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Introduction
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 a rework but in some cases may cause damage to the blasted surface.
This article guides you in choosing the appropriate media for a wet blasting cabinet.
Different abrasive media that can be used in a wet blast cabinet
The different abrasive media that you normally use in a wet blast cabinet are garnet, glass bead, aluminum oxide, crushed glass, and soda bi-carbonate
- Garnet (Almandine and Alluvial) : Garnet is a type of gemstone and has good natural abrasive properties. Garnet has a sub-angular shape, Mohs hardness number 7 to 7.5, comes in different grit sizes and it can remove coatings, old paint, rust, and mill scale from metal surfaces and can work on hard metals also. The coarser grit is suitable for working on aluminum, steel, stainless steel, and other metals and finer grit is suitable for materials like fiberglass. Garnet can quickly work through coatings and contaminants.
- Crushed glass: Crushed glass has Mohs number of 5.5 to 7 and is good for wet blasting aluminum and steel. The crushed glass has an angular shape and forms a sharp anchor profile; however, selecting a proper grit size and blasting velocity can give you the required anchor profile/finish. The advantage of crushed glass is it is a recycled material (environmentally friendly), cheap, and effective for many works. Crushed glass can be an economical media for the wet blasting of components like alloy wheels.
- Glass beads: A glass bead media consists of spherical glass particles and leaves a fine and polished finish on the blasted surface. The Mohs hardness of glass beads ranges from 5.5 to 6 and the particle size can be coarse to fine.The glass beads have a smooth surface all over and are safe for blasting soft and delicate materials. Glass beads are very popular for wet blast cabinets and by varying the blasting velocity a smooth anchored profile or a fine finish can be achieved.
- Aluminum oxide: The aluminum oxide has angular shaped particles; a Mohs hardness of 8 to 9 and the grit size of this media can be coarse to fine depending on the need. By varying the grit size and blasting velocity, aluminum oxide can be used for wet blasting hard metals (even with thin sections) and this is a good media for tough cleaning, extracting paint, coating, and rust. This is useful for finishing and edge rounding of surfaces. The high recyclability of aluminum oxide makes it an affordable media.
- Soda bi-carbonate: The soda bi-carbonate used for wet blasting is not the baking soda used in our kitchen. There are companies like Armex and Natrium that manufacture a variety of soda bi-carbonates specifically for the soda blasting process. The size of soda bi-carbonate particles may vary from 70 to 270 microns. Soda bi-carbonate is not an abrasive material and hence it cannot be used for surface profiling. Soda bi-carbonate is used as a media when the surface needs a smooth finish without any profile.
Factors to be considered for selecting the right abrasive media for wet blasting
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 depressions per unit area result in a finer surface finish.
Abrasive 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.
Typically, the minimum and maximum mesh size of the abrasive media normally used in a wet blast cabinet is 320 mesh and 80 mesh.
- 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
| No | Blasting media | Mohs hardness | Density/specific gravity | Recyclability |
| 01 | Garnet | 7.5 to 8.5 | 3.5 to 4.3 | Up to 5 times. |
| 02 | Crushed glass | 5 to 6 | 2.5 | Depends on the grit size. |
| 03 | Glass beads | 5 to 6 | 2.5 | Up to 30 times. |
| 04 | Aluminum oxide | 9.0 | 3.94 to 3.96 | Up to 50 times |
| 05 | Soda bi-carbonate | 2.5 | 2.2 | Single-use |
- 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.
Selecting the correct abrasive blast media for your job on hand.
Selecting the abrasive media depends on the existing condition of the surface to be wet blasted in the wet blast cabinet and the type of surface finish or coating/painting desired by the customer.
Let us consider some examples and find out the suitable media for wet blasting in the cabinet.
| Condition of the component received for wet blasting and expectation of the customer | Wet blasting process in the wet blast cabinet | Suggested media |
| The surface of the component has old coating/painting, rust, and contaminants. The customer wants a nice-looking painted or powder-coated finish. |
The old layer of painting/coating, rust, and contaminants are to be stripped off completely and a suitable anchor profile is to be created. | The suggested media is crushed glass, garnet, or aluminum oxide. Crushed glass may be suitable for aluminum alloys. Garnet or aluminum oxide may be required for a tough metal like steel. |
| The surface of the component has a thin layer of coating, grease, and grime. The customer wants the surface to be cleaned with a fine polished finish. |
The old layer of coating and the contaminants are to be cleaned and the surface finish should be good. | The suggested media is fine or medium grit glass beads. By using appropriate blasting velocity the surface can be completely cleaned with a fine, shiny, and smooth finish. |
The thumb rule for selecting the correct media can be:
When the surface is to be stripped of old paint, old coating, rust, and contaminants and a new anchor profile is to be created for re-painting or re-powder coating, select an angular-shaped media like crushed glass, garnet, or aluminum oxide. Make a choice based on the metal to be wet blasted.
When the surface is to be stripped off a thin layer of coating, grease, and grime, and the final finish required is shiny and smooth, select fine or medium grit glass beads.
When the surface is to be cleaned by removing grease and grime and a very fine and smooth finish is required, soda bi-carbonate can be a good choice. Alternately, fine-grit glass beads can also be used.
Conclusion
Selecting the correct abrasive media for a wet blast cabinet is important and it is worth spending some time on this. Noting down the component details, surface condition, abrasive media used, and the finish achieved and building up practical data will help when similar jobs come up again.