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Powder Coating

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

 Characteristic Epoxy Epoxy/Polyester Polyester
Appearance Excellent Excellent Excellent
Properties Excellent Excellent Excellent
Heat Resistance Poor Good Excellent
Weather Resistance Poor Good Excellent
UV Light Resistance Poor Fair Excellent
Colour and Gloss Stability Chalking Good Excellent
Gloss Yes Yes Yes
Granular Texture No Yes Yes
Smoothness No Yes Yes
Hammered No No Yes
Matt Yes Yes Yes
Metallic No Yes Yes
Satin Yes Yes Yes
Textured No Yes Yes
Class 1 Use Yes Yes Yes
Class 2 Use Yes Yes Yes
Class 3 Use No Yes Yes
Class 4 Use No No Yes
Acetone Limited No No
Acetic Acid No No No
Nitric Acid Limited No No
Phosphoric Acid Excellent Excellent Excellent
Ethyl Alcohol Excellent Excellent Excellent
Ammonia Limited No No
Sodium Chloride 20% Excellent Excellent Excellent
Sea Water Excellent Excellent Excellent
Tap Water Excellent Limited Excellent
Distilled Water Excellent Limited Excellent
Petrol Excellent Limited Limited
Sodium Hydroxide 30% Excellent No No

Reference 24

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.

Stoving Temperatures

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.

Reference 2