Abrasive Blasting

What is Abrasive blasting?

Abrasive blasting is the operation of forcibly propelling a stream of abrasive material against a surface under high pressure to smooth a rough surface, roughen a smooth surface, shape a surface, or remove surface contaminants. The first abrasive blasting process was patented by Benjamin Chew Tilghman on October 18, 1870.

There are several variants of the process, such as bead blasting, sandblasting, shot blasting and soda blasting.

Operations:

Abrasive 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 used. Common terms include bead blasting, sandblasting, shot blasting and soda blasting.

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 by 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.
• Automated Blasting: A fully automated blasting provides surface preparation and coating applications under cover to minimize the effects nature can have on prepared steel, with minimum or no impact on the environment.
• 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 a 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.

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. Tungsten carbide is the most popular nozzle liner material for mineral abrasives. Silicon carbide and boron carbide are more wear resistant and are for use with harder abrasives such as Aluminium oxide. Inexpensive abrasive blasting systems and smaller cabinets use a ceramic 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 similarly 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 allow 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 micrometers up to about 150 micrometers 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.

The most commonly 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 abrasive blasting usually consists of but is not limited to a hand-held nozzle that directs a stream of abrasive particles or other material towards a workpiece. The abrasive is mixed with air in a mixing chamber in order to transport it to the nozzle where it is subject to a high-velocity air stream that propels it towards the workpiece.

Dry ice blasting is a form of abrasive blasting, where dry ice, the solid form of carbon dioxide, is accelerated in a pressurized air stream and directed at a surface in order to clean it.

The method is similar to other forms of abrasive blasting such as sand blasting, plastic bead blasting, or soda blasting but substitutes dry ice as the blasting medium. Dry ice blasting leaves no chemical residue as dry ice sublimates at room temperature