After fasteners are manufactured, coatings are often applied to provide enhanced properties.
Fastener coatings are used to provide rustproof screws and corrosion-resistant nuts and bolts and decrease the torque required during installation, among other advantages. Examples are fluoropolymer coatings/lubricant blends that decrease friction during installation. Molybdenum disulfide serves a similar purpose but is suitable for high-pressure loads. Epoxy coating is used to provide superior impact resistance in addition to corrosion and abrasion protection. Inorganic zinc coating provides excellent corrosion resistance on steel fasteners ideal for outdoor applications. Phenolic coatings provide the best protection in low pH, high-temperature environments. Phosphate ferrous metal coatings provide anti-galling and minor corrosion resistance. There is a coating available for nearly every fastener application.
Since steel and aluminum fasteners are typically heat treated, the coating is applied afterward. The coatings can be water- or solvent-based, depending on the formulation. In order to provide a consistent finish and maintain desired production rates, the coated fasteners are typically cured in a batch or conveyor oven at temperatures of 95-205°C (200-400°F) for a period of 15-45 minutes. The time and temperature is dependent on the coating itself and is specified by the coating manufacturer.
Low VOC (volatile organic compound) coatings are now available that provide excellent results while reducing the release of volatile vapors during the application and curing processes. Low-VOC coatings simplify cleanup, reduce disposal costs and eliminate the need for pollution control.
For example, this electrically heated belt conveyor oven (Fig. 1) is used to quickly dry a coating on titanium and stainless steel fasteners. The design features a stainless steel wire mesh belt to carry the parts on trays. Fasteners are processed at a rate of 1,300 parts per hour, running two shifts per day for a total production of over 5 million fasteners per year. The oven uses a small footprint to optimize floor space.
The conveyor oven has a maximum temperature rating of 205°C (400°F) and a work chamber of 1,372 mm wide x 2,210 mm long x 152 mm high (4.5 feet wide x 7.25 feet long x 6 inches high). The recirculation system utilizes a 10,200 m3/hour (6,000 cfm) direct-driven blower and top-down airflow, with a 72-kW heating system to maximize heating rates and temperature uniformity of the product. The conveyor belt includes a variable-speed drive to allow changing of the heating times. A digital paperless recorder is included to record and archive the oven operating temperature using Ethernet communication.
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