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.