Abrasive blasting is widely used in the preparation of surface contaminated metals, prior to their protection, by a conversion or powder coating.
During the cleaning process particles are projected at a high velocity onto the substrate and the resulting kinetic energy impact abrades the contaminating scale, corrosion and/or particulate dirt present on the product. At the same time, the particulate impact also creates a degree of surface etching which subsequently provides a keyed surface for coating adhesion.
When it comes to abrasive blasting, there are many suitable types of material available from the relatively soft mineral slags or glass beads through to the very hard steel shot or silicon carbide. The selection of the most appropriate material is based on four factors :-
- The shape of the blasting material – round particles are ideal for the removal of brittle contaminants like mill scale, whereas angular materials tend to scour a components surface and are therefore employed to remove friable materials like rust, dirt or old coatings.
- Hardness – hard particles are best for removing heavier contaminants like rust, whereas soft abrasives are better for oil and grease removal
- Density – the degree of surface impact of a particle is directly proportional to its kinetic energy applied and thus the denser the blasting particle the greater the impact energy
- Size – the best abrasive materials contain a range of particle sizes; coarse particles to remove heavier deposits such as mill scale and finer particles to remove the friable materials
As a result, at Ridgewood Powder Coating, we specifically use Garnet ( an Aluminum Silicate) which has a hard irregular shape and also has a high density. This material is ideal as it enables us to provide a light ‘whip’ blast to a substrate so as to remove surface oxides from new substrates, through to creating a Swedish Standard (SIS 055900) – Sa2.5 to Sa3 finish (i.e. substrate cleaning to white metal), as required by many protective coating specifications.
Oils and greases are probably the second biggest headache for powder coaters after “the kiss of death” – Silicone Oils. They can be extremely difficult to remove from internal areas or cavities in the product. As a result the residual materials tend to boil or leach out during the powder curing process – this causes blow outs in the coating, aesthetic issues such as pinholes, fisheyes or unsightly brown marks.
Customers are therefore encouraged to talk to us about their methods of fabrication, such that we can work together to eradicate these problems.