Giving both good corrosion resistance and attractive appearance, powder paints have excellent covering capacity. The thicknesses deposited vary from 40 to more than 60 microns. Application is either by manual electrostatic gun or by automated equipment either direct to degreased castings or following passivation (phosphate or chromate). The latter pre-treatments are used for parts intended for class 3 and 4 exposure.
There are three families of powder paints currently used for zinc alloy castings, epoxy, epoxy/polyester and polyester. Their performances are clarified in the table below.
Characteristics of the Three Types of Powders
|UV Light Resistance||Poor||Fair||Excellent|
|Colour and Gloss Stability||Chalking||Good||Excellent|
|Class 1 Use||Yes||Yes||Yes|
|Class 2 Use||Yes||Yes||Yes|
|Class 3 Use||No||Yes||Yes|
|Class 4 Use||No||No||Yes|
|Sodium Chloride 20%||Excellent||Excellent||Excellent|
|Sodium Hydroxide 30%||Excellent||No||No|
Powder coating is not especially even, tending to thin at sharp external corners, whilst internal surfaces of cup and box shapes are often poorly coated because of the Faraday cage effect. Casting designers need to bear these factors in mind.
The stoving conditions required for each of the different powder coat material groups is:
Epoxy 140 – 160°C for 8 to 20minutes
Epoxy/Polyester 160 – 180°C for 10 to 15 minutes
Polyester 160 – 200°C for 10 to 15 minutes
(It is preferable to stove zinc alloy diecastings below 170°C because this radically reduces the risk of the casting blistering)
The diecaster must be informed of the finishing process route requirement at the earliest possible moment because the high temperatures at which some powder coatings, particularly polyesters, are cured demands particularly sound castings if blistering during stoving is to be avoided. This has implications on the design of the component because cavity shapes that cannot be filled without trapping pockets of air will inevitably cause problems when the subsequent powder coating is cured. Vacuum diecasting can overcome some of these problems particularly in thin wall components, but in most cases it is an unnecessary complication. Whilst the surface finish of the casting is not critical it must not have defects that tend to entrap any pre-treatment and cleaning fluids, as this will lead to defects such as “sissing”. It is well also to remember that the surface area to volume ratio of the casting will effect the rate of temperature rise during stoving and hence the speed of curing of the coating. For heavy sections it is advisable to preheat the casting before coating in order to avoid the build up of a severe temperature gradient in the coating during stoving. The effect of such a temperature distribution is to cause the outside of the coating to cure before all the air has escaped from below, which results in pin holes appearing in the coating surface.