Four Steps to Properly Identify and Eliminate Paint Defects
Over the years and out of necessity, Axalta’s Mike Withers has developed strategies for finding problems on a coating line and discovering solutions to those problems.
Q: We have been having a hard time with defects in our finishes. What are some processes we can use to help solve these issues?
A: Being in the powder coatings business for over 17 years, I am sometimes asked for help in troubleshooting paint defects. Unfortunately, many powder salesmen solely blame the pretreatment process for all issues and leave the customer to figure it out for themselves. Instead of taking that route, I decided to develop practices to methodically identify and eliminate defects.
Step 1: Recognizing the Film Build: What is the film build? Low film builds can look grainy like dry spray, while high film builds can have more orange peel and cause loss of mechanical properties.
Step 2: Getting the Right Tools: Identifying what is defect is a good start. When I started in powder coating, I was given a 10x handheld microscope. Though it helped, it limited my investigative abilities. Luckily, I was able to find a $50 handheld digital microscope online that varied from 10x-150x, improving my detection capabilities.
Step 3: Understanding the Appearance of a Defect: Next, I needed to know what a defect looked like under a microscope. What does a lint fiber from cotton gloves look like after it has been through the oven and the powder is cured? What does flash rust look like after powder is cured? What does a misted alkaline deposit on an already clean and dry part look like in the cured film. I started building my library. The great thing about digital is you can save the pictures of defects for future use. To fix a problem, you must find it. Where is the defect:
- On all parts or not?
- On all sides or not?
Step 4: Investigate the Problem: My way of starting is to use a laboratory pretreated panel. You can get them from ACT or Q-Panel or your pretreatment supplier. I would also purchase clean-only panels. I would hang a pretreated panel in front of the powder booth then let it run through the rest of the system. If I needed a larger surface area, then I would build a panel rack and hang a wall of panels on it. A local home improvement store will have 4' pieces of steel that are angle iron with holes predrilled every few inches. With a few of these, and some bolts and nuts, I could make a panel rack that I easily take apart later to store in my trunk.
If the defect appears on the panel that went through only the booth and oven, then you know it is either the booth, powder or oven. Next, I coat a pretreated panel with virgin powder using a well cleaned powder gun. I use a toaster oven to cure the panel. You can use an IR thermometer to ensure you don’t overbake the panel. This removes the customer oven and booth from the process. If the defect appears then it is the powder. If not, then it is either the booth or oven. Next, to check the booth, I will take powder from the feed hopper and, with a well cleaned gun, I will spray another panel and cure in the toaster oven. If I see the defect there, then it is in the booth. You will have to clean out the hopper(s) and discard the powder.
If I don’t see the defect in the booth then it’s probably the oven. Coat another panel from the reclaim powder and run it through the oven. If the defect shows now, then it is the oven. Lots of older ovens out there don’t have filtered make up air. Ovens should be cleaned from time to time. However, after cleaning you will likely see more contamination initially as you disturbed the system. Running textured powder coatings will hide the increased contamination.
Now, if the defect didn’t show up on the already pretreated panel that passed through the booth and oven, then the defect is occurring because of either pretreatment or the dry off oven. Using the IR thermometer, you can measure the temperature of parts going through the dry off oven. You can run a clean-only panel through the pretreatment system and then dry in the toaster oven, then hang back on the line in front of the booth. If the defect appears then it is in pretreatment. Always check the titration, pressures and temperatures. Probes go bad so verify that you are in spec. If not, get in spec and see if the problem goes away. Walk the washer after its pumps are off. Many issues come from clogged nozzles or broken risers. You can “poke” a plug back into the nozzle. As soon as the pump is turned on it will clog again.
You can eliminate stages of your pretreatment system. For example, hang a clean-only panel and let it go through pretreatment only. Remove from the line then rinse with distilled water. Then hang in front of dry off oven and let it run through the rest of the system. Repeat by eliminating stages until you find the root cause.
Your pretreatment powder coating supplier can help you troubleshoot problems, but they aren’t always quickly available. Improving your troubleshooting skills can help get you running again quickly.
Mike Withers is the architectural segment leader and electrocoat segment leader in the Powder Group at Axalta Coating Systems.
Emerging technologies can save energy, ease environmental concerns
Metal fabricators that laser-cut with oxygen take steps to prepare parts better for powder coating.
Powder coating is one of the most durable finishes that can be applied to industrial manufactured products, and offers excellent corrosion protection and is very safe because of its lack of volatile organic compounds.