How to Choose the Right Abrasive for the JobUpdated: January 16, 2023
Abrasive blasting is the general term applied to finishing processes that entail the high-pressure propulsion of a stream of abrasive material onto the surface of a workpiece. Abrasive blasting can serve many surface finishing purposes, such as removing contaminants or previous coatings, altering the shape and smoothing or roughening the surface.
There are many types of abrasive blasting applications, each of which requires the use of different types of blasting materials, referred to as blasting “media.” Finishing Systems has assembled this abrasive blasting media guide to help you select the most appropriate material for your specific finishing processes.
One note: the terms “sandblasting” and “abrasive blasting” are often used interchangeably. A conventional “dry” sandblasting process that uses traditional sand is not recommended for most applications. Blasting with sand generally requires twice the amount of material, making it cost-prohibitive for many companies.
Sand also contains significant levels of silica, which when broken down and inhaled by operators can result in serious respiratory disorders. Consequently, the process of ‘sandblasting’ is typically conducted utilizing alternative types of abrasive material:
Types of Abrasive Material
- Glass Beads: Glass is not as aggressive a blasting media as other materials, such as steel shot or silicon carbide. However, it is an excellent choice for applications that require a softer, brighter finish. It is well suited for stainless steel applications. Glass beads can also be recycled multiple times.
- Aluminum Oxide: Aluminum oxide is characterized by its superior hardness and strength. It can be found in applications ranging from anti-slip surfaces, industrial applications as a blasting media, and as a raw material in refractories. Aluminum oxide is designed for abrasive pressure blasting of almost any type of substrate: glass, granite, marble, and steel. Due to its ability to deeply etch it is used in the preparation of surfaces prior to painting or coatings.
- Plastics: Plastic abrasive is a dry thermoset cleaning media made from crushed urea, polyester or acrylic. Each varying and available in a range of hardness and particle size. Plastic is generally regarded as the best abrasive material for mold cleaning, blasting of plastic parts, or in applications where the removal of the substrate material is not permitted. Common industries that utilize plastic media blasting include automotive, aviation, boating, electronics and industrial applications.
- Silicon Carbide: Silicon carbide is the hardest abrasive blasting material available, making it the best choice for your most challenging surface finishing applications. It is available in various colors and purities. Its primary use ranges from bonded abrasive tools, lapping, polishing, glass etching and general-purpose heavy-duty blast cutting applications.
- Steel Shot & Grit: Steel abrasive is a cost-effective alternative to other abrasives due to its toughness and high recyclability. It can be used on a variety of surfaces to effectively remove contaminants, texture a surface for proper adherence of a final coating, or in peening (hardening) applications. The correct size, hardness and shape play a significant role in the proper media selection.
- Starblast: Starblast™ is a mined loose blend of coarse and fine staurolite sands with extremely low levels of silica making it an ideal general purpose blasting abrasive. It is perfect for removing scale and corrosion from steel surfaces while offering a low dust level for improved visibility.
- Walnut Shells: Walnut shell abrasive is a hard naturally occurring material made from crushed walnut shells. It is the harder of the soft abrasives, available in a variety of sizes for blast cleaning and polishing softer surfaces that could incur damage from harsher abrasives. Typical applications include polishing of soft metals, fiberglass, wood, plastic and stone. It can also be used in tumbling operations for polishing gems and jewelry.
- Corn Cobs: Corn cob abrasive is a granular abrasive manufactured from crushing the dense woody ring of a corn cob into various grit sizes. It is the softer of the naturally occurring abrasives making it ideal for cleaning, deburring, burnishing and de-flashing applications. Common industries include jewelry, cutlery, engine parts, fiberglass and the removal of graffiti or debris from wood, brick or stone.
What to Consider When Choosing an Abrasive Media for Your Project
Knowing what abrasives are available is the first step in choosing the right one for your project. However, you also need to understand the demands of your project and how the characteristics of an abrasive will support those demands. Consider the following factors.
The hardness of minerals is defined by the Mohs hardness scale. Abrasive media like silicon carbide has a high hardness value, while a plastic abrasive has a much lower hardness. The softer your abrasive is, the finer your finish will be. Harder abrasives are best for tougher jobs, like removing rust and corrosion.
Shape and Tread
Abrasives come in different shapes that affect how much of the material comes in contact with your surface. Angular and sub-angular abrasives like crushed glass and plastic urea have jagged edges and angular shapes. These types have less contact with your surface, but have a more aggressive cutting ability.
Sub-rounded and rounded abrasives are mostly or completely smooth, like walnut shells and glass beads. These abrasives are far less aggressive in their cutting ability but easily create a refined scratch pattern.
Density relates to how tightly the atoms of a substance are packed together. High-density abrasives have more impact on a small area and create deep profiles on your surface. If you want less force, you need a less dense material. For example, steel shot has a relatively high density, and walnut shell has a low density.
Coatings have depth profile requirements to ensure optimal bonding. Make sure your abrasive can create the appropriate peaks and valleys in your surface before coating. Consider size and shape when making this consideration.
Use of Machinery
Abrasives will only perform their best when you use your machine correctly. For example, lower grit abrasives work best with a lower RPM. If you run an abrasive at a higher speed when it has a lower grit, you may experience resin transfer.
Can You Reuse Sandblasting Media?
In an effort to minimize costs, many operations are evaluating whether they can reuse their sandblasting media. In many cases, the answer is yes — if you’re using abrasive blast cabinets, abrasive media can be used over and over again.
When it comes to recycling, the answer depends on a few factors. Sand cannot be reused after sandblasting, but it can be recycled into other materials, such as cement, while harder materials such as garnet, steel shot, and glass beads can be screened and separated to recycle. However, sandblasting media can’t be recycled when used in portable blasting applications that have no containment method.
When reusing blasting media, the two pieces of information you need to be successful are the nozzle flow (pounds/minute) and the volume inside the cabinet — this data will let you determine how many times the abrasive can be reused. The ideal rate for blasting reuse is for direct pressure to reach the maximum abrasive impact velocity at half the blasting pressure of the siphon.
Single Pass vs. Multiuse Media
Some types of blasting media have a low rating on the Mohs’ hardness scale and are therefore categorized as single-pass media, which means that these materials can only be used once. When blasted, this media disintegrates into particles too small to recycle.
On the other hand, media that is higher on the hardness scale can be reused, and are therefore categorized as multiuse media. The harder the material, the longer it is effective. This abrasive blasting media can be passed multiple times before it breaks down into particles that are too small to work.
How Long Does Blasting Media Last?
The life span of blasting media varies based on a number of factors, including the application and use patterns such as the following:
- Starting with larger and harder abrasive results allows longer reuse and vice versa.
- Blasting below the abrasive’s maximum impact velocity will allow for more uses.
- Always use a separator with your blasting cabinet — not doing so will contaminate the recycled abrasive and prevent further use.
Using a blasting media past its ideal recycle rate will slow down processing. Be sure not to overuse an abrasive — it will need to be replaced eventually, even if there is still some left.
The Role of Sandblasting Recovery Systems in Blast Media Recycling
If you’re recycling blast media, properly cleaning it with a blast recovery system before reusing it can be extremely beneficial. First, it can greatly reduce consumable expenses. Abrasive media like steel grit can be used hundreds of times, saving hundreds to thousands of dollars. In order to recover blast media for reuse, there are several options available.
Abrasive Blast Cabinet With a Vacuum Recovery System
There are two types of vacuum sandblasting recovery systems, one of which allows you to blast and then use a vacuum attachment to pick up the spent media. This allows for more media recovery and is one of the quickest recovery methods. However, this system uses a lot of compressed air and is therefore limited to less abrasive media. This system is the fastest and best for portable media recovery.
The other method uses vacuum recovery that operates continuously to suction media. While this method is slower than the other because the system is blasting and vacuuming at the same time, it prevents media spillage and is best for smaller or indoor work that must keep sandblasting dust generation to a minimum.
Mechanical or Pneumatic Blast Media Recovery System
Similarly, there are also two mechanical and pneumatic recovery systems. The pneumatic system delivers the sandblast media to a reclaimer through the movement of air. It then filters out anything too small or foreign for the blaster and returns the reclaimed media to a sandblast pot so it can be blasted again. These systems are great for abrasive media such as glass beads, plastics, aluminum oxide and smaller steel grit sizes.
A mechanical sandblasting recovery system is most commonly used with steel grit, as it is able to easily deliver the heavy media to a blast pot. This option is best when recycling highly abrasive blast media.
LEARN MORE ABOUT YOUR ABRASIVE BLASTING MEDIA OPTIONS
As a recognized leader in the metal finishing industry for more than 40 years, Finishing Systems has the experience and expertise to help you select the best media for your sandblasting applications. Contact us to learn more about the various types of abrasive blasting media we offer today.