Blasting (abrasive cleaning)
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.

	Additional regulations
	Prevention, SDS, column-model, etc.
	Producers, suppliers
	Optimisation potential
	References

back	List of all database-processes with this method	forward