How do I prevent outgassing when powder coating aluminum sand castings? D. M.
I have answered this question at least a dozen times during the last 23 years. However, since the recommended corrective action does not always work reliably, the problems with outgassing continue to plague the powder coating industry.
Gases are often trapped in the metal during the pour process. This is inherent to all castings whether they are sand castings or die-castings. If this problem becomes excessive, it can lead to casting porosity that can affect the strength of the casting. While most foundries take great care to ensure that the casting porosity never approaches this catastrophic condition, they spend little time in controlling entrapped gases that can affect the powder coating process. This brings me to my first point in reducing an outgassing problem: “Review the problem with your foundry to see if they can take steps to improve the castings and reduce the entrapped gases.” The foundry can redesign the molds or patterns to include chills, vents, etc. to reduce outgassing. Changing the metallurgy can also help, if you are allowed to do so.
Next, you should ensure that all surface contaminants are removed in the pretreatment process. Some surface contaminants can volatize (evaporate) in the cure oven, causing pinholes like outgassing. Powder coating a clean part will ensure that this problem is not being confused for outgassing from the casting.
Try pre-baking your parts at a temperature higher than your cure temperature for the same or longer time frame of your cure cycle. This will drive the bulk of the entrapped gas out.
Using an outgassing powder coating formulation will allow some of the entrapped gas to escape during the cure process before the powder coating begins to harden. This will eliminate some of the pinholes in the resultant coating. Just ask your powder formulator for a recommendation of which product they have that can help in this area.
Discuss casting impregnation or sealants for the castings with your foundry. There are several processes that employ polymers in a vacuum to seal a casting before it is delivered to you. These processes are normally compatible with most powder coatings, but try some test parts before committing all your production quantities.
Lastly, you can change your curing process from convection heating to either infrared or ultraviolet radiation heating systems. Both will allow you to cure the powder coating without heating all the part’s mass. If the bulk of your entrapped gases are deep within your part, it may eliminate the pinholes in the powder coating entirely. Good luck.