The forming of metal by the early armorer, metal craftsman and skilled blacksmith was an art, making everything from shields to swords harder and stronger by cold hammering. Today, the art form has become a controlled mechanical process and effective surface preparation technique. Originally called "shot blasting" by Zimmerli, the process was later termed "shot peening" by Almen. Blasting may be classified in many ways but, most simply put, it can be roughly grouped into two main types—air blasting and centrifugal blasting.

Air blasting uses compressed air to project the blasting material and can be employed in both indoor and outdoor operations. Materials used in air blasting are mainly siliceous sand and other mineral products as well as slag that has been pulverized and sized to the required grain size.

Centrifugal blasting, on the other hand, is a method whereby blades revolving at high speed are hit with blasting material, which is projected onto the surface of the work, creating the impact force. This method is typically employed inside a special blasting cabinet. Centrifugal blasting uses steel shots, steel grits, and other such materials that can be recycled for use in subsequent blasting operations.

Air Blasting Basics
Air blasting is a dry or wet mechanical finishing method that uses compressed air and an abrasive (media). The abrasive is mixed with compressed air in a hose and exits through a nozzle that controls the pressure stream towards the workpiece. The result is a very effective method of cleaning, deburring, deflashing, etching, honing, and peening.

Air blast equipment contains and meters abrasive into a compressed air stream through conveying hoses and nozzles to the workpiece. The part’s surface is eroded by a mass of abrasive particles until it is firm and clean. This process has been widely accepted to remove mill scale, rust, paint and other contaminants, due to its effectiveness in cleaning metals. It has proven to be more efficient than other removal methods such as manual wire brushing, hand sanding, chemical stripping, knife scraping and acid etching, all of which can be very time consuming.

Various abrasives are used in the air blasting process. Selection of the most suitable abrasive for a given application is a major factor in cleaning speed, surface etch and coating adhesion. The trend is to a finer size of abrasive because of increased cleaning speed on new or lightly rusted steel; a coarser size of abrasive is used for more corroded steel or harder-to-clean surfaces. Paint coating manufacturers have found a uniform etch with a clean surface much more effective for coating adhesion than an overly smooth surface of similar cleanliness. It is important to maintain a proper size of abrasive for effective blast cleaning. Respiratory protection is required for the operator and workers in the blast cleaning area to protect from airborne particles of spent abrasive and contaminants removed from the part surface.

In a suction blasting system, a partial vacuum is created to carry the air past the blast media, picking it up and carrying through the hose to the nozzle.

Types of Air Blast Equipment
Suction Blasting is a type of blast equipment that uses suction to move the media from its source to the blast nozzle. Media and air travel through separate hoses and are mixed just prior to passage through the nozzle. Media is then carried to the nozzle under a partial vacuum caused by the air passing by the media source. The recommended blasting pressure for this method is 30-80 psi. Suction blasting is used when a shallow anchor pattern is required. Cleaning speed is approximately 1/3 slower than that of pressure blast cleaning with similar size nozzles. Its use should be limited to touch-up or spot cleaning jobs, where high speed cleaning is not a factor.

Advantages include a low initial investment, low yearly maintenance costs, the fact that it is least expensive system to automate, and a more uniform finish than pressure blasting on large flat areas.

Pressure Blasting uses gravity to feed media directly into the compressed air flow. Both air and media then travel through a single hose to the nozzle. Pressure blasting is the most forceful form of abrasive blasting and is generally used where a deep etch is required. It is three to four times faster than suction blasting.

Pressure blasting uses gravity to mix the media into the compressed air flow.

In a pressure-type abrasive blast system, the abrasive machine is under the same pressure as the entire system (including the abrasive blast hose and nozzle). This cleaning method is the most productive of abrasive blast cleaning. The efficiency is largely dependent on actual nozzle pressure, which should be in the 10- to 100-psi range. The pressure blast machine or "pot" varies in size but must be under pressure for an even flow of abrasives. Velocity of the abrasive in the pressure method is greater than the abrasive velocity found in suction equipment.

Advantages include the fact that it requires less compressed air and labor per unit of area treated, it cleans two to three times faster than a suction system, it operates at as low as 5 psi for delicate work, and blasts deep inside cavities for complex or deep geometries.

Wet Blasting is the "throwing" of a water and abrasive mixture (slurry) through a blasting gun by means of compressed air. Water, as the carrying medium, allows the use of an extremely wide choice of abrasives—quartz, novacite, aluminum oxide, silicon carbide, glass beads—ranging in particle size from 60-5,000 mesh. This process is conducted in a completely enclosed cabinet so that the abrasive-water mixture may be continually reclaimed and re-used. In simplest terms, all wet blasting equipment consists of the overall enclosing cabinet (containing a "sump" where the main slurry of the water and abrasive takes place), plumbing lines of both stainless steel and high pressure hose (for the transmission of both free air and the abrasive-water mix), and operating controls.

In a wet blasting system, water can carry a variety of abrasive media. An enclosed cabinet allows continuous reclamation of the abrasive-water mixture.

In manually operated units, the operator, who inserts his/her arms through glove-ports in the front of the cabinet, holds the process gun. Work is usually admitted through the side or front work-loading doors. Work progress is observed by the operator through a large viewing window equipped with a rinsing device, which keeps it free and clean of abrasive splash. PC

Rosler Metal Finishing USA, a supplier of surface finishing equipment and consumables, offers mass finishing and shot blast equipment, plastic and ceramic grinding and polishing media, compounds for mass finishing, and waste water treatment systems. For information, phone 269-441-3000 or go to rosler.us.