Sandblasting Material GuideDecember 12, 2018
Sandblasting is an extremely useful procedure in a broad array of applications and industries. Whether a material needs to be cleaned, deburred, prepped for powder-coating, de-rusted, shot-peened or otherwise just have its paint removed, sandblasting is the process for the job.
These devices are useful in the auto industry, in ship and rail yards and in a range of industrial applications. Certain degrees of skill and safety training are required to effectively operate a sandblaster, depending on your intended application.
When a surface is coated in grease, paint or another undesired veneer, a sandblaster can completely remove the residue and restore the item to its original condition. When machining leaves sharp burrs on an object, sandblasters can smooth it until it is safe to handle. Shot-peening is another useful application in which a metal is subjected to a barrage of small but powerful impacts — think of what a ball-peen hammer does — in order to make it more malleable. This gives the surface of the metal a compressive, plastic quality that is less likely to develop tiny micro-fractures.
Let’s look at how sandblasters work as well as the different sandblasting media types.
How Sandblasting Media Works
What does a sandblaster do and how does it work?
A sandblaster uses pressurized air to hurtle a beam of tiny projectiles, which range from walnut shells to glass beads and tiny rock particulates.
It all starts with two components: the blast pot and the pressurized air intake. The blast pot is a large container that holds the blast media, whether it is glass beads, corn cob, steel shot, plastic or other material. It funnels the media downward through a set of valves that allow the amount entering the system to be controlled.
This media then joins the pressurized air as it zips into the chamber. Note that air pressure is entirely responsible for the process of sandblasting — it is the force that whips the media around. Adjusting it in turn adjusts the speed of the particles. Sandblaster air requirements are quite strict as a result — you need a remote-control system for both precision and safety.
The air and media travel through the blast hose and out of a nozzle. At this point, they are inside the sandblasting chamber, whether that is a small compartment or an entire room, and usually come out the nozzle of a handheld gun. The particles travel at high velocities and impact the object, stripping it of whatever is on its surface. Depending on the size, texture and density of the particles, they will also leave tiny divots in the material. These divots allow future coatings to adhere to the object’s surface.
What goes in must come out. For the chamber to avoid turning into a pressure bomb, air must escape at the same rate it enters. This is where an abrasive trap comes in handy. At the point where air escapes the chamber, a trap is in place to catch the abrasive media — otherwise it would fly out of the chamber as well.
These two basic components apply to both cabinet and portable sandblaster models.
Let’s move on now to the most important component of sandblasting: the media itself. There are lots of variables in sandblasting media to give users plenty of options to choose from.
Sandblasting Media Guide: What Type of Media Should You Consider?
Looking at a list of types of sandblasting media can be daunting at first. The particles can be organic, metal, silicate, plastic or stone. Why is there such a wide array of options? How do you begin choosing one?
The reason why so many media exist is because they each perform different tasks extremely well. This is due to a number of key abrasive properties of the medium: shape, size, hardness and density.
One important result of sandblasting is what is called the “anchor pattern.” When a particle impacts the surface of a metal, it forms a small crater. This tiny crater will be lined by ridges, which are pressed up on all sides of it. The depth of this crater is typically measured in thousandths of an inch and affects how well the coating will bond to the surface of the metal. The deeper the craters, the better the bond.
However, if the craters are too deep, the ridges may poke out through the surface of the coating. This can cause rusting and weakness in the coating, which means another round of sandblasting.
Let’s look at the abrasive properties of sandblasting media and how they affect the anchor pattern.
As you might have guessed, round particles are less abrasive than coarse ones. The sharpness of angles in a particle determines how deeply it will cut into a metal. A sharp particle will dig farther into a metal surface than a round particle of the same size.
The four categories of shape are angular, sub-angular, sub-rounded and round. Some of the coarsest media are coal slag and crushed glass, which would fall into the angular category. Olivine, garnet and plastic are less sharp and are examples of sub-angular media. Even less abrasive are sub-rounded walnut shells and staurolite, while smooth glass beads and silica sand are examples of rounded media.
Remember to check the shape of the media you choose. It will have a big impact — literally! — on the anchor profile.
We measure particles according to their “mesh” size. This indicates what size mesh the particles will fit through. A mesh size of 10 has 10 holes per square inch, which would fit relatively large particles, whereas a mesh size of 300 would indicate much smaller particles. If the size is given in a split format such as 20/40, this means that a size 30 mesh will accommodate 95 percent of the particles, but not a size 40.
Quick logic might have us conclude that bigger particles do more work than small ones. While it is true that a bigger particle may carry more momentum and therefore leave a bigger impact crater, there is also a drawback. Big particles make more localized impacts on a metal’s surface. That is, there is a lower number of particles per cubic inch than if smaller particles were used, so the result will not be as thorough.
To create a thorough finish and a uniform depth profile, the answer is to use the smallest media possible that will allow you to gain the required anchor pattern.
How hard a medium is also plays a role in how deep the particles will penetrate into a surface. Using softer media will leave a very slight anchor pattern, or none at all, depending on how soft the abrasive is. This is useful for removing gunk, paint and grease.
While it may seem logical to use the hardest material possible for efficient sandblasting, there is a catch to extremely hard abrasives. Abrasives that are too hard can also be brittle, which causes them to fracture on impact. When a particle breaks during collision, a portion of its kinetic energy goes into breaking. This means less energy is available for the surface.
Keep in mind when choosing an abrasive that you want to aim for the minimum hardness needed to accomplish your goal. It is easier to do more sandblasting than to undo it.
The reason the density of a particle is more important than its mass is because density tells us a lot more about how the particle will react upon impact. A dense particle has more mass per unit volume. That means more pressure upon impact, meaning it will dig deeper than a larger particle of the same mass. Think of dropping a steel marble onto your foot versus a dropping a large foam ball that weighs the same.
Additionally, denser particles transfer more of their kinetic energy into impacts. Softer, less-dense particles will compress when they collide with the object, meaning they absorb the collision and transfer their momentum less efficiently. These softer particles have their uses, though, such as paint removal and light surface cleaning.
Types of Sandblasting Media
Here are some of the most common sandblasting media available:
- Steel shot and grit: When you need to do heavy-duty work, steel shot and grit is an excellent choice. Steel shot is round and grit is angular. This material is made from steel and comes in several varieties of hardness, size and shape. It is often used for deburring, shot-peening and for removing tough coatings and preparing for epoxy coating. It is an important media in prepping structural steel for use.
- Glass beads: Glass beads also come in a variety of sizes and are made from soda-lime that puts very little stress on the substrate. They are excellent for preparing metal surfaces for painting and powder-coating as well as for removing unwanted surface blemishes like burrs, rust, paint and scale.
- Black beauty: This is a special type of coal slag abrasive material. It is extremely coarse, making it perfect for auto and industrial maintenance and repairs. It also strips paint and mill scale off surfaces, as well as removes rust.
- Aluminum oxide: This material is synthesized to be both hard and reusable. It is angular, durable and low in cost, making it a popular choice for deburring jobs and grinding. It can round off edges and comes in a variety of sizes, meaning it can contain a high number of particulates per cubic inch. This smaller grain size is great for polishing and smoothing surfaces.
- Silicon carbide: Silicon carbide is the hardest medium available for sandblasting today. Because it is so hard, it requires less time than comparable media. Silicon carbide comes in several sizes, ranging from coarse grit to fine powder, which is useful for polishing hard rock like granite. This abrasive is useful for grinding glass, removing mill scales and any other application where aggressive abrasives are required.
- Staurolite: Staurolite is an abrasive that is mined from metamorphic rocks. Useful for removing thinner coatings of paint, mill scale and rust, it is also a go-to abrasive for stripping off coatings that have weakened on steel and mechanical components. DuPont Starblast abrasives are made from this substance. Staurolite also produces less dust than other abrasives, which makes it easier for operators to see what they are doing on the job.
- Plastics: Plastic grits have a wide range of uses in sandblasting. This is due to the fact that plastics are variable in their size, hardness, shape and density, and can be manufactured to suit different needs. Typically, plastic abrasives come from polycarbonate, polystyrene or other similar compounds.
- Walnut shells: As it so happens, black walnut shells make for excellent abrasive media. Because it is naturally occurring and not as hard as some of its metal and plastic counterparts, walnut is useful for more delicate applications like cleaning and polishing. However, it can also be used in more aggressive applications like barrel tumbling and deburr jobs.
- Corn cobs: Similar to walnut shells, corn cobs can be ground up and made to serve as another sandblasting medium. It is safe to use on delicate components and will remove grime, grease, water and coatings while leaving the underlying surface largely unscathed. Its uses also extend to polishing and deburring. Additionally, it is resistant to breakdown and is therefore reusable.
- Sand: We mention sand because it is popularly thought to be a major component in sandblasting. In fact, it is used less and less these days. This is largely due to its silica content, which, when breathed by operators, can lead to serious illness of the respiratory system. Additionally, sand holds moisture and transfers it to the sandblasting equipment, which leads to premature aging.
How to Choose the Right Sandblasting Medium
These general sandblasting best practices cover how to choose the right media for your desired results:
- When in doubt, “soft” wins out: You can always add a harsher abrasive, but it is significantly more work to undo the work of an abrasive that was too hard. Particularly if you are blasting something soft like wood, it is a great idea to start with corn cobs or walnut shells. They will not damage the underlying substrate.
- Glass is shiny, but it also shines surfaces: If you need a polished look, glass beads are a great place to start. They are a fantastic finisher and will leave surfaces looking smooth and bright. As a reusable abrasive, they are also a money-saver.
- Aluminum oxide strips paint: Do you need to take the paint off of an object? With its hard, angular structure and reusability, aluminum oxide is the abrasive for the job.
- If you need to strip paint off fiberglass, choose plastic: Plastic is great for applications where fiberglass needs to be cleaned. Plastic is much softer than metal and will remove paint without damaging the fiberglass underneath it.
- For quick and aggressive etching, use silicon carbide: Silicon carbide is the toughest abrasive on the market. It makes a quick job of etching as well as removing unwanted rust and paint.
- Steel comes in round and angular forms: If you are removing paint from steel surfaces, be sure to choose the right kind of steel abrasive. If you get round steel shot, it is better used for shot-peening and polishing, while grit is better for paint removal.
Let Finishing Systems Provide You With Your Sandblasting Media and Equipment
At Finishing Systems, we have the sandblasting media, equipment and solutions you need to complete your project. We have been leaders in our industry since 1972 and have been a go-to source for companies like Caterpillar, Mack Trucks and Harley Davidson.
Our custom-designed systems can include blast preparation equipment, washing, painting and curing. The result is a complete turnkey finishing solution. Today, Finishing Systems is recognized throughout the industry as a top provider of finishing solutions and products for large and small companies across the country. Let us help you meet your needs. Contact us by phone or online form today to learn more.