What is Sand Blasting and How Does it Work？

What is Sand Blasting?

Sand blasting is a process that uses the impact of a high-velocity abrasive stream to clean and roughen a substrate surface. It employs compressed air as the power source to generate a high-speed jet, which propels abrasive media such as copper slag, quartz sand, or aluminum oxide onto the surface of a workpiece. This action alters the surface finish or shape of the workpiece.

The impact and cutting action of the abrasive media on the workpiece surface achieve a specific level of cleanliness and a defined surface roughness. This process improves the mechanical properties of the surface, enhancing the workpiece’s fatigue resistance and increasing the adhesion between the workpiece and subsequent coatings. Consequently, it extends the durability of the paint film and aids in achieving better coating layers and aesthetics.

Sand Blasting Process

The pre-treatment stage involves preparing the workpiece surface before the application of a protective coating or plating. The quality of this sand blasting pre-treatment significantly influences the coating’s adhesion, appearance, moisture resistance, and corrosion resistance. Poor pre-treatment allows corrosion to continue spreading beneath the coating, leading to large-scale flaking. Comparative exposure tests have shown that carefully cleaned surfaces can have a service life 4 to 5 times longer than those with only simple cleaning.

The sand blasting stage utilizes compressed air to generate a high-velocity jet that propels abrasive media onto the workpiece surface, altering its external finish. The impact and cutting action of the abrasive media achieve a specified cleanliness level and various roughness profiles, thereby improving the mechanical properties of the workpiece surface.

Types of Sand Blasting Equipment

Sand blasting machines are generally divided into 2 main categories, they are dry blasting machines and wet blasting machines or liquid blasting machines. Dry blasting machines are further classified into suction and pressure types.

(1) Suction Blasting Machine

A complete suction dry blasting machine typically consists of 6 systems: the cabinet/structure, the media power system, the piping system, the dust collection system, the control system, and the auxiliary systems. Suction-feed systems use compressed air as the power source. The high-speed airflow creates a negative pressure inside the blast gun, drawing the abrasive media through a feed hose into the gun. The media is then accelerated and ejected through the nozzle onto the workpiece surface to achieve the desired processing effect. In this type of machine, compressed air serves as both the media feeding power and the jet acceleration power.

(2) Pressure Blasting Machine

A complete pressure dry blasting unit typically consists of four main systems: the pressure pot/tank, the media power system, the piping system, and the control system. Pressure systems utilize compressed air to build up working pressure inside the pressure pot. This pressure forces the abrasive media through the metering valve into the blast hose and out through the nozzle, projecting it onto the workpiece surface for the desired processing result. In this system, compressed air acts as both the media feeding power and the jet acceleration power.

(3) Wet Blasting Machine

The most significant advantage of the liquid blasting machine over the dry blasting machine is its effective control of dust pollution during the blasting process, improving the operator’s working environment. It typically consists of five systems: the cabinet/structure, the slurry power system, the piping system, the control system, and the auxiliary systems. The liquid blasting machine uses a slurry pump to feed the abrasive mixture. The slurry pump delivers the uniformly mixed slurry to the blast gun. Compressed air is used as the acceleration power, entering the gun through an airline. Inside the gun, the compressed air accelerates the incoming slurry, which is then ejected through the nozzle onto the workpiece surface to achieve the intended processing objective. In this system, the slurry pump provides the feeding power, and compressed air provides the acceleration power.

Sand Blasting Cleanliness Grades

There are two representative international standards for surface cleanliness after sand blasting: the American SSPC standard, established in 1985, and the Swedish Sa standard, established in 1976. The Sa standard, which is the internationally accepted common standard, is divided into four grades: Sa1, Sa2, Sa2.5, and Sa3. Details are as follows:

(1) Sa1 Grade

Equivalent to US SSPC-SP7. This grade is achieved using simple manual methods like wire brushing or sandpapering. It is the lowest of the four cleanliness grades, offering only slightly better coating protection than an untreated workpiece. The workpiece surface must be free of visible oil, grease, residual mill scale, rust, and residual paint. Sa1 is also referred to as brush-off blasting.

(2) Sa2 Grade

Equivalent to USA SSPC-SP6. This grade is achieved using abrasive blasting (or shot blasting). It is the lowest grade for actual abrasive blasting and represents a general requirement, but it offers significantly better coating protection than manual cleaning. The workpiece surface must be free of visible oil, grease, dirt, mill scale, rust, paint, oxides, corrosion products, and other foreign matter, apart from stains (blemishes). However, these blemishes are limited to no more than 3% of the surface area and may include slight shadows, minor discoloration due to blemishes/pitting/rust, and light mill scale or paint blemishes. If the original surface had pits, slight traces of rust and paint may remain at the bottom of these pits. Sa2 is also referred to as commercial blasting.

(3) Sa2.5 Grade

This is the most used grade in the industry and often serves as the acceptance technical requirement and standard. Sa2.5 is also referred to as near-white blasting. The requirements are the same as the first half of the Sa2 standard, but blemishes are limited to no more than 5% of the surface area and may include slight shadows, minor discoloration due to blemishes/pitting/rust, and light mill scale or paint blemishes.

(4) Sa3 Grade

Equivalent to US SSPC-SP5. This is the highest treatment grade in the industry, also referred to as white metal blast cleaning. The requirements are the same as Sa2.5, but the 5% allowance for shadows, blemishes, rust, etc., is not permitted.

Applications of Sand Blasting

Sand blasting removes all contaminants, such as rust and mill scale, from the workpiece surface and creates a crucial surface profile, commonly referred to as an anchor pattern or “tooth”. By adjusting the abrasive particle size, various roughness levels can be achieved, improving the bonding strength between the workpiece and the applied anodizing or plating layer. It also ensures stronger and higher-quality adhesion for bonded components.

Cleaning and Polishing

Sand blasting effectively removes all contaminants from the surface of cast and forged parts and post-heat-treated components, such as mill scale, oil, and other residues. It polishes the surface, enhancing the surface finish and revealing the uniform, consistent natural color of the metal, resulting in a more aesthetically pleasing appearance.

Deburring and Surface Finishing

Sand blasting can remove minute burrs from the surface of CNC machined parts, resulting in a flatter, smoother surface. This eliminates the detrimental effects of burrs and improves the overall quality of the workpiece. Furthermore, blasting can create small, precise radii at surface intersections, making the part look more refined and precise.

Improvement of Mechanical Properties

After sand blasting, a uniform, fine-grained surface texture is created, capable of retaining lubricating oil. This improves lubrication conditions, reduces noise, and extends the service life of mechanical components.

Decorative Finishing

For certain specialized workpieces, sand blasting can produce specific reflective or matte finishes as required. Examples include achieving a satin/matte finish on stainless steel parts, polishing plastics, buffing jade, mattifying wooden furniture surfaces, creating patterns on frosted glass, and roughening fabric surfaces.

Considerations for Sand Blasting

 Before starting work, all protective gear must be worn. Working bare-armed is prohibited.

• The work must be carried out by no less than two people.
• Air storage tanks, pressure gauges, and safety valves must be checked and calibrated periodically. The air tank should be drained of accumulated dust/moisture every two weeks, and the filter in the abrasive pot should be inspected monthly.
• Check that the ventilation ducts and the sand blasting machine doors are properly sealed.
• The ventilation and dust extraction equipment must be switched on at least 5 minutes before starting blasting. Sand blasting is prohibited if the ventilation and dust extraction equipment is malfunctioning.
• The compressed air valve must be opened slowly, and the air pressure must not exceed 0.8 MPa (approximately 116 psi).
• The abrasive particle size should be appropriate for the job requirements, typically within the 10 to 20 mesh range, and the abrasive media must be kept dry.
• When the sand blasting machine is in operation, unauthorized personnel are prohibited from approaching. The machine must be shut down before any cleaning or adjustment of moving parts is performed.
• Compressed air must not be used to blow dust off the body or for horseplay.
• After the work is completed, the ventilation and dust extraction equipment must continue to run for another 5 minutes before being shut off to exhaust residual dust and maintain a clean work area.

About Getzshape

When your project demands globally compliant finishes, you need a partner who understands these industrial standards inside and out. Getzshape delivers globally compliant CNC machined parts and surface finishes, including precision sandblasting solutions. Our commitment to the highest Sa and SSPC cleanliness grades ensures your components meet performance and durability requirements. Contact us to explore our capabilities.