Mechanical Finishing Clinic: Pressure Blasting for Heavy Mill Scale Removal
Q. We are blasting heavy mill scales off of medium-sized parts in our suction blast cabinet with 80-grit aluminum oxide. We want to continue to blast by hand in a cabinet but would like to be more efficient. Can you suggest a quicker process?
A. Change your blast delivery system to a pressure pot blast cabinet to deliver the media at higher velocities, and also change your media to a heavier 36- to 60-grit aluminum oxide. Air blasting utilizes air compressor energy to deliver air/media mix at high velocities and large volumes, impacting the part being processed. There are two types of air blast media delivery systems: suction and pressure.
Suction blasting uses a venturi principle of siphoning media from a hopper. The air jet is ½ the ID of the nozzle. As the air stream is passed through both, it creates a low pressure. This siphons the media from a hopper up and into the air stream. The media acceleration distance is approximately 4 to 12 inches from the nozzle to the workpiece. The suction systems work well and can be blasted continuously as long as blasting media is in the hopper. Suction systems do not deliver media well at low pressures (5 to 15 psi), with limits on the length of the suction feed hose. Large, heavy blasting medias are difficult to convey into the air stream with suction blasting. Most industrial blast cabinets are suction systems and work with most medias.
Pressure blasting utilizes various sizes of ASME-approved pressure vessels, called pressure blast pots that contain the media, and as it is energized with compressed air, it pressurizes the pot. When the air/media mix is released from the pot, it accelerates from the pot through at least 5 to 10 feet of hose and then even faster as it travels through the venturi of the nozzle. The acceleration rates of air/media mix are much higher (3-4 times) in pressure blasting than suction blasting. When the pressure pot is depleted of the media, it has to be depressurized in order to refill the pot.
Pressure blasting is more productive than suction systems, can blast all medias regardless of weight or size and can deliver medias at low pressures.
Originally published in the November 2016 issue.
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