Finishing
Electropainting
Cathodic electropaint is a tightly adherent, decorative and corrosion resistant thin coating that can be evenly applied over complex shapes. These attributes make the process especially suitable for zinc alloy diecastings because it can protect all areas of complex castings without adversely affecting dimensional tolerances. The actual thickness of coating applied is controllable between around 10 to 40 microns. Typically it is around 20 microns. Excellent corrosion resistance is achieved with such coatings.
The castings are racked prior to treatment and during the process they are immersed in the water-soluble electropaint bath. Hence the casting designer must pay attention to racking points and drainage capability.
It is common to pretreat the casting with either a chromate or phosphate based passivation treatment.
At the end of the process the castings are stoved to cure the coating, a temperature up to around 180°C is used which is usually insufficient to cause any problems with the casting.
A fair range of colours can now be applied via electropainting and both semi matt and gloss finishes are available.
Because the coating is applied so evenly, the surface finish of the diecasting prior to electropainting is important. Any surface defects in the diecasting are reproduced in the paint surface. (They are not exaggerated, as they would be by electroplating)