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11/1/1998 | 5 MINUTE READ

Fasteners and Finishes

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While the trend both domestically and globally is toward reducing automotive fastener finishes, niche coatings will always have a place.


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While the trend both domestically and globally is toward reducing automotive fastener finishes, niche coatings will always have a place. Special conditions will always exist that require special finishes. Electrical grounding, fuel splash, heat, wear, color and unique applications are a few situations that place unusual demands on the coating industry.

Grounding applications now require corrosion protection in addition to electrical conductivity. Zinc plating is an old standby, but modern conditions have raised the salt-spray-hour requirements beyond the thickness that can be effectively plated. Zinc thicknesses greater than five tenths are usable in many conditions but often interfere with installation of mating parts. White corrosion product builds up to a point where it insulates and prevents electrical conductivity. The ideal grounding coating is thin, conductive, easily applied, adapted for bulk processing and, as always, inexpensive.

Assembly techniques have also been refined, and fasteners are now installed with tooling that requires a consistent friction factor. Several coatings are available today that meet most of these conditions. A great interest has developed in tin/zinc alloy plating as a coating that meets many of the grounding requirements. Few coaters have been able to produce the required coating consistently, and hours to failure vary widely from lot to lot for some applicators. Another coating, Sermagard's® 400C base coat from Teleflex Inc., shows 400 hours corrosion resistance with excellent properties in heat areas as well as conductivity.

Fuel splash has the effect of removing the black topcoats that many automotive customers now demand on their fasteners. In addition to gasoline and diesel, reactivity to alternate fuels such as alcohol must be considered. Since fuel splash fasteners are used outside of the weather-strip area (the line that separates five year corrosion requirements from lesser values inboard), high corrosion resistance is an added factor. The ideal fuel resistant coating is black, inexpensively and easily applied, thin and has 400 hours or more of corrosion resistance, since it is visible every time the fuel door is opened. While there is a trend for some users to go to black oxide coated stainless steel in many fuel area applications, the cost of the metal and the fact that the black oxide soon washes off keeps this from being the solution of choice. Of the viable products on the market for this area is a process that includes a zinc-nickel alloy base layer of a multiple layer package. If the organic topcoat is lost, the underlayment effectively prevents rust although the part is then argent in color. One group of organics that has had excellent acceptance is the dip-spin black PTFE-based products that are inert to solvents. However, thread and recess filling are serious problems with these coatings ifapplicators overfill barrels and underspin to reduce costs. Several varieties of organics with varying percentages of PTFE are available today and have met with mixed success.

Heat is a difficult condition to overcome in finish durability. The temperature of automotive systems has steadily risen over the past 40 years. For example, the standard radiator solder has been replaced with a high-melting-point type because the cooling systems now run at temperatures higher than the melting point of the old material. The temperature increase is partially due to noise reduction improvements that consist of enclosing the engine within a noise shield, which effectively encloses heat as well. Many vehicle systems now operate at 300-350F. Exhaust areas, with catalytic converters, run from 600 to more than 1100F. Few coatings survive in these environments. Generally, the use of stainless steel fasteners has alleviated some of the most severe attachments, but at the cost of strength. In other areas, metallic-based coatings with additives can be made to work. Sermagard's® line of high-temperature coatings offers corrosion at heat levels of 1000F and greater. Metal Coatings International's Dacromet 320® line of products and its new non-chromate global product, GeometTM, offer protection to temperatures in the 750F range with proper functionality.

Dependent upon the exact temperature, several of the metallic based dip/spin products on the market are capable of existing in moderately high (between 350-500F) temperatures. Users of these types of products should consult the manufacturers to determine at what temperature range the compounds degrade.

Wear has always been a factor in the determination of correct finish selection. While little has changed in usual conditions of use, new situations have arisen. The computer age has spilled over into automotive design. Location systems, computerized ignition, internal comfort controls, even seat positioning are dependent upon microchips. Finishes that have tendencies to flake off particles that may "short" out silicon pathways are forbidden. Because of this flaking problem, many system boards are now held in place with plastic fasteners or are snap fitted into tabs and slots. Acceptable finishes on metallic parts, where necessary, are required to be hard, dense, non-metallic coatings (organics). Appearance and wear properties are still favorably produced by hard chromium plating and some new formulations of zinc-nickel alloys. However, future restrictions of hexavalent chromium in global markets (Germany and Europe after 2002) will limit their use in vehicles with worldwide marketing.

Special situations exist where most standard finishes do not meet the mark. Marine environments demand much more resistance to weather and environment than most standard finishes can deliver. While salt spray, especially with an alternate submersion condition, is considered a rigorous corrosion test in some industries, it's a way of life in ocean front structures such as drill rigs, naval docking, and all the steel that functions at the seaside. Doing service in this area for years are the Whitford product lines of Xylan® PTFE and FEP-based compounds that offer excellent outdoor exposure and marine resistance.

Color has plagued the finisher forever. The requirements of matching surrounding parts, accounting for sunlight fade, durability and meeting all the other requirements of a finish (no-oils against plastic, non-oily to touch, cheap, easy to apply, etc.) are a drawback to color coordinating fasteners to their substrates. Further, the economical fact of filling up a unit with several dozen gallons of paint to do a few hundred pounds of parts is extremely costly and time consuming. This leads many coaters to decline color jobs since they probably will get complaints and returns on anything that is run. Still, the customers will continue to request color. Formerly, color was not much of an issue, black or argent sufficed for most conditions and identification. Where necessary, color was accomplished with dyes in chromates (the colors had only to be orange-like, pink-like, etc). While some coloration is available today, matching to a paint panel is still very difficult and requires a paint that is workable and doesn't change during processing. It is a good idea to have some disclaimers on any color match jobs that are run. On parts other than fasteners, flow coating has been able to match color requirements fairly well. The thickness of such coatings should be considered by potential users and should be a point that the plater makes to his customers before starting a job.

Any more niches out there? Write or e-mail the PF Staff and pass on your comments.