Advantages of Shot and Grit Blasting
Blasting with abrasive media is used to remove scale, rust, ingrained dirt, paint coatings, oil stains, and oxide films from metal parts, while also preparing the surface for welding or protective coatings. The principle is simple: a particle accelerated at high speed strikes the metal and knocks off contaminants, while the metal surface acquires a rough texture where coatings adhere well and hold securely.
Hard and fast-moving shot can not only remove burrs after mechanical processing but also strengthen the metal surface, improving fatigue resistance and wear resistance.
It is also an environmentally friendly process, as the cleaning is purely mechanical and does not require chemical reagents.
Shot is accelerated either by compressed air (grit blasting) or by centrifugal force generated by a rotating rotor (shot blasting). There are technical differences between these methods that should be considered.
Differences Between Grit Blasting and Shot Blasting
Grit blasting technology is essentially similar to sandblasting: a compressor pushes an air-abrasive mixture through the system, which is further accelerated and focused through a nozzle. If we replace sand with shot (metal, ceramic, or glass) and increase the nozzle diameter, we get grit blasting, where compressed air at 5–7 bar accelerates the shot to 20–30 m/s. The advantage of grit blasting is flexibility: using a hose, the operator can reach difficult areas and process parts of complex shapes and various sizes.
Shot blasting uses a centrifugal principle. The core of a shot blaster is an impeller, to the axis of which a feeder supplies the shot. The impeller picks up the shot, accelerates it, and throws it out through a fixed opening in the distribution chamber. Under centrifugal force, the shot slides along the blades to the periphery and exits in a fan-shaped stream. Adjusting the window position controls the spray width and particle speed.
Turbiners accelerate the shot to 60–100 m/s (up to 360 km/h). Mechanical shot blasters produce a stable, uniform stream of particles, ensuring even surface treatment.
Shot blasters are highly productive. For comparison, a conventional shot blaster has a spray area of 150×800 mm, consuming up to 160 kg of shot per minute (some turbines consume up to a ton per minute). In contrast, a standard grit blaster ejects about 25 kg/min of abrasive material. The more particles hit the surface and the higher their speed, the faster the parts are cleaned.
Disadvantages of shot blasters include blade wear, stationary operation, and unsuitability for complex-shaped parts.
About Shot
The quality of any shot or grit blasting depends on the abrasive chosen. Shot can be metallic, ceramic, glass, or plastic.
Cast iron and steel shot are used for effective cleaning and surface strengthening but are not suitable for all metals; for example, aluminum requires aluminum granules to avoid galvanic corrosion.
Electrocorundum is an extremely hard abrasive made from aluminum oxide, providing high-speed and quality surface treatment.
Glass abrasives are used for precision parts and soft alloys.
Plastic shot is used for delicate surface treatment of soft metals, plastics, glass, wood, and ceramics.
Shot has two major advantages over sandblasting: it can be reused, making it economical, and it strengthens the metal surface. The principle of "micro-peening" is that when the shot impacts the surface, the metal is locally compacted. This induces compressive stresses, increases surface hardness and fatigue strength — especially beneficial for high-stress parts like gears, crankshafts, and connecting rods. It also reduces stress peaks around notches, holes, and scratches, significantly extending the service life of parts.
Types of Shot and Grit Blasting Machines
In industrial settings, shot blasting is often used for cleaning similar parts, so industrial machines typically have multiple grit or shot blasters operating automatically.
All industrial systems include shot recycling mechanisms: spent abrasive, along with debris, falls through a grid, is collected, separated from sand, scale, and other contaminants, and returned to the hopper, ensuring continuous abrasive supply. Separation efficiency can reach 99%, meaning less than 1% of dry residue remains. Proper separation is crucial, as more than 3% sand or dust increases equipment wear by at least 30%.
There are five common types of installations, which can be equipped with grit or shot blasters: in-line, suspended, drum, carousel, and tunnel.
- In-line installations consist of a working chamber, several blasters, and a conveyor (or suspension) that moves parts from entry to exit. Depending on chamber size and number of blasters, they can process small items or entire wagons/containers.
- Suspended installations are for parts that cannot be rotated, have complex geometry, or are sensitive to impact. Parts are hung on a rotating hook and rotated around a vertical axis. Blasters positioned above and below treat all sides. One cycle can clean one large or several smaller items.
- Drum-type machines handle a large number of small parts that can withstand collisions. Items are loaded into a cylindrical chamber and moved as the drum rotates, achieving uniform cleaning.
- Carousel installations (rotary table type) are similar to suspended systems. Parts rotate during processing, exposing all sides sequentially. This is suitable for items where the base does not need cleaning. Tables can be segmented for multiple parts.
- Tunnel installations are used for cleaning pipes, metal profiles, or sheet metal. Pipes are rotated for full surface coverage in one pass. These installations may include dust removal units, preparing parts for immediate painting.
There is also specialized equipment, such as mobile shot blasters, which remove contaminants from flat surfaces like concrete, asphalt, stone, and metal. They can also roughen surfaces for better coating adhesion or reduced slip. Mobile units consist of a blaster, abrasive cleaning mechanism, and a cart for manual movement. Some are handheld, weighing around 10 kg, allowing wall processing.
Chambers for Grit Blasting
Grit blasting is similar to sandblasting, but to reuse the shot, it must be collected. Small workshops or auto repair shops use manual grit blasting chambers. Operators insert their hands into full rubber gloves and use a grit blasting gun while viewing through a window.
Larger options include walk-in grit blasting chambers. Operators wear protective suits and helmets with breathing air supplied via hoses. Parts of any complexity, including large items, can be processed. Chambers have powerful ventilation to remove dust obstructing view.
More advanced walk-in chambers are robotized: a manipulator with 6–8 degrees of freedom moves the nozzle around the part at 10 bar and a 12–19 mm nozzle, working faster than a human operator. Speed and productivity can match in-line systems, excluding loading/unloading. Robots improve accuracy, uniformity, and surface strengthening, providing significant economic advantages.
Wet Grit Blasting Machines
These are the same as regular grit blasting machines but use water. A high-speed air stream (0.1–1.0 MPa) carries a suspension of water and 20–30% shot.
This method achieves the same goals: surface cleaning, roughening, and strengthening. Differences include no surface heating (dry peening can reach 600°C) and no geometric distortion. Wet blasting is gentler, produces a smoother micro-relief, increases microhardness by 30–35%, and modifies a 200–300 μm surface layer.
Wet blasting productivity is 1.5–2 times lower than dry, but surface quality is higher. It is economically unsuitable for large-scale cleaning of railcars or large metal structures, but ideal for high-tech industries such as gas turbine manufacturing, precision engineering, and aerospace.
Practical Recommendations for Using Shot Blasting Machines
To ensure proper cleaning, surface roughness, and equipment longevity, it is recommended to:
- Monitor turbine speed,
- Ensure even shot feed,
- Use uniform, high-quality abrasives,
- Allow only trained operators to handle the equipment,
- Regularly maintain machines and replace or repair worn parts promptly.
