Complete coverage of powder coating is always a worthwhile goal, both in terms of application and knowledge about the subject.
More metal fabricators are finding themselves in a position where they are expected to take on responsibility for finishing metal parts. Either OEM customers want them to take on powder coating activities, or at the very least they want the shops to coordinate the finishing process in addition to the fabrication.
Of course, in this economy, many metal fabricators aren’t in a position to say no to such requests. Fortunately, powder coating technology has advanced to the point where it’s more user- and production-friendly than ever.
It’s one of the younger metal finishing technologies on the marketplace, having appeared on the manufacturing scene in a large way in the 1970s. The technology grew rapidly in the 1980s as key industry segments, such as appliance and architectural, gravitated to it, and powder coating usage grew with international expansion in the 1990s. The growth of the technology is not as robust as in past decades, but this lull has put pressure on the material and application equipment sector to develop new products in an aggressive manner.
The basic premise for powder application remains the same. Dirt, oils, and lubricants have to be removed from the material with chemicals, which also help prepare the metal surface for powder application and improve powder adhesion. These pretreatment chemicals usually are applied in a spray process, but sometimes a submersion method with several tank stages is used. Once the parts are dried (see Figure 1), they enter a booth where an application gun shoots electrostatically charged particles at the grounded metal parts (see Figure 2). The powder adheres to the parts as long as electrostatic charge remains on the powder, which is usually more than enough time for the parts to travel to a curing oven. The oven provides the elevated temperature necessary to melt the powder and cause the material to flow out, creating a skin over the metal part.
The powder coating industry is not one marked by regular technological revolution, but that doesn’t mean noteworthy advancements haven’t taken place over the last five to 10 years. Here are five developments you should be aware of if your company is increasing its involvement in powder coating.
Read more: Noting advances in powder coating
Q: We struggle to avoid light coating in some areas of our parts. Our application is all manual so we have focused on working with operators to help them understand the critical areas and work harder at getting enough powder in the inside corners and tight spaces, but they still have some failures. We also have some heavy coating at times and that may be related to the effort to avoid the coating. Any suggestions on how to better control film build?
A: This is an age-old question that has been considered a few times in this and other columns. There are many factors that impact film build control so the answer is not simple. Start by considering the key word in the sentence — “control.” Control implies a systemic approach to a problem by implementing clear and repeated methods and techniques. The things that need to be controlled include powder fluidization, flow rate, gun-to-target distance, stroke pattern and speed, parts racking, line speed and presence of good contact to the ground.
It starts with racking. Do you have the right number of parts per minute traveling through the booth? If you have too many, the coating will vary toward the light side. If you give the parts too much space, the coating can be heavy in some areas. Spacing needs to be adequate for good access without leaving too much empty air in the pattern. Parts should be close enough for good efficiency without limiting access to all areas of the part. The position needs to assist vision and ergonomic access to the part surface. Parts need to be held steady and consistently. Hooks need to be clean and in good repair. The line speed and amount of part surface needs to be comfortable for the operators so they can cover all surfaces without racing to keep up with the line.
Next, make sure that the powder is flowing smoothly and consistently from the gun tip in adequate volume for the amount of part surface that is traveling through the booth. Control the velocity and pattern to allow a high percentage of efficiency and minimize overspray.
Work on consistent patterns so the parts get the same coverage all the time. Use some research and trials, if necessary, to make sure you have the best possible setup and spray pattern. Measure the film thickness and help the operators see what is happening to each area of the part.
Finally, work on standard methods for all of these operational variables so you can improve on consistency. Standard operating procedures and well-conceived racking arrangements can provide the improvement you are looking for in film-build control.