There are two types of blast media used in abrasive blasting, these being; grit and shot. Grit is angular or subangular abrasive that can either be expendable (single use) or recyclable (multiple uses). Whether the media is expendable or recyclable, is dependent on many factors such as how hard the abrasive is and nozzle velocity, to name a few. Grit typically has a faster cleaning action over shot, due to its angular profile. Upon impact with a surface, grit “cuts” into the substrate contributing to its rapid cleaning action. This process removes more matter but generates more dust. Additionally, while grit is an aggressive abrasive, it cannot resist impact shattering as well as spherical media, thus it is less resilient.
Shot abrasive is a spherical grain that is wholly recyclable, however, the number of times the media can be recycled is dependent on the factors previously mentioned. Instead of cutting into the substrate, shot dimples the surface, working on the mechanism of plasticity rather than abrasion. This process removes less matter from the substrate. While shot and grit abrasive blasting can be used for surface preparation and treatment, shot blasting is also used to “peen” surfaces. Shot peening is the process of using spherical media dimple the surface of any metal or composite to relieve tensile stress and increase compressive strength. Some examples of components that would require shot peening are gear parts, engine blocks, and camshafts.
Both shot and grit can be used in wet and dry abrasive blasting. When wet blasting, the media is mixed with vapor or water to create a slurry. Dependent on the machine, the slurry is then propelled at high velocity by either compressed air or water pressure onto the workpiece. These processes are coined “vapor blast” and “hydro-blast”. To prevent the settling of abrasive inside the machine’s components, it is kept in continuous agitation. “Air” or “Dry” blasting only utilises compressed air to project abrasive onto a surface. In general terms, media travels through a pressurized container into a blast hose and is ejected to the point of operation. Dry abrasive blasting operations can take place in both closed-looped facilities and outdoors. However, wet blasting only takes place outside.
Reclaimable media is used exclusively in closed-loop blasting facilities, these include cabinets, blast booths, and centrifugal wheel systems.
Blast cabinets are a small closed-loop system, where the operator processes small components by manipulating an internal blast nozzle using integrated rubber gloves. The cabinet itself has four modules that make up the system, these being the containment, blast media, a set of media resistant gloves, and the recycling unit. The cabinet also comes with a window where the operator views the component they are processing, and a foot pedal to mediate the flow of the blast nozzle. There are two variants of blast cabinets: suction and direct pressure systems. Direct pressure cabinets operate at higher pressures, while suction systems draw the media into an airstream and blasts at lower pressures.
Blast booths are an upscaled version of a blast cabinet. Within these rooms, operators manipulate a blast hose and nozzle to abrade heavy industrial components. To operate in these rooms staff must be trained in blasting technique and safety. Blast rooms can come in a range sizes and can accommodate very large or uniquely shaped objects, such as rail cars, commercial or military vehicles, construction equipment, ship sections, and aircraft. Like cabinets, blast rooms function on the same premise of the four-module system with some additional features and can be designed to fit bespoke needs.
Also known as an “Airless blasting”, wheel blasting machines are self-contained units that use reclaimable abrasive. Wheel blasting machines utilise a rotating centrifugal wheel to propel abrasive at exceptional speeds. These machines come in a range of sizes and are fitted with conveyors and generally automated.
Typical operations where blasting is utilised:
- Surface cleaning by removing mill scale, rust, and paint.
- In surface preparation for painting, welding, or other processes that require a clean surface.
- Deburring, removing tooling marks, and finishing.
- Peening and changing metallurgical properties.