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The medium used in a cleaning or peening operation is the most important
factor. It is therefore vital to select the correct size, shape and hardness for the particular
requirements specified.
Size
The abrasive supplied by the manufacturer is predominantly one size
but, in use, an ‘operating mix' develops which contains a range of particle sizes.
The operating mix contains the balance between ‘Impact' from the new and larger
particles to remove larger contaminants and ‘Coverage' from the medium and smaller
partly worn-down particles to remove the smaller and/or remaining contaminants.
The initial size selection, therefore, is critical to ensure the correct size
grading within the blasting process to achieve the balance between ‘Impact' and
‘Coverage' and hence produce the required surface finish at the lowest operating
cost.
Shape
The particle shape is determined by surface finish and profile requirements, type of contaminant to be removed and prevention of distortion through peening. The shape of an abrasive when new is not always the same as when in the operating mix. Whereas steel shot remains round throughout its life and iron grit remain angular, steel grits may loose angularity depending upon their initial hardness. GP grit changes from angular to round when in use, GL grit remains angular, but loses its sharp edges and GH grit remains angular. Cut wire pellets are cylindrical when new, but round off during service.
The shape of the abrasive is therefore an important factor in selection, particularly in respect of how that shape develops during service.
Hardness
The degree of hardness required is determined by the speed of clean required, the type of contaminant to be removed, the surface finish and profile required and the operational costs involved. Abrasive hardness is important because hardness, by definition, is resistance to deformation. So the harder the pellet, the less it deforms and the more it performs useful work in deforming the work surface. However, |
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energy is wasted in the fracturing of pellets so there is no advantage in using an abrasive which is hard, but also extremely brittle. Maximum hardness combined with a low breakdown rate is required for optimum results. Hardness is also an important factor in quality and cleaning efficiency, particularly when the work is intricate and has deep cavities. The harder the abrasive shot, consistent with lack of brittleness, the greater its ability to ricochet and hence clean areas that cannot be cleaned by a direct blast.
Abrasive hardness, therefore, is an important factor in blasting economy because it determines the length of cleaning cycles and the quality of the resulting finish as well as having a bearing on maintenance costs. |
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Choosing the correct abrasive, with the right shape, size and hardness,
and maintaining the optimum operating mix are the most important elements in
controlling costs. There are, however, other factors that must be taken into account
if maximum efficiency is to be achieved.
For further information contact Metabrasive Technical Services
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Metabrasive Limited, Creative Industries Centre, Wolverhampton Science Park, Glaisher Drive, Wolverhampton, WV10 9TG
Tel: 01902 792610 Fax: 01902 712058 |
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