We recently sent a truckload of galvalume tubing to be powder coated. The finished product appears to have a bumpy surface that looks like fish eyes. This galvalume consists of 43.4% zinc, 55% aluminum and 1.6% silicon. The coating thickness on this product is approx 1.0 mils, while the powder coating thickness is approx 1.1 mils. Could it be the silicon in the matrix of galvalume?
--Richard Vivian, Bolton Steel
Hello Richard,
One issue that catches my eye is the powder coating thickness - 1.1 mils seems thin. Typically I would recommend a minimum thickness of 1.8 mils for a general industrial application. The thicker film provides increased protection of the substrate and a smoother, more aesthetically pleasing appearance. A 1.1 mil thickness of most powders provides a somewhat orange-peeled (textured) surface. Too thin a film may not be enough to cover the surface of the metal.
I think that it’s also important to differentiate between “bumpiness” and “fish eyes.” Bumps obviously involve raised spots of film. Fish eyes are depressions caused by contaminants that possess a significantly lower surface tension than the powder coating. In the worst scenarios, the defects extend down to the surface of the substrate. Silicones, oils, lubricants, mold release agents and forming compounds are known to cause fish eyes in powder coatings.
Here’s the course of action I recommend:
- Re-inspect the defects under low-level magnification to determine whether they are raised or depressed.
- Increase your film thickness to a minimum of 1.8 mils.
- If you are fairly certain that the defects are fish eyes, rigorously clean the galvalume surface and then apply the powder. By rigorously, I mean scrub/abrade the surface and then apply a strong solvent cleaner with a clean cloth. This should remove any contaminant and eliminate the fish eyes.
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Dear Joe,
We recently began powder coating operations here at our facility. What is the proper way to dispose of the spent powder that we can no longer use? A couple of our powder vendors have told us to bake the powder until it forms a solid and then throw it away with our normal everyday refuse. How would we go about baking it if that is a viable option? I know the material is non-hazardous; however if we threw the material away without solidifying it, wouldn’t the material create an inhalation hazard?
--Sandy Forsyth, McLaughlin Body Co.
Dear Sandy,
I’ve managed a number of powder coating manufacturing sites. Occasionally we would incur an accumulation of obsolete or unwanted powder. In all cases we were able to dispose of the powder in our regular dumpster that was destined for the landfill. Our trash haulers had one stipulation - they wanted to avoid creating a plume as the material was landfilled. We prevented this by securely packaging the obsolete powder in fiberboard drums with tight lids.
Long ago I worked for a major powder coating manufacturer that implemented the solidifying procedure that you mentioned. I lived close to the plant, so at midday I regularly dined at home. Upon my return one day, I encountered a number of fire vehicles encircling my place of employment. Our solidifying process for waste powder had caused a fire in the oven, and hence a visit by the local fire brigade. Large volumes of powder coating can ignite during baking under certain conditions.
I would check with your local waste hauler(s) and get their input before implementing my recommendation. Local regulations may apply, and the waste hauler should be up-to-date with what options exist.
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Dear Joe,
We recently installed a powder line for automotive parts. Now we would like to install an in-line stripping process for our racks. Can you help us?
--Jeremy Chung and American Showa, Inc.
Hi Jeremy,
Your best choice is a burn-off system. In this process, the coating actually ignites and burns off the rack. These systems operate in-line at temperatures ranging from 1000 to 1200ºF. The high temperature is needed to burn the accumulated coating in a relatively short period of time.
It’s important to use this process only if the racks are sturdy enough to take the high heat; light-gauge racks will invariably warp under these conditions. The other important consideration is ensuring that the coating residue is adequately removed from the racks prior to hanging new parts on the line. Otherwise you run the risk of contaminating parts with bits of ash. Residue removal is usually accomplished by pressurized water streams.
The best place to locate suppliers of this technology is to peruse Finishing Today’s Buyers Guide. You can access it on our website at www.finishingtodaymag.com by either clicking on the “Buyers Guide” button in the left-hand column or performing a search using the fields located at the top of our website. There are a number of companies listed that specialize in burn-off stripping technology.
Ask Joe Powder is a regular feature of Finishing Today magazine. Please send your questions to askjoepowder@yahoo.com.
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