Powder Application Methods

Article From: Products Finishing, , from Powder Coating Institute

Posted on: 7/1/2000

Concurrent with the technological advancements made in the formulation of powder coatings are new and innovative ways to apply powder and improve powder application efficiency.

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Concurrent with the technological advancements made in the formulation of powder coatings are new and innovative ways to apply powder and improve powder application efficiency.

Pre-Assembly Coating

Coil Coating. Coil coating technology for powder continues to be developed. Coil coating is the coating on one or both sides of flat metal sheets or strips on a continuous production line basis that are then cut and post-formed. Coil coating with powder can produce deep textures in a single pass and unique visual effects. It is used for a variety of products, such as appliances, lighting fixtures, window and doorframes, solar sunscreens and other building components.

Blank Coating. Powder coating of precut metal blanks that are then post-formed prior to final assembly remains a strong growth area, particularly in the appliance market. This process allows for complete edge wrap, uniform film thickness and high transfer efficiency as blanks can be spaced only inches apart on the coating line.

Curing
Radiation Curing cures a coating by exposing it to electromagnetic waves or particles such as infrared, ultraviolet or electron beam. Radiation curing opens up new applications for the powder coating of heat-sensitive substrates such as wood, wood-based materials, plastic parts and assembled components with heat-sensitive details by reducing the curing temperature below 250F. The coating of metal substrates can also benefit from this technology with lower energy and investment costs, shorter curing times and higher line speeds in powder coil coating plants.

Infrared Curing ovens use radiant energy to cure the powder on the part, sometimes in as little as 30 sec. The high heating rates possible in an infrared oven allow the surface to be heated without heating the entire substrate, providing a rapid heat-up and quick cool-down.

UV Curing is achievable using specially formulated powders. The powder needs to be exposed to enough heat so it is molten when exposed to UV energy. The heat source is typically infrared, but convection heating can also be used. The coating is then exposed to a lamp that directs UV light onto the product. The photoinitiators, part of the coating, absorb the UV energy from the ultraviolet light source and initiate a series of chemical reactions that rapidly convert the molten film to a solid cured finish in a matter of seconds.

Near IR Curing technology uses specially formulated powders, high-energy light sources and high focusing reflector systems, completing the powder coating and curing process within several seconds. Heat-sensitive assembled parts can benefit from near-infrared curing. This process also allows swift curing times on metal substrates with no film thickness limitations, especially with colors like yellow or red that are difficult to cure in higher film thicknesses with current UV powder coatings.

Powder Coating Wood Products

Wood products offering the greatest potential for powder coating are engineered wood materials like medium-density fiberboard (MDF). MDF is used to manufacture office furniture, ready-to-assemble furniture for the home and office, cabinets for kitchens and bathrooms and store fixture and displays. Barbecue trays made of MDF are also currently powder coated.

Some woods and wood products have enough moisture content to provide sufficient conductivity and can be coated directly. Wood parts can be blown off with compressed air to eliminate any surface contaminants. To enhance electrostatic attraction, wood can be pretreated with a liquid coat to improve adhesion and the part can be preheated.

Curing powder on MDF can be accomplished by infrared, a hybrid of infrared and convection ovens or UV light in conjunction with infrared or convection ovens using specially formulated UV-curable powders.

Powder coating is more environmentally friendly than other traditional MDF coating practices. Current lamination techniques also rely on edge-banding methods, difficult to do with odd shapes and angles. As office furniture manufacturers are moving away from simpler shapes, such as squares and rectangles, to more curved edges and interior "holes" to allow for drop-through of computer cords, powder coating will become an even more desirable coating option. Powder coating also offers a wide array of colors.

Process Innovations

Electromagnetic Brush Technology is a new application process that makes it possible to apply powder coatings at differential speeds and thicknesses on flat substrates. This process is similar to that used in photocopiers and laser printers with "toner" particles triboelectrically charged against the ferromagnetic "carrier" particles and transported with mixing rollers to a rotating shell or drum, which has stationary magnets within it. The ferromagnetic particles form a chain as directed by the magnetic field lines, also called a magnetic brush. The substrate becomes powder coated as it passes by the magnetic brush and the electrostatic field is turned on. Thicker layers can be achieved by passing the substrate by the magnetic brush more than once. Potential application areas include the coating of coil, blanks and even wood.

In-Mold Coating. An in-mold powder coating process has been developed in which powder coatings are sprayed onto a heated mold cavity before the molding cycle begins. During the molding operation, the powder coating chemically bonds to the molding compound and produces a product with a coating that is chip and impact resistant.

In-the-Field Powder Coating. Thermoplastic powders can be applied in the field, provided the substrate is clean and preheated properly. Bridge support columns and steel sidewalks have been coated successfully. Also, pipe joints coated in the shop have been shipped to the field, welded together and a flamespray powder applied to the weld seams and pipe joints, creating a seamlessly coated, corrosion-resistant pipe.

Color Changes. High-production powder systems apply more than 20 different colors, with several color changes per day. Recent and ongoing developments in the equipment used for powder application have significantly reduced the time, effort and capital cost required for color changes. Efforts to increase application efficiency, streamline the powder delivery system and redesign the booth have all contributed to speeding up the color change process. Properly designed, operated and maintained powder systems can change colors in minutes. Powder booths with plastic walls that repel rather than attract the powder, automated belts and sweepers that brush powder particles on the floor to recovery systems and powerful bursts of air through the pumps and guns to clean the system have all helped to reduce color change times, sometimes to 15 min or less.

Combining Technologies. Some finishing operations have developed ways to use powder coating in tandem with liquid finishing lines, using powder base coats and liquid topcoats for instance. This combined technology not only provides a more environmentally compliant finishing process than is achievable with a liquid-only finishing operation but also can result in new colors or effects not available in liquid or powder alone.

Robotics. Advances in microprocessors and robotics are also allowing increased production in powder coating facilities. Robots are typically used where an operation must be repeated for each workpiece on the line. The clearcoat being applied to BMW automotive body panels benefits from a complex series of twists and bends of a robotic gun, programmed to even shut the car door during the clearcoat application. When combined with analog power output and voltage controls, robots can adjust powder delivery settings in the midst of coating a part, maneuvers too difficult to be accomplished manually.

Other Innovations. Application equipment manufacturers are also working to develop equipment that will deliver lower film builds to increase yields, increase first-pass transfer efficiency and further automate the powder coating application process.

Recent research and development has been directed toward on-line process monitoring as a prerequisite for automatic control of powder coating lines, which will become more important with the increased demand for thin-film powder coatings. All of these advances, plus the inherent advantages of working with powder, ensure that powder coatings will have a permanent and ever-increasing share of the finishing market.



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