We are in the process of starting up a new e-coat system and are getting “bleed-out.” Do you have any suggestions? Any help you could provide would be appreciated. B. L.
Bleed-out is a term that is used for a multitude of defects or concerns and come from a variety of sources. Bleed-out is usually an area of excess film thickness in the form of a “baked out drip.” It varies from difficult-to-see to a very pronounced crusty, porous, glob. Bleed-out is usually adjacent to a hem, seam, weld or hole (if deep and/or blind).
The actual cause of the bleed-out varies and can be a minor concern to a very major concern as an indication of potential contamination. The minor concern is when the actual bleed-out material is just excessive e-coat being squeezed out of a hem or seam during the curing process. Remember, e-coat is an immersion process and an electrical deposition process. Therefore, the e-coat material wants to get into any opening (by immersion) and tries to get into recesses (by excellent throwing power) when the parts contain these configurations. If welds are not continuous or tubular components are not welded airtight or have adequate drain holes, these are other sources of bleed-out.
Several things can be done to minimize bleed-out if part design cannot be changed. Sometimes, part design can reduce or eliminate bleed-out. These include:
- If hems cannot be eliminated, forming the hems with a metal thickness (or less) gap in the hem to allow liquid to flow through the hem rather than get trapped in it, can help. The inside of the “open” hem will also get coated better than a “closed” hem.
- Spot weld or projection weld seams are very difficult to make airtight. The only change here is to make the contact area as small as possible (narrow flanges) and weld as close together as possible without causing wrinkles.
- Threaded holes are more difficult to drain than smooth holes. Can holes be tapped after coating or can self-tapping fasteners be used?
- Blind holes are a major concern and should be eliminated (drilled after coating) or converted to through holes (even smaller than the main hole).
- Tubular parts can be a major source of bleed-out and contamination—these were addressed in a previous clinic (March 2001 www.pfonline.com/articles/clinics/0301cl_ecoat2.html) question.
If the bleed-out is the result of fabrication lubricants (oils, grease, dry lubes, machining fluids, etc.) or washer cleaner that is not being completely removed from the recesses during the pretreatment process, this is a major concern. First of all, this is a source of contamination in the washer and the e-coat system (coating tank and post rinse system). The resulting bleed-out is also of more concern as the areas are potential points of corrosion (little if any pretreatment) and will be more of the crusty, porous, glob type of defect because they are the result of a chemical reaction between the contaminant and the e-coat material itself.
If part design cannot be used, other process changes can be made but are expensive in labor or energy. These include:
- Wipe/wash the fabrication lubricants off the areas that will be formed into hems on seams before hemming or welding (spot or projection)
- Make sure all welds are airtight or adequate drain holes are provided in tubular products
- Add a dry-off oven between the pretreatment system and the e-coat system. Run samples of the worst parts to see if this helps. Cool-off after the dry-off will also be required to prevent carrying heat into the e-coat tank
While some bleed-out is the “nature of the beast,” others can be a major source of contamination or potential corrosion problems. You need to identify how serious a problem you have and how it can be corrected.