Liquid Coatings Q&A: Electrostatics on Wood and Plastics
Q. Can I use electrostatics to spray non-metallic parts, such as wood or plastics?
A. Electrostatics work on both wood and plastic parts. There are hundreds of manufacturers that use electrostatics to coat a variety of what we classify as “non-conductors,” including sheet glass, bottles and fiberglass.
The fundamentals of the electrostatic process involve applying a negative electrical charge to the coating material as it is being atomized. The negatively charged particles are then attracted to a workpiece, which is at ground potential. The benefit is increased transfer efficiency—more of the coating material is going on the workpiece and less is being wasted. Electrostatics also offer the benefit of “wrap,” the attraction of coating to the edges and backside of the workpiece, so there is efficiency gained in the application process as less passes are needed. Ensuring that the electrostatic charge that is imposed on the part can quickly dissipate to ground is the key to a successful electrostatic application.
In the case of wooden parts, the wood’s inherent moisture content can provide good electrostatic attraction. Typically, with a moisture content of 7 percent or higher, good electrostatic attraction can be achieved. In some cases, wooden parts can be passed through a humidity or mist chamber prior to the electrostatic application. A conductive sensitizer or prep coat applied to the wood can also improve attraction.
Methods/techniques to make plastic parts conductive for the purpose of electrostatic applications include:
- Conductive sensitizers
- Conductive adhesion promoter
- Conductive primer
- Imaging techniques
The method used varies based on the type of substrate coated and the coating materials that are used. In some cases, one or more of these methods may be combined. For instance, in the Tier-One automotive industry, plastic bumper fascia are placed over a cast aluminum buck or ground image. This serves two purposes: It helps the part maintain its shape through the curing process and acts as a “ground image” behind the part. We refer to this as an “imaging technique” where we place a ground image behind a non-conductive part. In this industry, a conductive adhesion promoter may also be applied to promote adhesion between the paint and the plastic surface. Once this is applied, good electrostatic attraction will be obtained.
We have conducted many trials in a lab environment, as well as in customers’ facilities. The results are consistent: Non-electrostatic applications will yield a transfer efficiency of 30 to 45 percent, and an electrostatic application will yield a transfer efficiency of 55 to 75 percent on plastic parts, depending on the equipment and process used.
Workholders and fixtures should be kept clean to ensure that a good path to ground can be established between the product being coated and the workholder. The point of contact between the fixture and the substrate coated is one of the most important areas to prep coat or prime, and this is the point where ground contact is made.
Originally published in the April 2016 issue.
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