Fasteners and Finishes - Part V

Article From: Products Finishing, from Technical Presentations Co

Posted on: 8/1/1998

In the early 1980s, major OEMs began using electrodeposited high-build primers to increase corrosion protection on body sheet metal and reduce or eliminate the surfacer/finisher coat.



Photo courtesy of Forest City Technologies, Inc.

In the early 1980s, major OEMs began using electrodeposited high-build primers to increase corrosion protection on body sheet metal and reduce or eliminate the surfacer/finisher coat. These hard, thick coatings replaced the softer cathodic primers, which had served industries for decades.

With this innovation came many new problems. The coating would plug the threads of installed weld and pierce nuts, which jammed mating parts. It would freeze floating cage nuts, causing under-loaded joints through premature installation-tool turnoff via "chatter" of the tool (Most installation tools are set to turn off when the torque reaches a set level. Chatter causes a spiking of values, fooling the tools into thinking that they're at their turnoff point). And it allowed a slow creep to the attached joint. The high-build coating acted as a thick hard layer that would slowly evaporate. After a period of time, usually about 72 hours, creep would cause loads to be decreased by almost one-third, contributing greatly to loose joints or, at least, under-loaded ones (a prime cause of fatigue).

Many solutions were tried. Plugs worked, but at a terrible price and only when they were put in, stayed in and were taken out before assembly. Paper masks did not last a cycle. Neither did increasing installation torques when it was found that the electrodeposited coating varied greatly from the sides (nearest to the electrodes) to the center of the parts (thinnest areas). What finally solved the problem was a PTFE-based compound that insulated the threads from the electrodeposition of the primer. PTFE compounds are similar to Teflon®. The compound successfully kept the primer from sticking to the surfaces to which it was applied, while not interfering with the installation of mating fasteners. Since the introduction of this idea, all the OEMs in the automotive industry, as well as several other major manufacturers, have instituted similar specifications for the PTFE masking compounds. Untold millions of dollars have been saved, as well as making an improvement in the engineering and quality fields of manufacturing.

Because of the special properties of PTFE compounds, the masking compound was found to be a good weld splatter preventive. Hot metal, from incorrectly set welders, sputters out and adheres to the threads of internal nuts, tapped holes and so on. The welded droplets cause interference problems with assembly, ranging from poor assembly and galling of fasteners to a complete inability to assemble and the need for on-line repair and re-tap. The PTFE coating repels most weld splatter, allowing the nodules to fall harmlessly off the threads. Obviously, very large pieces of molten metal would burn through the coating and adhere, but few situations like that were encountered.

Many finishers and small job shops now do their own primer coatings. Most OEMs outsource bracketry and small jobs with a requirement that they be coated with the high-build primers for corrosion protection. Finishers should be aware that these primers can cause later complaints that their parts do not accept mating bolts, do not gauge and may have weld splatter in the threads. To avoid this potential situation, the finisher should ask if a thread-masking compound could be used on high-build primer coated parts.

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