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Home > Parts cleaning > Processes > Blasting / CO2 dry ice blasting
Blasting
/ CO2 dry ice blasting
Abrasive cleaning uses small sharp particles propelled by an
air stream or water jet to impinge on the surface, removing
contaminants by the resulting impact force. A wide variety of
abrasive media in many sizes is available to meet specific needs.
Abrasive cleaning is often preferred for removing heavy scale
and paint, especially on large, otherwise inaccessible areas.
Abrasive cleaning is also frequently the only allowable cleaning
method for steels sensitive to hydrogen embrittlement. This
method of cleaning is also used to prepare metals, such as stainless
steel and titanium, for painting to produce mechanical lock
for adhesion because conversion coatings cannot be applied easily
to these metals.
Both pressure blast and suction blast nozzle systems require
high power to generate the compressed air or pressurised steam
that is used to accelerate and propel the abrasive. Pressure
blast nozzle systems generally rely on a 685 kPa (100 psig)
air supply to propel abrasive through a special nozzle. Airblast
nozzles are used in a variety of shapes. Most systems have replaceable
nozzles of metal alloys or nozzles with wear-resistant ceramic
inserts. All types of abrasive may be handled with the pressure
blast system in a variety of environments.
Generally considered the simplest form of abrasive blast equipment,
suction blast cabinets may be used manually or have fixed or
oscillating nozzles. The nozzle in the suction cabinet is an
induction nozzle that creates a blasting mixture by the siphon
effect of the air discharged through the nozzle body.
Airless abrasive propelling wheels that use blades or vanes
require about 10 % of the horsepower required by air blast systems
to throw equal volumes of abrasive at the same velocities. The
life of the wearing parts of the blast wheel assembly (impeller,
control cage, wheel blades, housing liners) is influenced primarily
by the type and condition of the abrasive medium and contaminants
picked up in the cleaning process. Clean steel shot provides
the longest useful life of wheel and guarding housing liners.
Much greater wear results from the use of non-metallic abrasives
such as sand, aluminium oxide, and silicon carbide. Relatively
little wear is caused by glass beads and nonferrous shot.
Centrifugal wheel -type blast machines may be relatively simple,
having a single blast wheel, a simpler work conveyor, an abrasive
recycling system, and a dust collection device.
DRY-BLAST CLEANING
Virtually all metals can be cleaned by at least one of the available
abrasive blasting processes, but the abrasive medium must be
carefully selected for soft, fragile metals and their alloys,
such as aluminium, magnesium, copper, zinc, and beryllium.
Stationary equipment for dry blast cleaning
Dry blast cleaning is probably the most efficient and environmentally
effective method for abrasive cleaning and finishing - proper
ventilation helps maintain a clean work area, dust collectors
provide simple dust disposal. To enusre adequate ventilation
of abrasive blast cabinets, a fabric filter dust collector is
generally used with properly designed duct work. The fabric
filters are generally equipped with exhaust fans on the clean-air
side of the dust collector.
Several types of equipment are available for dry blast cleaning,
and equipment selection is primarily based on the type of parts
to be blasted and the relative throughput required:
a) Cabinet machines: a cabinet houses the abrasive-propelling
mechanism, holds the work in position, and confines flying abrasive
materials and dust. Cabinet machines may be designed for manual,
semiautomatic, or completely automated operation to provide
single-piece, batch, or continuous-flow blast cleaning.
b) Continuous-flow machines: equipped with proper supporting
and conveying devices, continuous-flow machines are used for
continuous blast cleaning of steel strip, coil and wire. These
machines are also used to clean castings and forgings at a high
production rate. Continuous-flow machines incorporate abrasive
recycling facilities and an exhaust system for removing dust
and fines.
c) Blasting-tumbling machines: those consist of and enclosed
endless conveyor, a blast-propelling device or devices, and
an abrasive recycling system. These machines simultaneously
tumble and blast the work. As the conveyor moves, it gently
tumbles the work and exposes all workpiece surfaces to the abrasive
blast. This equipment is not used for cleaning parts after machining,
because tumbling damages machined surfaces.
Portable equipment for dry blast cleaning
When parts to be cleaned are too large to be placed in blasting
machines, portable equipment, such as air blast equipment, can
be brought to the workpiece. A low-cost sand usually is used,
because it is difficult to reclaim or recirculate the abrasive
with portable equipment.
Portable recycling equipment is a new development in air pressure
blasting. This equipment uses a pressurized media hose contained
within a larger, evacuated hose. After impact, the media are
returned through the outer hose to the central unit for reclaiming
and recycling. With this equipment, large external jobs may
be done with specialized media without environmental problems.
Microabrasive blasting is another portable air blasting method.
Both the abrasive particle size (10 to 100 µm) and nozzle openings
(0,4 to 1,2 mm in diameter) are very small. Microabrasive blasting
is normally a hand-held operation for precision deburring, cleaning,
or surface preparation. Dryness and uniformity of particle classification
are very critical, and abrasives cannot be reused.
WET BLASTING
Wet blasting differs from dry blasting in that the abrasive
particles used are usually much finer and are suspended in chemically
treated water to form a slurry. The slurry, pumped and continually
agitated to prevent settling, is forced by compressed air through
one or more nozzles, which are directed at the work.
In further contrast to dry blasting, wet blasting is not intended
for the gross removal of heavy scale, coarse burrs, or soil,
but is intended to produce only relatively slight effects on
the workpiece surface. Many small parts, including hypodermic
needles and electronic components, are deburred by wet blasting.
In most instances, pre-cleaning must precede wet blasting to
prevent contamination of the recirculating slurry. Pre-cleaning
methods include conventional degreasing methods, heavy rust
and dry soil may be removed by dry blasting.
Many different kinds and sizes of abrasives can be used in wet
blasting. Sizes range from 20-mesh (very coarse) to 5000-mesh
(ultrafine). Among the types of abrasives used are organic or
agricultural materials, baking soda, silica, quartz, aluminium
oxide, etc.
The liquids most commonly used to carry the abrasive particles
are water-containing additives such as rust inhibitors, wetting
agents, and anticlogging and antisettling compounds. In a few
applications, petroleum distillates have been used as abrasive
carriers for removal of oil residues and fine chips or burrs.
Petroleum distillates, however, can be used only with specially
designed wet blasting units because of the fire hazard.
Although equipment for wet blasting is often of special design
for a particular application, several basic types of machines
have been developed for general use, including:
- cabinet-type machines;
- horizontal-plane turntable machines with tables of various
diameters;
- vertical wheel-type machines;
- chain or belt conveyor machines;
- shuttle-type cabinets with cars and rail extensions;
- car-mounted, self-contained turning mechanisms for shafts
or tubular parts.
These basic types may be equipped with strippers, takeoff conveyors,
and wash-rinse-dry stations.
CO2 dry ice blasting
A form of abrasive blasting, in which hard pieces of frozen
CO2 pellets are shot at a surface with air or other gases. CO2
pellets can strip paints and remove grease and oil. Some parts
may be sensitive to thermal changes from the pellets and should
be tested first. Thin parts may be damaged by pellet impact.
The pellets can be generated in different sizes and propelled
at various speeds to increase cleaning rates or reduce surface
damage.
Alternatively, soft "snow flakes" of frozen CO2 gas to clean
surfaces are used. CO2 snow is very effective at removing particles.
It has been used for removing small particles from optical components,
gyroscopes, thin film mirrors, and other delicate surfaces.
Some sources have reported success in removing thin fluid layers,
flux, and fingerprints. It will not remove rust, paint, greases,
or heavy oil layers. The process is best suited for line-of-sight
cleaning.
The advantage of the CO2 pellets and snow is that they sublimate
on contact with the material to be cleaned. This leaves the
operator with only the soil to dispose of.
Safety considerations include ventilation and protection of
people from extended contact with the cold snow. Safety glasses
also should be worn.
Noise levels may reach between 60 and 120 db. Meanwhile special
nozzles have been developed achieving reductions of ca. 20 db.
For related laws and regulations see under 'Blasting media'
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