Abrasive blasting is a technique that has been in existence since 1870 when the first abrasive blasting process was patented by Benjamin Chew Tilghman.
Abrasive blasting is the operation of forcibly propelling abrasive material against a surface under high pressure in order to achieve the following:
- Smoothing of a rough surface
- Roughening of a smooth surface
- Shaping of a surface
- Removal of surface contaminants
To propel the material, pressurized fluid or air, a centrifugal wheel is used. Today, there are many variations to this process, such as sandblasting, sodablasting, shot blasting, and bead blasting.
The key features of wet abrasive blasting include the ability to use very fine or coarse media with densities ranging from plastic to steel and the ability to use hot water and soap to enable synchronous degreasing and blasting to eliminate dust and remove hazardous material or waste (asbestos, radioactive, or other poisonous products) without danger to surfaces and structures, thereby achieving effective decontamination.
Wet abrasive blasting process can be applied using all traditional blasting formats such as walk-in booths, hand cabinets, automated production machinery and total loss portable blasting units. Process speeds can be varied according to the requirements. For example, it can be performed as fast as conventional dry sand blasting while using similar size and type of media. Surfaces are not damaged as water acts as a buffer between the media and the surface. Thus, wet abrasive blasting has twin advantages of decreasing media breakdown rates and preventing foreign materials from sticking to the surface. Coating or bonding operations that are performed after wet blasting are much better than after dry blasting based on the level of cleanliness.
Wet blasting techniques include wet abrasive blasting, high-pressure water blasting, high-pressure water and abrasive blasting, and air and water abrasive blasting. The type of wet blasting method is chosen according to the application.
Most wet blast devices are of the cabinet mounted type that can be modified by using take-off conveyors, auxiliary strippers, and wash-rinse dry stations. The wet blast device usually contains the following parts:
- Vertical wheel
- Horizontal plane turntable
- Shuttle with rail/car extensions
- Chain/belt conveyors
- Self-contained turning mechanisms for shafts and tubular parts
- Combination tumbling-cabinet machines.
How Does Wet Abrasive Blasting Work?
There are several variations of wet abrasive methods, however the two most commonly used ones are highlighted below:
- Hydro-blast process: Sand is mixed with water and propelled by water pressure
- Vapour-blast process: The abrasive is suspended in a liquid projected at high velocity by a forceful flow of compressed air
Wet blasting involves high-velocity compressed air propulsion of a slurry towards a surface. The slurry consists of fine abrasive mixed in chemically treated water, and is continuously agitated to prevent the abrasive from settling. As most wet abrasive blasters mix water with the abrasive, this causes the surface cleaning rate to be lower than that of dry abrasive blasting. To rectify this issue, a retrofit device has been developed to fit over the end of conventional abrasive blast nozzles.
The compressed air is sent into a vessel containing the mixture at a pressure equal to that fed to a blast hose. The reason for equalizing the pressure is to enable the abrasive mixture to be fed via a mixing valve into an air line. The flow of abrasive is controlled by a full flow valve located between a hopper and a mixing chamber.
The three ways the slurry can be propelled against a surface are as follows:
- By a flow of compressed air to raise the slurry via a siphoning action and then project it via a specially designed nozzle
- By a flow of compressed air to propel a gravity-fed slurry to a gun
- By use of a high-pressure centrifugal pump to produce the needed speed of projection for the slurry
The swirl chamber and the exit nozzle are important parts of the wet blasting device. The swirl chamber is provided with a tangential water inlet, so that the incoming water swirls via the inside of the chamber and then out the exit nozzle. The centrifugal force makes the water to form a hollow cone pattern around the abrasive blast stream. The angle of the water cone is controlled by the shape of the exit nozzle and centrifugal forces.
After wet blasting, the surface has to be rinsed to remove any used abrasive. It is recommended that the rinse water should contain a rust inhibitor in cases where the blasting water also does. Rinsing is not an easy task as the surface can contain a number of ledges, and water, abrasives, and residues collect in these places. Once the surface is completely dry, coating can be applied.
Applications and Products of Wet Abrasive Blasting
Wet abrasive blasting process can be applied in the following areas:
- Burr removal without damage to substrates
- Surface preparation for hard chrome coatings, bonding coatings, bonding composites, and tin and other tool coatings
- Aviation engine rebuilding
- Etch glass for decorative or functional purposes
- Peen surfaces for a bright, clean appearance
- Automotive overhaul and engine rebuilding
- Clean plastic injection molds
- Hole cleaning for printed circuit boards
Benefits of Wet Abrasive Blasting
Key benefits of wet abrasive blasting are listed below:
- Extremely fine media can be used
- Media impregnation into substrate is not possible
- Deeply cleans irregular surfaces
- Eliminates the need for hand-sanding
- Eliminates the need to use toxic chemicals
- Reduces dust
- Closed cabinet recycles media/slurry
- No heat warping with thin parts
- Final finish is appealing with a satin feel
- Provides an excellent surface for plating
- Provides an excellent surface for bonding
- Holds tight tolerances