What Is Media Blasting? | Uses & Effects on Various Surfaces
Over time surfaces become blemished due to a number of environmental contaminants, such as rust, or applied finishes such as paint. Removing rust or paint and other stuck-on materials is a job too tough for scrubbing or power washing and too intricate for sanding, so another method was developed, called media blasting.
While everything from glass to walnut shells has been used for blasting, the original media of choice was silica sand. But blasting needs to be safe for the blasting operators because all blast media creates dust and inhaled dust of any type is a common carcinogen.
Safety equipment and particular materials are thereby favored so that jobs can be completed without harming both the blasting specialist and the work surface beneath the corrosion, grime, or other build-up that must be preserved.
Surface restoration is just one use of media blasting. Other application purposes include creating compressive stresses on workpiece surfaces, deflashing, deburring, edge rounding, descaling, roughening and simply smoothing a rough surface.
Various industries rely on media blasting to prepare products for additional stages in the manufacturing process, including trimming metal parts and blanks and preparing, for example, dental restorations for bonding (their minimal surface area makes it essential that optimal bonding conditions are achieved, often through sandblasting).
Various Blast Media Types
The word “media” refers to the type of material launched at the surface via a spinning wheel or compressed air. Various media types exist to optimize the different goals of the blasting in question, with certain characteristics prized for unique applications, among them: hardness, friability (the ability of grains to break down into sharp edges), shape, surface profile, speed, pressure, and recyclability. These characteristics determine how well a particular type of media strips target material.
Earlier walnut shell was mentioned as a media type, and while most people would never think of nut shells being repurposed, walnut shells do an excellent job cleaning and stripping without causing damage to the substrate. Steel shot smooths and polishes, while steel grit removes paint quickly and ground glass is best for removing paint before repainting or recoating. These are just a few examples of the types of media, with blasting specialists getting creative to produce the best outcome for the goal of the project at hand.
Reusing Blast Media
When media is blasted at a surface—for example a rusty tow hook—using compressed air and a sprayer, the media is lost to the environment. Single-use applications are not environmentally friendly or cost-effective, but they are versatile in that they can handle any sized object, such as a large metal fuel tank (although for large applications, blast rooms are available, even complete with recovery floors which allow blast material to not be lost to the surrounding environment, reducing cost and contamination).
The Science Behind Media Blasting and Its Effect on Different Surfaces
You can think of each piece of blasting media material as the head of a tiny hammer, hitting and chipping a piece of surface corrosion, paint or other coating from an object. But the shape of different types of media is not all the same due to unique friability; the way a material breaks when made small enough to become blasting media determines the shape of its edge, making it more or less corrosive as it is projected against the surface in question, although this is only one characteristic of blasting media.
Characteristics of Abrasives
The ability of materials to scratch one another, developed by Friedrich Mohs. Plastic is less hard than silicon carbide, for example.
Peaks and valleys form when an abrasive media is shot at a work surface, and the roughness degree of these characteristics determines how well a coating adheres.
How a material breaks down, what its edges look like once this occurs, largely determines the characteristic of shape.
Different shapes contact work surfaces to different degrees. For example, rounded abrasives contact the surface more but cut less aggressively due to having soft edges; angular shapes have less contact but more aggressive cutting.
Not every type of blast media can be recycled, or sent through gravity-fed or centrifugal airwash reclaimers that separate the dust created by blasting from the abrasive media that is suitable for continued use.
The surface being blasted is a critical consideration when choosing the type of blast media to apply. If you use the wrong blast media, or apply it with the improper amount of pressure, you can cause damage to the surface. Damaging an expensive aircraft component could result in major losses for a blasting business.
Iron fixtures such as fences and artwork are often blasted in order to restore them to their original luster.
These components become caked with grease and dirt which is nearly impossible to clean without the aid of blasting media that penetrates every small crevice, often without requiring disassembly, blown clear from the components via the power of compressed air.
Plastic is much more malleable than metal, so when it’s blasted with media there’s a greater risk of warping, melting, scratching and other forms of damage.
Blasting Machine Types
Siphon Blast Cabinets
For smaller applications, blast cabinets are helpful; they contain handheld blasting nozzles and arm-entry holes whereby the operator can apply blasting media in a closed environment, ensuring that media is reused. Suction guns pull blast media through a hose to a blast nozzle which directs the media to the target, such as an engine block needing corrosion cleaning. Blasting is so effective that even the blast hose nozzles are subject to wear as the abrasive material passes through and onto the surface of the target object, ready to be replaced once the amount of wear expands the nozzle opening 1/16” of its original size.
Direct Pressure Blast Cabinets
As its name implies, direct pressure works at a higher speed than siphon blast cabinets, appropriate for heavy-use industrial blasting applications. Productivity is increased and heavier media is an option, usually doubling the stripping time of caked-on residue.
Small industrial components are tumbled while a rotary wheel blaster runs overhead, spraying the components with media. Often uses include shot peen, cleaning, deburring, and descaling castings.
Rotary Table Machines
Alloys, forgings and castings are treated within these machines which feature a rotary pedestal and turning table. Centrifugal blast equipment if for high volume production. Each blast wheel can throw hundreds of lbs of shot per minute
These enclosed rooms prevent blasting media from leaving the area and may include systems for reclaiming and reusing media by separating the dust from the media. These rooms can be mobile, weatherproofed, and customized to accommodate certain components like wind turbine blades.
Advantages of Abrasive Media Blasting Over Other Surface Preparation Methods
Blasting operators need to stay safe as inhalation of virtually anything risks an otherwise safe substance turning into a carcinogen once it becomes imbedded into lung tissue (silicosis is caused by inhaling large amount of silica dust, a condition that plagued earlier sand blasting operators who had no respirators to protect themselves.) Chemicals pose their own risks when used to strip materials, as anyone who has used powerful household cleaners such as degreasers knows first-hand. Without exposure to chemicals, and while wearing proper personal protective equipment, blasting operators remain free from risk.
Not only is blasting machinery reusable—along with many blasting materials themselves—naturally-occuring blasting materials like corn cob and walnut shell are harmless to the environment and those who operate blasting equipment (as long as these materials are not inhaled).
Sand was first used as a blasting material due to its extremely low cost, but great pressure has largely discouraged the use of sand as a blast media due to safety concerns. Blast media specialists look instead to increase value, such as superior greater durability and less scrap, in exchange for the use of blast media that cost more.
While working within a blast cabinet or blast room, blast media operators keep the media contained and in many cases suitable for reuse.
While it seems like chemical cleaning would be quick, such as a dip of a metal part into a bath of stripping product, a second manual step often slows down the process when the residue created by the chemical stripping must be scrubbed clean. Most blasting media however don’t leave a residue; why replace one residue you’d like to remove with another? Disposal of used chemicals can also be a costly problem.
When you use chemicals to strip materials the substrate can be damaged. This isn’t always so obvious at first as chemical damage can crop up long after the cleaning job is complete. Reversing this damage increases costs or, in the worst cases, changes the surface of the material rendering it unusable.
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