Selecting blast media

You’ve spent a great deal of time – and money – selecting just the right blasting room or blast cabinet for an abrasive blasting operation. And you’ve carefully chosen the actual blast equipment to execute the operation, from nozzles to reclaim systems, to dust collectors.

Now, pick the blast media, and you’ll be up and running in no time.

Stop! You should put as much thought, time, and energy into selecting the most appropriate and effective blast media for your operation as you did in choosing your equipment. There’re many options, but they exhibit differing levels of hardness, shapes, sizes, and other properties, resulting in a substantial performance difference.

For aircraft refurbishment, the abrasive blasting process is primarily for two purposes:

⮚ Cleaning and abrading a surface (e.g., fuselage), which removes all dirt, grime, oils, etc. from the surface; roughens the surface to improve adhesion for a new surface finish of paint, powder coating, or polyurethane.

⮚ Cleaning and deburring a surface without producing a profile or removing materials from the surface (e.g., partsin jet turbines), where removal of minute amounts of metal could affect performance and safety.

Before evaluating blast media, answer two critical questions to make the best choice:

⮚ What are you blasting (carbon steel, stainless steel, aluminum, iron, carbon fiber)?

⮚ Is it old or new material? This can make a difference in the integrity of the material and will determine choice of blast media and pressure.

Now you can evaluate the wide array of blast media – and select the one most effective for the application. Five primary properties of blast media to understand before choosing the one best for the job:

⮚ Size: Small particles will clean faster, provide better coverage, produce a more uniform profile; Use the smallest particle possible to obtain the desired profile

⮚ Shape: Angular particles will produce a deeper profile, rounded ones don't create a profile; rounded media will provide an even, peened surface

⮚ Hardness: The harder the particle, the deeper the profile it creates; softer organic and plastic media remove dirt, oil, grease, paint without creating a profile (ideal for when no new coating is to be applied)

⮚ Density: Denser particles impart more kinetic energy over a smaller surface area, give a deeper profile with less deformation

⮚ Reclaimability: Some materials are easily recovered and reused, even hundreds of times; single-use abrasives can't be recovered; reclaimable materials are more expensive but present a better value

The coating you chose will provide specs for preparation of the material being coated. The technical data sheet (TDS) or product data sheet (PDS), will state the profile needed to create a specific depth.

Aluminum oxide – The most common media for use in blast rooms and cabinets, it's an angular, durable blasting abrasive often used in sandpaper (along with silicon carbide). The lower the mesh or grit, the more abrasive the material is (i.e., 40 grit significantly erodes a surface; 200 grit provides a smoother finish). Extremely hard and aggressive, aluminum oxide is often used to remove paint and rust from metal parts.

Recommended uses: Heavy-duty surface prep jobs, usually involving steel, where a rough profile for excellent coating adhesion (paint, polyurethane, powder coating, etc.) is needed. It's used on the steel parts of an airplane, especially landing gear.

Garnet – This gemstone material is a little softer than aluminum oxide and not as reclaimable, but is fast-cutting, low-dust producing, and low-consuming. A good general outdoor surface preparation abrasive that's excellent for removing tough coatings, paint, rust, and mill scale from steel.

Recommended uses: Depending on mesh size, it can clean aluminum, fiberglass, and steel. Inert and non-toxic, it's used in places where water contamination is a concern.

Steel grit – Expensive but has high reclaimability, it's a top blast-room material offering fast stripping to remove rust, paint, or scale from steel surfaces. It's frequently made from stainless steel so no rust appears on the finished product and it removes contaminants from a surface and creates a blast profile, providing good adhesion for new coatings.

Recommended uses: For removing contaminants (rust, paint, or scale) from steel surfaces and creating a well-defined profile so new coatings adhere more effectively. Steel grit is softer than aluminum oxide and doesn't fracture easily.

Steel shot – Extremely hard, heavy, expensive, but highly reclaimable, so it provides high value. Generating very little dust, steel shot creates a peening action producing smooth surfaces. Its heavy weight gives greater impact and hammering action for cleaning heavy forgings and removing heat-treating scale. Because of the peening action, shot can improve the compressive strength of critical metal parts, such as the root of engine turbine blades and other substantial turbine components. Steel shot peening is used on aircraft and helicopter components to strengthen them and make them less susceptible to fatigue.

Recommended uses: Steel shot is used for deburring and peening of stampings and fabrications and for removing corrosion from the surfaces of machinery and equipment. It's also used for removal of epoxy/adhesive and rubber build-up removal.

Glass beads – Often used in a blast cabinet or reclaimable blast operations for cleaning or peening, it's perfect for stainless steel equipment that doesn't get coated. Available in various sizes and made from soda-lime, it puts little stress on the substrate. Create smooth, matte finish different from the highly polished look of angular abrasives.

Recommended uses: General cleaning, peening, and cosmetic finishing of sensitive metal surfaces (especially aluminum and stainless steel) and useful for deburring without part damage. Can also be used for peening to improve tensile strength. It's commonly used on turbine blades.

Plastic – A soft, light abrasive that creates no profile and is good for stripping paint and mold from sensitive surfaces (it won't remove rust) while being a less-hazardous alternative to chemical stripping and faster than hand-stripping. Because it's extremely soft, it's ideal for removing paint from fiberglass parts. Using plastic for blasting produces low dust levels.

Recommended uses: Removing paint and for aluminum and composite aircraft skins, as well as for blasting delicate parts. Harder plastics can be used on harder aluminum aircraft parts and landing gear.

Organics – Walnut shells and corn cobs are two popular organics used to clean brass, wood, or other soft materials without causing surface damage. They’ll remove dirt, grease, oil, carbon, scale, burrs, and paint without changing the underlying substrate. Starch can also be used on very thin-skinned aluminum, composite flight controls, airframes, and blades.

Recommended uses: Cleaning electric motors and aircraft engines, wooden surfaces, dies and molds, and removing graffiti. Starch can remove tougher aircraft coatings without damaging delicate substrates.

Another point to consider is the effect abrasive blast media have on blasting equipment. Harder, coarser materials will provide highly aggressive action and can produce an excellent profile for coating. But that same hardness can also take a toll on nozzles, blast hoses, and reclaim equipment. Keep that in mind as you make your final selection.

There's blast media perfect for the job. Do your homework up front to ensure you get the best performance while minimizing or eliminating product damage.

Titan Abrasive Systems

About the author: Brandon Acker is CEO of Titan Abrasive Systems. He can be reached at [email protected].