EW January 2011

technical article

4.2 Chemical principles of the new coating

To conclude, both wire manufacturers and wire users benefit from the use of polymer coatings. This results in a competitive advantage for both parties.

The newly developed polymer coating is a mix of high-molecular compounds that are free from heavy metals, boron compounds, mineral oil, chlorine or sulphur-containing lubricant additives (such as molybdenum disulfide and tungsten disulfide) or com- pounds based upon fluorine, for example, Teflon ® . 4.3 Benefits for the customer 4.3.1 Benefits of a polymer coating for the wire manufacturer or the pre-treater of rod sections. Using a polymer bath instead of a soap bath will provide the wire manufacturing plant with the following benefits: Polymer coatings need no soap powder • in the drawing die. This avoids the problem of dust formation affecting the operators. It reduces costs, as there is no need to purchase soap powder or to provide for its disposal. It also improves the reliability of the drawing machines because less soap dust settles on the machines and drives Improved forming means improved • productivity Clean wire is beneficial for subsequent • processing operations Improved corrosion protection helps • the product store for longer, providing customers with a better product 4.3.2 Benefits of a polymer coating for manufacturers of fasteners or cold extruders For wire processors, such as fastener manufacturers, this coating offers the following benefits: Improved extrusion accuracy due to • cleaner material Higher speed of the bolt heading • machines Longer tool life • Manufacture of parts with complex • geometry that were not possible before Less frequent changes of oil in the • bolt heading machines, as no soap is carried over to contaminate the oil re-circulation system

5 Electrolytic

phosphating – a heavy metal-free coating process

Calcium phosphate coat prior to forming

Calcium phosphate coat after forming

5.2 Benefits Free from heavy metals so the effluent • becomes easier to manage No sludge for disposal • Process temperature of approximately • 25°C Treatment time of 2–5 seconds allows • for relatively short plants Required coating weight of 5–15g/m² • can be adjusted via the current density (A/dm 2 ) Higher drawing speeds for wire, and • lower pressing and ejection forces in cold heading and cold extrusion No heavy metal-containing effluent in • the de-phosphating of fasteners prior to heat treatment 5.3 Outlook A future treatment process for cold heading wire and solid parts could combine the benefits of an electrolytic, heavy metal-free phosphating with those of a polymer coating. Figure 5 shows such a plant. This will contribute to reducing the treatment times for surface treatment from around one hour to less than one minute. Consequently, the quality and productivity will appreciably improve for both the surface treatment plant, and the bolt manufacturing or cold extrusion plant. n This paper was presented at Istanbul Cable & Wire ’09 and is reproduced here by kind permission of the IWMA. Figure 4 ▲ ▲ : Cold heading wire prior to and after forming

If a conversion coating (zinc phosphating) is required, then the problems of disposal of the produced sludge and the heavy metals that contaminate the effluent stream must be addressed. The solution to this problem is a calcium phosphate coating applied electrolytically. 5.1 Electrolytic application of a calcium phosphate coating A calcium phosphate coating is also a conversion coating but it differs funda- mentally from the conventional zinc phosphate coating used to date. The coating solution is free from heavy metals such as zinc or nickel and so, therefore, is the coating. The coating can best be described with the formulation CaHPO 4 . The principle of coating deposition on a conductive surface is illustrated in Figure 3 . Since the part acts as a cathode, pickling attack on the iron material does not occur. no phosphating sludge will form so this application process does not generate sludge. Figure 4 shows phosphated and drawn cold heading wire. It can be clearly seen that the calcium phosphate coats are white prior to forming. After forming, the wire exhibits a regular grey colour. Without this pickling attack

Figure 5 ▼ ▼ : Diagram of a continuous line treatment plant

Continuous line treatment of cold heading wire or solid parts

Cold Extrusion

Chemical

Calibration draw of cold heading wire

Jude Burke Chemetall UK – UK Fax : +44 1908 373939

Cleaning

Calcium phosphate

Polymer

Email : ukinfo@chemetall.com Website : www.chemetall.co.uk

Cold Extrusion

Mechanical

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EuroWire – January 2011