Q. We run aluminum extrusions using a seven-stage pretreatment system with a chrome treatment conversion coating. We began running aluminum die-cast parts through the same system and are seeing small blisters and holes in the coating. Our coating supplier tells us that it is caused by “outgassing.” How can we stop this?
A. Outgassing is the release of trapped air pockets in the cast material during the cure cycle and it is a problem for many powder coaters, “the curse of the casting.” Castings are porous and the air that is near the surface can expand and blow a small hole in the coating during the melt phase of the cure process, leaving a small, crater-like blemish.
Powder can be formulated to adjust the flow cycle and make the powder more forgiving when the released air blows up through the molten film.
You can also look at lower cure temperatures. All powder materials have some flexibility for lower temperatures if you can allow more time. You have some limitations because you are running powder material for AAMA 2605 compliance, presumably fluoropolymers, so some low-temperature formulations may not be suitable. The basic advice is that lower temperature at longer cure cycle times will allow some of the gas to escape without causing a problem.
Some companies preheat the cast material to a temperature exceeding the ultimate cure temperature by around 50°F and then cool and coat. This will drive off some of the trapped air before the cure cycle.
Keep in mind that all of these ideas can help, but there is no guarantee that they will be adequate to give you good parts all day long. They are helpful ideas and will improve the issues without major system changes.
You may want to X-ray some castings. An X-ray will show you just how bad the problem is by highlighting the porosity in the casting. If you have more than one source for castings, you should X-ray them all to compare different die casters and see if one is superior. If they are, you can shift more work to them and you will have less gassing.
If you cannot solve the problem with any of these efforts, you have two options left. You can set up a primer operation and add a powder primer to the process. The gassing will occur during the primer operation but the topcoat will seal over the blemishes, reducing their impact from an appearance perspective and making sure that corrosion cannot start where the film is flawed.
There is also a process called impregnation. You can hire a service to have them do it for you or you can do it yourself. The casting is placed inside a chamber and exposed to a resin that fills and seals the cast surface prior to coating. This process has been effective for some manufacturers that demand a good surface and sealed part.
Originally published in the April 2017 issue.