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 |
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e.g. SA (2 ½ ) |
Roughness |
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Ra – Rt – Rz –Rmax., etc. |
Structural shape |
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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
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