Abrasive Blasting
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There are two methods of abrasive blasting, wet and dry, each with their pros and cons; fugitive dust control, however, remains the primary consideration with dry blasting.
Wet Abrasive Blasting
Advantages Over Sandblasting
A wet process offers a distinct advantage over sandblasting whose widespread usage around the world has led to strict workers’ health and safety regulations in several countries, notably in Europe. You can use fine to coarse media with different material types ranging from plastic (PVC), glass (silica) to metal (stainless steel).
Spotless Surfaces
Water / detergent solutions allow effective degreasing and blasting of foreign material that adhere to substrates leaving surfaces spotless with little or no embedded or secondary contamination originating from process materials.
Precision Finishing
Wet blasting is mostly used as a precision finishing operation that involves air-blasted slurries of fine abrasives suspended in chemically treated water. Hand cabinets are typically employed to clean small, delicate workpieces.
Dry Abrasive Blasting / Shot Blasting
Media Types
Media used in dry blasting processes vary day to day among manufacturing plants and are dependent on the specific needs of the customer. Media type ranges from crushed nut shells, mineral ore (aluminum oxide, silica) to metallic shot (aluminum, steel).
Common Applications / Equipment
- Bench-mounted hand cabinet
- Rotary basket
- Rotary table
- Tumbler
- Automated feed booth
- Barrel
- Oscillation unit
- Blast room
Substrate Surfaces And Process Media
Cleaning presents a serious risk for workers’ health and safety. Dust collection equipment selection must consider airborne particulate matter from both the substrate surface and the blast or process media. These materials represent as much as 95% of the collected particulate matter in the dust collector and, as such, dust collection requirements must take into account hazards and risks associated with fire, explosion, corrosion, and/or chemical toxicity.
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Feed Materials
Wheel (Airless) Blasting:
This technique employs a wheel and thereby a centrifugal force to propel the abrasives against an object. As the name implies, it is an airless blasting operation since there is no gas or liquid propellant. It is typically found in heavy production where there is a need for an automated system with little human interaction.
Operation / Efficiency
Structural parts or components are either mechanically conveyed or manually loaded to the blast machine. The size of the equipment, and the number and power of the wheels may vary considerably depending on the parts to be cleaned as well as on the expected result and efficiency of the operation. These machines produce heavy grain loading conditions as a function of the quantity of particulate matter generated.
Direct Pressure:
With precise system controls, this technique of propelling a stream of abrasive material under high pressure gives a uniform surface finish to substrates in a surprisingly short period of time, and is thus well suited for heavy finishing work.
Operation / Efficiency
Examples include removal of heavy mill scale or paint, cleaning hard to reach recessed areas, odd material shapes or large workpieces as well as high-speed production parts. This technique offers a coarser, heavier grain loading condition.
Induction (Suction):
This cleaning technique is used on occasion for light to medium production, where limited space or budget constraints prohibit large scale production. Since some pressure is lost to media suction, this method is not as efficient as the direct pressure technique.
Operation / Efficiency
It is usually employed within cabinet-type blasting units and does not generate high grain loading. However, grain loading may significantly increase should the exhaust come from a combined series of cabinets.
Dust Collection Strategies
Consider locating hoods at a safe, reasonable distance away from the blasting process, but maintain satisfactory removal of particulate matter from the immediate area of the blasting operation.
Dust Loading Conditions Play A Critical Role
Collector dust loading plays a large part in collector behaviour and performance. Even when blast media is not particularly abrasive, such as sharp-edged grains, a large quantity of particulate matter entering at any given time may foul and ultimately obstruct filter elements. Consider the material and working environment, and adjust inlet conditions accordingly to reduce dust loads handled by the collector.
Locate Collection Hoods Away From The Blasting Process
There are two advantages to maintain the collection hood at a distance:
- To avoid clean blast media entering the dust collector; and
- To reduce abrasion within the collector, since only dust should be collected and conveyed within the pneumatic system.
Volumetric Flow Rate Design
An adequate volumetric flow rate to be used is a function of several criteria, namely process equipment type, surface substrate construction / condition, blast media characteristics and any specific customer requirements.
Opportunities for Improvement
Visible clarity
requests define the frequency of system air replenishment for a given volume. They are usually associated with the time it takes the blasting environment to become cluttered with airborne particulate matter and thereby represents a health and safety hazard.
Recommendations
We recommend 60 - 100 cfm/ft2 of floor or ceiling space, and 100 cfm/ft2 of wall surface. Cabinet blasting, on the other hand, requires at least 20 air volume changes / minute.
Dust containment
requests involve cabinet or automated blasting processes. Required air volumes are based on velocities across the open area of the enclosure that will keep fugitive dust emissions at bay.
Recommendations
We typically recommend 500 f/m across cabinet entrances and 200 cfm/ft2 across rotary table openings.
Cross draft velocity
requirements concern blast rooms with collector size based on velocity across a given cross sectional area.
Recommendations
We recommend 150 - 200 f/m across the room and work piece at a reasonably safe distance from the operator.
Warning - blasted substrate surfaces: Bear in mind the hazards and risks associated with fire, explosion, corrosion, and/or chemical toxicity of the blasted material in the design and configuration of your dust collectors.
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