Whereas the type of the abrasive used to follow clearly from the term “sand blasting”, nowadays difference has to be made between different kinds of abrasives. The silica sand that was used in former times can be applied now only with special permit due to the risk of silicosis; these special permits have to be applied for at the competent authorities. Silicosis is a disease (also called black lung) caused by finest silica sand entering the lung and there it clogs the air sacs.

By using the blasting method various kinds of work can be performed now. The number of abrasives that can be adjusted to the respective purpose of work is therefore high. There is no universal abrasive. The selection of the respective grain sizes is decisive besides the selection of the abrasive. Bigger particles transfer higher kinetic energy and are thus well suited to e.g. break open thick layers. Quicker and steadier cleaning results are, however, obtained by a big number of smaller particles.

Slag, corundum, steel shot and glass have proven to be suitable apart from some very special abrasives.

3.1 Slag abrasives

In principal, slag abrasive can be used wherever people used to work with silica sand. Retrofitting of the existing facilities is not necessary. Because slag abrasive will shatter into particulate matter by 50% an application with subsequent processing requires much work. Due to the related high percentage of particulate matter any workplace outside has to be covered.

3.2 Corundum

Corundum is considered to be the most important synthetic abrasive and does not contain iron. It consists, among other things, of aluminium oxide and silicon dioxide. Silicic acid only exists in bounded form. Because it can be produced with different Al₂O₃–contents – by this the hardness can be varied – it can be used for different kinds of work. Furthermore, corundum is supplied in various qualities. Difference is made between an inexpensive regenerate, electro corundum and expensive high grade corundum.

Corundum can be used for recycling uses. Due to its hardness normal corundum of medium quality can be used about 20 – 25 times in rotation. This reduces the necessary amount of abrasives to about 1/20 compared to slag abrasives. Also after several runs the angular corundum does not lose its shape. Due to its geometry it is not suitable for applications in shot-blasting wheels.

3.3 Metallic abrasives

In case of stationary blasting metallic abrasives like steel shot are very important because they can be processed and the operational costs can be therefore kept at a low level. The abrasive is made by granulating molten metal with a water or vapour jet. Metallic abrasives can also be supplied at different levels of hardness. Hard cast iron shows the biggest hardness and abrasivity, but it is also the brashest abrasive with a respectively short durability.

Metallic abrasive can be used about 100 up to 200 times at compressed-air blasting before it will decompose into particulate matter. The fact that the kinetic energy is focussed on one point during the impact leads to high blasting performances. The application of ball-shaped abrasive is necessary when used in shot-blasting wheels because of high degree of wear and tear.

Wire cuts can also be used as an alternative to steel shot and white cast iron granular material. This is cut wire having a cylindrical grain shape. Steel wire has got a very long durability. But as it is considerably more expensive and it does not round off as quickly as cast steel it has not yet gained acceptance. The market share amounts to about 15%.

3.4 Glass abrasive

Glass abrasive is free of silicosis and thus harmless. It mainly consists of bound silicon dioxide. Angular glass convinces by good abrasive performances, it, however, breaks up quite quickly. Due to high prices glass abrasives are only used where another abrasive cannot be applied due to its chemical composition or mechanical properties.

The application of glass beads is more important which are obtained from molten mass. This ball-shaped material is available at various diameters up to 0.63 mm. Because of the low grain mass glass beads are suitable for surface finishing or cleaning processes. Shot peening can be carried out by this as well.

4. Efficiency assessment of abrasive blasting

An efficiency assessment of all blasting methods with abrasives can be made through the generated kinetic energy E_K and its output at the impact on the surface.

In order to accelerate the body and to keep it going at a possibly high speed work has to be performed. This one then exists in the form of kinetic energy in the body.

If

E_k = kinetic energy of the body (Nm or Joule)

m = mass of the body (kg)

v = velocity of the body (m/s)

then applies

This shows that

   the change of the mass has linear and

   the change of the velocity quadratic effects.

The kinetic energy of the abrasive increases linearly at the same speed at increasing mass.

The kinetic energy of the abrasive increases squarely at the same mass at increasing velocity.

Influence of the grain size of the abrasive on the blasting result

By reducing the medium grain size the number of grains increases exponentially.

“Maximum coverage is achieved with fine abrasive grain.”
“Maximum roughness height is achieved with rough abrasive grain.”

Coverage is a measure for the impacts per mm².

The abrasive weight thrown off (M) per unit time

The medium grain weight (m)

Result:
If the diameter of the grain doubles

then
   the weight of the grain will increase by eight times
   the kinetic energy will increase by eight times
   the coverage will reduce to one eighth.