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Wire & Cable ASIA – September/October 2007 91 Wir & Cable ASIA – March/April 11
Less frequent changes of oil in the bolt heading •
machines, as no soap is carried over to contaminate
the oil re-circulation system
To conclude, both wire manufacturers and wire users
benefit from the use of polymer coatings. This results in a
competitive advantage for both parties.
5 Electrolytic phosphating – a heavy
metal-free coating process
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 fundamentally 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
CaHPO4. 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.
Without this pickling attack 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.
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/dm2)
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.
Jude Burke
Chemetall UK – UK
Fax: +44 1908 373939
Email:
Website:
Consequently, the quality and productivity will appreciably
improve for both the surface treatment plant, and the bolt
manufacturing or cold extrusion plant.
This paper was presented at Istanbul Cable & Wire ’09 and
is reproduced here by kind permission of the IWMA.