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The time it takes to change color in a traditional powder coating system has long been a shortcoming of powder coating technology. For more than two decades, however, the economical, environmental and finish quality benefits of this technology far outweighed its color change limitations. Long color change times were a relatively small price that finishers were willing to pay for the ability to reclaim powder overspray and achieve material use efficiency as high as 99%. Recently, rising consumer demand for greater color variety, combined with a shift to just-in-time operation, short lead times and custom-color production runs, has turned color-change time of conventional powder coating systems into a significant obstacle to the continuous growth of this environmentally friendly finishing technology.

In response to increasing market demands, leading powder coating equipment manufacturers have invested heavily in research and development to discover new technological solutions that shorten color change times to as little as 10 min.

To better understand how new technology can significantly reduce color change time in a powder coating system, let us first look at the equipment and techniques traditionally available to powder coaters.

Conventional Color Change Methods

Several coating systems solutions have traditionally been available to powder coaters working with multiple colors and types of coating materials. The selection of a solution most suitable for each particular application is based on several key considerations: system efficiency; number of colors; frequency of color changes; budget; and available space. In most cases, a powder coater faces multiple tradeoffs between these five parameters.

Regardless of whether a cartridge- or cyclone-based powder recovery method is used, color change time in a powder spray system is strongly affected by several key factors.

System Size. A larger booth takes longer to squeegee clean all internal surfaces. The more spray guns used, the more powder is circulated through the system, and, therefore, more application hardware needs to be cleaned.

Frequency of Color Changes. The duration of operation with each color and the frequency of color changes play an important role. Even in a small hand gun system, color change time will be strongly affected by how long a given color is sprayed. The longer a system runs with one color, the more residual powder accumulates inside the delivery tubing, pump and spray gun. And, with many powders, the probability of impact fusion inside the delivery and recovery system components also increases.

Application System Efficiency. More efficient powder application on parts generates less overspray, making it easier to clean the booth.

Application System Design. Several important factors must be considered. How easily can the system be disassembled for cleaning and inspection? One must also consider the powder path design and construction inside the pump, delivery tubing and spray gun. A greater number of internal powder path components exposed to the direct impact of powder particles makes impact fusion on those components more likely. A greater number of parts that must be connected together increases the chance of gaps between the powder path components where powder can accumulate.

The material that internal, powder contact components are made from is a consideration. Improperly selected materials can either cause rapid wear of internal spray system components or make them prone to impact fusion with a variety of powders.

Spray Booth Construction and Design. Material for the booth wall and ceiling strongly affects the degree of powder accumulation as well as the efforts required to remove that powder during color changes. Also, the smoothness of internal surfaces and the number of seams between booth panels have an effect on color changes.

Aerodynamics inside the booth also affect color changes. Improper air flow inside the booth can reduce the application efficiency of a powder spray system and increase overspray, as well as create areas inside the booth where oversprayed powder tends to accumulate.

Color Change Resources. Not only is the number of people important, but also their skill level, familiarity with color change procedures and their ability to work as a team.

Color Change Quality. This is also one of the most important factors. Color change time can vary significantly depending on the tolerance of the finished product to color contamination. A manufacturer of warehouse shelving components may be more tolerant to color contamination and may follow a more simplified color change procedure than a supplier of kitchen appliances where any color contamination constitutes a reject.

When changing from beige to light brown, many powder coaters can afford to be less than meticulous about color change quality. Thus, if the production schedule allows for gradual transitions from light to dark colors, color change time can often be reduced.

In general, the time it takes to perform a quality color change in a conventional cartridge- or cyclone-based automatic powder coating system can typically range from 30-120 min, depending on the combination of factors listed above.

Figure 3 illustrates four types of system color change solutions traditionally available to coaters, which depend on two main factors: 1) the number of colors sprayed and 2) the frequency of color changes.

Cartridge Filter System. Cartridge-filter-based powder recovery systems have become the industry standard because these systems capture and recover 99% of oversprayed powder and can deliver overall system efficiency of more than 96%, even when powder coating wire goods, where application system efficiency rarely exceeds 20% (Fig. 2).

A small, single-booth cartridge-based powder coating system can be used in multi-color operations if the frequency of color changes does not exceed about three changes per shift and downtime, cost permitting. Typically, single-booth cartridge systems are used in operations where up to three to four different types of powder are automatically recovered and the colors are not changed more than every 3 to 4 hr.

The decision about how many designated color modules to invest in and how many different powders to reclaim is relatively easy. It is typically based on the payback period calculation and the amount of floor space available for storing multiple color modules.

The volume of each powder used typically declines (assuming overall production capacity is unchanged) with each color change; therefore, the payback period on each designated-color cartridge module becomes longer. When the number of reclaimed powder types increases, the use of cyclone-based recovery systems often becomes economically attractive.

Cyclone Systems. In contrast to cartridge collectors, cyclones do not collect powder; they separate it from the air stream making it available for reuse. Because cyclones do not retain powder material, no designated color powder recovery equipment is necessary.

Conventional cyclone systems typically do not offer any considerable color change timesaving. However, systems using the cyclonic powder recovery method often become economically attractive in applications where more than four colors are automatically reclaimed.

When deciding to select a cyclone-based powder recovery system, one should consider that cyclone separator efficiency typically ranges between 85-95%. This is considerably lower than the 99% efficiency delivered by cartridge collectors. Thus, depending on the application and recovery system efficiency, production volumes with each color, price of powder and cost of designated color modules, the number of colors at which cyclone systems become economically advantageous compared to cartridge systems can range from four to seven colors.

Although cyclone powder recovery systems are suitable for powder coating operations where multiple colors are reclaimed, the type of powder recovery method does not result in color-change time reduction. When powder coaters want to recover multiple powder materials and change colors frequently, every 20 min of every hour for example, a conventional, single-booth powder coating system can't meet the demand.

Roll On/Off Multi-Booth Systems. Historically, the solution has been multi-booth systems. The use of multiple booths delivers the fastest color change times. One booth is rolled offline and the booth with a different color is rolled online. This can typically be done in less than 5 min. Then while the one booth is in use, the other can be readied for yet another color.

The other way to implement a multi-booth system is by using a power-and-free conveyor to route production through designated color booths. Although this approach is quite expensive, it does provide on-demand production flexibility in systems working with relatively few colors.

Although virtually no production time is lost with multi-booth powder coating systems, there are a number of shortcomings as well. First, they require significant capital investment in equipment and floor space. Second, it still can take up to 2 hr to change colors in an offline booth. Thus, if the colors must be changed every hour, one would require three spray booths and a crew of two to three people continuously changing colors one booth after another.

What if one wants to work with a few dozen colors and be able to change colors on-demand every 30 min? Just a couple of years ago the answer would be spray to waste. Quite an expensive solution. Most recently, however, several fast color change systems have been introduced. Some of these systems have reduced the time it takes two people to change color down to 10-15 min.

Fast Color Change Systems

To achieve a dramatic reduction in color change time, a system approach is required. Equipment suppliers must look at a complete powder coating system:

• Determine all sources of potential color contamination;
• Evaluate how much each system component contributes to color change time and to potential color contamination;
• Establish acceptable tradeoffs and recognize possible limitations;
• Redesign each subsystem to optimize it for cleanability;
• Closely integrate all system components;
• Automate as many tasks as economically justifiable;
• Establish precise color change procedures and communicate them to the end user.

Powder Application System. Cleaning a powder application system is typically the first step in a color change process. A powder application system consists of the powder pump, powder delivery hose and spray gun. Experiments show that the powder delivery hose is not only the most difficult part of the system to clean, but is a major contributor to color contamination when changing between high contrast colors. Thus, one of the main tasks in reducing color change time and improving the quality of color changes in powder coating systems is to reduce the length of powder delivery tubing, optimize hose routing and select the tubing material that reduces powder retention and impact fusion inside the hose.

Most leading brands of powder spray guns and pumps are already streamlined for easy cleanability. Each individual spray system does not take a long time to disassemble and clean. The task becomes quite time consuming and laborious when manual cleaning of a 24-gun spray system is required. This fact, as well as the simple, streamlined construction of some spray systems, has made them prime candidates for automated cleaning.

In modern color change systems, pulsed bursts of compressed air are frequently used for automated cleaning of powder spray systems. Some modifications to application system components are often required to enable pulse cleaning and to ensure system integrity and tightness of joints after the impact of compressed air pulses.

In addition to automated cleaning of internal components, modifications to the external design of spray guns are often required to minimize powder accumulation on gun surfaces and enable automated cleaning.

Depending on the required color change quality and contrast of powder colors, the application system cleaning can be either fully automated or require minimal operator involvement. In either case, application equipment color change takes only a few minutes.

In applications demanding very high color change quality between high contrast colors, the use of two sets of powder delivery tubing (one each for dark and light colors) is recommended.

Powder Spray Booth. To optimize booth construction and design for color change, several factors must be considered: booth size and surface area requiring cleaning; canopy material and construction; and airflow inside the booth.

To minimize the surface area requiring cleaning, equipment suppliers have to optimize the total system layout. Equipment positioning around and inside the booth should ensure maximum operating efficiency and easy access for cleaning. In addition, all ancillary systems must be closely coupled with the booth to reduce the length of powder delivery and recovery paths.

Construction and booth canopy material has a major influence on the ease and speed of color changes. It is generally accepted that the use of dielectric plastic and composite materials for the booth canopy significantly minimizes powder attraction to booth walls and often contributes to higher powder application efficiency.

However, even when working with dielectric wall materials, there is a significant difference in cleanability and powder retention of different plastics and composites. Some materials allow powder coaters to simply blow small amounts of powder from booth walls with compressed air while others still require a squeegee.

Because of the variety of materials that can be used for booth canopy construction, multiple theories and claims can be made highlighting the benefits of one solution over another. Due to complexity of material science and electrostatic phenomena, prospective fast color change system users should simply try cleaning different booth materials to determine which solution works best.

Powder attraction to booth walls and how easily it can be removed is not the only criteria for selecting booth material. It is also important to minimize the number of panel joints inside the booth. Even a smooth, initially well-filled joint can become a potential powder trap after the booth has been in operation for a while. Therefore, every attempt should be made to select a booth wall material that allows manufacturers to minimize the number of wall joints.

Optimization of airflow inside the booth has dual importance. First, sub-optimal booth airflow can create "pockets" inside the booth where oversprayed powder will accumulate. Second, airflow in the spray application area has to be minimized for increased application efficiency and reduced powder overspray.

In addition to optimizing booth layout, construction and design, several technical solutions are available to minimize the amount of powder remaining on the booth floor at the time of color change. Some systems use moving floor belts that continuously remove oversprayed powder from the booth. Others use automatic floor sweepers that travel along the booth floor. The use of these and other types of booth cleaning devices can be important in applications with a large number of spray guns and long intervals between color changes.

Powder Recovery System. For powder recovery systems, we should not only consider that portion that separates the powder from the air stream, but also the equipment for transfer and screening of the recovered material.

Since fast color change systems are used in applications working with more than four to five different colors, cyclone-based powder recovery systems prove to be most suitable and are implemented with all currently available fast color change systems.

Although most cyclone systems look similar, there are numerous designs that affect powder separation efficiency, the amount of powder that can remain inside the cyclone after color change and the impact fusion of powder inside the cyclones. In addition, depending on the required color change time and quality, consideration should be given to how closely coupled to the booth the recovery unit is, the presence and length of ductwork and accessibility to the inside surfaces of the system for inspection and cleaning.

Once oversprayed powder is separated from the air stream, it has to be transferred back to the feed system and screened for mechanical contaminants. In conventional powder coating systems, transfer pumps are used to move powder from the recovery unit to the sieve/screener and the feed system. With this method, significant volumes of compressed air are used to transport powder through transfer hoses. Due to some specifics of powder screening and feed systems used in fast color change application, the volume of compressed air used for powder transport should be minimized. This requirement has led equipment suppliers to find alternative methods for reclaimed powder transfer.

The way recovered powder is screened in fast color change applications is also different from conventional powder coating systems. Most conventional high-volume automatic powder coating systems use rotary screeners (or sieves) for screening and conditioning of reclaimed powder. Rotary sieves are field-proven, high-throughput technology, but they are virtually impossible (within reasonable limits) to color change.

Thus, vibratory screeners are widely implemented in fast color change systems. Depending on system design and layout, a screener is typically either integrated into a powder recovery unit or is part of the feed system. For increased throughput capacity, vibratory screeners are sometimes ultrasonically assisted.

Powder Feed System. To meet the demands of fast color change applications, powder feed systems have undergone dramatic transformations. Even terminology has changed. What once was a plain feed hopper is now a feed center or color kitchen.

Feed centers from different equipment suppliers differ slightly in design, features and degree of automation. However, almost all of them serve three main purposes: enable quick change of powder containers; assist in pulse-cleaning of powder spray systems; and provide a clean operating environment. Depending on the frequency of color changes, a feed center can either feed powder directly out of the original shipping boxes or use a modified version of a fluidized hopper.

A feed center in which powder is fed directly out of a box is most appropriate for applications with frequent color changes since the volume of powder in the box is limited, and return of reclaimed or addition of fresh powder to an open box has its limitations.

For systems with longer intervals between color changes, a feed center with a fluidized hopper may be preferred, since it provides more consistent powder delivery, simplifies the return of reclaim and the addition of fresh of powder into the system. It also provides for good reclaim/virgin powder mixing and conditioning.

Most powder feed centers also contain a set of nozzles used for pulse cleaning of the feed system.

Feed center enclosures are typically kept under a negative pressure. This ensures that any airborne powder resulting from powder transfer or cleaning operations is drawn either into a designated set of cartridge filters or the after filter section of the reclaim system to provide for clean operating environment.

In this article we reviewed traditional color change methods used in powder coating systems, analyzed their limitations and the critical factors affecting color change times. We also discussed new technical solutions implemented in modern fast color change systems. PFD