The first abrasive blasting process was patented on 18 October 1870 by Benjamin Chew Tilghman and, in essence, the process has used the same basic technique ever since.
While there are variations in media type, blasting techniques and equipment, all abrasive blasting involves a material being propelled at a surface for the purpose of surface treatment or cleaning.
The properties of the blasting media used depends on the application; from softer, less abrasive media like walnut shells through to various sands, silicon carbide, alumina or emery particles, and steel shot.
The blasting media is fired through a nozzle at the surface to be treated. Compressed air, liquid (usually water) or vapour (usually steam) streams, or mechanical projection methods (such as rotating paddles) are used to propel the media.
The process can be done using a mobile abrasive system, in a blast cabinet, or even in a specially installed blast room.
Coarser abrasive media can result in sparks being given off during the blasting process. These sparks vary in colour, size and glow depending on type, with steel shot blasting producing heavy and bright orange sparks and garnet abrasives producing a very faint blue glow.
Blast media comes in different forms, offering different levels of abrasion. During the early 1900s it was felt that sharp-edged grains were the most effective media, but this has been shown to be incorrect, depending on factors such as the desired outcome and the surface being treated.
Minerals
Mineral abrasives include silica and garnet. Silica, however, breaks up quickly, creating dust that, if inhaled, can lead to silicosis, a debilitating lung disease. Silica can be coated with resins to help control the dust but, despite this, it is banned from use in countries including Belgium, Germany, Russia, Sweden and the United Kingdom.
Garnet is more expensive than silica sand, but offers equivalent production without producing as much dust and having none of the safety hazards associated with ingestion. Other minerals used in abrasive blasting include magnesium sulphate or kieserite.
Agricultural / Organic
Agricultural or organic blasting media include nutshells and fruit kernels. Being soft abrasives, these are used when it is important to avoid damage to the underlying material, such as when removing graffiti, cleaning brick or stonework or removing coatings from circuit boards due for repair.
Synthetic
Synthetic media types range in their levels of abrasiveness. The softer synthetic abrasives include corn starch, wheat starch, sodium bicarbonate (baking soda) and dry ice. These are used for similar activities as those for agricultural media, with soda blasting with sodium bicarbonate, for example, fragmenting on impact causing surface materials to explode away without damaging the substrate. More abrasive synthetic media includes process by-products (such as copper, nickel or coal slag), engineered abrasives (like aluminium oxide, silicon carbide, glass beads, and ceramic shot) ad recycled products (including plastic abrasives and glass grit).
Metallic
Metallic abrasives include steel shot, steel grit, stainless steel shot, aluminium shot, copper shot, cut wire and zinc shot. These types of media are among the more abrasive types available, although they vary depending on the qualities of the metals being used.
There is a variety of blast equipment available that falls into one of three types:
Portable or Mobile Blast Equipment
Portable blast equipment can either be designed for dry or wet blasting.
Dry blasting systems typically use a diesel air compressor which provides high-pressure air to single or multiple blast pots. These blast pots are pressurised containers filled with the abrasive media. The blast pots allow an adjustable amount of media into the blasting line, with greater volumes of air from the compressor allowing for the use of more pots simultaneously.
Mobile wet blasting systems introduce the abrasive media into a pressurised stream of liquid (typically water), to create a slurry. Wet blasting systems reduce the amount of dust produced by the system.
Portable blast systems are often mounted on semi-tractor trailers, allowing them to be moved to and around sites. Other portable systems are lightweight hopper-fed types.
Portable applications may or may not recycle the abrasive materials after use.
Blast Cabinets
A blast cabinet is an enclosed system that allows the operator to recycle the abrasive following blasting. They consist of a cabinet to contain the process, the abrasive blasting system, a recycling system and dust collection. Blast cabinets can be manual, whereby an operator would place their arms in gloves attached to the cabinet to direct the blast gun while viewing the operation through a viewing window and turning the blasting media on and off with a foot pedal or treadle. Automated blast cabinets can be used to process large quantities of the same component without direct operator interaction, and may include several blast nozzles and a system for conveying the parts through the cabinet.
As with portable blast equipment, blast cabinets can either be used for dry or wet blasting. Dry blasting systems include siphon and pressure systems. Siphon (or suction) blast systems use compressed air to create a vacuum in the blast gun. This negative pressure pulls the abrasive media into the blast gun from where it can be directed through the blast nozzle and at the workpiece with the compressed air. Pressure systems require the abrasive media to be stored in a pressure vessel with a blast hose attached. Both the vessel and the hose are pressurised to the same pressure and the abrasive media is then fired from the blast nozzle with compressed gas. Wet blasting systems inject the abrasive/liquid slurry into a compressed gas stream and is typically used where heat produced by the friction of dry blasting could damage the workpiece.
Blast Rooms
Blast rooms are larger versions of blast cabinets. They can be large enough to accommodate very large items such as vehicles, construction equipment, or even aircraft. These rooms are used like any other blast system to roughen, smooth, or clean surfaces of an item, depending on the needs of the finished product.
Blast rooms can include a range of equipment, but will typically include some similarities. Each will include an enclosure or containment system (often the room itself) to prevent the blast media from escaping. The abrasive media will usually be contained in a pressurised blast pot and there will also be a blasting system to propel the abrasive media, commonly used systems include wheel blasting and air blasting systems. A dust collection system will filter the air and prevent any particulate matter from escaping. Once the blast process is complete, a material recycling or reclamation system will collect the abrasive media so it can be reused. These collection systems can be automated, mechanical or pneumatic systems installed in the floor or may even simply involve sweeping or shovelling the blast material back into the blast pot.
Additional equipment in blast rooms include overhead cranes to move the workpieces into place and sound-dampening materials to reduce noise levels during blasting.
Nozzles
Blast nozzles come in a variety of shapes, sizes, and materials. Tungsten carbide is commonly used for mineral abrasives. Boron carbide or silicon carbide nozzles are more wear resistant, so they are often used with harder abrasive media. Smaller or less expensive blasting systems may use ceramic nozzles.
Different types of blasting are separated by the types of media they use or the actual process itself (such as automated or wet blasting). Here we present some common forms of abrasive, and non-abrasive, blasting:
Sandblasting
Sandblasting is often used as a generic term for abrasive blasting. This involves the cleaning, smoothing or shaping of a hard surface by striking it with solid particles at high speeds. The effect is similar to that of sandpaper except that it offers a more even finish and can reach into difficult areas. Industrial sandblasting is done using compressed air but can also occur naturally when wind picks up sand particles, leading to Aeolian erosion. Artificial sandblasting was first patented by Benjamin Chew Tilghman on 18 October 1870.
Automated Blasting
Automated blasting is the automation of a blasting process rather than a specific process in its own right. It is often part of a larger automated procedure that can involve other surface treatments like coating and preparation. Automated blasting can be part of a wider production process whereby a production line of similar parts are delivered to the blasting chamber in succession.
Bead Blasting
Bead blasting uses fine glass beads to clean away deposits from surfaces, remove paint or other applications where a uniform finish is required for machined parts. Widely used in the automotive industry, bead blasting is also used to clean mineral specimens with a Mohs hardness of seven or less and would thereby be damaged by harsher sandblasting methods.
Bristle Blasting
Unlike other blasting methods, bristle blasting does not use a separate blast media, instead using a brush-like rotary tool made from dynamically-tuned high-carbon steel wire. The rotating bristle tips create a localised impact, rebound and crater formation to clean and coarsen surfaces.
Dry-Ice Blasting
Dry ice blasting uses a mixture of air and dry ice to dislodge surface contaminants. Frozen carbon dioxide particles strike the surface of the workpiece causing slight shrinkage as a result of freezing that disrupts adhesion bonds. Dry ice is a relatively soft material that is less destructive to the underlying material when compared to other methods. Since the dry ice sublimates, there is no residue other than the removed material to clean up after the process.
Hydro-Blasting
Also known as water blasting, hydro blasting is not a form of abrasive blasting since no abrasive materials are used. Instead, the process uses a high-pressure jet of water to remove old paint, chemicals or other build-ups without damaging the surface of the workpiece. Hydro blasting has the benefit of reaching difficult to reach areas as well as reducing waste by recapturing the water and using it again.
Micro-Abrasive Blasting
Micro-abrasive blasting, also known as pencil blasting, is a dry abrasive process that uses small nozzles to deliver a fine stream of abrasive to a small area or part. The nozzles are typically 0.25-1.5mm in diameter and the area covered by the blast is just 1mm2 to a few cm2 at the most, with the abrasive media particles being between around 10 and 150 micrometres in size. The abrasive jet is fine and accurate enough to write on glass or cut a pattern in an eggshell, although high pressures are usually required.
Wet Abrasive Blasting
Wet abrasive blasting was developed as an alternative to dry blasting, using a fluid (typically water) to help propel the abrasive media at the workpiece. The water offers the advantage of trapping dust and lubricating the surface as well as cushioning the impact of the abrasives and thereby reducing the unwanted removal of underlying materials. Wet abrasive blasting can be undertaken with portable equipment or blast cabinets and blast rooms. It can also be automated and delivers advantages including the ability to use both fine or coarse media ranging from plastic to steel. The reduction in dust makes it safer for use with siliceous materials and for the removal of hazardous materials like asbestos, radioactive or poisonous products. Wet blasting can also use hot water and soap to allow for simultaneous degreasing during the blasting process.
Wet blasting is not as fast as dry blasting when using a comparative size and type of abrasive material. This is due to the liquid creating a lubricating cushion between the abrasive and the substrate, protecting the surface and reducing breakdown rates. However, with a reduced impact on the surface, dust reduction and the elimination of static cling, wet blasting can create very clean surfaces. The lack of surface recontamination means that the same equipment can be used for different blasting operations, although blasting mild steel will create immediate ‘flash’ corrosion on the surface of the steel substrate due to the presence of water.
Vacuum Blasting
Vacuum blasting offers the advantage of generating very little dust and spill, since the blast tool simultaneously blasts and collects used media and loose particles. Because the media is automatically separated from the dust and loose particles it can be reused several times, reducing blast media consumption.
Vapour Blasting
Vapour blasting is a form of wet blasting except, in this instance, pressurised air is added to the water in the nozzle to create a mist or vapour. This leads to a process that is milder than wet blasting allowing mating surfaces to be cleaned without removing their ability to couple together.
Wheel Blasting
Wheel blasting uses a spinning wheel to propel the abrasive at an object. This is an ‘airless’ operation since no gas or liquid propellant is used. The size and number of blast wheels varies depending on the required results of the process. Wheel blasting is typically performed in a chamber where the abrasive can be recycled, with media including steel or stainless steel shot, cut wire, grit, or similarly sized pellets being used. Blast machines can be used to propel plastic abrasives for deflashing rubber and plastic components. The blast wheel was first patented in 1932 by Wheelabrator, and the first blast wheel was built in China in the 1950s.
When performed correctly with the right type of blast media and process, abrasive blasting offers several advantages over other surface cleaning and treatment processes.
It is an easy and fast process so long as the correct precautions are taken. It can also be a process that requires very little machinery (as is the case with portable options), although equally, it can also involve a wide range of machinery in the case of blast rooms, for example.
Abrasive blasting is ideal for a wide range of applications and industries to remove unwanted deposits, clean surfaces, or alter the shape or properties of a surface.
Abrasive blasting can be used to clean a variety of different materials, including delicates ones with the correct media.
The disadvantages to abrasive blasting are typically safety-related.
If not done correctly, small particles can enter an operator’s airways and lead to lung diseases such as silicosis. Some organic blast media, like walnut shells or corncobs, can cause an allergic reaction in some individuals, possibly leading to anaphylactic shock.
Of course, the blast media can also be dangerous to naked skin when propelled at high speeds, so care needs to be taken not to expose your skin to the blasting process.
You can see more about safety and abrasive blasting below.
Abrasive grit blasting is widely used by a range of industries to clean or modify the surface properties of materials.
Common applications include:
- Surface preparation ahead of coating or bonding operations
- Removal of paint, rust, sand or scale from surfaces
- Roughening component surfaces before spray coating
- Removing burrs and edge profiling for machined components
- Creating a matte cosmetic finish to surfaces
- Removing mould flash from plastic components
- Creating surface texturing to alter the appearance of stamped or moulded products
- Creating compression in metal surfaces, for example, to reduce stress corrosion cracking in aluminium alloys
Different media types are used for different applications, depending on the requirements of the process, as follows:
- Glass Beads: These are widely used for applications that need a soft, bright finish, such as with many stainless steel applications
- Aluminium Oxide: The superior hardness and strength of this blast media means it can be used for applications on almost any type of substrate, including glass, granite, marble and steel. It is widely used to roughen surfaces prior to coating
- Plastics: Plastic abrasives vary in hardness and particle size, but are frequently used for mould cleaning or for applications where the substrate material needs to be kept intact. Industries that use plastics for abrasive blasting include automotive, aerospace, marine and electronics
- Silicon Carbide: Being the hardest abrasive blasting material, silicon carbide is used for the most challenging surfaces. It is used for a range of applications, including lapping, polishing, glass etching and heavy duty blast cutting
- Steel Shot / Grit: Being tough and easily recyclable, steel abrasives are a cost-effective option. They are used on a variety of surfaces to provide texturing, remove contaminants or for peening (hardening) applications
- Walnut Shells: Made from crushed walnut shells, this abrasive is one of the softer options, although it is harder than corn cobs (see below). Applications include blast cleaning or polishing surfaces that may suffer damage from harsher abrasives. These surfaces include soft metals, fibreglass, wood, plastic and stone. Walnut shells can also be used to polish gems and jewellery.
- Corn Cobs: A soft media, corn cobs are ideal for burnishing, cleaning, deburring and de-flashing applications. This media is commonly used for jewellery, cutlery, engine parts, fibreglass and removing graffiti or debris from brick, stone, or wood.
Abrasive grit blasting is used for a wide range of applications to clean or dress surfaces as well as for altering the shape of materials in a variety of industries.
While it is used for cleaning the hulls of boats, removing graffiti from brick and preparing surfaces ahead of coating or welding, there are also some more unusual examples where blasting processes are used.
The lettering and engraving of most modern-day cemetery monuments and headstones are created by abrasive blasting. Abrasive blasting is also used to produce three-dimensional signage in both metal and wood. Sandblasting is also used for store fronts and interior designing on clear acrylic glass and glazing.
Other applications include building refurbishments and to create works of art from glass.
As shown in the disadvantages (above), there are safety issues that need to be considered when undertaking abrasive blasting.
Abrasive blasting can create a large amount of dust from both the blast media and the substrate. While most abrasives (such as steel shot, aluminium oxide and glass beads) are not hazardous, there are others (such as silica sand and copper or nickel slag) that can be hazardous if inhaled. Other airborne, particle-based dangers include when blasting lead-based paint, which can be harmful to the nervous system.
Abrasive blasting can also present dangers for operators associated with projection burns, falls from walking over round shot, heat exhaustion, explosive atmospheres and exposure to high noise levels.
Before the creation of the first sandblasting enclosure in 1918, operators were at risk of lung damage as a result of dust inhalation and injury from flying materials. Modern blasting rooms, cabinets and portable blasting equipment have been adapted to mitigate against such dangers, including the use of exhaust fans to draw dust away.
Aside from the direct operational risks, there can be secondary risks associated with the process such as falls from scaffolding or carbon monoxide poisoning from using gasoline-powered engines in confined spaces.
Abrasive blasting is now regulated in many countries so that it can only be performed in a controlled environment with the correct ventilation, protective clothing and air supply for breathing. Many of these safety features are incorporated into blast rooms but additional equipment may be required when using blast cabinets or portable blast systems.
Typical safety equipment for mobile abrasive blasting operators includes:
- Pressurised Blast Hood or Helmet: The hood or helmet will include a viewing window with a replaceable or protected lens and an air-feed hose
- Air Supply: The air feed hose from the blast hood or helmet is usually attached to a grade-D pressurised air supply. Grade‑D air supplies are mandated by OSHA to protect operators from hazardous gases and will include a pressure regulator, air filtration system and a carbon monoxide monitor/alarm. Alternatively, a self-contained oil-less pump can be used to feed pressurised air into the blast hood/helmet. This method does not need an air filter or carbon monoxide monitor/alarm, as the pressurised air is coming from a source that cannot generate carbon monoxide
- Hearing Protection: Ear plugs or ear protectors
- Protective Clothing: This can vary but generally will include gloves and overalls or a leather coat and chaps for steel abrasive use. Operators may also wear a specially-designed blast suit.
Abrasive grit blasting, also known as abrasive blasting or sandblasting, is a common procedure in a wide range of industries. Using different blast media for different applications and results, blasting can be controlled manually or automated. Blasting equipment varies in complexity from simpler portable systems to whole blast rooms fitted with additional features.
The blasting material or media can be propelled against a surface using fluids (for wet blasting), vapour streams, compressed air or by a centrifugal blast wheel.
Blasting is used to clean surfaces (for example, prior to welding or coating), to smooth rough surfaces, roughen smooth surfaces, provide a desired finish or etch into materials, and for otherwise shaping a surface.