The Introduction of Sandblasting

The term sandblasting describes blasting abrasive material against a surface by using compressed air. Although sandblasting is often used as an umbrella term for all abrasive blasting methods, it differs from shot blasting where the abrasive media is propelled by a spinning wheel.

Sandblasting is used to remove paint, rust, debris, scratches and casting marks from surfaces but it can also achieve the opposite effect by etching surfaces to add texture or design.

Sand is rarely used in sandblasting today due to health risks and problems related to moisture content. Alternatives like steel grit, glass beads and aluminium oxide are now preferred among many other types of shot media.

Sandblasting uses compressed air to propel abrasive materials, unlike shot blasting, which employs a wheel blast system and centrifugal force for propulsion.

What Is Sandblasting?

Sandblasting, often also called abrasive blasting, is a method used to remove surface contamination, smooth rough surfaces, and also roughen smooth surfaces. This is a rather low-cost technique thanks to its inexpensive equipment, and it is simple while delivering high-quality results.

Sandblasting is considered a gentler abrasion blasting technique compared to shot blasting. However, intensity can vary depending on the type of sandblasting equipment, the pressure of the compressed air, and the type of abrasive media used.

Sandblasting offers a wide selection of abrasive materials that are effective in different applications, such as removing paint and surface contamination that is lighter in intensity. The process is also ideal for cleaning sensitive electronic components and corroded connectors delicately. Other sandblasting applications that require greater abrasive blasting power may use a high-pressure setting and a more abrasive shot media.

How Does the Sandblasting Process Work?

The sandblasting process works by propelling sandblasting media to a surface through the use of a sandblaster. The sandblaster has two main components: the blast pot and the air intake. The blast pot holds the abrasive blasting media and funnels the particles through a valve. The air intake is powered by an air compressor that applies pressure to media inside the chamber. It exits the nozzle at high speeds, impacting the surface with force.

The sandblast can remove debris, clean surfaces, remove paint, and improve the surface finish of the material. Its results greatly depend on the type of abrasive and its properties.

Modern sandblast equipment has a recovery system that collects the used media and refills the blast pot.

Sandblasting Equipment

Compressor – The compressor (90-100 PSI) provides a pressurised air supply that propels the abrasive media to the surface of the material. Pressure, volume, and horsepower are often the key factors to take into account when selecting an appropriate sandblasting compressor.

Sandblaster – Sandblasters (18-35 CFM – cubic feet per minute) deliver the abrasive media onto the material using compressed air. Industrial sandblasters require a higher volumetric flow rate (50-100 CFM) as they have a larger area of application. There are three types of sandblasters: gravity-fed, pressure blasters (positive pressure), and siphon sandblasters (negative pressure).

Blast cabinet – A blast cabinet is a portable blasting station that is a small and compact enclosed system. It usually has four components: the cabinet, abrasive blasting system, recycling, and dust collection. Blast cabinets are operated by using glove holes for the operator’s hands and a foot pedal for controlling the blast.

Blast room – A blast room is a facility that can accommodate a variety of equipment which are typically used for commercial purposes. Aircraft parts, construction equipment, and automotive parts can be comfortably sandblasted in a blast room.

Blast recovery system – Modern sandblasting equipment has blast recovery systems that recoup sandblasting media. It also removes impurities that may cause media contamination.

Cryogenic deflashing system – Low temperatures from cryogenic deflashing systems allow for safe deflashing of materials, such as diecast, magnesium, plastic, rubber, and zinc.

Wet blast equipment – Wet blasting incorporates water into the abrasive blasting media to reduce overheating from friction. It is also a gentler abrasion method compared to dry blasting since it only scrubs the target area in the workpiece.

Sandblasting Media

As the name suggests, earlier forms of sandblasting primarily used sand due to its availability, but it had its drawbacks in the form of moisture content and contaminants. The major concern with sand as an abrasive is its health risks. Inhaling silica dust particles from sand can cause serious respiratory diseases, including silicosis and lung cancer. Thus, nowadays sand is rarely used and a wide range of modern abrasive materials have replaced it.

The blasting media varies depending on the desired surface finish or application. Some common blasting media include:

Aluminium oxide grit (8-9 MH – Mohs hardness scale) – This blasting material is extremely sharp which is perfect for preparation and surface treatment. It is cost-effective as it can be reused many times.

Aluminium silicate (coal slag) (6-7 MH) – This by-product of coal-fired power plants is a cheap and dispensable media. The oil and shipyard industry uses it in open-blasting operations, but it is toxic if exposed to the environment.

Crushed glass grit (5-6 MH) – Glass grit blasting uses recycled glass beads which are non-toxic and safe. This sand-blasting media is used to remove coatings and contamination from surfaces. Crushed glass grit can also be used effectively with water.

Soda (2.5 MH) – Bicarbonate soda blasting is effective in gently removing metal rust and cleaning surfaces without damaging the metal underneath. Sodium bicarbonate (baking soda) is propelled at a low pressure of 20 psi compared to regular sandblasting at 70 to 120 psi.

Steel grit & steel shot (40-65 HRC) – Steel abrasives are used for surface preparation processes, such as cleaning and etching, due to their rapid stripping capability.

Staurolite (7 MH) – This blast media is a silicate of iron and silica sand which is ideal for removing thin surfaces with rust or coatings. It is generally used for steel fabrication, tower construction, and thin storage vessels.

In addition to the aforementioned media, there are plenty more available. It is possible to use silicon carbide, which is the hardest abrasive media available, and organic shots, such as walnut shells and corn cobs. In some countries, sand is still used to this day, but this practice is questionable as the health risks are not justified.

Shot Media Properties

Each type of shot media has these 4 main properties that operators can consider when selecting what to use:

Shape – Angular media has sharp, irregular edges, making it effective in removing paint, for example. Round media is a gentler abrasive than angular media and leaves a polished surface look.

Size – Common mesh sizes for sandblasting are 20/40, 40/70, and 60/100. Larger mesh profiles are used for aggressive application while smaller mesh profiles are for cleaning or polishing to produce a finished product.

Density – Media with higher density will have more force on the metal surface as it is propelled by a blast hose at a fixed velocity.

Hardness – Harder abrasives generate a larger impact on the profile surface compared to softer abrasives. Media hardness for sandblasting purposes is often measured through the Mohs hardness scale (1-10). Mohs measures the hardness of minerals and synthetic materials, characterising the scratch resistance of various minerals through the ability of harder materials to scratch softer materials.