Richard Besant from Powdertech (Corby) provides tips on how to achieve the best possible coating appearance and integrity in the metal finishing process during design and manufacture.
As with so many things, beginning with the end in mind can pay dividends, and this is particularly true of metal fabrication design. Incorporating the finishing and coating process in design review will enable you to achieve the result that you envisaged.
Within even superb fabrication there can be small features that prove detrimental to a successful finish of any type, including powder coating. With a little bit of knowledge of the powder coating process, and how best to design for it, you can avoid unnecessary complications and achieve the best possible result.
Venting is key
Your fabrication will be immersed in molten zinc if it is to be galvanized, or pre-treatment chemicals if aluminium. Suitable ventilation holes need to be drilled into hollow sections in the correct places to allow air out so the object sinks rapidly.
Steel and zinc have roughly the same densities so a hollow steel fabrication will float in a bath of zinc, much as an empty plastic bottle would do in a bath of water. Ventilation holes allow zinc to flow in and air to flow out but the smallest amount of air trapped in the section will cause it to float and burn. There will also be incomplete internal protection and a poor surface finish. Well-designed drainage makes the fabrication lighter, cheaper and safer.
Your design should avoid entrapment of pre-treatment chemicals which can leech or boil out, causing bubbling in the final coat. Riveted or stitch welded fabrications can also trap chemicals so it is wise to avoid this where possible.
To allow molten zinc to flow uninterrupted along the surface of the steel, the corners of internal stiffeners at the web should be cropped to avoid pockets of air and ash.
Avoid sharp edges
Sharp edges caused by laser cutting, shearing, sawing and punching are a problem in both steel and aluminium. They occur at the trailing edge of a perforation, sheared edges of sheet and the edges of expanded mesh. Sharp edges cause localized thinning of all coatings, including zinc and powder coating. On galvanized steel they can also increase fracturing of the zinc coating along an edge, leaving the area vulnerable to rust. The solution is to round off or chamfer sharp edges, to allow the molten zinc or powder coating to flow around it.
Avoid deep recesses
Deep recesses in a fabrication can be problematic for powder coating. Electrostatically charged powder will resist penetrating a recess that is, as a rule, deeper than it is wide and the coating may be incomplete.
Consider the installed location
In harsh environments such as a marine or heavy pollution, it is advisable to avoid pockets where contaminants may become trapped. Water should always be allowed to flow away.
Secure joining of metal pieces
For bonded items such as butt straps the adhesives must be capable of withstanding powder coating curing temperatures in excess of 200ᵒC. Faces to be bonded must be clean and grease-free. Welding must be continuous to prevent leeching of acid, flux and pre-treatment chemicals from between the weld resulting in incomplete galvanizing and powder coating.
Suitability for the coating process
The completed size of your metal design may determine the type of finishing process you will need to use. Check with your metal finisher / galvanizer to make sure you do not exceed the dimensional and weight limits of their plant. Talk to your supplier or to a trade organisation, to determine how to achieve the best design/metal finishing solution.
Suitable hanging (jigging) points need to be incorporated into the design, in positions that will support the weight of the material. For harsh environments hanging holes must not be in the exposed face.
Unclear specification can lead to lengthy and costly delays in the construction programme and all parts of a building should contribute towards the whole, in appearance just as much as function. Specifications should be consistent for similar elements and applied throughout building. Specification should include applicable industry application standard, powder coating material standard, colour reference and gloss level.
Cleaner metal means better finish
Low tack film residue burnt into metal, welding flux and spatter, as well as grinding and cutting lubricant, silicon, swarf, laminated steel and grinding marks will affect both the appearance and performance of a coating.
Powder coated surfaces offer a high degree of protection and stunning visual appeal which is optimised when these simple design and fabrication tips are addressed.
Richard Besant is sales director at Powdertech (Corby)