Not all blast cabinets are created equally and abrasive selection is very much dependent on the application and machine at hand. Whether you’re doing aerospace engineering or cleaning a car part, using the right abrasive in the right machine is key to the success of your project. If you aren’t sure you have the right machine, check out our buying guide to confirm you have the right cabinet.
Abrasive Types: Cutting and Non-cutting
The two basic types of abrasives are cutting and non-cutting abrasive. Glass beads, plastic, soda and even steel shot are considered non-cutting abrasives. Glass beads are by far the most used non-cutting abrasive type being that they do not reduce the size of a part and remove only the surface coating be it paint, rust, etc. When used correctly, glass beads can actually make the size of a part appear to grow by making a small crater in the part surface that measures larger unless you are using a profilometer, a measuring instrument used to measure a surface’s profile, in order to quantify its roughness. When used properly, glass beads impact the surface while spinning, making a small indentation in the part surface and sealing the surface. In some cases, depending on the part material, the surface is sealed with a luster. The larger the bead the more luster the part has after blasting.
There are other non-cutting abrasive have special applications. Plastic abrasive is commonly used for aerospace applications because it can provide chemical-free cleaning without changing the metallurgy of the part. Soda, which cannot be recycled, is another non-cutting abrasive that is capable of cleaning parts that are covered in oil and other deposits. Steel shot is also a non-cutting abrasive, and its weight overcomes most coating adhesions by simply overpowering the coating. The part being blasted must be able to withstand such strong abrasive, and many thin parts are distorted by shot peening.
In cases such as hard anodizing, the surface created by glass beads is detrimental. Hard anodizing requires an open part surface for adhesion, which is why cutting abrasives are used. The most used cutting abrasive type is aluminum oxide – it opens surfaces, has great recycling rates, can produce a gloss surface with the correct abrasive mesh size for powder coated surfaces and allows the best possible adhesion for many other critical coatings. Aluminum oxide has many different types but know the most used is brown fused aluminum oxide, which contains a small amount of iron. Being that some applications do not allow for iron, iron-free white aluminum oxide is also available. On the Mohs mineral hardness scale Aluminum Oxide has a hardness of 9.0. Another even harder commonly used cutting abrasive is silicon carbide, which rates at 9.5 on the Mohs scale. To put this into perspective, sand rates about 4.0, and for this reason it’s not used in cabinets, as it would explode on impact under nearly any pressure.
One important factor to always keep in mind is that blasting cabinets use different kinds of abrasive than portable blast equipment because the later doesn’t recycle abrasive. For example, softer much less expensive cutting abrasives like sand, slag and garnet should never be used in cabinets because these abrasives will overload any size dust collector and quickly create a huge mess.
Assuming you have the right cabinet, there are a few things to keep in mind when you’re selecting abrasive:
- All abrasive types have different cleaning speeds. When graphed, the ratio of abrasive size to speed takes on the appearance of a bell curve where large abrasive and small abrasive are at either end and take the longest time to clean. At the top of the curve are the fastest abrasives with enough mass and particle density to produce the fastest cleaning speed. For Glass Beads the fastest is about 70 to 100 mesh or a number 8 glass beads with good size or diameter and good particles per square inch, and for aluminum oxide the fastest is about 60 to 80 mesh but know that aluminum oxide has a much larger minimum/maximum spread.
- Naturally the cabinet’s gun size can affect cleaning speed. Smaller machines with smaller guns often compensate by using cutting abrasive to speed up the cleaning process. Small guns also often use abrasive beyond maximum impact velocity, which means it explodes on impact, ripping and tearing the surface clean. Unfortunately, once abrasive has exploded it cannot be recycled and may plug your dust collector. If speed is vitally important to your application, you may want to consider purchasing an industrial-grade cabinet that can handle a large gun and abrasive with much higher frictional heat.
- In Industrial Manufacturing, specifications already exist to texture, de-burr, prepare for hard anodizing, anodize, paint, powder coat, remove aircraft coating, shot peen, create a chemically clean surface, etc. The abrasive to be used – everything from abrasive mesh and grit size to processing pressures – are indicated. If this pertains to you, and you’re looking to increase productivity by changing your abrasive, you’re out of luck. The best we can offer is recommending a more powerful machine; capable of prolonged, daily operation or possibly a large cfm gun size but the abrasive you use will remain the same.
- For the first time user or the manufacturing company not already doing the part preparation internally, the right abrasive is determined by the specific work needing to be done. We can help guide you to the cabinet model that is best for your application. Know that almost every model of abrasive blasting cabinet is capable of running almost all abrasive types, some are just much more efficient than others, and this is why Media Blast manufactures a complete line of wet, dry, siphon, gravity feed, direct pressure and micro blast cabinets. We manufacture special models for plastic, steel shot and soda abrasive, and pay special attention to the Sandcarving and Automotive markets.
- Some cutting abrasives are just not suitable for cabinets that are designed to recycle abrasive. Materials such as garnet; slags with great names like black beauty and copper slag are designed to be used once and are therefore inexpensive. Some companies claim these are safe for blasting cabinets but they should never be used in machines that recycle abrasive, they will plug almost any dust collector size so buyers beware.