Controlling Dirt on Coated Parts
What can be done to better control the amount of dirt in a powder system and on coated parts? Powder expert Rodger Talbert weighs in on this and other questions.
Q. What can we do to better control the amount of dirt in our system and on our coated parts?
A. Dirt and particulate is an age-old problem that plagues many coating systems, even when they are fairly new and well-designed. There are three different areas of system design and operation on which to focus in order to control dirt: general design practices, air management and housekeeping.
Dirt can come from processes within the system or from the surrounding plant environment. The system should be designed to isolate cleaned and coated parts from these sources as much as possible. Start with the chain in an overhead system; a sanitary pan should be installed under the rail to prevent oil or other contamination from falling on parts. The powder application area should be enclosed in a room that has an HVAC system to control temperature and humidity. The room should be slightly pressurized to limit infiltration of dirt from the plant. Temperature should be controlled between 60°F and 80°F, and relative humidity (RH) should be kept between 40 and 60 percent. An additional level of dirt control can be achieved by requiring that nylon coveralls be worn in the room, creating an entrance area to put on those coveralls and limiting access to the room.
Also avoid locating other processes near the coating operation that can generate particulate. Grinding, welding, sanding and similar operations should not be located where the byproducts of their operation can be introduced to the coating area.
Air management is another critical part of dirt control in a powder coating operation. Powder systems are exposed to exhaust from ovens and washers, and the air that is exhausted needs to be added back into the plant using air makeup units (AMU) so that the air movement around the system is consistent and predictable. If air is allowed to infiltrate the building through doors and cracks, it will carry a lot of unwanted particulate that can wind up on coated parts. An AMU can deliver the right amount of filtered and tempered air into the plant without excess velocity that could cause problems.
Housekeeping and maintenance also are extremely important. Surfaces should be cleaned on a regular basis. Accumulation of dust and dirt should be avoided. Ovens need to be cleaned; spray equipment should be cleaned frequently. Racks and hooks should be stripped often enough to avoid excess powder buildup that can chip off and land on coated parts.
Finally, organize the parts that been rejected because of particulate and try to identify the type of “dirt” on them. Where is it in relation to the depth of the film? Near the surface or under the coating? Does it have a color, or is it black or white? Look at it under magnification and see if you can find characteristics that lead to a source. If you can identify the source, then you can work on specific methods to limit the impact of the source. Once you know the source, you can apply common-sense methods to fix the problem and end the rejects it causes.
Q. We seem to be getting a lot of rejects because the coating coverage is not complete. Any advice for how we can better control this problem?
A. Film-build control, and light or heavy coating is related to fundamental practices and training. When you are plagued by light coating, several things are going wrong. For starters, do you check earth ground on a regular basis? Are your hooks clean and making good metal-to-metal contact with the parts? Loss of earth ground has a big impact on film build and control of the deposition.
Do you have good gun-delivery setup? Smooth, consistent, well-distributed spray patterns are essential to coating uniformity and coverage. If you have too much velocity or a poor spray pattern, it will make it hard to develop uniform and complete coverage.
Are you using the correct voltage and amperage settings? High voltage and micro-amperage are effective if you are coating flatter surfaces and the gun is not too close to the part, but higher amperage on complex shapes may make it hard to cover Faraday cage areas. If the gun is less than 3 inches from the part, higher amperage will cause back ionization and uneven coverage.
Are your operators trained? You should have at least one operator who has a deep understanding of electrostatics and gun settings, and who can teach the newer operators and show them how to control the application. If you do not have such a person, you need to send someone to get that training at a hands-on training program, or have your powder supplier come in and provide guidance. You cannot expect someone to learn on the job and always give you a perfect part. The guns settings and spray technique must be optimized to provide consistent high yields.
Make sure you have good ground; adjust the spray pattern to maximize deposition and minimize irregularities; keep the velocity down; set the voltage correctly; control micro-amps; and make sure your operators understand how to use the gun. That should minimize your light coats.
Q. What should we consider in testing and selecting a robot for our powder coat application?
A. Robotic application of liquid paint is common in many industries. There also are some powder coating operations that use robots effectively, and many companies have considered using them. The obvious advantage is the labor reduction, and associated reduced cost and efficiency. Robots are costly up front, but the payoff is good. The main obstacle is the racking method. Robotic application requires much more precise racking arrangements than manual application. The cost of racking and the maintenance of racking can be significantly higher.
Consider the number of racks you use and their design. Look for ways to make the racks sturdy, and remember that parts must be racked in consistent positions to get the most out of the robot. Do some testing and see how it goes. If you can ensure steady and consistent rack positions, it may be a great benefit for you. If you have a lot of different racks designs and parts, and it is hard to make them precise for positioning in front of the robot, they may not be a good option for you.
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