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Start | Basics Blasting Technology



5. Influence of compressed-air blasting on the surface of the workpiece

Abrasive blasting, that’s the cleaning of surfaces during which a granular material is thrown on a surface at a high speed in order to remove paint residues, scale, sand and other contaminations. The result is called cleanness.


During the blasting process the background (the surface of the workpiece) is also deformed, this deformation (roughness height) can show very low, but also high roughness. But the structural shape can also be influenced.

We make a difference between

Cleanness  ----  e.g. SA (2 ½ )
Roughness  ----  Ra – Rt – Rz –Rmax., etc.
Structural shape  ----  number of peaks per measuring length,
steepness of the peaks, etc.


Energy is transferred to the steel grain during the blasting process. This energy is influenced by:

     the amount of air of the compressor
     the generated pressure
     the nozzle diameter
     the abrasive   shape of the grain
                         specific weight
                         size of the grain
                         hardness of the grain
     the impact angle
     the distance nozzle – surface

The kinetic energy that is transferred to the abrasive breaks down into

     kinetic energy of the rebounding grain
     deformation energy of the grain
     deformation energy of the surface (only this one does
     the cleaning work)

The rate of distribution depends on the following as regards these kinds of energy:

     the hardness of the surface to be blasted
     the impact angle of the blasting grain
     the physical properties of the blasting grain like:
          hardness
          size
          specific weight
          elasticity

During the blasting process contaminations are removed from the surface and at the same time this surface is prepared for new coating.

It is the task of abrasive blasting to remove contaminations from the surface. These ones can consist of different materials like for instance:
     - scale
     - rust
     - built-ups of sand (foundries)
     - welding residues
     - old paint coats, etc.

The required energy to clean a coated surface depends on the old coats to be removed, i.e. their
     - composition
     - hardness
     - thickness/toughness
     - condition


5.1 Cleanness

The durability of a coating depends on the cleanness of the surface on which the coating was made. The required degree of cleanness is determined according to the load of the coated surface.

To determine the degree of cleanness the Swedish standard SIS 055900 is used in Europe. This standard has been included in DIN 55928 in Germany. Here the degree of cleanness is determined on the basis of comparative lists.

When dealing with these lists it is of utmost difficulty to evaluate the surfaces. If the blasting process is performed with different abrasives or by using different grain diameters this will result in completely different degrees of brightness and structures.

It should be a matter of course not to blast the surface cleaner than asked for by the customer, because unnecessary extension of the blasting time reduces considerably efficiency.


5.2 Roughness and structural shape

When preparing a surface for coating it is important – apart form cleanness – to get a structure of the surface that offers optimal adhesion of the coating. This adhesion is ensured by obtaining a suitable degree of roughness and structural shape.

In case of high roughness heights the consumption of paint is higher than for lower roughness heights. It should also be taken into account that the applied coating thickness of the paint should correspond to the obtained roughness height.

Roughness and structural shape depend on:

- the abrasive:
     - shape of the grain
     - composition of the operational mixture
     - hardness of the abrasive
     - toughness of the abrasive

- the blasting unit:
     - nozzle (shape of the nozzle)
     - pressure – discharging speed

- the material to be blasted:
     - hardness
     - intensity of the jet

- the blasting device:
     - impact angle
     - distance between workpiece – nozzle
     - nozzle movement and guidance.

Example: Different abrasive grains (steel shot)

Ra = medium roughness value
Rmax = maximum roughness height
Rz = averaged roughness height