How fast can I change colors? That’s one concern holding back some finishers from switching to powder coating. What the question really means is, “How long does it take to clean my system so there’s no old powder left to contaminate the next color?”

The added labor cost and lost production time of lengthy color changes can mean the difference between success and failure for paint operations where frequent color change is a fact of life.

Not long ago color changes could take a couple of hours. Today, changing from one reclaim color to another can be done in a matter of minutes.

What has made this dramatic reduction in color-change time possible? Certainly a great deal of attention has focused on making equipment easier to clean. Following is a small sampling of some innovations that shave valuable time off the process.

Figure 1. Powder route on a powder coating line

The Color-Change Road Map
Any place that old powder can come in contact with new powder is a potential hot spot for contamination.

Take a look at the route powder follows on your powder line, and you’ll understand where improvements have been needed along the way (Figure 1).

First, powder is delivered to the system in boxes and must be fluidized and pumped to the spray guns. Most of the powder sprayed from the gun goes on the parts—but some doesn’t. This over-sprayed powder must be collected and recirculated back to the delivery system.

Figure 2. The feed center centralizes all delivery apparatus so a single operator can easily inspect, clean and reconnect the delivery equipment for color change.

Changing colors means cleaning all of the previous powder from the powder feed equipment, booth and recovery system.

The modern feed center minimizes the number of surfaces required to deliver powder, and provides mechanisms for their easy cleanup (see Figure 2). By centralizing all the delivery apparatus, a single operator can easily inspect, clean and reconnect the delivery equipment for color change.

Figure 3. Hinged door opening on cyclone ductwork facilitates cleanup.

Equipment is made more accessible by mounting it so that it can pivot into a convenient cleaning position and be relocated precisely when finished.

The feed center can be equipped to supply powder from bags, boxes, totes or any other type of powder container.

The latest cyclone recovery systems are designed to funnel over-sprayed powder into a small chamber that is easy for operators to access for proper cleaning. Opening a hinged door in the booth wall can actually access the cyclone ductwork. This provision makes complete cleanup of the ductwork a simple matter (Figure 3).

New Powder Booth Designs
There has been an evolution in booth design. At first, designers concerned themselves with the mechanical aspects of the booth. They focused on how to make spray booths lighter, easier to install and less expensive to purchase. These booth designers experimented with a range of commercially available materials from stainless steel to plastics like PVC and polycarbonate.

In the next phase of booth design, engineers began to design booths that were easier to clean and provided greater visibility for painters.

Figure 4. Sloped surfaces, even curved booth walls, are integrated into new booth designs to eliminate hard-to-clean areas.

Everybody realizes that it’s easier to clean something that’s free of tight corners, seams and ledges where powder can accumulate. Existing booth materials were fashioned into better shapes to eliminate hard-to-clean areas. Sloped surfaces, even curved booth walls, have been integrated into many of the latest booth designs (Figure 4).

Figure 5. New booth designs use a single wall for both the collector and the booth wall.

The latest generations of booths literally break down the walls between the booth and ductwork by creating designs that use a single wall for both the collector and booth wall (Figure 5). An operator opens a hinged panel inside the booth and has complete unfettered access to clean the collector.

An area of booth design that has benefited from advances in computer-aided design tools is airflow inside the booth.

Air in a powder booth creates a complex set of problems. On one hand, airflow must contain powder within the booth and keep it from escaping through booth openings. Air can be used to draw powder to the collector so reclaim is enhanced. However, airflow around the part should be as gentle as possible since it can draw powder away from the part, decreasing system transfer efficiency. Designing for better airflow has two objectives. First, creating a “quiet” zone around the gun and the part so electrostatics can draw powder to the part without being disrupted by air currents. And, airflow should help to direct over-sprayed powder to the collection system.

New Materials for Booth Construction
The latest development in booth design is in actually creating new booth materials that have the most desirable combination of physical properties and cleanability.

Figure 6. Different spray booth materials have varying physical properties and, therefore, degrees of cleanability.

What role does booth material play in color change? Figure 6 shows various common booth materials that have been sprayed with powder and then blown off with compressed air just as they would in a color-change operation. Each panel shows how much powder is left behind.

Powder that’s left behind is a reject waiting to happen—since the powder will ultimately become airborne and contaminate a part’s finish.

Another test performed on common booth materials reveals that wiping down the walls of a booth may not remove enough powder to prevent contamination. In this test, booth wall sections made of single-skin and double-skin PVC and proprietary composite material were wiped with a cloth after being blown clean of powder.

Figure 7. Powder weight of each cloth after wiping a supposedly clean booth.

The rag was then analyzed for powder contamination. The chart shows the weight of powder of each cloth after wiping a supposedly clean booth wall surface (Figure 7).

Borrowing from the success in other applications, composite materials now offer the best of all worlds.

The latest materials have been engineered to have low attraction to powder coatings while exhibiting all the desirable properties of a booth material.

Figure 8. The proprietary composite material is actually designed around the structure of an I-beam for increased durability.

In the plant, booths can take a beating—from tow motors to falling parts. Some materials (like stainless steel) are extremely resistant to impact while others (like polycarbonate plastic) crack and shatter easily with impact. New materials are composites designed for high-impact resistance. The proprietary composite material is actually designed around the structure of an I-beam (Figure 8). With two skins separated by a lightweight core, the wall is able to withstand great impact and flexing pressures.

Figure 9 shows a comparison of IZOD (ASTM D 256) test data for the proprietary composite material compared with standard impact data for various common booth materials.

Figure 9. Comparison of IZOD test data for the proprietary composite material compared with standard impact data for various materials.

An advantage of the I-beam construction is that lightweight materials combine to form an exceptionally tough panel. The proprietary composite material weighs only 1.38 lb/sq ft—less than half the weight of a hollow PVC panel built with conventional 3/16-inch skins. These lightweight panels are self-supporting, eliminating bulky, expensive and obtrusive framework usually required to support plastic canopies.

The light weight of the material also makes it possible to fabricate larger-sized panels so a booth can be built with fewer sections and seams. Mating of panels inevitably leads to places for powder to collect.

Another consideration in booth materials is lighting and visibility, since they play an important role in making good parts. Today’s booths are designed to be well-illuminated, as bright paint booths make it easy for operator to see what they are doing.

A few years ago, clear, see-though plastics became popular since they allowed light inside the booth. One problem with some see-through materials, however, is how quickly they become obscured with powder once the spray guns are triggered. This is especially true of materials that have a high electrostatic affinity for charged powder.

Another factor that reduces the usefulness of see-through materials is that they may be prone to becoming “sandblasted” during use. Powder coatings are abrasive and spraying them against a clear panel, wiping, squeegeeing them off, spraying and cleaning repeatedly wears the surface.

This wearing of the surface has two effects. First, it affects the cosmetic appearance since scratches reduce the clarity of the material. More importantly, the scratches create a more porous material for powder to cling to and create possible contamination. The ideal booth wall is smooth with no nooks-and-crannies for powder to collect in.

Table I indicates the hardness (measured using ASTM D 785-98) of various booth materials compared to a composite like the proprietary composite material formulated specifically for powder booth applications.

In summary, fast color change requires a booth that can be cleaned quickly and easily. Since the earliest days when a powder booth was a big metal box to spray into, advances have been made with better collectors and modern feed centers.

Now the booth itself has become the focus for improvement. Modern powder booths can optimize transfer efficiency through engineered airflow throughout the booth. New, specially designed composite materials offer the best combination of physical properties and cleanability. These composites can be fabricated into shapes tailored to provide the best clean-ability and larger sections to eliminate joints and seams and facilitate installation. PFD