Every small but successful coating operation inevitably faces the moment when existing practices are no longer sufficient to meet customer demand. Continued growth requires change. But how can you add automation to your business without compromising on flexibility?
A well-designed batch powder coating system can provide the answer. Encompassing part preparation, powder coating and curing, batch systems can allow you to retain the flexibility of a manual coating operation while increasing your plant’s throughput and customer response time.
Spray wand systems can be used on all metals; however, they are not able to remove heavy scale or rust.
Preparation
Parts are typically cleaned using solvent wiping, blasting, dip cleaning or a spray wand system. The selection of the pretreatment system depends on the type of substrate and soils encountered, the level of quality required, the plant’s budget, and the amount of space available in the plant.Solvent wiping is a low-cost method that requires little space. Typically used for cold rolled steel and nonferrous metals, it can remove light soils and achieves modest quality requirements.
Blasting can be used on all metals to remove heavy rust and scale. It achieves a fair quality but requires a significant capital investment and more space than solvent wiping.
Dip cleaning exposes the whole part to a chemical solution. Many different combinations of treatment can be used, and no mechanical cleaning is required. However, rinsing can be difficult.
Spray wand systems can be used on all metals; however, they are not able to remove heavy scale or rust. They provide good quality but require a significant capital investment and a substantial amount of space in the plant.
Wet parts are usually dried manually with air blow-off knives or in ovens, depending on the layout of the plant.
Guns usually are charged with corona charging (not shown) or tribo charging (above).
Coating
Following pretreatment, the substrate is ready to be coated. Small, low-cost powder booths can be used, but these can handle only limited part sizes, and the parts usually require a substantial amount of handling by personnel.Walk-in booths are more expensive and take up more space in the plant, but they allow for greater part flexibility, less handling and higher volumes. Most walk-in systems can handle complex-shaped parts, very long parts, parts requiring low or high film builds, and very large or small parts.
Installing powder coating booths in an environmental “clean room” can optimize the powder application and ensure dirt-free parts. Modern clean rooms usually are equipped with automatic climate control systems designed to maintain the environment at optimal coating conditions. All internal booth surfaces should be reasonably smooth for easy cleaning
The powder gun feed systems can either be feed or box hoppers. Guns usually are charged with corona charging, in which a static electric charge is induced on the powder particles by passing the powder through an electrostatic field generated by a high-voltage device; or tribo charging, in which the static electric charge is generated by creating friction between the powder particles and a nonconductive material.
Powder conditioning provides a controlled atmosphere for better powder coating application and a superior cured finish appearance. The air conditioning system should maintain the temperature between 65 and 75°F and humidity between 45 and 65% under a full load.
The powder room should be maintained under positive pressure to prevent airborne contaminants from entering the part openings. The design air velocity through the booth openings and door should be maintained at about 125 feet per minute (fpm). The supply registers should be distributed uniformly through the room and sized for air velocity less than 150 fpm, and the return air should be filtered to prevent powder as small as 2 microns from entering the exchanger coil.
Material handling equipment for powder coating systems can be monorail, power and free, spindle or cross-bar conveyors to allow for a multilevel, compact, space-saving design.
Convection ovens can accommodate a range of part sizes and shapes.
Curing
After coating, the parts are placed on hooks or racks for curing. Carts should be built for flexibility.Curing takes place in either convection or infrared ovens. Convection ovens, which typically are powered with natural gas, require a longer cure cycle than infrared systems. However, they can accommodate a range of part sizes and shapes, and they have a relatively low operating cost. Although the price of natural gas has been rising over the past several years, it is still less expensive than electricity in most regions.
Infrared systems, which typically operate on electricity, are smaller and use shorter cure cycles than convection ovens. However, they are limited in the part mass and geometry that they can accommodate, and they tend to have a higher operating cost. While combination convection/infrared ovens can be used for large production runs, they usually are not economical for small batch operations that handle a variety of parts.
Ovens for batch powder coating systems should incorporate an electronic timer to allow you to optimize the curing cycle for each batch. Most timers offer easy, one-button operation once the correct curing cycle has been programmed into the system. To optimize energy efficiency, ovens also should incorporate insulated panels that are a minimum of 6 in. thick.
Depending on the plant layout, the oven can be designed with a single door or a pass-through design, in which parts enter the oven on one side and exit on the other using an automated system. The oven’s heater box can be placed on the top, side or back of a single-door oven, or on the top or either side of a pass-through oven, based on the amount of space available in the facility.
One other consideration for oven design is the airflow. When curing powder coatings, the air should not impinge directly on the parts or it will adversely affect the finish. Some powder curing ovens incorporate a two-speed fan that uses low volume when parts are first placed in the oven and then switches to a higher volume after the powder has melted. The use of a dual-speed system can reduce cure times and increase efficiency.
To optimize efficiency further, some plants also use their curing ovens to dry parts after washing and before coating. If the ovens are otherwise idle, this can be a good use of equipment and can free up personnel to handle other tasks.
A Successful Operation
Batch powder coating systems offer a number of strategic advantages, including the opportunity to run a variety of part sizes, the ability to achieve a fast setup for color changes, and the ability to respond quickly to customer needs.To ensure a successful powder coating system, make sure you allow the proper amount of space in your plant, and invest in good-quality equipment that will allow you to retain or improve your coating flexibility. Once your new equipment is installed, be sure to follow good maintenance practices, seek work that fits your batch operation, and continue to improve the education and skills of your finishers. By installing the right system and seeking continuous improvement, you can successfully grow your powder coating operation.
For more information about finishing systems, visit www.kmisystemsinc.com.
Editor’s note: This article is based on a presentation from the FIN-X ’07 Conference Proceedings (©FIN-X ’07). Used with permission.
