19.57
Unknown
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. A pressurized fluid, typically air, or a centrifugal wheel is used to propel the blasting
material (often called the media). The first abrasive blasting
process was patented by Benjamin Chew Tilghman on 18 October 1870.[1]
There are several variants of the process, such as bead blasting,
sand blasting, sodablasting,
and shot blasting.
Types
One of the original pioneers of the wet abrasive (vapormatting)
process was Norman Ashworth who found the advantages of using a wet
process a strong alternative to sandblasting - which was banned in the 1950s.
Common features include: the ability to use extremely fine or
coarse media with densities ranging from plastic to steel, the ability to use hot water and soap to allow simultaneous
degreasing and blasting, elimination of dust—so silicacious materials can be
used without worry, hazardous material or waste can be removed without
danger—e.g., removal of asbestos, radioactive, or other poisonous products from components
and structures leading to effective decontamination.
The process is available in all conventional formats including
hand cabinets, walk-in booths, automated production machinery and total loss
portable blasting units.
Process speeds can be as fast as conventional dry sand blasting
when using the equivalent size and type of media. However the presence of water
between the media and the substrate being processed creates a lubricating
cushion that can protect both the media and the surface from excess damage.
This has the dual advantage of lowering media breakdown rates and preventing
impregnation of foreign materials into the surface. Hence surfaces after wet
blasting are extremely clean, there is no embedded secondary contamination from
the media or from previous blasting processes, and there is no static cling of
dust to the blasted surface. Subsequent coating or bonding operations are
always better after wet blasting than dry blasting because of the cleanliness
levels achieved. The lack of surface recontamination also allows the use of
single equipment for multiple blasting operations—e.g., stainless steel and carbon (mild) steel items can be
processed in the same equipment with the same media without problems.
Bead blasting
Bead blasting is
the process of removing surface deposits by applying fine glass beads at a high
pressure without damaging the surface. It is used to clean calcium deposits
from pool tiles or any other surfaces, and removes embedded fungus and
brighten grout color. It is also used in auto body work to remove paint.
Wheel blasting
In wheel blasting, a wheel uses centrifugal force to propel the
abrasive against an object. It is typically categorized as an airless blasting
operation because there is no propellant (gas or liquid) used. A wheel machine
is 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, and 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. The first blast wheel was patented by Wheelabrator in
1932.[2]
Hydro-blasting
Hydro-blasting, commonly known as water blasting, is commonly used
because it usually requires only one operator. In hydro-blasting, 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 into places that are difficult to reach using other
methods. Another benefit of hydro-blasting is the ability to recapture and
reuse the water, reducing waste and mitigating environmental impact.
Micro-abrasive blasting
Micro-abrasive blasting is dry abrasive blasting process that uses
small nozzles (typically 0.25 mm to 1.5 mm diameter) to deliver a
fine stream of abrasive accurately to a small part or a small area on a larger
part. Generally the area to be blasted is from about 1 mm2 to
only a few cm2 at most. 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.[citation needed] The abrasive media
particle sizes range from 10 micrometres up to about 150 micrometres. Higher
pressures are often required.
The most common micro-abrasive blasting systems are commercial
bench-mounted units consisting 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.
Automated blasting
Automated blasting is simply the automation of the abrasive blasting process.
Automated blasting is frequently just a step in a larger automated procedure,
usually involving other surface treatments such as preparation and coating
applications. Care is often needed to isolate the blasting chamber from
mechanical components that may be subject to dust fouling.
Dry ice blasting
In this type of blasting air and dry ice is used and with the help of a huge mass
and air pressure, the parent material is cleaned without destroying the
properties of the parent material. The dry icesublimates, leaving no residue to clean up.
Bristle blasting
Bristle blasting, unlike other blasting methods, does not require
a separate blast media. The surface is treated by a brush-like rotary tool made of dynamically tuned high-carbon steel wire bristles. Repeated contact with the
sharp, rotating bristle tips results in localized impact, rebound, and crater
formation, which simultaneously cleans and coarsens the surface.
Equipment
Device used for adding sand to the compressed
air (top of which is a sieve for adding the sand)
Portable blast equipment
Mobile dry abrasive blast systems are typically powered by a
diesel air compressor. The air compressor provides a large volume of high
pressure air to a single or multiple "blast pots". Blast pots are
pressurized, tank-like containers, filled with abrasive material, used to allow
an adjustable amount of blasting grit into the main blasting line. The number of
blast pots is dictated by the volume of air the compressor can provide. Fully
equipped blast systems are often found mounted on semi-tractor trailers, offering high mobility and easy transport from site to site.
Others are hopper-fed types making them lightweight and more mobile.
In wet blasting, the abrasive is introduced into a pressurized
stream of water or other liquid, creating a slurry. Wet blasting is often used in applications where the minimal
dust generation is desired. Portable applications may or may not recycle the
abrasive.
Blast cabinet
A sand-blasting cabinet
A blast cabinet is essentially a closed loop system that allows
the operator to blast the part and recycle the abrasive. It usually 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, 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 systems typically used in a blast cabinet. Two,
siphon and pressure, are dry and one is wet:
·
A siphon blast system
(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. The
abrasive mixture travels through a nozzle that directs the particles toward the
surface or workpiece.
Nozzles come in a variety of shapes, sizes, and materials. Tungsten carbide is the liner material most often used for
mineral abrasives. Silicon carbide and boron carbide nozzles are more wear
resistant and are often used with harder abrasives such as aluminum oxide.
Inexpensive abrasive blasting systems and smaller cabinets use ceramic nozzles.
·
In a pressure blast
system, 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
system that injects the abrasive/liquid slurry into a compressed gas stream.
Wet blasting is typically used when the heat produced by friction in dry
blasting would damage the part.
Blast room
A blast room is a larger version of a blast cabinet and the blast
operator works inside the room. A blast room includes three of the four
components of a blast cabinet: the containment structure, the abrasive blasting
system and the dust collector. Most blast rooms have recycling systems ranging
from manual 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.
Media
In the early 1900s, it was assumed that sharp-edged grains
provided the best performance, but this was later demonstrated not to be
correct.[3]
Mineral: Silica sand can be used as a type of mineral abrasive.
It tends to break up quickly, creating large quantities of dust, exposing the
operator to the potential development of silicosis, a debilitating lung disease.
To counter this hazard, silica sand for blasting is often coated with resins to
control the dust. Using silica as an abrasive is not allowed in Germany, United Kingdom,Sweden,
or Belgium for this reason.[4]
Another common mineral abrasive is garnet. Garnet is more expensive than silica sand, but if used
correctly, will offer equivalent production rates while producing less dust and
no safety hazards from ingesting the dust. Magnesium sulphate, or kieserite, is often used as an alternative to baking soda.
Agricultural:
Typically, crushed nut shells or fruit kernels. These soft abrasives are used
to avoid damaging the underlying material such when cleaning brick or stone,
removing graffiti,
or the removal of coatings from printed circuit boards being repaired.
Synthetic:
This category includes corn starch, wheat starch, sodium bicarbonate, and dry ice.
These "soft" abrasives are also used to avoid damaging the underlying
material such when cleaning brick or stone, removing graffiti, or the removal
of coatings from printed circuit boards being repaired. Sodablasting uses
baking soda (sodium bicarbonate) which is extremely friable, the micro fragmentation on impact exploding
away surface materials without damage to the substrate.
Additional synthetic abrasives include process byproducts
(e.g., copper slag, nickel slag, and coal slag), engineered abrasives (e.g., aluminum oxide, silicon carbide or carborundum, glass beads, ceramic
shot/grit), and recycled products (e.g., plastic abrasive, glass grit).
Metallic: Steel shot, steel
grit, stainless steel shot, cut wire, copper shot, aluminum shot, zinc shot.
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 glow (often invisible in sunlight or brightly lit work areas) from garnet
abrasive.
Safety
Cleaning operations using abrasive blasting can present risks
for workers' health and safety, specifically in portable air blasting or blast
room (booth) applications. Although many abrasives used in blasting rooms are
not hazardous in themselves, (steel shot and grit, cast iron, aluminum oxide, 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 shot scattered on the ground,
exposure to hazardous dusts, heat exhaustion, creation of an explosive atmosphere, and exposure to excessive
noise. Blasting rooms and portable blaster's equipment have been adapted to
these dangers.[citation needed] Blasting lead-based paint
can fill the air with lead particles which can be harmful to the nervous
system.[5]
The Occupational Safety and Health Administration (OSHA) mandates engineered solutions to
potential hazards, however silica sand continues to be allowed even though most
commonly used blast helmets are not sufficiently effective at protecting the
blast operator if ambient levels of dust exceed allowable limits. Adequate
levels of respiratory protection for blast operations in the United States is
approved by the National Institute for
Occupational Safety and Health (NIOSH).
Typical safety equipment for operators includes:
·
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 (or
self-contained oil-less air pump) – 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 monitor/alarm. An alternative method is a
self-contained, oil-less air pump to feed pressurized air to the blast
hood/helmet. An oil-less air pump does not require an air filter or carbon
monoxide monitor/alarm, because the pressurized air is coming from a source
that cannot generate carbon monoxide.
·
Hearing 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 blast suit
(unless blasting with steel abrasives, then they would use a leather suit).
In the past, when sandblasting was performed as an open-air job,
the worker was exposed to risk of injury from the flying material and lung
damage 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.[6]
Sandblasting also may present secondary risks, such as falls
from scaffolding and
absorption of lead particles when removing lead-based paint
from infrastructure.[5]
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.
Worn-look jeans
Many consumers are willing to pay extra for jeans that have the appearance of being used. To give the fabrics
the right worn look sandblasting is used. Sandblasting has the risk of causingsilicosis to the workers, and in Turkey, more than 5,000 workers in the textile industry suffer from silicosis, and 46 people are
known to have died from it. Sweden's Fair Trade Center conducted a survey among 17 textile companies
that showed very few were aware of the dangers caused by manually sandblasting
jeans. Several companies said they would abolish this technique from their own
production.[7]
In 2013, research claimed that in China some factories producing
worn-look jeans are involved in varied non-compliance with health and safety
regulations.[8]
Applications
The lettering and engraving on most modern cemetery monuments and markers is created by
abrasive blasting.
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 flat signs. These signs often incorporate gold leaf overlay and
sometimes crushed glass backgrounds which is called smalts. When
sandblasting wood signage it allows the wood grains to show and the growth rings to
be raised, and is popular way to give a sign a traditional carved look.
Sandblasting can also be done on clear acrylic glass and glazing as part of a store front or interior design.
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 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.
Posted in: blasting
0 komentar:
Posting Komentar