You hear so many different stories about powder coating, how it is tough or brittle, how it cracks or how it is flexible, how its long lasting but won’t last outside. I’m sure you’ve heard them all, and the tales and contradictions just go on and on and on.
I recently came back from an expo where one painter with a great reputation said that “powder is tough and flexible, but has hardly any bonding ability.” Another painter that specializes in graphics stated that “powder was way too brittle, and any flex will crack the powder, and it may just pop off.”
So I thought I would run a little test of my own and show you how four tubes that are coated with the same powder but with four different variables react differently. All four tubes were degreased and then sanded, and then each tube had been notched in its ends so I could easily tell them apart from each other after coating. The powder used was a black gloss, which we use on a regular basis.
This describes the scenario that each tube would go through: The #1 tube will have the proper thickness of powder applied and will be properly cured at the right temperature and time from the data supplied (see graph) by the powder supplier. The #2 tube will have the proper thickness and be cured properly, but the tube will be glass beaded. Several coating suppliers do not recommend glass beading, and one of these companies says it will void any warranties if done so.
ALL POWDERS WILL HAVE DATA SHEETS
RAW TUBES, THE 2ND ONE HAS BEEN GLASS BEADED.
The #3 tube will have the powder applied thicker than the data sheets, but will be cured correctly. What causes an object to have the powder to thickly applied? Multiple coats of powder, applying powder while the object is to hot (called a hot shot) or having a gun that doesn’t have the adjustments to apply varying powder thicknesses.
The #4 tube will have the proper thickness applied and will be cured at the right temperature, but not for the right amount of time. Don’t go by how many minutes in the heat, but rather by how many minutes it is in the oven AFTER the object metal reaches temperature. I always refer to the graph for the actual powder being applied for the heat and time required.
I use a commercial convection oven because of its even heating throughout on the powder coated object. It has been tweaked so that no area in the oven differs more than 10 degrees. The fan mounted inside the oven walls distribute the heat evenly around the objects for proper curing. The oven is preheated to 400 degrees an hour beforehand to allow the walls to retain even temperatures also.
Commercial convection oven
The least expensive powder gun I could find that would allow me to vary the powder density, powder charge and the conveying air was $3000. It also has a fluidizing hopper that has an air feed. This allows the powder to have better separation and to be evenly charged, which makes it easier to apply the proper thickness of material over the entire object.
I will do 4 different tests on the tubes. The first is a visual on the finish of the four tubes. As you can see, all four tubes look identical, with a smooth shiny black finish.
TUBES LOOK THE SAME AFTER COATING, BUT THEY ARE IN FACT QUITE DIFFERENT
The second test will be with a small smooth dent in the tube. The #1 and #2 tubes both had no cracks at all and no loss of adhesion. The #3 tube (which had the thick powder application) had cracks around the edge of the small dent. As you can see in the photo, the undercured #4 tube had the powder flake off to bare metal in the majority of the dent area.
The third test will be a dent with a sharp edge. The #1 tube showed no loss of powder or cracks. The #2 tube that was glass beaded showed cracking of the powder in the dent area and also had a very small amount of coating loss in the sharp edge, which means a loss of adhesion. The #3 tube had cracks not only in the dent area, but also in an area ½” away from the dent. The #4 tube, as you can see, had almost all the powder flake off in the dent, as well as an inch away, and showed numerous cracks.
1ST TUBE: NO CRACKING; 2ND TUBE: SOME LOSS OF BOND; 3RD TUBE: CRACKING; 4TH TUBE: NO BOND AND VERY BRITTLE.
The last test would be a scratch test. I used a razor knife and carved the powder on the tubes. The #1 and #3 had a coating that carved off in layers and had good adhesion. The #2 tube carved off in layers, but as you got close to the steel tube, you could tell adhesion was not as good as the previous two. The #4 tube, which was undercured, scratched off very easy. The powder was very brittle and had no adhesion at all.
THE UNDER CURED 4TH TUBE WAS BRITTLE AND HAD NO BONDING.
Remember that your frame has thick and thin areas. The neck tubes and axle plates always take longer to reach curing temperature. Get an infrared gun and check many areas of the object being coated. When it reaches temperature, start your curing timer.
As you can tell by these results, if your object is properly cleaned, the powder properly applied in thickness and then properly cured, it will be a very durable finish. A smooth and glossy look does not mean that you have a durable finish. Only your powder coater knows if all was done correctly. Sometimes that better deal may not be best in the long run.