The general design of the structure/part to be painted and the initial condition of the metals used must facilitate not only the surface preparation, painting and inspection but also the subsequent maintenance of the part. The aim is to achieve the structure’s durability potential (= service life) through carefully-considered design, an appropriate paint system and regular maintenance.
Good communication between the project manager, the metalworker (responsible for design) and the job coater (responsible for paint application) is required to select the best-suited corrosion protection system and to limit design faults, which can be detrimental to the quality of the coating and/or the durability of the structure.
In addition to taking into account the production rules set out in ISO 12944, the design should also consider:
- the dimensional aspect, with the aim of facilitating handling and transport (size and weight)
- the environmental constraints (intended use of the structure) for judicious selection of the metal used for production, surface treatments and paint system. Simple designs should be chosen, and joints should preferably be welded, and not bolted or riveted, before treatment (with the exception of large structures that are difficult to treat and handle).
Delivery of the parts
1. Initial conditions of the parts
The surface state of the metal used to produce a part plays a vital role in the aesthetics and durability of the powder coating, as surface defects can lead to marks and performance defects.
The metal used must therefore be free from:
- surface irregularities (swarf, deformations, mill scale, grinding defects, etc.)
- traces of marking/pairing (felt tips, paint markers, adhesives)
Tips : Tubular elements with longitudinal welds must be assembled so that the weld is not visible.Following design, the part should be stored in conditions that meet these requirements: designated area (to avoid impacts, marks, etc.), clean environment, protected from bad weather (corrosion). Some products used for forming metals (cutting fluid, stamping oil, forming oil, etc.) and for marking sub-assemblies can lead to defects after painting (cissing, marks, skipping). If parts are paired, metal labels or mechanical engraving on areas of the part not visible after installation should preferably be used for the purpose of identification.
2. Sharp edges
Depending on the thickness and/or the method used, the cutting phase can create sharp edges, which are often poorly protected by the paint system (inadequate coating = thinner areas). This can have a dramatic effect on the durability of the paint system depending on the environment in which the structure will be erected or how the part will be used (first stages of corrosion). Sharp edges must be rounded off or chamfered.
Some ‘oxidising’ cutting methods (CO2 laser cutting, oxy-cutting, etc.) result in the appearance of a HAZ (heat affected zone) and a non-adherent oxide layer to which the paint system bonds poorly (risk of flaking). In this case, mechanical treatment or strong pickling is necessary to remove this layer before painting, allowing the paint system to bond correctly. Blasting is not possible on thin metal sheets due to the risk of deformation.
If poorly performed, certain metal processing operations can lead to burrs (cutting, machining, perforating, etc.). After painting, these burrs create either unsightly thicker areas or thinner areas that are detrimental to the durability of the system. They must be removed by careful grinding by the metalworker to reduce aesthetic problems.
Painting perforated sheet is not recommended when corrosion resistance is sought. The sheared edge may not be protected, and forced punching can form a local protrusion that constitutes a probable starting point for corrosion in the long term. When the use of perforated sheet cannot be avoided in the design of the part, the following perforation specifications must be complied with the formula under picture on the right.
1. Combinations of materials
Combinations of materials of different types or compositions can result in differences in appearance after powder coating (e.g. a combination of steel and galvanised steel) and/or poor corrosion resistance (galvanic corrosion).
In all cases, the formation of galvanic couple must be avoided and, if applicable, surfaces in contact must be electrically insulated (insulating washers). Attention must be paid to add-on components, which can sometimes be incompatible with the powder coating process (resistance to heat and chemical or mechanical processes). To facilitate the polymerisation of the paint, mechanically welded parts must be designed so that there is as little difference in the thickness of their components (thin sheet/plate) as possible.
Welds must not have any roughness, icicles, porosities or craters or be oxidised. Ad hoc treatment can be used to eliminate any defects.
Weld beads must be as continuous as possible (taking into account the mechanical operational constraints of the part/structure), non-porous and free from craters in order to avoid bleeding of surface preparation fluids, which results in the risk of coating failure and subsequent starting of localised corrosion. Porous, discontinuous welds result in:
- the escape of gases during the paint curing stage; evaporation causes a bubbling defect,
- the trapping or running of shot and residual products such as surface treatments, particularly inside tubes or air gaps.
A smooth weld bead will be obtained more easily on correctly chamfered areas.
Slag and spatter must be eliminated. Products designed to prevent the adhesion of spatter (antispatter) must be free from silicon, which would be detrimental to painting, and appropriate surface preparation (chemical or mechanical) must be performed on these anti-adherent products.
For assemblies in close contact (stiffeners, reinforcements, etc.), the uprights and diagonals must be welded set back to allow the free circulation of treatment fluids and avoid bleeding and unprotected or poorly protected areas.
Welds made after galvanising must, if applicable, be treated and „reconditioned ".
Voids (narrow spaces, cracks, air gaps) can result in the start of corrosion, as the smallness of the space makes it impossible for the treatment and paint to enter.
Ideally, the design will avoid narrow gaps and failing this, the gaps will enable treatment (access space) or will be sealed (non-porous welds or filler).
They include, but are not limited to:
- lap joints
- volute connections
- the welding of hinges
- rosettes on flat sheet
4. Retention zones
The initial design of the structure/part will ensure that there are as few liquid retention/stagnation zones (tubes, hollows, flat areas) as possible, with a view to its use and also its treatment before powder coating. Vents (drainage of treatment fluids) will be provided when the part is made up partly or entirely of hollow bodies.
The entire perimeter of the join must be welded to prevent voids. Carefully considered design will restrict the stagnation of water or other deposits and allow for easy access for the application of the paint system.
6. Hanging points
Hanging points must be defined at the design stage, in order to ensure the handling of the part and effective treatment. In particular, hanging holes will be positioned so that they are hidden from view after installation of the part, and must also allow for the part to be tilted during surface treatment to avoid the effects of entrainment (pollution of downstream baths from upstream).
As far as possible, the point must be discussed with the applicator at the start of the design phase.
7. Moving parts
Parts that comprise removable moving parts can be powder coated separately, leaving a clearance that takes into account the thickness of the coating.
The final production stage before corrosion protection and/or painting is finishing. This operation includes the removal of burrs, the cleaning of weld beads and the correction of surface imperfections.
Protection and expected performance will vary according to the design of the part to be painted, the quality of the surface pretreatment and application and thickness of the coating system, as well as the maintenance program of the coated surfaces. This information is given as an indication. It is based on our experience and laboratory results, and does not constitute a commitment or any form of guarantee on our part.