Abrasive blasting is the operation of forcibly propelling a stream of abrasive material
against a surface under high pressure to make it smoother, remove surface contaminants
or to roughen it.
There are several variants of the process, such as bead blasting, sandblasting, shot
blasting and sodablasting.
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."
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 sodablasting.
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.
* 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 to 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.
o 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
+ 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.
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 the high
volume of dust created by the sand breaking when hitting the object.
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 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.
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 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
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 a leather coat and chaps. Professionals would wear a cordura/canvas
Sandblasting used to be 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 would cause silicosis after sustained
inhalation of the 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.
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).
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 is faster and 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.
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.
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
Other materials for sandblasting have been developed to be used instead of sand;
for example, carborundum grit,steel shot and grit, copper slag, powdered slag, glass
beads (bead blasting), metal pellets, dry ice, garnet, 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.
Many coarser media used in sandblasting often result in energy being given off as
sparks or light on impact. The colours and size of the spark or glow varies significantly,
with heavy bright orange sparks from steel shot blasting, to a faint blue glue often
invisible in daylight from garnet grit.
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
Sandblasting techniques are used for cleaning boat hulls, as well as brick, stone
and concrete work. Sandblasting is used for cleaning industrial as well as commercial
structures, but is rarely used for non-metallic workpieces.