more difficult with other guns.

Using additional sophisticated analytical techniques, the uniformity of the spray pattern is even easier to quantify. By the time particles reach the substrate, the cloud is an almost perfectly uniform, slow-moving distribution of charged powder particles—which is ideal for achieving optimum transfer efficiency.

There is a popular misconception that because powder coatings can be reclaimed, then the oversprayed powder is somehow “free.” In reality, of course, the picture is not so rosy. First, commercial powder systems are not 100% efficient. In reality, not all oversprayed powder even makes it to the reclaim system. It sticks to fixtures, hooks, booth walls, guns, and floors—which requires cleanup and disposal. Besides, even if the system was 100% efficient, the higher distribution of fine particles found in the overspray limits the number of times powder can be effectively recycled.

The cost of reclaimed powder includes the cost of the time and labor used to spray it in the first place. The cost of operating a line that is less than 100% efficient. That is why the impact of first pass transfer efficiency is so important to success and the bottom line.

Innovations that increase transfer efficiency carry with them other benefits. The new ED pump not only permits better charging and a softer cloud, but also uses less plant compressed air to achieve the same results. New, smaller-diameter hoses not only provide a denser powder package for transport and more efficient charging, but they also require less air for purging them during cleanup and color changes.

Less air means less money, less turbulence, and less waste. Today, at a time when competitive pressures make efficiency and profitability more important than ever, less energy consumption also means a cleaner, safer environment.

Bio

Tom Matthey, customer service manager, is a 13-year veteran of Wagner, with extensive experience in both liquid and powder coating systems. Tom's responsibilities include outside and internal customer service, customer training, and coordination of Wagner’s laboratory facilities.