Abrasive Blasting 101
Updated January 2022: The first dry blasting machines relied on siphon abrasive delivery method which uses an abrasive media gun to siphon, or pull, the abrasive to the blasting nozzle where it is then boosted in particle velocity and injected into the cabinet. Think of this like using a cue ball to hit the eight ball into the side pocket. While it gets the job done, some energy is lost when the two balls come in contact with each other. Similarly, while siphon delivery equipment is easy to make and simple to operate, it requires higher blasting pressures to achieve maximum abrasive impact velocity.
Dry blasting cabinets utilizing direct pressure abrasive delivery developed later. These machines use a pressure vessel or pot to pneumatically push the abrasive to the nozzle. With direct pressure, the abrasive has no delivery weight so it travels faster and faster inside the abrasive hose until it passes out the nozzle office at a much faster speed than siphon or compressed air quantity to orifice speed. To keep most abrasives from exceeding maximum impact velocity, the pressure is reduced by 50-percent compared to the siphon blaster delivery method. Because direct pressure has a more focused patter it can create higher frictional heat than siphon systems and delivers abrasive at about twice the speed of siphon delivery methods. Cabinets using direct pressure operate on less compressed air and produce more frictional heat than siphon types. This allows direct pressure to do things siphon is unable to do. Because direct pressure provides a more focused pattern of abrasive delivery it is better for removing heavy coatings, can push heavy steel shot and grits the siphon system can’t deliver and the direct pressure can push abrasive through drilled holes by using the part as the blasting nozzle. A siphon can’t continue abrasive delivery when the nozzle is held close to the part surface or against a drilled hole.
Why two types of abrasive cabinets? Think of Siphon and Direct Pressure machines as the tortoise and hare respectively. The Siphon, like the tortoise, is slower, but the abrasive continuously cycles through the machine. The Direct Pressure machine, like the hare, is much faster, but the pressure pot needs to re-pressurize and cannot run continuously.
So if Siphon delivery is slower, why do most automated machines use that abrasive delivery method? Because the slower siphon is actually faster when automated because it never needs to stop and re-pressurize. There are some Direct Pressure units that allow pressure pots to run without stopping, but they result in a brief moment where no abrasive is delivered – something that is unacceptable during automation. Click here to learn which abrasive delivery is right for you!