Forty years ago American industry began to take notice of a new finishing technique: painting with dry powder. The basic concept of powder coat painting had been around for years. However, no coating equipment had been designed that made the process precise and efficient enough for it to become a viable coating solution for most industries.
But back at the end of the 60’s and beginning of the 70’s two developments coincided. Together they sparked new interest in powder coating. The first development was the ecology movement. The second was new equipment application techniques that made powder coating more suitable for production application.
A finisher who uses conventional solvent based coatings today must install expensive, non-productive incineration or absorption equipment. Reduction or elimination of the sources of pollutants is a much better solution. Current alternatives include high-solids liquid coatings, water-based coatings, powder coatings, Ultra Violet (UV) coatings and radiation-cure coatings. Of these, powder is not only the most practical, but it is also the only one which solves the pollution problem almost totally.
However, powder coating would have remained a poor solution to the problem of solvent emissions if better powder application methods hadn’t evolved at about the same time the ecology movement began. Initially, most powder coatings were applied by fluidized bed. Heated parts passed through an air-suspended bed of powder particles which melted and fused to the hot surface. This process worked, but it left much to be desired. Coating thickness was difficult to control from part to part and often varied from thick to thin on the same part. It was almost impossible to produce a film less than 5 mils thick.
By the late sixties the powder application by electrostatic spray had been developed to the point of practical industrial usage. In this process, similar to the electrostatic application of liquid paint, powder particles are charged and sprayed on a grounded work-piece. The particles adhere to the part by electrostatic attraction and melt, fuse and cure in conventional bake ovens. Spray guns are similar to electrostatic wet-paint spray guns and may be manually or automatically operated. Coatings can be applied at controlled thickness from less than one mil to more than three mils in a single pass. Thanks to electrostatic spray concept, powder coating was readied to meet the environmental challenge.
In hundreds of plants powder coating systems, ranging from a single gun, manual installations to large multi-gun automatic systems, have met or surpassed the environmental requirements . New powders have been developed, and an ever increasing product variety is being decoratively coated. Powder coated products include tractors, sewing machines, furniture, bicycles, boating accessories, spreaders, freezers, toys, air conditioners, vending machines, playground equipment, pens, power tools, lawn mowers, sprayers, lighting fixtures, copiers, stoves, fire extinguishers and several segments of the automotive industry. If someone isn’t powder coating it, they’re thinking about it. And if they’re not, they should be.
Of course, none of these products would be powder coated unless powder was economical and produced a quality finish. It does. Material utilization is almost 100 per cent. There is virtually no waste or emission. Powder coatings are tougher than comparable liquid coatings. They do not drip, run or sag. Solvent pop is eliminated. Most powder coaters report reject rates cut fifty per cent or more compared to wet paint systems. In addition, without the solvent no flash-off time is needed and hazardous waste disposal is reduced extensively.
These are the reasons why so many manufacturers and countless custom coaters have switched to powder coatings within the last 40 years. Following the energy crises they found an even greater reason to be happy about their decision. Powder coating lets them ignore solvent shortages and saves precious energy.
Every liquid coating, even water-borne, contains some organic solvent which is restricted in its use and increasingly expensive. Solvent-free powder coating relieves the manufacturer of any problems stemming from solvents. Perhaps more important, powder coatings require less energy to apply than other types of industrial coatings.
Make-up air is not required in a powder application booth. Wet systems must exhaust most or all of the booth air. Because solvent vapours are present, air from wet booth must be exhausted outside the plant. In most locations properly filtered air from a powder coating booth can be returned to the plant. This eliminates a substantial portion of the air make-up required for the finishing line and related costs of heating or air conditioning the air.
The major source of heat loss from a bake oven is due to the ventilation required to remove volatile solvent vapours driven from the coating material as it cures. In conventional solvent enamel the organic solvent content may be as high as 70 per cent. With high solids paint it is about 20 per cent. With water based paint it may range from 10 to 20 per cent. The volatile content of a powder coating is usually 1 per cent or less. Therefore, although some powder coatings may require higher baking temperatures, bake ovens require less ventilation because of the powder’s low volatile content. This means reduced heat loss and energy savings.
Even though powder coatings’ initial entry was in the automotive industry as an “under-the-hood” application where only a single colour was required, the true impact on the market came when Singer Sewing Machine decided to convert from liquid to powder. With Singer’s selection of powder as a consumer accepted coating, Singer confirmed that the powder coating is as good as or better than any other, and this benefit added to all the other advantages. At the time Singer listed cost factors related to the finishing process that saved them 51 cents per machine. Singer not only used the concept but promoted it as well. Another early commitment from the appliance industry was for the coating of domestic range side panels by a major appliance manufacturer. The powder line was installed in 1974 replacing a two coat conventional liquid line. Some of the immediate advantages noticed at the time were:
1. 50% reduction in floor space for coating operations.
2. Reject rates reduced from the 15-20% range to less than 3%.
3. System payback in less than two years.
A lighting fixtures company examined a variety of alternatives to solvent enamel for coating fluorescent lighting fixtures. They hoped to increase production capacity, reduce maintenance, control pollution, and cope with the high costs and shortages of raw materials and energy. They considered exempt solvent coatings, solvent recovery by absorption, solvent incineration, water borne coatings, electro-coating and powder. After thorough examination of all the alternatives, they chose electrostatic powder spray. Powder was not only the best solution to their problem. It also gave them a better, higher reflectance finish. This manufacturer has since converted most of its plant to powder.
All the above mentioned installations are relatively sophisticated, high-production automatic systems. But powder coating systems needn’t be fancy. There are more manual systems in operation than automatic systems. One company coats fertilizer spreaders with a manual system. Another applies powder coatings manually to instrument housings. These are only two of several thousand manual systems in operations today. Most consist of standard design packaged systems and most of them employ already existing pre-treatment facilities and bake ovens.
Cleanliness is a major advantage of powder coating over wet paint. The powder is contained in a closed system, and nearly all the powder which does not cling to the part can be recovered for reuse. In the past powder overspray recovery was accomplished by large dust collecting systems. The large volume of air required for cleanliness and safety carries the over-sprayed powder out of the bottom of the booth. Large ducts carry this air-powder mixture to cyclones and/or bag filters or cartridge filters which separate the powder, and the clean air is exhausted back into the plant.
Although this technique worked, it was in a sense the Achilles heel of powder coating. Long ducts were required. Complex designs and heavy structures were needed to withstand the high static pressure required to move large volumes of air through the large dust collecting equipment. Floor space and headroom requirements were high. Also, constant recirculation of the powder at high velocity degraded the powder. Balancing air flow through the openings in the bottom of the booth in order to completely scour out all the powder and to prevent accumulations also presents a problem.
Fortunately better methods of overspray recovery were developed by Gema of Switzerland and Interrad (USA). The system they introduced separated the powder from the high volume air flow inside the coating booth. The air passes through an endless belt of filter cloth which runs along the floor of the booth. The filter belt removes the powder from the air. The air leaving the booth is clean and can be exhausted immediately and brought back into the room through HEPA Filters, without the need for large cyclones and/or cartridge filters.
The belt carries the over-sprayed powder to the end of the booth where a small but powerful vacuum system sucks it up off the belt. The small cyclone does the same job as large cyclones and bag filters used in the old systems. This is possible because although it operates at high vacuum; the small cyclone must handle only a few hundred cubic feet of air per minute compared to the several thousand CFM passing through the booth proper. The advantages of this simple system were enormous. The system required much less floor space. Headroom was no longer a problem. Booth air exhaust ducts were shorter and lighter. Powder could not build up in the bottom of the booth. Eliminating the bottom recovery design meant that the height of coating booths could be reduced noticeably and advantageously.
Simplification of returning recovered powder to the spray gun supply hopper was a major advantage of the new filter belt recovery system. Before that there were two basic powder return techniques. In the first, the cyclone was mounted above the supply hopper and the powder returned by gravity. This was seldom workable due to the high headroom required. Thus, most installations needed a separate pneumatic conveyor to transport recovered powder from the cyclone or bag/cartridge filter back to the supply hopper. With the new filter belt system, automatic return was an integral part of the recovery system. Because of its small size, the recovery cyclone could be easily mounted right above the supply hopper so that powder returned to the supply hopper by gravity flow. Thus, one system did both jobs simply and effectively. Development of the belt system had a major affect in advancing use of the powder coating process throughout industry as a whole.
Following the belt booth design, another new concept was developed in Germany that eliminated all the ducting and large cyclones and used cartridge filters in a module that is directly attached to the booth itself.
As more customers started to convert to powder it became clear that colour change is a major problem with powder coating. This is, of course, dependent on the number of colours required. Several companies used a different booth for each colour. A major appliance company had five belt booths in line with roll-on/roll-off capability to accomplish quick colour change. Another bicycle company had 7 booths inline to accomplish colour change. This concept is still in use today, two coating booths, one for white and one for red for example. Using duplicate booths, each with its own powder recovery system, colours can be changed in less than two minutes. Later with the development of the cartridge booth concept companies started to use cartridge modules to change colours. Today’s technology focuses on colour change booth design. Many new concepts are available to help expedite colour change in the same booth.
Powder coating is the metal finisher’s most complete solution to the pollution dilemma. It eliminates the dual problems of solvent emissions and solid wastes. But more than that, powders produce superior finishes practically and economically. Last, but certainly not least, powder coating conserves precious energy. Today Powder Coating is the fastest growing finishing concept.