Blasting Media - Choosing the Right Abrasive (VIDEO)
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Learn about the pros and cons of different blasting media types and which projects you should use them for -- including sand blasting, vapor blasting, soda blasting, and dustless blasting.
What is Abrasive?
Abrasive (also called media or abrasive blasting media) is added to the blast pot and sometimes mixed with water depending on the application. When abrasive blasting, you propel this mixture towards a surface at high speed, to remove paint, rust, and other coatings.
Types of Abrasive
Dustless Blasting or other sandblasting equipment allows you to use a wide variety of abrasives, either wet or dry. For wet blasting, any abrasive that sinks in water and is not water soluble can be used. However, abrasives that are dirty or have a very inconsistent particle size can cause problems, such as sputtering or clogging of the machine.
Here are just some of the abrasives that you can use with the Dustless Blaster:
- Recycled bottle glass
- Garnet
- Blast sand
- Sugar sand
- Coal slag
- Copper slag
- Glass beads
- Sodium Bicarbonate
- Walnut shell
- Corn cob
Our favorite abrasive is recycled bottle glass, because it's clean, inexpensive, environmentally friendly, and suited for a wide variety of jobs. Sometimes different abrasives are available simply based on geography.
A common question we get is "Can I use play sand?" The answer is NO. Play sand is not an industrial product, so it's not controlled and graded the way abrasives are. It won't behave predictably in your equipment, and you could wind up in loads of legal trouble, from the EPA to local municipalities. Most bags of play sand are explicitly marked "Not to be used for sandblasting".
Where can I buy media?
You can purchase blasting media at the Dustless Blasting Parts Store and have it shipped to you! We sell the 40/70 recycled glass for $10 per 50lb bag.
If you'd prefer to save on shipping costs, you can also purchase media from a local supplier.
Abrasive Shape
Basically, there are two different shapes: angular which has sharp edges, and round with no edges.
Angular abrasive has sharp edges which will cut into the substrate, leaving an anchor profile for the new finish to stick to. Examples of angular shaped media would be crushed glass, slags and garnet abrasives to name a few.
Round media is used mainly for cleaning or stripping while leaving the surface smooth. It will not rough up the surface or leave an anchor profile. Examples of round media would be glass beads, sugar sand and plastic pellets.
An anchor profile is a fancy way of describing the rough surface created during the blasting process. These peaks and valleys are usually measured in mils (1/ of an inch). One of the main causes of premature coating failure is an insufficient anchor profile. It's a good idea to understand from the customer what he/she expects when you are done. Some people prefer an anchor profile, while others will want a smoother surface.
For example, if you are stripping a car to be repainted, you'll probably want to leave an anchor profile for the new paint to adhere to. If you are simply cleaning calcium deposits from a pool &#; which will not be repainted &#; leaving a profile is unnecessary.
Abrasive Size
Most abrasive is measured with mesh size. During production, it gets shaken through various screens. These screens might have as little as 20 holes per square inch, or as many as 100. This means that 40/70 glass fits through the 40&#;70 holes per square inch screens.
20/40 glass is coarser than 40/70, and 60/100 glass is finer.
A common misconception is that the coarser glass will help you get through a job faster. It is true that 20/40 glass is slightly more aggressive than 40/70. However, because it's so coarse, you're only getting half as many particles of glass in the same size bag. You'll end up using twice as many bags to get the job done, which is not only expensive, but inefficient! It's a much better idea to increase abrasive density if you want to complete the job faster.
Abrasive Weight/Density
Understanding the weight or bulk density of the media you are using will also help decide which is best for the process. The heavier the media, the more impact it has on the surface you are blasting.
Imagine a golf ball and a ping pong ball. They are the same shape and size, but the golf ball is more dense. If you threw them at someone, the golf ball would hurt a lot more.
Crushed glass has a bulk density of 75-80 lbs per cubic foot&#; while garnet weighs around 145 lbs per cubic foot. So, the two abrasives at the same mesh size and blast pressure will have different results. A 40/70 crushed glass will be more &#;gentle&#; on the surface than the same mesh size of garnet at the same blast pressure.
The harder and heavier the abrasive is, the rougher the profile will be. Using larger, more coarse abrasives will decrease the run time in your machine. For example, if you are blasting with 40/70 crushed glass and decide to use a larger mesh size of 20/40 you will notice a decrease in run time. If both bags of abrasive are 50 lbs, there are fewer particles in the 20/40 mesh size bag than the 40/70. A larger mesh size will be more aggressive so it will have more of an impact to the surface you are blasting.
Comparison Chart
There isn&#;t necessarily a right or wrong way to remove a coating. The deciding factors would be speed and end result of the substrate you are stripping. Understanding the impact of different abrasives will help in deciding the best media for the job. The following chart will explain the characteristics of various media types.
Examples
To see examples of real-life blasting jobs and what abrasive you should use for them, download our PDF guidebook on Choosing the Right Abrasive!
In Conclusion
Choosing an abrasive is like choosing sandpaper for a project. More coarse sandpaper like 60 grit or 100 grit will strip quicker and have a rougher surface, while finer mesh like 400 or grit will be slower and have a smoother finish. Finer meshed abrasives are great for softer substrates like fiberglass, concrete and wood, while larger mesh abrasives work best for thick steel or thick coatings.
To sum it up, 40/70 crushed glass is a great general, all around blasting abrasive. If crushed glass is struggling to remove a coating then switching to a heavier abrasive like garnet will work better. Just determine what abrasive the substrate can handle and the thickness or toughness of the coating you are stripping.
Other factors that affect your results
We've covered how abrasive choice can affect your blasting, but you should also think about:
Air Pressure: Air pressure or PSI is how hard we are throwing the abrasive. The harder the substrate and coating, the more aggressive you will want to be. Learn more about choosing the right blast pressure here.
Distance from the surface you are blasting: This is very simple and will come naturally. The further from the surface you are blasting the greater the energy will dissipate. For example cleaning graffiti from a wall, you might be 2-5 feet away from the surface. Removing 80 mil of tank liner you might only be 4-10 inches from the surface. See how standoff distance affects the blast pattern in the nozzle control video.
Reclaiming Media
We often get the question "Can I reclaim or reuse blast media?"
Crushed glass is the most commonly used media in the Dustless Blaster, and it's generally not worth reclaiming. Because the glass particles shatter upon impact, they will actually end up being smaller than 40/70 after they have been used. The glass is only $10 per 50lb bag, so it's usually not worth the effort to gather, clean, and re-sift the media.
That being said, other abrasive such as glass or plastic bead are more expensive, and may be worth reclaiming, as long as they don't shatter on impact. In this video, one of our business owners used a vacuum to reclaim and reuse glass bead he used on a pool tile line.
Helpful Guides
Choosing the Right Abrasive (PDF Guidebook)
Abrasive Characteristics (Reference Sheet)
Main Levels of Surface Preparation (Reference Sheet)
Tech Talk: Choosing the Right Abrasive (Video)
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Various types and sizes of garnet are available for abrasive cutting applications. When the right size and type is introduced into the high pressure waterjet stream, they balance the aggressiveness of the cut with the cut finish. However, when it comes to choosing the right garnet for your cutting system, or the right garnet for your application, it can be overwhelming.
There are multiple types of garnet on top of different grades and quality choices. What garnet you choose depends on your application; the thickness of the material you are cutting, its composition, the desired speed of the cut and the cut finish requirements all factor in. It is extremely important to get the right garnet, but how do you know what&#;s right? Let&#;s explore.
What is Garnet Abrasive?
Garnet is a naturally occurring crystallized aluminum or calcium silicate mineral captured in igneous and metamorphic rocks. The garnet, as it is mined and collected, is crushed to finer grains. Pieces which are larger than 60 mesh (250 micrometers) are normally used for sand blasting. The pieces between 60 mesh (250 micrometers) and 200 mesh (74 micrometers) are normally used for water jet cutting. The garnet abrasive crystals are typically rated between 7.0 and 7.5 on the Mohs scale for hardness. For reference, a diamond is a 10 on the Mohs scale and talcum powder is a 1 (for more information on the Mohs scale, check out this post https://geology.com/minerals/mohs-hardness-scale.shtml). While there are synthetic materials available, Garnet crystals are the most common abrasive used as they do not chemically react with the water stream or any materials being cut.
There are two main categories of garnet:
Alluvial Garnet/River Garnet
- Alluvial Garnet, giving the appearance of a pinkish sand, is naturally crushed, formed over years as beaches and rivers break down the mineral.
The largest source of abrasive garnet today is garnet-rich beach sand which is quite abundant on
Indian
and
Australian
coasts and the main producers today are Australia and India.
Since the material has been naturally crushed and ground, the material is normally available in fine sizes only: 80 mesh with ranges from 56 mesh to 100 mesh size.
Rock Garnet
- Rock garnet, possessing more of a dark brown to reddish tone, has the longest usage history and is produced in America, China and Western India. The crystals are mined and then crushed in mills and purified. Rock Garnet has the sharpest edges and provides for a more aggressive cut than other types of garnet.
Mesh Grades (Size) and What They Mean
The key attribute used to distinguish the different types of garnet is mesh grade. Mesh grade is essentially a size rating of the individual pieces of garnet. In water jet machine applications garnet sizes of 50 to 120 are most common. The higher the mesh grade, the finer (smaller) the garnet abrasive is and vice versa. For example, a 100 mesh rating is a finer and smaller garnet than a 70 mesh abrasive.
When selecting your grade, an 80 mesh is usually the starting point being it is the single most commonly used mesh grade. To increase cut speed on denser materials such as stainless steel, test the application with a more aggressive mesh between 40 and 60 mesh. To improve cut finish, test the application with a higher mesh rating between 100 and 120 mesh.
Since the volume of water required to move the garnet varies based on mesh size, review the nozzle and orifice combination to optimize water flow with your garnet type.
Other Attributes to Look for When Choosing Garnet
There are a few other factors you should consider when choosing the right garnet for your cutting application and we have outlined them here.
Purity Rating
- Higher purity typically means added processing to sort out non-garnet products. While higher purity ratings cost more, they deliver superior cutting results.
Particle Size Consistency
- Consistent particle size is important as oversized particles risk clogging the orifice and bringing your cutting system down. Double sifted garnets are higher quality garnets in the sense that all the garnet is more uniform in size for better consistency and accuracy during cuts.
- Abrasive Removal and Recycling
- When an abrasive removal system is used in the cutting application, spent garnet is captured. During the cut process, the garnet breaks down as they shatter and smooth. Garnet recycling is often down at a centralized location under the supervision of your garnet supplier and is increasingly a consideration in choosing your garnet type. If this is important to your operation and/or facility, make sure to inquire if your garnet is recyclable before purchasing.
In general, the payout in cut performance, cut quality and machine uptime supports the additional investment to purchase a quality abrasive product.
Talk to Your Water Jet Machine Manufacturer
Waterjet cutting is a confluence of many factors in addition to garnet, including the nozzle/orifice combination selected, pump pressure, abrasive delivery rates and cut speed. Talk to the manufacturer of your machine to assist in selecting the combination of components and garnet that optimizes the cutting in your shop.
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