Abrasives and their correct choice, is the most important and difficult part when establishing a blasting or shot blasting process. It is necessary to know the type of abrasive to use, for example, at the time of obtaining a certain superficial finish, a cleaning, pickling or engraving within an industrial process, to obtain perfect finishes and free of residues or materials. pollutants, etc. There are many types of abrasives but, for sandblasting or shot blasting, there are materials that are well known in our sector, such as for example:
– Glass beads (without free silica)
– White and brown corundum (aluminium oxide)
– Ceramic abrasive (zirconium)
– Plastic abrasive
– Vegetable abrasive (walnut and almond shells)
– Steel shot
– Stainless steel shot
– Chrome steel shot
The great absent in this list of abrasives is the “silica sand”, because its use produces silicosis, a serious disease of the respiratory tract by inhalation of free silica particles that are released into the dust generated, when the dust is broken during the blasting process. ABSHOT in its Safety Policy and respect for the environment, has never had among its products the silica sand.
All the abrasives shown in the above list are free of free silica and any contaminants. ABSHOT has the technical and safety data sheet of the abrasives distributed, and all the necessary documentation provided by the manufacturer.
THE FACTORS FOR A CORRECT FINISHING.
It is just as important to achieve the desired surface finish as to achieve a balance between the costs and the effectiveness of the abrasive blasting treatment. There various factors related to the abrasive that, together with the blasting equipment or to use, determine the final finish:
The size of the particles will determine the roughness obtained. If we set the pressure and the flow of air, to vary the size of the grain will vary us the roughness obtained by the impact of the abrasive particle on the surface. This means that the bigger size of the particle, eaves the larger print on the surface.
The abrasive manufacturers use various nomenclatures and numbers to define the size of each grain, but they are normally governed by the FEPA standard that sets the size standards.
Abrasives have two typical morphologies: spherical and angular. As a general rule, a spherical abrasive is less worn for the sandblasting or shotblasting equipment, the finished surface is softer to the tact (since the surface roughness has no peaks but it’ s in the form of “waves” due the impact of the spheres), and the process is usually slower.
On the other hand, an angular abrasive wears the blasting equipment much more, the surface finish is rough to tact (as the surface roughness is in the form of a “saw” due to the impact of the abrasive with edges) and the process is usually faster, as it’ s much more aggressive than with a spherical abrasive.
As a summary, we can say that the angular abrasive works best when it comes to removing or pickling layers of paint and corrosion. The spherical abrasive, on the other hand, is better for thinner thicknesses and fine finishes, satin finishes and shot peening processes to eliminate surface tensions in the pieces.
Density is the weight of the abrasive by volume. This is the least determining characteristic that must be taken into account when carrying out blasting or shot blasting work, except if the difference in density between the chosen abrasives is very wide.
The denser the material, the greater the energy with which it is impacted against the surface. In other words, one heavier abrasive than another can leave a deeper imprint than another much lighter abrasive.
The hardness of the abrasive will determine its effect on the surface to be sandblasted or shot-blasted.
If the abrasive is harder than the surface of the material to be sandblasted, pickling (or deformation) of the surface to be treated occurs, as the abrasive is more resistant than the material against which it impacts.
Conversely, if the abrasive is softer or softer than the surface of the material to be sandblasted, the abrasive will only remove the coating on top of the surface (paint, rust, etc.) since the abrasive is less resistant than the material against which it hits and bounces, after such impact, without leaving a superficial trace.
Therefore, an excessive hardness of the abrasive to be sandblasted with respect to the hardness of the surface to be treated can cause deformations in the piece and even premature wear in the blasting or shot-blasting equipment.
The hardness of the abrasive is measured according to the Mohs scale, with 1 being the scale value as soft as talc, and 10 being materials as hard as diamond. If we classify the abrasives from higher to lower hardness, we see that white corundum and brown corundum have a hardness of 9 Mohs, followed by ceramic shot with about 7 Mohs, glass microspheres about 6 Mosh, like aluminum silicate, and then come the vegetable abrasive and plastic shot with about 3 Mohs approximately.
Steel and stainless steel abrasives are normally measured in Rockwell (HRC). Steel shot is differentiated by low (46-51 HRC), medium (53-57 HRC) and high hardness (>64 HRC). Stainless steel shot has a source hardness of 30 HRC and, as it is used, achieves an operating hardness of 45 HRC. Stainless steel shot is also available up to 63-65 HRC hardness.
WHICH ABRASIVE SHOULD WE USE?
ABSHOT has been a manufacturer of sandblasting equipment for over 40 years, and distributor of shot blasting equipment from a prestigious international company.
We have a warehouse and factory in Cervelló with a stock of more than 250 tons of abrasives, to guarantee the its supply to our customers.
Our pilot test plant is at your disposal for carry out blasting, shot-blasting and degreasing tests in order to determine the process and the most suitable abrasive for your needs.