Choosing the correct abrasive media for wet blasting is very important because an inappropriate media choice will not provide the desired finish. It can not only cause rework but in some cases may cause damage to the blasted surface.
The factors you have to consider when selecting an abrasive media for wet blasting are the grit size, shape, hardness, and density of the media, and blasting velocity. Factors like friability (tendency to break into smaller particles under force), recyclability, and cost are also important.
Size: The blast media comes in different grit sizes. Larger media particles will make deeper depressions but lesser depressions per unit area resulting in a rough surface finish. Smaller abrasive particles make shallow depressions but more impacts per unit area resulting in a finer surface finish.
Abrasives media is measured by the mesh size through which it was passed and the mesh can be as coarse as 20 holes per square inch or as fine as 100 holes per square inch. 40/70 media indicates that it was passed through a mesh having 40 to 70 holes per square inch. Hence a 20/40 media is coarser compared to 40/70 media, and a 40/70 media is coarser than a 60/100 media.
It is not possible to use a coarser media and complete the work faster. 20/40 media can blast more aggressively than 40/70 media. But, if you compare the number of media particles in one pound of 20/40 media and one pound of 40/70 media, one pound of 40/70 media has more media particles. Hence, when you use 20/40 media less number of particles are in action and when you use 40/70 media, more particles are in blasting action. Hence, the time taken for blasting will be almost the same, but there will be a difference in the anchor profile or finish obtained. Also, the cost of using coarser media will be more since more media is consumed.
Shape: The shape of the abrasive media particle can influence the outcome of the surface profile and the shape can be angular, sub-angular, sub-rounded, or rounded. Angular media particles like crushed glass or garnet have a rough surface with jagged faces and can strip coatings, rust, and contaminants faster. They can leave a deeper anchor profile/pattern on the surface.
Sub-angular or semi-irregular media is similar to the angular type but has less jagged faces and sharp points compared to the angular type.
Sub-rounded abrasive media is almost smooth and has only a few jagged edges and sharp points.
Rounded abrasive media has a smooth surface all over and it can even be spherical. Glass bead is a good example of rounded abrasive and is used for removing mill scales, and thin coatings, and to get a good finish or a peened surface.
Hardness: The hardness of the abrasive media is indicated as Mohs numbers (Mohs hardness scale) and the hardness number ranges from 0 to 10. The number 10 indicates the hardest and 0 indicates the softest.
Hard particles will create a deeper surface profile and softer particles will create a smooth and fine finish. A softer media is preferred for cleaning grease, oil, and grime and removing light coatings, and when an anchor profile is not required on the substrate. On the other hand, a harder media is used for removing corrosion, and rust, or for creating an anchor profile on the substrate.
Density and velocity: The density and velocity of the abrasive media also have an influence on the blasting. Denser abrasive particles blasting the surface at higher velocity will leave a deeper anchor profile compared to abrasive particles with lower density blasting at lower velocities.
An abrasive media with higher density can store more kinetic energy in it and hits the surface with high impact to form a deeper surface profile, whereas an abrasive media with low density can store less kinetic energy, and hits the surface with low impact to form a lesser deep profile.
Using the same abrasive media, the surface finish can be slightly modified by varying the blast pressure, but this will not affect the basic characteristic of the media.
Let us summarize the characteristics of the abrasive media in a table
|7.5 to 8.5
|3.5 to 4.3
|Up to 5 times.
|5 to 6
|Depends on the grit size.
|5 to 6
|Up to 30 times.
|3.94 to 3.96
|Up to 50 times
Different types of anchor patterns and surface profiles- When the blast media impacts the surface, it creates a profile in the shape of the media on the substrate. A media in a spherical shape create a surface with mini dimples (like the surface of a golf ball) and this finish is called peening.
Angular and sub-angular abrasive media can create deeper and well-defined anchor profiles on the substrate. If you observe an anchored profile under a lens, you can find a surface full of valleys and peaks. The indentations created by the media are the valleys and the surface that is pushed up due to the indentations is the peaks. Each type of coating requires a particular anchor profile or surface roughness for best results. You can say that during powder coating the coating substance fills the valleys and covers the peaks of the anchor profile. If the peaks are too high, they will start showing up and can catch rust. And, if the valleys are not sufficiently deep, then they cannot hold the coating (the coating cannot adhere properly).
Recycling and cost of abrasive media- Some abrasive media have a high initial cost and if it can be recycled more times, then its ability to recycle will offset its high initial cost.