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.
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.
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.
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 HardnessAluminum Oxide9Crushed Glass5 to 6Glass Beads5 to 6Garnet7.5 to 8.5Silicon Carbide9 to 9.5Nickel Slag7Copper Slag7Steel Shot8Steel Grit8Plastic Abrasive3 to 4Staurolite7 to 7.5Sodium Bicarbonate2.5Walnut Shell4 to 5Corn Cob4 to 4.5The 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.
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 1.35Corn Cob1.2 to 1.4Plastic Abrasive1.5Sodium Bicarbonate2.2Glass Beads2.5Crushed Glass2.5Copper Slag2.8 to 3.6Nickel Slag2.8 to 3.8Silicon Carbide3.2 to 3.22Staurolite3.6Garnet3.5 to 4.3Aluminum Oxide3.94 to 3.96Steel Shot4.8 to 7.8Steel Grit4.8 to 7.8There are four basic shapes in abrasives: angular, sub-angular, sub-rounded, and rounded.
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.
Sub-angular abrasives have some jagged edges, but less than angular types. Some abrasives that fall in this category include garnet and plastic urea.
Sub-rounded abrasives are not quite smooth, and the jagged edges are almost gone. This type of abrasive include staurolite and walnut shells.
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.
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.
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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.
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.
You can also find a more complete grit size to microns to mesh to inches chart here.
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 TypeAbrasive TypeMax Impact Pressure (Siphon)Max Impact Pressure (Direct Pressure)Cold Roll Steel Part HardnessRecycle Uses AverageGlass Beads80 PSI40 PSIB72 to B usesAluminum Oxide90 PSI45 PSIB72 to B usesSilicon Carbide80 PSI40 PSIB72 to B to 100 usesSteel Shot130 PSI100 PSIB72 to B usesPlastic Urea80 PSI40 PSIN/A30 usesWalnut Shells70 PSI35 PSIN/A4 to 5 usesSodium Bicarbonate70 PSI40 PSIN/ASingle UseSlag50 PSI25 PSIN/A2 to 3 usesGarnet60 PSI30 PSIN/A4 to 5 usesThere 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 SizesFriabilityRecyclesCostSourceAluminum Oxide93.94 to 3.96Angular / Sub Angular12 to 325Low to Med50$$$MfgCrushed Glass5 to 62.5Angular30 to 325Med1 to 3$MfgGlass Beads5 to 62.5Rounded30 to 325Med30$$$MfgGarnet7.5 to 8.53.5 to 4.3Sub Angular / Sub Round8 to 150Med4 to 5$$$NatSilicon Carbide9 to 9.53.2 to 3.22Angular8 to 325Low70 to 100$$$$MfgNickel Slag72.8 to 3.8Angular12 to 80High2 to 3$B-PCopper Slag72.8 to 3.6Angular12 to 80High2 to 3$B-PSteel Shot84.8 to 7.8Rounded7 to 120High100+$$$MfgSteel Grit84.8 to 7.8Angular / Sub Angular10 to 200High100+$$$MfgPlastic Abrasive3 to 41.5Sub Angular12 to 80Med to High30$$$MfgStaurolite7 to 7.53.6Sub Rounded20 to 120Med2 to 3$$NatSodium Bicarbonate2.52.2Sub Rounded70 to 220Very HighSingle Use$$MfgWalnut Shell4 to 51.2 to 1.35Sub Rounded / Sub Angular6 to 100Med-High4 to 5$$$B-PCorn Cob4 to 4.51.2 to 1.4Sub Angular8 to 40Med-High4 to 5$$B-PIf you have any questions, please contact us, and we&#;ll be happy to help you make the best choice for your needs.
Which should you choose? Your choice involves balancing the results you want with the cost you're willing or able to pay. If initial price is your primary consideration, choose good abrasives, but be aware that a lower purchase price up front may not end up being economical in the long run. When you're running high-productivity applications and have to keep initial abrasive prices in mind, it's time for better abrasives. Consider the best abrasive products when maximum productivity and lowest total cost are critical to your operation.
When testing abrasive products to determine the best solution for your operation, there are three objective measurements to consider; one of which must remain constant between products being tested. These measurements are time, material removed, and product wear. By holding one of these constant and measuring the other two, you can come up with an objective comparison between multiple products. For example, you could grind for 10 minutes with each product, and measure how much material was removed and how much of the abrasive products were lost; this is often measured by weight. Alternatively, you could run each product to the end of its life, while measuring how much material it removed, and how long it lasted. Finally, you could remove a set amount of material while measuring the time and abrasive wear. Each business has unique priorities; following these test methods will allow you to select the right choice to meet yours.
Take a look at this chart when choosing an abrasive for welding and metal fabrication. Each application lists the relevant products in the order of best, better and good.
APPLICATIONABRASIVE PRODUCTSFORMULATIONSCut-off applications:
Grinding:
Blending:
The first step in using any grinding product safely is choosing the right abrasive product for each machine. Check the manufacturer's catalog to see what products are designed for which machines. Some catalogs use icons to make the machine-to-abrasive match clear and easy.
Check the product packaging as well. It's tempting to ignore product packaging, but it provides a wealth of information, especially about safety. Read and follow the safety information on the product packaging, and mount it on your machine so your operators always have access to it.
Safety Tips
We all want higher productivity and lower cost, but safety comes first. Start by making sure your operators use their grinding and sanding equipment as if their lives depend on their safety precautions. Then use the correct abrasive products to get the job done right.
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