Can powdered metal parts be electrocoated? What kind of performance can be expected? J.G.
Yes, powdered (sintered) metal parts can be electrocoated, however, they do create some challenges. The two main challenges are the voids or porosity in the parts and the lubricants trapped in these voids.
Similar to castings, voids can force air to escape during the curing of the electrocoat, causing blisters similar to "solvent pop" with liquid coatings. Outgassing is another term for this defect caused by the voids. There are four possible methods of dealing with the porosity of powered metal parts:
1. Ignore the voids as long as the outgassing is not a visual or corrosion problem. While the blistering will vary and is unsightly, most of the corrosion resistant properties of the electrocoat will be maintained.
2. Pre-bake the parts before electrocoating, similar to what is done to some porous castings. This pre-bake must be at a temperature above the electrocoat cure temperature and will also force out most of the lubricants trapped in the voids. While pre-baking will force gas, air and lubricants out of the part, the voids are still there and may refill with air or pretreatment chemicals. This refilling will probably result in some blistering, but much less than without pre-baking.
3. Impregnate the powdered metal parts before coating. Impregnation is usually done with a steam-cured organic resin such as polyester. The chosen resin needs to be tested to see that it is compatible (after curing) with the electrocoat, and that the electrocoat cure does not effect the impregnating resin other than discoloration. Since these resins are non-conductive, they will not be coated with the electrocoat and will show up as brown specks in/on the surface of the part. Unless the electrocoat is the same color as the baked resin, these specks may be aesthetically unacceptable. From a corrosion resistant standpoint, the impregnation process should show a significant improvement.
4. If the brown specks in/on the surface of the part are not acceptable, the parts can be burnished after impregnating to remove the excess resin from the surface. This is another added step to the process that will not enhance the corrosion resistance of the part but will improve its appearance.
I would suggest you review the aesthetic and corrosion requirements with your customer and/or the end user before committing to any or all of the above options. Samples coated by a custom coater or an electrocoat supplier lab will also help this discussion.
Lubricants used during the fabrication of the powdered metal parts can also create problems similar to the voids. The lubricants can be handled the same as the options suggested for voids but need other considerations:
1. If large quantities of powdered metal parts are to be electrooated, make sure your pretreatment chemical and electrocoat suppliers are aware of the lubricants involved to determine their ability to be cleaned from the part and/or contamination of the bath.
2. If the parts are pre-baked, some of the lubricants can be "burned off" if the time/temperature is adequate. However, if the time/temperature is not adequate, the lubricants might only be charred and create a cleaning problem.
I would suggest samples be processed using different time/temperature pre-baking, and then be electrocoated by a custom coater or an electrocoat supplier lab.