Siphon

Siphon machines use an injector gun to create suction on the abrasive supply hose. The abrasive hose is used to convey the mixture of abrasive and suction air to the gun where the compressed air supply line and air jet are used to accelerate the abrasive. All siphon type machines include two hoses attached to the abrasive blast gun assembly.

Siphon machines are the most common type of blast machine found in the market today. All Media Blast siphon machines include the abrasive media-mixing valve for proper adjustment and flow of the blast media to the gun. Consider this feature the same as a mixing control on any painting system; beware of units that claim “No mixing valve necessary”. Machines operating without mixing valves cannot regulate the amount of media being delivered to the gun assembly. Because not all applications use the same abrasive type and or size and siphon systems operate best with a 20-25% abrasive to air mixture, a mixing valve is required. This eliminates the troublesome abrasive surge associated with units having no abrasive mixing valve, often called the “Splotch Blast” or “Splatter Blast” with no abrasive behind each pulsing surge!

Siphon machines operate with less frictional heat and higher operating pressures but it must be stated that almost all abrasives used can reach maximum impact velocity in any siphon system operating at pressures of 80-90 psi.

Wider patterns make siphon systems correct for general surface cleaning on larger open areas and normal parts. Siphon style delivery will create a more uniform surface pattern on the part surface than direct pressure systems. The larger the siphon gun, the more uniform the surface texture.

Easier to operate, less to purchase and reduced maintenance are all reasons to purchase a siphon type delivery model.

Direct Pressure

Direct Pressure machines use a pressure vessel to pneumatically push the blast media through a single abrasive hose and out the blast nozzle. An easy way to understand is the direct pressure systems drop the abrasive into an air hose and pneumatically push the abrasive out through an air jet. These two items are often referred to as abrasive hose and blast nozzle but they are really an air hose and air jet due to the direct pressure delivery configuration.

Direct delivery systems concentrate the abrasive which creates greater frictional heat in any one area. Because the direct pressure system pneumatically pushes the blast media, it is possible to clean into blind holes and pockets. The nozzle can often be held against the part, example cross-drilled crankshaft, and the part simply becomes the nozzle allowing the pneumatic push of the direct pressure system to clean the ID.

All direct pressure machines double the particle velocity over the siphon systems at any given pressure. Processing with the direct pressure system requires lowering the blasting pressure by 50% over siphon to prevent exceeding the maximum abrasive impact velocity. Because of the concentrated pattern, the direct pressure system operates with more frictional heat on the part. Often advertisements indicate that this system is faster but keeping the compressed air usage the same and the abrasive velocity the same, the result is almost identical pounds per minute delivery to the part being cleaned. Taking pot refill into consideration and the smaller pattern, the siphon system with continuous run time can often out perform the direct pressure model.

Direct pressure models do something that siphon can’t and the applications listed below are reasons the choice would be direct pressure.

• Heavy abrasives or large abrasives, steel shot for shot peening etc.
• Heavy thick coatings that require additional frictional heat on the surface to be removed
• Blind holes or deep pocket cleaning
• Maximum abrasive velocity using maximum available compressor line pressure