7 Factors for Choosing the Right Abrasive Media

Choosing Abrasive media for blasting

Why is choosing the right abrasive media for the job so important?crush

If you use a mismatched abrasive for a blasting job, you may end up having to redo the job, because the anchor pattern is incorrect, or cause unwanted damage to the blasted surface.

There are several types of abrasive blasting applications: cleaning, finishing, and surface prep for coatings, to name a few. By using the appropriate type of abrasive media, along with the right type of delivery, wet or dry blast, direct pressure or siphon blast, you will set yourself up for a successful, profitable blasting job.

Table of Contents

  1. Surface Prep
  2. Mohs Hardness
  3. Density and Specific Gravity
  4. Particle Shape
  5. Anchor Pattern
  6. Mesh Size
  7. Number of Recycles
  8. Bonus: Comparative Chart of Factors

Blasting for Surface Preparation and Finishing

Selecting the right abrasive media for surface prep is vital. This is because different media types have different characteristics, and create different surface profiles.

The anchor pattern that an abrasive creates will determine how effectively a coating bonds to a prepared surface.

What type of anchor pattern you need to create to apply a coating depends on the surface being treated.

Factors to Consider When Selecting An Abrasive

Some of the things you should consider when choosing an abrasive are hardness, density, shape of the abrasive, the type of profile the abrasive will leave, friability, how many times you can recycle the abrasive, and overall cost.

Mohs Hardness

One of the things abrasives are measured by is the Mohs Hardness Scale. This hardness scale was created by German mineralogist and geologist Freidrich Mohs.

How the scale works is the lower the number, the softer the mineral. The higher the number, the harder the mineral. Talc is a 1, diamond is a 10. Higher number materials can scratch or etch lower number materials.

Here’s the Mohs hardness for several popular abrasives.

AbrasiveMohs Hardness
Aluminum Oxide9
Crushed Glass5 to 6
Glass Beads5 to 6
Garnet7.5 to 8.5
Silicon Carbide9 to 9.5
Nickel Slag7
Copper Slag7
Steel Shot8
Steel Grit8
Plastic Abrasive3 to 4
Staurolite7 to 7.5
Sodium Bicarbonate2.5
Walnut Shell4 to 5
Corn Cob4 to 4.5

The softer the abrasive, the finer the finish will be. Harder abrasives are useful for removing certain types of corrosion and rust. Softer abrasives are best for cleaning up grease, grime, and removing light coatings. Softer abrasives are preferred when you do not want to leave an anchor pattern in the substrate.

Also, the more pressure you blast with, the velocity and force the abrasive will be thrown with. You can always adjust the blasting pressure, but you cannot change the characteristics of the abrasive itself. For that reason, it is important to look at the shape, as well as the hardness of an abrasive.

Density Plays a Role in Abrasive Blasting

Blasting abrasives each a different density. In simplest terms, the more dense a molecule is, the tighter the atoms are packed together. The denser the substance, the more kinetic energy it can hold.

The denser the particle, the more impact it has over a smaller area, meaning a deeper surface profile. Less dense particles create a less deep profile, since they hit with less force. (Speed + mass = force of impact).

Density is measured in specific gravity, with water being a 1.0 SG. The more dense a substance is, the higher specific gravity it will have.

AbrasiveDensity (Specific Gravity)
Walnut Shell1.2 to 1.35
Corn Cob1.2 to 1.4
Plastic Abrasive1.5
Sodium Bicarbonate2.2
Glass Beads2.5
Crushed Glass2.5
Copper Slag2.8 to 3.6
Nickel Slag2.8 to 3.8
Silicon Carbide3.2 to 3.22
Garnet3.5 to 4.3
Aluminum Oxide3.94 to 3.96
Steel Shot4.8 to 7.8
Steel Grit4.8 to 7.8

How The Shape of an Abrasive Affects the Surface Profile

There are four basic shapes in abrasives: angular, sub-angular, sub-rounded, and rounded.

angular abrasive
Angular abrasives have many facets, and several jagged faces and vertices. Examples of angular abrasives are crushed glass and certain types of slag. Angular abrasives are good for stripping corrosion and rust.

subangular abrasive
Sub-angular abrasives have some jagged edges, but less than angular types. Some abrasives that fall in this category include garnet and plastic urea.

subrounded abrasive
Sub-rounded abrasives are not quite smooth, and the jagged edges are almost gone. This type of abrasive include staurolite and walnut shells.

rounded abrasive
Rounded abrasives are smooth, or may even be spherical. Glass beads or steel shot would be examples of rounded abrasives. Usually, rounded abrasives are used for removing mill scale (from hot rolled steel) or thin factory coatings.

Types of Anchor Patterns and Surface Profiles

As the blast media hits the surface, it creates substrate profiles in the shape of the abrasive.

Rounded abrasives create a dimpled profile, like the surface of a golf ball. This is also known as peening.

Anchor Pattern

Angular and sub-angular abrasives create deeper, more pronounced anchor patterns. When the blasted abrasive hits the surface, two things happen: an indentation is made in the surface, and part of the surface is pushed up around the indentation. For every action, there is an opposite and equal reaction.

The indentations in the surface are known as valleys; the part of the surface pushed upwards are known as peaks. The distance between the top of the peak and bottom of the valley is known as a depth profile.

Each coating has a specific depth profile that must be achieved for optimal bonding. This is the surface roughness, or anchor pattern that best allows the coating to set. Ideally, the coating will cover the peaks and fill the valleys.

If the peaks are too high, they may push up through the coating, and rust will prematurely form. If the depth profile is not deep enough the coating may not adhere correctly, and prematurely erode. Ideally, your applied coating will cover the peaks and fill the valleys.

The size of the abrasive, as well the shape, has an effect on the resulting anchor profile.

Considering The Mesh Size of the Abrasive

Generally speaking, it is best to use the finest abrasive available that will create the necessary surface profile.

Abrasives are usually measured in grit size, microns, or mesh size.

Grit Size to Microns to Mesh

Grit SizeMesh USSMicrons (Avg)Inches (Avg)

You can also find a more complete grit size to microns to mesh to inches chart here.

Recycles and Total Cost

Some blasting abrasives, like aluminum oxide or silicon carbide, cost more up front, but are economical in the long run, because they can be recycled many times.

Other types of abrasive blast media, like crushed glass or metal slag, are primarily used for outdoor blasting, and meant to be used and cleaned up. These are usually cheaper than other blast media, but only good for a few recycles in a blast cabinet or blast room.

Some abrasives are in the mid-range of cost and recycles. Glass beads or plastic urea both fit this description.

Consider what types of abrasive blasting jobs you do most frequently, and what type of media you need to stock to be profitable and efficient.

Abrasive Recycle Rate by Media Type
Abrasive TypeMax Impact Pressure (Siphon)Max Impact Pressure (Direct Pressure)Cold Roll Steel Part HardnessRecycle Uses Average
Glass Beads80 PSI40 PSIB72 to B8630 uses
Aluminum Oxide90 PSI45 PSIB72 to B8650 uses
Silicon Carbide80 PSI40 PSIB72 to B8670 to 100 uses
Steel Shot130 PSI100 PSIB72 to B86100 uses
Plastic Urea80 PSI40 PSIN/A30 uses
Walnut Shells70 PSI35 PSIN/A4 to 5 uses
Sodium Bicarbonate70 PSI40 PSIN/ASingle Use
Slag50 PSI25 PSIN/A2 to 3 uses
Garnet60 PSI30 PSIN/A4 to 5 uses

Abrasive Media Comparative Chart

There are many types of abrasives for many different types of blasting applications. This chart compares the most common abrasives for a variety of factors.

AbrasiveMohs HardnessDensityShapeMesh SizesFriabilityRecyclesCostSource
Aluminum Oxide93.94 to 3.96Angular / Sub Angular12 to 325Low to Med50$$$Mfg
Crushed Glass5 to 62.5Angular30 to 325Med1 to 3$Mfg
Glass Beads5 to 62.5Rounded30 to 325Med30$$$Mfg
Garnet7.5 to 8.53.5 to 4.3Sub Angular / Sub Round8 to 150Med4 to 5$$$Nat
Silicon Carbide9 to 9.53.2 to 3.22Angular8 to 325Low70 to 100$$$$Mfg
Nickel Slag72.8 to 3.8Angular12 to 80High2 to 3$B-P
Copper Slag72.8 to 3.6Angular12 to 80High2 to 3$B-P
Steel Shot84.8 to 7.8Rounded7 to 120High100+$$$Mfg
Steel Grit84.8 to 7.8Angular / Sub Angular10 to 200High100+$$$Mfg
Plastic Abrasive3 to 41.5Sub Angular12 to 80Med to High30$$$Mfg
Staurolite7 to 7.53.6Sub Rounded20 to 120Med2 to 3$$Nat
Sodium Bicarbonate2.52.2Sub Rounded70 to 220Very HighSingle Use$$Mfg
Walnut Shell4 to 51.2 to 1.35Sub Rounded / Sub Angular6 to 100Med-High4 to 5$$$B-P
Corn Cob4 to 4.51.2 to 1.4Sub Angular8 to 40Med-High4 to 5$$B-P

If you have any questions, please contact us, and we’ll be happy to help you make the best choice for your needs.

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