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Introduction to Powder Coating
Powder coating is a dry finishing process in which free-flowing, thermoplastic or thermoset powder material, rather than a liquid suspension, is applied to a surface, melted, and then allowed to dry and harden into a protective coating. Typically, the powder is applied via electrostatic spray deposition (ESD) to an electrically grounded metal substrate. Once the powder coating is completely applied to a part, it is then heat cured in an oven until the powder melts and flows together. Powder coatings can also be applied by dipping preheated parts into powder material within a fluidized bed. The powder melts and fuses with the part’s heated surface upon contact. In either case, as the part dries and cools, the cured powder forms a smooth, hard finish.
Powder coating services are suitable for various materials, including metals, plastics, glass, and medium density fiberboard (MDF), and can provide both functional and decorative surface coatings in a wide range of colors, styles, finishes, and textures that are not easily achievable by conventional liquid coating methods. Powder coatings are also comparatively durable, cost-effective, and environmentally-friendly. However, they may not be suitable for all manufacturing applications, such as for low budget, thin film, or large part coating applications.
While each coating method has its advantages and disadvantages, this article focuses on powder coating. It explores the benefits and limitations of the powder coating process and provides comparisons between the powder coating and liquid coating methods.
Advantages and Benefits of Powder Coating
Compared to conventional liquid coating methods, there are several advantages and benefits of powder coating. These include:
- More durable coatings
- Capabilities for thicker and specialty finishes
- Less environmental impact
- More efficient curing and drying processes
- Greater material utilization
The powder coating process produces a long-lasting and durable, hard finish that is both protective and decorative. Demonstrating more resistance to impact, moisture, chemicals, and extreme weather than most liquid coatings of similar thickness, powder coatings offer a part or product greater protection from scratches, chipping, abrasions, corrosion, fading, and general wear. They can also act as electrical insulators and can withstand hundreds of hours of salt spray contact. For colored powder coatings, even after being subjected to extreme environments, the color remains brilliant and vibrant. This durability makes powder coatings well-suited for abrasive, high traffic, or high use areas, as well as diminishes the need for future coating touch-ups or replacement.
Powder coatings can be applied to a variety of metal and non-metal materials in a wide range of thicknesses, colors, finishes, and textures that are not readily achievable through other coating methods.
They can be easily produced in much greater thicknesses compared to conventional liquid coatings without running, dipping, or sagging. Additionally, since powder coatings are composed fully of solid coating material, while liquid coatings contain only a percentage of solid material, producing a coating of a specific thickness can be achieved with fewer passes of powder than with the use of a liquid coating. This efficiency not only decreases the turnaround time for a finished product but saves on the amount of coating material required and the energy costs of curing and drying between coats.
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Powder coating material can be formulated in virtually any color with various protective and decorative physical and mechanical characteristics. Available finishes for powder coatings include, but are not limited to, flat, satin, glossy, metallic, clear, iridescent, and glittered, with available textures ranging from smooth to wrinkled and matte to rough. Textured coatings are suitable for hiding imperfections on a material’s surface, such as recesses, depressions, scratches, and dimples, as well as for diminishing the need for further finishing steps.
With contemporary social and legal attitudes towards ecological awareness, pollution control is an increasing concern in all areas of manufacturing. For example, manufacturers are usually required to file for permits and follow strict environmental regulations on the amount of solvent and volatile organic compounds (VOCs) emitted into the atmosphere or else face penalties and fines. The use of powder coatings offers an alternative that minimizes the amount of air pollution control equipment required while facilitating compliance with these regulations and restrictions.
Compared to liquid coatings, powder coatings do not require solvents to maintain the coating suspension. Solvents contain VOCs which are detrimental to the environment. In liquid coatings, the average VOC content is between 3.5 to 5.5 pounds per gallon, which translates to approximately one-third to one-half of each gallon of the liquid coating being emitted into the environment and atmosphere during the application process. Since powder coatings do not contain any solvents, they emit little to no VOCs throughout the finishing process.
Some other environmental advantages of powder coatings include less carbon dioxide emissions, no hazardous waste requiring disposal, and no need for the use of a primer before the coating application. For aluminum, this is especially beneficial since the primers used on aluminum material usually contain chrome, which is toxic and has carcinogenic properties.
Curing and Drying Process
Compared to liquid coatings, powder coatings require much shorter curing, processing, and drying times. Liquid coatings generally require several hours without heat, and forty-five minutes to an hour with the use of heat, before they are sufficiently cured to be handled in assembly operations. Once assembled, the coated parts are usually left to dry completely overnight before being packaged for shipment. Powder coatings, on the other hand, can be fully cured, on average, in ten minutes. Once cooled enough to handle, the part can be assembled and then packaged immediately. This further decreases the production time for a completed product or assembly, as well as reduces the number of parts that are left in staging areas to be assembled, packaged, or shipped.
Powder coatings also generally require less air and air exchanges within the spray booth and curing oven. Due to safety and environmental regulations, liquid coating equipment requires higher volumes of air and air changes within the equipment to filter out VOCs. The spray booths also need to vent outside of the work area due to the presence of solvents in the coating material. Since powder coatings do not contain solvents or VOCs, the heated air can be recycled, and then merely filtered and exhausted into the workshop. This advantage saves on energy usage and the overall costs for the curing and drying process.
In powder coating applications, nearly all coating material is utilized. Unlike with the application of liquid coatings where the oversprayed coating material is immediately lost and wasted, undamaged and uncontaminated powder overspray can be reclaimed and recycled for future coating applications. Additionally, as powder coatings do not contain solvents, the material is not lost to the atmosphere during application, leading to a powder coating material utilization rate of close to 100%.
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Limitations and Disadvantages
While powder coating demonstrates several important advantages over liquid coating, there are also some limitations to the process. These include:
- The range of suitable materials
- Difficulty producing even, thin coatings
- Lead times for custom coatings and color change
- Longer and more costly drying/curing for large parts
- Greater start-up costs
Powder coatings are primarily used for metal materials, such as steel, stainless steel, aluminum, and brass, but they can also be applied to a variety of non-metals, including glass or fiberboard. The range of materials that are suitable for powder coating is limited by the fact that powder coatings must be heat cured. Therefore, any materials, especially non-metals, undergoing the powder coating process need to be able to withstand the curing temperatures of the oven without melting, burning, or deforming.
Additionally, while metals can be electrically grounded and powder coated via electrostatic spray deposition, non-metals require a fluidized bed powder coating application. For the latter powder coating method, a consistent and even coat is much harder to achieve than with the former. The need for a fluidized bed also increases the overall equipment costs for the establishment of a complete powder coating facility.
Thin Film Coatings
While the powder coating process is excellent at achieving smooth and even thick coatings, it is difficult to produce the same with thin (<6mils) coatings. This limitation results from how powder coatings are applied, as it is challenging to control the amount and speed at which the powder is applied to the substrate. Additionally, while thicker coatings can more easily hide surface imperfections, thinner coatings tend to display more orange-peeled textures. This condition can be attributed to inadequate pre-treatment, too much, too little, or improperly applied powder, and insufficient curing times or temperatures. In any case, when a smooth, thin coating finish is required, it is much more difficult to achieve the result with the powder coating method than with the liquid coating method.
Custom Coating Material and Color Change
The powder coating color is determined by the plastics that are ground to create the powder material, and the materials are usually produced in minimum consignments, which may not prove to be cost-effective for custom, small batch coating needs. Additionally, powder coating material cannot be mixed together to create different colors, as each formulation is custom-made to exhibit specific physical and mechanical characteristics. This fact will affect lead time and turnaround time for a completed part or product.
Switching between colors during the powder coating process also affects lead and process times. Since overspray is usually reclaimed and recycled for future powder coating applications, the spray booth area much be meticulously emptied and cleaned between each color change. Otherwise, there is a risk for cross-contamination between different powder materials, which can affect the final color and finish of a part.
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Curing and Drying Process for Large and Thick Parts
Generally, powder coatings require much shorter curing, processing, and drying times than liquid coatings. However, for thicker, larger, and heavier parts, the curing and drying time may increase and require more energy. This increase is because the curing process is highly dependent on the bulk temperature of the part, meaning that the entirety of the part needs to be sufficiently heated for the surface temperature to melt and cure the powder coating. Achievement of this desired temperature requires larger volumes of heated air and exhaust and takes more time to complete, which may make conventional powder coating of large parts less cost-effective than using the liquid coating method.
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Powder Coating Cost
In the long-term, powder coating may prove to be cost-effective due to the durability of powder coatings, shorter processing and curing times, smaller environmental impact, and greater material utilization rates. However, the initial powder coating cost and the investment in powder coating equipment can be more substantial than that of liquid coating.
At a minimum, liquid coating applications only require a spray gun and spray area. For powder coating, however, a workshop needs at least a spray gun, spray booth, and curing oven in order to complete the powder coating process. The addition of the curing oven adds significantly to the initial start-up costs for powder coating. It can also be a limiting factor to the maximum part size that can effectively be handled; larger, thicker, and heavier parts require a larger and more powerful oven, which further adds to the overall equipment costs.
Powder Coating – Summary
Outlined above are the benefits and limitations of the powder coating process. Table 1, below, illustrates the various advantages held by the powder coating and liquid processes as described in the previous sections.
Table 1 – Comparisons between Powder Coating and Liquid Coating
Capabilities for Specialty Finishes
Thick Film Coatings
Thin Film Coatings
Range of Suitable Materials
Processing and Drying Time (general)
Processing and Drying Time for Large Parts
Coating Material Utilization
Color Change Lead Time
While powder coating offers itself as a viable alternative to conventional liquid coating methods in regards to durability, specialized finishes, environmental impact, turnaround time, and long-term costs, specific applications such as thin film, color changing, or large part coating operations are still more suited for liquid coating methods. These are just some of the considerations that may be taken into account by manufacturers and machine shops when deciding whether powder coating is the most optimal solution for their particular coating application.
For more information on domestic commercial and industrial suppliers, visit the Thomas Supplier Discovery Platform, where you will find information on over 500,000 commercial and industrial suppliers, including over 1,500 powder coating service providers.
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