Abrasive blasting

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Abrasive blasting is the operation of cleaning or preparing a surface by forcibly propelling a stream of abrasive material against it. Usually explained as the use of a material against another material to make it smoother, remove surface contaminants or to roughen a surface. It is also the appropriate term for what is known as glass bead blasting, sandblasting, sand carving, sodablasting or shot blasting.

Contents 1 Surface cleanliness and roughness 2 Operations 3 Abrasives 4 Setup and Equipment 5 Safety 6 Sand blasting 6.1 Media 6.2 Applications 7 References 7.1 Bibliography

[edit] Surface cleanliness and roughness

Abrasive-blasted surfaces are characterised by two kinds of information that are both essential but fundamentally different, although sometimes mistaken for each other: cleanliness and roughness.

Cleanliness reflects the degree of presence of undesirable residual contaminants on the surface, whereas roughness refers to the micrometric shape of the surface, called the surface profile.

[edit] Operations

Abrasive blasting: Abrasive blasting or air blasting is a method of propelling abrasive using a compressed gas (typically air) or pressurized liquid (typically water) as the propellant. There are numerous generic terms for this application usually related to the abrasive media in use; common terms include sand blasting, shot blasting, grit blasting, bead blasting and blast cleaning.

The large variety of applications creates the need for a diversity of equipment:

• Portable blast equipment (aka blast pot): Dry abrasive blasting applications are typically powered from a diesel air compressor. Most applications involve a pressurized vessel that contains the abrasive and meters it into the compressed air stream. Wet blasting is accomplished by injecting the abrasive into a pressurized water stream or creating a slurry of abrasive and water that is pressurized or introduced into a compressed air stream. Wet blasting is often used in applications where the minimal dust generation is desired. Portable applications may or may not recycle the abrasive and portable dry blasting generally does not attempt to contain or minimize the dust generated from the operation.

• Blast cabinet: A blast cabinet is essentially a closed loop system that allows the operator to blast the part and recycle the abrasive. A typical blast cabinet consists of four components; the containment (cabinet), the abrasive blasting system, the abrasive recycling system and the dust collection. The operator blasts the parts from the outside of the cabinet by placing his arms in gloves attached to glove holes on the cabinet, viewing the part through a view window and, typically, turning the blast on and off using a foot pedal or treadle. Automated blast cabinets are also used to process large quantities of the same component and may incorporate multiple blast nozzles and a part conveyance system.
  • There are three types of blast systems used in a blast cabinet. Two systems (siphon and pressure) are dry and one is wet:
    • A siphon blast system (aka suction blast system) uses the compressed air to create vacuum in a chamber (known as the blast gun). The negative pressure pulls abrasive into the blast gun where the compressed air directs the abrasive through a blast nozzle.
    • A pressure blast system incorporates a pressurized vessel. The abrasive is stored in the pressure vessel then sealed. The vessel is pressurized to the same pressure as the blast hose attached to the bottom of the pressure vessel. The abrasive is metered into the blast hose and conveyed by the compressed gas through the blast nozzle.
    • Wet blast cabinets use a slurry system that injects the slurry into a compressed gas stream. Wet blasting is typically used to create a surface profile when the frictional heat of dry blasting would damage the part.

• Blast room: This is a larger version of a blast cabinet with the exception that the blast operator works inside the room. A blast room always has three of the four components of a blast cabinet: the containment, the abrasive blasting system and the dust collector. Most blast rooms have recycling systems ranging from manual recycling (sweeping and shoveling the abrasive back into the blast pot) to full reclaim floors that convey the abrasive pneumatically or mechanically to a device that cleans the abrasive prior to recycling.

Wheel blasting: Wheel blasting or shot blasting is typically categorized as an airless blasting operation because there is not a propellant (gas or liquid) used to propel the abrasive. Rather, a centrifugal wheel is used to propel the abrasive against the substrate. Wheel machines are a high-power, high-efficiency blasting operation with recyclable abrasive (typically steel or stainless steel shot, cut wire, grit or similar sized pellets). Specialized wheel blast machines propel plastic abrasive in a cryogenic chamber; this type of wheel blasting is usually used for deflashing plastic and rubber components. The size of the wheel blast machine, and the number and power of the wheels vary considerably depending on the parts to be blasted as well as on the expected result and efficiency.

Hydro-blasting: Hydro-blasting, commonly known as water blasting, is a common abrasive blasting operation because it is very effective and, in most cases, will only require one operator. Hydro-blasting is the process by which a highly pressured stream of water is used to remove old paint, chemicals, or buildup without damaging the original surface. This method is ideal for cleaning internal and external surfaces because the operator is generally able to send the stream of water in places that previously were deemed unreachable. A major benefit of hydro-blasting is the ability to recapture and reuse the water, thus eliminating waste and the impact on the environment.

Micro-abrasive blasting: A dry abrasive blasting process. Typically pressure blast systems as this allows the feed of abrasive media to be controlled independently of blast pressure but some systems are siphon blasters. Micro-abrasive blasting uses smaller nozzles (typically 0.25 mm to 1.5 mm diameter) to provide a fine stream of abrasive accurately to either a small part (mm size) or a small area on a larger part. Generally the area to be blasted is from about 1 mm to only a few cm at most as abrasive blasters with larger nozzles are faster for larger areas. Micro-abrasive blasting uses media with particle sizes from 10 micrometres up to about 150 micrometres and usually higher pressures than most of the larger blasters 40 psi (300 kPa) to 150 pounds per square inch (1,000 kPa) deliver sufficient energy to these small particles. The abrasive media is generally not recycled as the particles either shatter on impact or lose their sharp edges. Also known as pencil blasting, the fine jet of abrasive is accurate enough to write directly on glass and delicate enough to cut a pattern in an eggshell.

[edit] Abrasives

Silica sand: Silica sand is the most commonly used abrasive and typically has the most negative impact. The silica sand quickly breaks up and creates large quantities of dust, which when exposed to the operator can cause Silicosis, a debilitating lung disease. When silica sand is sold for the purpose of blasting, it is often coated with resins to control the dust from the breaking sand. Note: Most larger silica sand companies will not sell sand for the purpose of sandblasting due to high volume of dust creating by the sand breaking when hitting the object. i.e. U.S. Silica Co.

Coal or smelter slag: Coal or smelter slag contains a low silica content and therefore does not cause Silicosis. However, these abrasives have been reported to release other Hazardous Air Pollutants (HAP's) into the air.

Metallic, synthetic, and mineral: These other types of abrasives are growing in popularity due to the low dust creation and reclamation of material. Each of these abrasives report significantly less dust and waste than sand and coal/slag abrasives.

Organic: Typically ground nut shells, fruit kernels, or baking soda. These soft abrasives are used for cleaning of brick or stone work or the removal of graffiti without damaging the underlying material. Also used for the removal of conformal coatings from printed circuit boards for rework. Additionally, organic media such as nut shells and ground corn cob are often used for removing failing finish from log homes.

[edit] Setup and Equipment

The most common used are commercial bench-mounted units. The setup consists of a power supply and mixer, exhaust hood, nozzle, and gas supply. The nozzle can be hand-held or fixture mounted for automatic operation. Either the nozzle or part can be moved in automatic operation.

Equipment for sandblasting usually consists of but is not limited to a hand-held nozzle that directs a stream of sand particles or other material toward a workpiece. The sand is mixed with air in a mixing chamber in order to transport the sand to the nozzle where it is subject to a high velocity air stream that propels it toward the workpiece.

[edit] Safety

Cleaning operations using abrasive blasting can present risks for workers' health and safety, especially in air blasting applications. Although many abrasives used in blasting booths do not have a hazardous component (steel shot and grit, cast iron, aluminum oxide [aka corundum], garnet, plastic abrasive and glass bead), other abrasives (silica sand, copper slag, nickel slag, and staurolite) have varying degrees of hazard (typically free silica or heavy metals). However, in all cases their use can present serious danger to operators, such as burns due to projections (with skin or eye lesions), falls due to walking on round shots scattered on the ground, exposure to hazardous dusts, heat exhaustion, creation of an explosive atmosphere, and exposure to excessive noise. Blasting booths and portable blaster's equipment have been adapted to these dangers. OSHA (Occupational Safety and Health Administration) mandates engineered solutions to potential hazards, however lobbying has allowed silica sand to continue to be used even though most commonly used blast helmets are not sufficiently effective at protecting the blast operator (respiratory protection is approved by NIOSH - National Institute for Occupational Safety and Health).

Typical safety equipment for operators that are open air blasting or blasting in a blast room include the following:

• Positive pressure blast hood or helmet - The hood or helmet includes a head suspension system to allow the device to move with the operator's head, a view window with replaceable lens or lens protection and an air feed hose.
• Grade D air supply - The air feed hose is typically attached to a grade D pressurized air supply. Grade D air is mandated by OSHA to protect the worker from hazardous gases. It includes a pressure regulator, air filtration and a carbon monoxide alarm.
• Ear protection - ear muffs or ear plugs.
• Body protection - Body protection varies by application but usually consists of gloves, and overalls or apron (minimal), leather coat and chaps, or cordura/canvas blast suit.

Originally, sandblasting was always performed as an open-air job, which exposed the worker to risk of injury from the flying material and lung injury from inhaling the dust. The silica dust produced in the sandblasting process caused silicosis after sustained inhalation of dust.

In 1918 the first sandblasting enclosure was built, which protected the worker with a viewing screen, revolved around the workpiece, and used an exhaust fan to draw dust away from the worker's face.^[1]

Several countries and territories now regulate sandblasting such that it may only be performed in a controlled environment using ventilation, protective clothing and breathing air supply (as shown in the top image).

[edit] Sand blasting

Sandblasting or bead blasting is a generic term for the process of smoothing, shaping and cleaning a hard surface by forcing solid particles across that surface at high speeds; the effect is similar to that of using sandpaper, but provides a more even finish with no problems at corners or crannies. Sandblasting can occur naturally, usually as a result of particles blown by wind causing aeolian erosion, or artificially, using compressed air. An artificial sandblasting process was patented by Benjamin Chew Tilghman on October 18, 1870.

Sandblasting equipment typically consists of a chamber in which sand and air are mixed. The mixture travels through a hand-held nozzle to direct the particles toward the surface or workpiece. Nozzles come in a variety of shapes, sizes, and materials. Boron carbide is a popular material for nozzles because it resists abrasive wear well.

[edit] Media

Historically, the material used for artificial sandblasting was sand that had been sieved to a uniform size. In the early 1900s, it was initially assumed that sharp-edged grains provided the best performance though this was later demonstrated to not be correct.^[2]

Other materials for sandblasting have been developed to be used instead of sand; for example, carborundum grit, steel shots, copper slag, powdered slag, glass beads (bead blasting), metal pellets, dry ice, garnet^[3], powdered abrasives of various grades, and even ground coconut shells, corncobs, walnut shells, and baking soda (sodablasting) have been used for specific applications and can produce distinct surface finishes. Some commercial grade blasters are specially designed to handle multiple blast abrasives. These blasters are commonly referred as multi-media blasters.

[edit] Applications

Sandblasting can also be used to produce three dimensional signage. This type of signage is considered to be a higher end product as compared to the flat signs. These signs often incorporate gold leaf overlay and sometimes crushed glass backgrounds which is called smalts.

Sandblasting can be used to refurbish buildings or create works of art (carved or frosted glass). Modern masks and resists facilitate this process, producing accurate results.

Sandblasting technique is used for cleaning boat hulls, bricks, and concrete work. Sandblasting which is also known as blast cleaning is used for cleaning industrial as well as commercial structures. However, sandblasting is rarely used for nonmetallic workpieces.

[edit] References

[edit] Bibliography

• Manufacturing Processes Reference Guide by Robert H. Todd, Dell K. Allen, and Leo Alting--1st ed.