With the invention of powder coating about 60 years ago, a lot of work began in the coatings and finishing industries to leverage this new technology. The porcelain enamel industry was an early adopter of electrostatic powder coating, but a lot of development was required from both the material and equipment aspects. By the 1980s, porcelain powder coating was taking place in many factories, primarily for flatware but also in several lines for cavities, like ovens and dishwashers.
The physics of powder coating of porcelain powder is the same as for traditional powder paint, although there are some differences in behavior, etc. One of the most noticeable is the transfer efficiency; porcelain powder is about 40 percent efficient, so much more powder travels through the recovery system in comparison with traditional powder paint. Additionally, porcelain powders are often applied in a thicker film; usually six to eight mils. Almost all of the electrostatic dry powder porcelain enamel application is for major appliances; for exterior parts it is usually a two coat/one fire process, and for interior (non-appearance) parts it is a one coat/one fire process. In two coat/one fire processing, the first coat is a special ground coat enamel (designed to adhere to the steel) which is applied to a thickness of one to two mils, directly followed in a second application booth with a cover coat (for the final color of the part) at a thickness of five to seven mils. Both layers are melted/cured at about 1500 degrees Fahrenheit in a single pass through the furnace. Firing times are typically dependent on the thickness of the metal; for successful enamel coatings the time at peak temperature is two to three minutes.
For electrostatic powder enameling, the benefits are similar to powder paint systems. They include full recovery of overspray, coating thickness control, film uniformity, better edge coverage, gloss, and smoothness of finish after firing, plus the advantages of the automation and control of today’s powder application systems. The primary limitation for electrostatic powder enameling is the choice of colors; this is because the color must be smelted into the frit. The use of frit plus pigment is problematic due to differences in particle size, density, and electrical charging parameters – thus, recycling is difficult since the various constituents apply at different rates. Color change requires either multiple booths or significant clean-up time.
We have seen recent growth in the combination of robotics with powder application, which is improving production repeatability and efficiencies as well as reducing costs. R&D on electrostatic powder enamel chemical formulations and grinding technologies is also continuing with the objectives of improving application parameters and reducing defects.
This article comes from PCT edit released