Roughness and the right blasting media: advice and knowledge

You probably visit our site because you are interested in Straaltechniek as a (potential) partner for your projects. Or because you have doubts about the blasting media. “Which blasting media do I really need?”, “How do I know which media to use?”. “How does the roughness relate to the speed of the blasting?”. Hopefully you will find the answer to your question below. But don’t hesitate to ask us, because every application is different!

The first purpose of your blasting media and installation

You almost always use blasting to clean. It can be rust or mill scale, or to remove unnecessary remnants from 3d printed products. Castings are also often post-blasted.

The second purpose of blasting: the right roughness

You have already thought about creating the right roughness. Roughness is often necessary for a protective layer for your product. Wet paint, powder coating or perhaps metallization. If you are going to do shoot peening, you should have another goal, which is to increase fatigue resistance. Click here to learn more about this. But do you already know what roughness you need?

Which roughness and which blasting media?

Below you will get a feel for what is involved in achieving the right roughness values.

The figure above clearly shows what is involved in selecting the correct abrasive. Here you can see schematically what a so-called anchor profile is. It ensures that the paint adheres. In the case of the figure above, the primer layer thickness must in any case be greater than the roughness.

This will give you a quick idea of how to determine the desired roughness. Note: these are all rules of thumb. Let us personally advise you and also by your supplier of the paint or coating.

Akzo Nobel shows in its epoxy spec sheet which anchor profile is desired. Click here for an example. It can be seen here that the roughness in this case is 75 microns at a typical (dry) layer thickness of 200 µm.

Desired Roughness = 25-30% of the total dry film thickness of the coating.
Blasting media size = 10 x the required roughness.

This rule of thumb is by no means sacred, but it gives a feeling and understanding of what approximately happens when blasting.
It would be ideal to have some type of blast media selection program. Unfortunately this does not exist. Fortunately, there is literature in this area, which is again very valuable, for understanding how the process works. Below you can see indicative measurement results for different types of abrasives and the resulting roughness.

There are 5 things of enormous importance for the end result of your blasting process: the blasting media, the blasting installation, the blasting process, the product to be blasted and clear specifications of, among other things, the roughness to be achieved.

Whichever abrasive you choose, the following matters are important.

Blasting media: the shape

The shape is of course of enormous importance for the desired roughness of the product to be achieved. Some shapes result in special added properties of the surface of the end product. Think of shot peening.

The shapes you may choose from are:

  • Angular grit
  • Sub-angular grit
  • Sub-rounded shot
  • Rounded shot

With angular grit you can think of “crushed glass” and slag. Slightly less angular media is the corundum and plastic. Walnut shells and staurolite are again not as rounded media as real spherical shot or glass bead.

Blasting Media: the size

The effect of the size of the blasting media on the resulting roughness has already been indicated above. The diameter of the media varies roughly between 0.1 and 1 mm.

The size is often expressed in the so-called mesh size. This figure represents the number of sieve threads per square inch. The diameter of the media varies roughly between 0.1 and 1 mm.

Sometimes the blasting media is described with 2 numbers. For example 50/70. This means that 95% of the blasting media falls through a 50 sieve, but not through the 70.

Blasting Media: the hardness

The hardness of different types of media is shown in a table below. The hardness largely determines the roughness of the product, but it is often also very much related to the dust production. Especially with blasting media of steel, shape and hardness are available in a wide range. Steel media is described with the Rockwell hardness scale.

straalmiddel hardheid (Moh) levensduur (cycles)
Aluminium oxide (Korund) 9 15-25
‘Crushed’ glas 5-6 1-3
Glasparels 5-6 5-35
Garnet 7,5-8,5 2-5
Silicium carbide 9-9,5 25-40
Koperslak 7 1-2
Stalen shot 8
Stalen grit 8
Plastic straalmiddel 3-4 8-10
Stauroliet 7-7,5
Sodium bicarbonaat 2,5
Walnootschaal 4 4-5
straalmiddel hardheid (HRC) levensduur (cycles)
Stalen grit GP 50 1600
Stalen grit GL 55 900
Stalen grit GH 64 300
Stalen shot 49 2200
Geknipte draad (cut wire) CW 44
Geknipte draad (cut wire) CCW, SCCW 44
Gietijzer (cast iron grit) 57 150
RVS shot CN 30 5000
RVS stalen grit CG 58 500

Blasting media: dust production

In general, a harder blasting medium is also much more brittle, so it disintegrates into dust much faster. This property therefore has an enormous influence on the extraction installation of the blast cabinet. Media consumption will therefore also increase enormously with a higher hardness. The tool life or lifespan is described by the number of “cycles”. One-time blasting media can be used 1 “cycle”, while steel shot lasts up to 2500 cycles.

Blasting media from Straaltechniek: with us you blast more competitively

The trick is to use a blasting media with high hardness and low dust production. With steel blasting media, Straaltechniek has access to media that combines the “best of both worlds”. Tests were carried out in December 2020 by a customer blasting 240,000 m2 per year. Straaltechniek came out as the very best buy. At least 30% higher blasting speed is achievable, with a tool life that is up to 3x higher. Thus, this saves media costs, while the higher production speed directly reduces labor costs. But also the production time, so that more money can be made with the same facility! So you become more competitive with our media.

Blasting media, product, compressor and compressed air consumption

During compressed air blasting a significant amount of compressed air is consumed. The compressed air consumption is enormously dependent on the media used for blasting and the nozzle diameter. Straaltechniek can predict how much compressed air is needed, what blast speed will result and what roughness the end product will have. We do this based on our experience, but as validation we can always perform tests for you. Below you will get an idea of the compressed air consumption with different nozzle diameters.

Compressed air consumption in cubic meters per minute
1 bar 2 bar 3 bar 4 bar 5 bar 6 bar 7 bar 8 bar 9 bar 10 bar
Ø 3 mm 0,08 0,17 0,25 0,33 0,4 0,5 0,6 0,7 0,8 0,8
Ø 4 mm 0,15 0,3 0,4 0,6 0,7 0,9 1,0 1,2 1,3 1,5
Ø 4.8 mm 0,21 0,43 0,6 0,9 1,1 1,3 1,5 1,7 1,9 2,1
Ø 5 mm 0,23 0,5 0,7 0,9 1,2 1,4 1,6 1,9 2,1 2,3
Ø 6 mm 0,33 0,7 1,0 1,3 1,7 2,0 2,3 2,7 3,0 3,3
Ø 6.4 mm 0,38 0,8 1,1 1,5 1,9 2,3 2,7 3,0 3,4 3,8
Ø 7 mm 0,5 0,9 1,4 1,8 2,3 2,7 3,2 3,6 4,1 4,5
Ø 8 mm 0,6 1,2 1,8 2,4 3,0 3,6 4,1 4,7 5,3 6,0
Ø 9 mm 0,8 1,5 2,2 3,0 3,7 4,5 5,2 6,0 7,0 7,0
Ø 9.5 mm 0,8 1,7 2,5 3,3 4,2 5,0 6,0 7,0 8,0 8,0
Ø 10 mm 0,9 1,9 2,8 3,7 4,6 5,5 6,0 7,0 8,0 9,0
Ø 11 mm 1,1 2,2 3,4 4,5 5,6 7,0 8,0 9,0 10,0 11,0
Ø 12 mm 1,3 2,7 4,0 5,3 6,7 8,0 9,0 11,0 12,0 13,0
Ø 12.7 mm 1,5 3,0 4,5 6,0 7,0 9,0 10,0 12,0 13,0 15,0
Ø 13 mm 1,6 3,1 4,7 6,0 8,0 9,0 11,0 12,0 14,0 16,0
Ø 16 mm 2,4 4,7 7,1 9,0 12,0 14,0 17,0 19,0 21,0 24,0
Ø 19 mm 3,3 6,7 10,0 13,0 17,0 20,0 23,0 27,0 30,0 33,0

The media recycling

If you know with which blasting media and at what pressure you are going to blast, it still has to be determined which type of media recycling to use.

There are 2 kinds of media recycling:

  • Direct recycling from the hopper.
  • Recycling with elevator, air sifter and silo.

For Corundum media recycling, there are different choices apply than for stainless steel shot recycling.

To determine the correct recycling and the correct extraction, we use key figures such as blasting speed and life of the abrasive. We occasionally test a few parameters, so that we know for sure which category is best suited, and which settings and modules should be used.

From blasting media to the blast cabin extraction installation

The extraction system in a blasting installation can be the source of many problems if it is not chosen carefully.

  • Too low a flow rate of the extraction installation causes the pipes to fill up with expensive blasting media or dust. Choosing the wrong pipe work diameter can also lead to this.
  • Too high a flow rate, or an incorrectly dimensioned labyrinth can also suck up expensive blasting media.
  • If the refresh rate is too low, there is not enough visibility in the blast cabinet, so that effective blasting cannot take place.
  • The wrong choice of the filter cloth in the extraction system can clog the extraction.
  • The use of a too small number of square meters of filter cloth can result in too high dust emission or that the filter cartridges must be replaced early.
  • Choosing the wrong fan can result in too high electricity consumption or too much noise or for motors that jump into thermal protection.
  • Finally, metrics chosen too conservatively can result in a too much expensive solution.

So. Now you have a good idea of what a blasting installation must achieve and what the impact of the desired roughness or media can mean. Of course you still have many questions. We believe that we can be of added value in providing advice on all of the above subjects. We certainly don’t know everything about blasting. But together with you we always come to the optimal solution! Call quickly to consult one of our advisors!

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