The main purpose of pre-treating the metal surface prior to powder coating is to:
Thoroughly remove all foreign matter such as iron filings, grease, cutting fluid, dirt, welding slag, etc.The surface is treated to make it suitable for painting.Through the pre-treatment process, the entire surface of the treated workpiece is made uniform regardless of the source of the metal and the contaminants attached.As with other methods using organic surface treatments, in order to fully exploit the potential of powder coatings, special attention must be paid to the pretreatment process.The surface pretreatment process may vary with the specific end use requirements of the workpiece, including: from simple cleaning operations to multi-stage pretreatment of the conversion coating deposited on the metal surface.To apply a static particle coating to a grounded metal surface, the metal surface must be free of any high resistance components. Any insulating film on the surface of the workpiece to be sprayed will limit or even prevent the deposition of powder.
Substrate
Steel, aluminum, copper, zinc alloys and galvanized steel are common metal substrates for coating powder coatings. In many cases, a thoroughly cleaned metal can achieve a satisfactory coating effect if the coating conditions are normal.
steel
For iron/steel surfaces, optimum corrosion and salt spray resistance is achieved by zinc phosphate conversion coatings.
aluminum
For aluminum and its alloys, although the clean surface is easy to apply and the adhesion of the coating is excellent, a proprietary chromate conversion coating can still be used to improve performance.
Zinc alloy
For all zinc-containing substrates, such as Zinc,galvanized steel, a suitable phosphate coating is recommended.
Porous castings and “sandblasting” surfaces
Due to entrained air, these surfaces may have serious “stomach” problems with powder coatings. Therefore, the shape of the metal and the thickness of the coating must be strictly controlled. In some cases, warming up for a few minutes can overcome this problem.
Remove oxides and scales
This can be achieved by mechanical friction, wire brush friction, or for larger surfaces, sandblasting can be used. In the UK and many European countries, the use of sand as an abrasive has been banned.
A disposable coarse abrasive or recyclable metal abrasive that replaces sand, now adds a range of ultra-fine abrasives, from 600 mesh fused alumina (as fine as talc), soft shell abrasives such as walnut shells and peach kernels. A tiny glass sphere of less than 25 μm in diameter. With these very fine abrasives, a very uniform surface can be obtained. Obviously, when using ultrafine abrasive grains, the iron filings are removed slowly, but the use of coarse abrasive grains causes the surface to be rough, so that during the drying stage, the powder fluidity is limited, resulting in loss of gloss and surface contour. Extremely rough.
To know the relative surface roughness of the blasted steel surface, the “peak-to-valley” measurement of the blasted steel surface should be about 100 μm. If fused alumina (180/220 grade) is used, it is 3-5 μm; if glass beads are used, it is 1-1.5 μm.
Remove grease
This is usually the first step in pre-painting of metal surfaces. There are a variety of methods of operation, which are described in more detail below.