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Dr. Dmitri Kopeliovich
Abrasive blast cleaning is method of the surface preparation by abrasive particles forced by either compressed air or water jet. The particles impact the surface at high velocity and produce the effects of cleaning, roughening and activation.
The parameters of abrasive materials, which should be taken into account for their selection:
Hardness. Hard materials are used when aggressive abrasive actions are required: removal of a significant amount of the contaminants from the surface, producing deep roughness profile.
Particles size. Larger particles produce deeper surface profile, which may be excessive for thin
coatings. Fine particles blasting results in a shallow profile adversely affecting the
adhesion strength of the coating. The size of abrasive particles may vary between the mesh size 10 (0.0787”/2000 μm) to 270 (0.0021”/53 μm).
Particles shape. Sharp particles provide more aggressive cutting action and form deeper surface profile. Blasting of rounded particles creates shallow surface profile.
Blasting method (dry or wet). Wet blasting (slurry blasting) utilizes fine particles forming
suspensions in water (walnut shells, silica,
alumina, glass beads).
Cost. Abrasive cleaning may be performed either outdoors or in a blast cabinet (room). The abrasive materials for outdoors applications can not be re-used. Therefore relatively cheap materials like sand and slag are used outdoors. The abrasive particles operating in confined spaces (cabinet, room) may be re-used multiple number of times therefore more expensive abrasives are commonly used there (alumina, metal shot and grit, glass bead, etc.).
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Silica sand is an inexpensive material used mostly in outdoors applications. Silica has a high breakdown rate causing substantial dust formation. Crystalline silica dust is carcinogenic.
Alumina sand is a
hard and
tough material (low breakdown rate) producing low amount of dust.
Slags (coal slag, copper slag, nickel slag) are byproducts of other processes (coal power plants,
Extractive metallurgy). Crushed slags are used as inexpensive disposable blasting material.
Iron grit is a crushed chilled
cast iron combining moderate cost with high hardness and sharp shape of particles (high rate of breakdown). The particles break in the impact action and form new sharp edges.
Steel shot is a durable and stable abrasive media in form of spherical particles. Steel shot processing is clean (no dust) and consistent. Besides cleaning operation steel shot is used also for
Shot peening, which is a hardening cold work process.
Garnet - crushed and screened minerals combining hardness, relatively high density, moderate cost and low toxicity (low content of crystalline silica).
Glass beads are spherical glass particles producing a smooth surface quality without dimensional changes. Glass beads do not contain crystalline silica and produce no dust.
Plastic particles are used when a gentle cleaning action is required. The particles do not damage the substrate surface.
Walnut shell abrasive is made of crushed walnut shells. It is used mainly for de-burring, polishing and gentle cleaning of small and delicate parts. Walnut shell is also used for graffiti removal.
Corn cobs is made of the hard rings of corn cobs. The properties and the applications of corn cob are similar to those of walnut shell.
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A typical abrasive blasting system consists the following components:
Air compressor produces pressurized air at a certain pressure and flow rate. The air stream produced by the compressor serves as a carrier for the abrasive particles. Commonly the air pressure used for abrasive blasting is about 100 psi (690 KPa). The flow rate is determined by the orifice diameter of the nozzle. The larger the nozzle diameter, the more powerful compressor is required.
Blast pot is a container filled with an abrasive material. The compressed air is supplied to the blast pot where it mixes with the abrasive particles and flows to the nozzle.
Nozzle accelerates the abrasive particles and directs them towards the workpiece surface. A strait barrel nozzle accelerate the stream to 318 ft/sec (97 m/sec). The speed provided by venturi nozzles is 660 ft/sec (200 m/sec). In order to reduce a
wear of the nozzles their surface is coated with a
ceramic (e.g. tungsten carbide,
silicon carbide)
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