The What, Why, and How of Powder Coating
A Practical Approach to Powder Coating Justification
What is powder coating, why should I consider using it, and how will it work in my operation are questions that every finisher needs to be asking today. Briefly, these questions can be answered as follows: 1) powder is a dry, clean finish that gives a highly durable coating, 2) in the long run, powder will save you money, and 3) a powder coating system can be designed to meet just about any requirement your operation may have. The purpose of this booklet is to explain quickly and simply how powder coating delivers these highly desirable solutions to many finishing problems, and to provide you with a practical, worksheet approach, to help you decide if moving to powder is the right move for you.
Over the past decade powder coating has been increasingly accepted as the preferred finishing process for the future.
Click HERE for a Glossary of Powder Coating Terms
The reasons for this conversion from wet to dry can be attributed to three major forces: economy - the high cost of energy and materials require a more cost effective and less wasteful process; excellence - consumers, and other end users, are demanding higher quality and more durable finishes; and ecology - progressively more stringent regulations are being aggressively enforced in an effort to control air pollution and hazardous waste disposal. Which in turn goes back to economy. The cost of complying with the Regulations and the disposal of toxic and flammable waste are constantly rising.
Many companies have found that it is less expensive to convert to powder than to bring their wet systems into compliance!
With strong financial arguments providing the necessary stimulus, therefore, powder coating technology has evolved quickly. The quality and variety of powders available have grown as an increasing number of companies get involved with its production; and the technology for spraying the powder, collecting it and reusing it, is constantly improving transfer efficiencies and material utilization, and reducing color change times. Early perceptions that powder was too difficult to control, too costly to install, too time consuming for color change, or did not provide enough choice of color are largely obsolete today.
But the move to a totally new technology can still be confusing. This brochure is designed to make your decision easier, providing both explanations and justifications to help determine the right solution for you. So let's start at the beginning and work through the questions.
What is Powder Coating?
Powder is a dry coating. Instead of being dissolved or suspended in a liquid medium, such as solvent or water, powder is applied in a granular form. This material is finer than ground pepper but coarser than flour, and is applied directly to the surface to be coated.
The powder is created by blending the various components (binders, resins, pigments, fillers and additives) and processing them through an extruder into a continuous mass. This homo-genous mass is cooled and broken into small chips, which are then ground into the powder. Each powder particle contains within it the necessary components for reforming into the finished coating. After the powder is applied to the part, typically using an electrostatic spray process, the part passes through an oven and cures, melting into a smooth film on the surface of the part.
There are two distinct types of powder, Thermoset and Thermoplastic. The Thermoset powders are reactive, which means that under cure conditions there is a chemical "crosslinking", so that, once cured, the coating will not remelt. Epoxies, acrylics and most hybrids are examples of Thermoset powders, making up over 90% of the current powder market.
Thermoplastic powders do not "crosslink" when cured, but simply melt and flow over the surface of the part. The film hardens on cooling, but if it is reheated it will remelt. Vinyls, nylons and fluorocarbons are examples of thermoplastic powders.
Enamel powder is a specialized formulation used in applications that previously used liquid porcelain enamel. It utilizes glass in its formulation and is cured, or fired, at a very high heat. The result is a finish that is particularly resistant to heat, scratching and harsh chemicals, and is typically used in appliances, such as washers, dryers, ovens and ranges.
How is the Powder Applied?
The application process involves applying a charge to the dry powder particles and spraying them onto a grounded substrate. The substrate, or part, is typically grounded through the conveyor or hanger holding the part. The powder, once attracted to the part, is then held on the surface until it is melted and cured into a smooth coating film in the bake oven. The spray process takes place inside a booth designed to contain the oversprayed powder and makes it possible to collect the overspray and ultimately reclaim it for reuse.
The powder is fed pneumatically out of the powder container, or hopper, into the powder applicator, or gun. As the powder exits the gun, a low amperage, high voltage charge is applied to the powder particles, causing them to be attracted to the grounded workpiece. This attraction may even cause the powder to "wrap" around the piece, coating the back side.
The oversprayed powder, suspended in the air contained inside the powder booth, is then passed through a separation process that permits the powder particles to be retrieved from the air. The clean air is fed back to the work environment, eliminating the need for air make-up. The reclaimed powder is mixed with a proportionate amount of fresh, virgin powder for reuse achieving consistent results and up to 98% material utilization.
How Will Powder Coating Benefit My Operation?
Reviewing the potential benefits of powder coating will give you an incentive to proceed with the quest for the best finish for your operation.So let's look at the three "E's" of powder coating in more detail.
- Material utilization is much higher with powder, making your material costs much lower. 92% to 98% of the powder you buy will be applied to the parts you are finishing versus an average of 60% with an electrostatic liquid system (the other 40% is waste and must be disposed of!)
- Since most of the material is used on the part, there is very little waste to be disposed of. And powder is not considered hazardous waste, so the cost of disposal is minimal compared to the high cost of toxic waste disposal.
- Air used to exhaust the powder spray booth is returned directly to the plant, eliminating heating and cooling costs for the make-up air required when air is vented outside the plant.
- Air loss from the curing oven is minimized as there is only a very small amount of volatile substance that must be exhausted. The cost of maintaining oven temperatures is therefore minimized also.
- Powder is simpler to spray, so less skilled labor is needed, training is easily done and less errors are made in coating. All of which saves scrap, labor and, ultimately, operating costs.
- Most powders require no primer, providing more savings in time and materials.
- The cured powder finish is less susceptible to damage than a liquid finish. There is less need for repair work on the finished item, and packaging is less elaborate, saving time and cost on rework and packaging.
- Epoxy, acrylic and hybrid powders provide excellent adhesion and hard-ness for improved resistance to chipping, abrasion, corrosion, and chemicals; and it's flexible enough to be formable without cracking.
- Polyester powders provide additional advantages in ultraviolet and weathering resistance.
Powder is the overwhelming preference of the EPA, eliminating:
- Solvent fumes and VOCs from spray booth and oven exhausts that pollute the air.
- Potentially toxic sludge and water that can contaminate the earth and must be disposed of as hazardous waste.
How Will a Powder System Work in My Operation?
Powder systems are available in all degrees of technical sophistication, and can be designed to meet a wide range of requirements for perfor-mance, cost and space constraints.
From a basic manual, one gun operation with a batch booth, to a highly complex multi gun, totally automated configuration, there are guns, booths and other peripherals for all occasions.
The guns are all essentially similar in their function of spraying the powder. Each gun has a control unit that regulates the voltage being generated and the rate at which the powder is delivered from the hopper. Areas to review when selecting the application equipment are the efficiency of the charge, the consistency of the powder flow, and the accuracy with which both can be adjusted to provide the right level of performance. Once the optimal settings have been established it is important that they can be repeated systematically each time they are used.
Booths and Recovery
Powder booths are also basically similar, but use two distinct types of recovery equipment, cartridge filters or a cyclone separator. Each style is particularly suitable for a different type of application. To select the appropriate system you should consider the production batch size, and the number of different types or colors of powder being used and the frequency with which they are changed. The finished quality of the desired coating should also be considered when making the selection.
Therefore, prior to starting your search for the perfect powder system, you should have a clear picture of what you need and what factors are important to your operation. If you expect to change powders frequently, then a fast color change time will be high on the list of necessary features; if, however, you do long runs using the same powder throughout, then a highly efficient reclaim system will be more critical. If the parts are all the same then automatic guns may be cost effective, but if they are intricate structures that are difficult to coat, manual operators may make more sense.
Now that we have covered the basics you should have a better idea of what powder is all about. On the following pages are some work sheets that will show you just how powder can deliver savings to your operating costs. After working through them, you may be surprised at how quickly a powder system may pay for itself - and it will keep you in compliance, no matter how much more stringent the regulations become!