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Powder on Plastic Parts

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Q. We have heard that some companies are applying powder coating to plastic parts. Is this true? What kind of plastic parts are being powder coated? Is this a common process?

A. There are some plastic materials and other heat-sensitive products that can be powder coated. There are really two options, depending on the particular substrate material and type of product. One option is the use of low-bake powder materials. Powders can be formulated to cure between 250° and 325° F, depending on the particular chemistry. A plastic or material that has a high enough deflection temperature can be cured in either infrared ovens or convection ovens. In some cases, a polymer can have the deflection temperature raised by adding glass fiber and become better-suited for powder coating cure. Examples include Acetal Copolymer, Nylon 6 and Polyethylene Terephthalate (PET). The addition of 30-percent glass fiber to these polymers will raise the deflection temperature enough to allow it to hold up during the cure process. Fiberglass pultrusions for window frames are examples of products that are powder coated.

The other option for powder coating heat-sensitive substrates is ultraviolet (UV)-curable powder coatings. The powder is melted at 200° F and then cured by exposure to the UV light. This takes a special powder that includes a reactive photoinitiator in the formulation. The cure takes place very rapidly once the molten powder is exposed to the UV lamps, and the part is never raised above the melt point of around 200° F. UV powders can be used for any surface that can be readily exposed to the light energy from the lamps. In some cases, lamps can be mounted on a robotic arm to reach inaccessible areas of an assembly.

Curing is only part of the challenge. The other is how to apply a uniform coating with an electrostatic device over a nonconductive surface. In some cases, the plastic surface is cleaned and treated with a conductive material that will provide electrostatic attraction. This creates the necessary electrostatic background so that the powder can be applied to the treated surface. In other cases, the part might be exposed to some heat to allow the powder to adhere. Preheating of pultrusions, for example, has been very effective in several cases.

Application of powder on plastic surfaces is not common at this point but has grown and does have several benefits compared to any liquid coating option. It reduces the emissions and often the number of coats required as well as the waste stream, all while providing a very durable finish. Application of powder on heat-sensitive substrates is very viable. It will require some testing and research, but it can work very well.

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