Q&A: Gap Size Specifications

Q. I cannot find a standard or a design rule that specifies the minimum gap necessary for complete coating during electrocoating. We design complex, geometrical metal fabrications, and many times we have to leave small gaps between components of the assemblies. R.D.

A. There is no formal standard or design rule that I know of for determining minimum gap sizes. Whether or not your electrocoat would properly deposit in gaps will depend on your specific electrocoat technology, application and operating conditions of voltage; percent solids; temperature; agitation; and dwell time. Whether or not your machine is a square transfer or continuous monorail could also have an impact on the outcome of coating inside minimum gaps.

The ability of electrocoat paint to deposit in tight spaces is called throwpower. Most electrocoats will build inside gaps of 8-10 mm to a depth of 1-3 inches, with a decreasing film thickness pattern from the outside to the inside. As the gap size increases, the thickness profile will equalize between the outside and inside. For gaps around 25 mm, typical electrocoats could deposit more than 400 mm.

Testing several gap sizes in probe assemblies would indicate the minimum distance or gap for your specific geometry and conditions in regards to not only film deposition, but also for adequate cleaning and phosphating. In my experience, open gaps of 7-8 mm or smaller are acceptable if they are open in both ends, but could be too small for liquids to freely enter and exit if the gap is closed on one end. In many cases with small gaps of less than 1 mm, liquids enter via capillary action and get trapped deep inside. These conditions create unacceptable quality defects due to kick-outs and boil-outs.