What is Abrasive Blasting ?
Abrasive blasting – commonly known as sandblasting – is a surface treatment process used to smooth out, shape, or clean a hard surface by propelling abrasive media at a very high velocity on its surface by the use of compressed air.
This technique has been used for over a century now, but has greatly evolved with the introduction of new designs, technologies, and media materials.
Blasting operation results may vary by controlling process parameters such as abrasive media (physical properties, particle size, angular VS spherical shape, etc.), velocity of abrasive stream, angle of blast gun, distance from the workpiece, and throughput time. The condition of the workpiece – shape, presence of cavities, hardness level, contamination and/or deterioration level caused by rust and corrosion, coating layers, etc. – and the desired end result are usually the main considerations when it comes to defining the abrasive blasting parameters.
Check out the Six Factors Impacting Blast Cleaning Performance guide for more information on blasting process parameters that need to be taken into consideration.
Common Abrasive Blasting Equipment
Typically, abrasive blasting systems consist of the four components described below:
The delivery method is a very important factor to consider when developing a process as it could produce a very narrow stream for precision applications or, on the other end, a very wide stream increasing the process efficiency, while reducing the precision level.
Sandblasting can be achieved in a dry environment – which is usually the case – but can also occur in a humid environment, called Wet Blasting. In Wet Blasting, water is added to abrasive media mix to obtain a smoother finish, reduce the amount of dust created during the process, or obtain a less aggressive action on delicate surfaces. There are three types of delivery systems:
- Air Blasting (Pneumatic) – Blast media flow is driven by the use of pressurized or vacuumed airflow usually produced by an air compressors
- Mechanical Wheel (Airless Blasting) – Abrasive media (usually shot or a mix of angular-shape and spherical-shape media) is propelled onto a surface by means of centrifugal wheels similar to the ones we use to scatter fertilizer on the lawn.
- Wet blasting (Pneumatic or Hydraulic) – In wet blasting, the water and media are mixed in a heated hopper. The media and water mixture is then propelled onto the workpiece by using the Venturi principle “suction system” or by a pump “pressurized system”.
We have a very comprehensive Blasting Technical Guide discussing these delivery systems’ pros and cons in detail.
Abrasive blasting is more efficient and safe when operated in an enclosed environment to control dust and flying particles produced by media impact. Depending on the size of the workpiece, a containment system can be a sandblast cabinet with a see-through window and openings in which the operator can slide his arms, or a blast room enclosure where operators equipped with proper safety equipment can move freely around the parts. A negative air pressure is applied to the enclosure system, which is also well-sealed to make sure no dust can escape into the work environment. However, some applications require an outdoor environment for sandblasting – i.e. shipyards, construction sites, buildings, civil engineering structures, and other large-size workpieces. Depending on the wind velocity and direction, dust can be a problem for surrounding areas and activities. For these reasons, open pit blasting operations are usually prohibited without formal authorization by municipal authorities.
Media Reclaim System
An abrasive media reclaim system function is meant to reclaim and recycle used abrasive media when it exits the process. A media reclaimer allows for significant savings on abrasive media consumption, and is also very beneficial for the environment by considerably reducing abrasive media waste to be disposed of. It usually consists of three major components:
- A means to collect used abrasive contaminated with dust and scraps generated during the sandblasting process – usually a hopper at the bottom of the cabinet or in-floor hopper excavated underground of the blast room.
- A conveyor to carry used abrasive to the media reclaimer – either a pneumatic duct for relatively lightweight media, an endless screw for heavier media, or a mechanical loader for extremely large sized blasting facilities.
- A means to sort reusable abrasive from dust and shattered media – IST has designed an air wash separator and a cyclonic media reclaimer which can be tuned for different types and sizes of abrasive media.
While outdoor applications make it more complicated to implement an abrasive media reclaim system, some wise operators use tarpaulin to collect used media and reuse them in further processes.
Dust Collection System
Finally, a dust collector located next to the abrasive blasting equipment or outside of the building allows for the collection of dust and fine particles generated during blasting operations. Sucking dust away from the containment system provides the operator with a better visibility and reduces the risks of fire associated with combustible dust. Dust collectors create the negative air pressure assuring that no dust comes out of the blast enclosure. Refer to the NFPA Standards Regarding Combustible Dust for more information on safety measures and devices that could be undertaken to comply with NFPA best practices and reduce the risk of fire or explosion in the shop.
Abrasive Blasting Media
A wide variety of blast media can be used in abrasive blasting operations. Abrasive media comes in different sizes, compositions, and hardness levels. Organic or industrial by-products are usually preferred for their low cost and properties. Here are some examples of common abrasive media used in abrasive blasting:
- Steel Grit or Steel Shot – both produced by hardening, crushing, and tempering carbon steel scrap. These are preferred for aggressive cleaning or peening applications due to their superior hardness and durability.
- Aluminum Oxide – obtained from a chemical reaction between aluminum and oxygen and known for its hardness and strength properties. Mostly used for effective blast cleaning.
- Coal Slag – a by-product of coil burning plants, and known for its low price and effectiveness in multiple blast cleaning or surface preparation applications. Its weakness is that it fractures upon first impact and produces a lot of dust during blasting operations.
- Glass Beads – produced from molten glass – usually sourced from sorting plant glass debris – and preferred for blast cleaning applications on various substrates.
- Nutshell– manufactured by crushing nutshell scraps from nut manufacturing plants, and ideal for soft cleaning application on delicate parts that cannot be contaminated by ferrous metal (such as car engines and components).
- Garnet– Produced from natural almandine garnet, a mineral found in nature, and known for its natural hardness, durability, and aggressive properties.
There are numerous other types of abrasive media produced from industrial by-products or minerals found in nature. According to the application, a spherical or angular shaped media can be chosen to obtain the best results and an efficient process. We strongly advise against the use of silica sand or other products containing silica because they could be harmful for both the operator and system components.
Common Applications of Abrasive Blasting
Many industries employ abrasive blasting to manufacture products or components, and also to give a second life to various objects by reconditioning their surface. Sandblasting is used as a surface preparation technique prior to applying coating on the surface of an object, and can also be very efficient when removing a coating layer or cleaning the surface of an object to extend its life. Among the many different abrasive blasting applications available, these are the ones most commonly used by our customers:
- Blast Cleaning – The action of removing any stubborn contaminants from a surface – such as mill scale, rust, salt, etc. – with or without very limited modifications of the workpiece to clean the surface of metal products.
- Surface Preparation – The action of creating an anchor profile in a workpiece in preparation for a subsequent coating or painting process.
- Surface Finishing – The action of creating a smooth surface or a rugged pattern on the surface of an object for aesthetic purposes.
- Shot Peening – The action of inducing internal compressive stress on the structure of a metal piece in order to increase its service life and reduce the risk of fatigue and cracks.