We are in the process of powder coating zinc die-castings and are having a problem with outgassing. We use an epoxy powder coating and have tried several things to reduce the problem. We have tried pre-baking the parts and sanding after pre-baking, but the more we pre-bake, the larger the outgas hole. We even tried to coat them while they were still hot, but without success. These parts require a smooth class A finish, and we have had trouble with them for over five years. Recently we tried using a wet primer, followed by "scotch-brite" pads and hand wiping. This seems to work best, but we still have defects of at least 25%. Any suggestions? C.O.
Outgassing in die-cast parts is a normal occurrence when using powder coatings. The reason for this is oven temperatures used with powder coating will often be high enough to release trapped gasses from within the casting material, piercing the coating. The resultant defect is a pinhole or bubble in the coating. This doesn't mean that outgassing can't be reduced to an acceptable level (less than 25%). It does, however, require effort from all the suppliers in the process (die-caster, powder supplier, and custom coater). Let's look at these separately.
The die-casting supplier can work to reduce the trapped gasses by modifying the tooling to reposition or redesign vents, gates, chills, etc. You must first identify the areas where the gas escapes so the die-casters know where they must concentrate their efforts. They can also add vacuum to certain areas of the die to reduce the gas. Controlling the amount and use of mold release lubricants will also prove to help. Using a good metal mix to cast a quality part can help tremendously.
Operations performed after the casting has been removed from the die can also impact outgassing in die-cast parts. A die-cast vendor once told me that a die-cast part is like a loaf of French bread; it has a hard "crust" on the outside. If this "crust" is left intact, it will help hold the entrapped gasses inside the part. Once this "crust" is removed, the entrapped gasses can come out freely, causing pinholes in powder coatings. Therefore, if this "crust" is removed by trimming dies, sanding, polishing, or the like, then the amount of outgassing will increase. Precautions should be taken to ensure that the "crust" stays intact.
There are processes that can be performed after the part has already been die-cast. Pre-baking is one of these processes. But like you already stated, this may not improve the situation much if the casting is acting like a sponge, reabsorbing the gas it released when hot after it has cooled. The other post-casting process that has proven to help is impregnation. This process involves injecting a sealing material into the casting under vacuum and then curing the sealant. Choosing the proper sealant material is important to ensure that it does not become an impediment to adhesion and that it is inert once cured to prevent interaction with the powder coating.
You can select a powder coating that is of the outgassing variety. These formulations increase the flow time of the powder to allow the entrapped gasses to escape and then flow back over the pinhole. These powder coating formulations can be effective if there is not an abundance of entrapped gasses that continue to puncture the coating as it beginning to gel and eventually harden. Check with your powder supplier for the availability of these powder formulations.
Lastly, as the custom coater, you must be sure that outgassing and not improper cleaning of the substrate is responsible for the pinholes in the coating. Residual soils that volatize in the oven can often cause pin holing through the powder coating. This situation has nothing to do with outgassing, so the previously mentioned solutions will not eliminate this problem.
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