How Sandblasting and Bead Blasting Change Metal Surface Roughness in Abrasive Blasting

During the metal die casting, welding, or machining process, deposits and oxidation scale can attach to the component. If these surface imperfections are not removed, the part’s surface quality may be affected. One widely used technique by engineers to remove these deposits is abrasive blasting.

There are several abrasive blasting methods to choose from, depending on cost, the desired surface finish, and the material to be sandblasted. However, whichever method is selected, the final texture achieved depends on several factors, such as substrate material, geometry and size, impact angle and blasting pressure. This guide provides an overview of sandblasting and bead blasting and how they change metal surface roughness.

What is Abrasive Blasting?

Abrasive blasting is the process of using pressurized air, combined with either dry abrasive particles or abrasive particles mixed with water, to shoot at a part or surface. The use of abrasives during blasting creates a hammering effect, which can remove rust, mill scale, lacquer and other coatings from the surface. This cleans the object and creates a surface structure that can serve as an adhesive layer for a new coating. There are several types of abrasive blasting, including sandblasting, bead blasting, wet blasting, vacuum blasting, centrifugal blasting and shot blasting.

In sandblasting, silica sand is used as the abrasive, while in bead blasting, fine glass or plastic beads are used as the abrasive medium. Bead blasting produces relatively gentle cleaning compared to sand blasting. Wet blasting combines abrasive with water to improve process safety by reducing dust. Centrifugal blasting uses a blade wheel to propel abrasives at high velocity towards the surface being cleaned.

What happens to metal surface roughness during abrasive blasting?

The high-velocity, repeated impacts of particles on the component change the surface. Different abrasive materials have different techniques for changing the surface. The particles can cut the surface, peen, fracture or deform. Consequently, the final surface has microscopic peaks and valleys, different from the original machined part.

How does sandblasting increase surface roughness?

The term “sandblasting” has conventionally been used to refer to sandblasting irrespective of the abrasive media used. In reality, sandblasting is a type of abrasive blasting that historically used silica sand, but modern industry now uses safer angular abrasives like aluminum oxide or garnet due to health regulations. During the sandblasting process, these angular particles (which have sharp edges) strike a metal surface, concentrating impact forces on small areas of the surface. These impacts remove microscopic materials from the part’s surface, leaving new valleys. As a result, an irregular surface profile is created.

Compared with bead blasting, sandblasting can increase the surface Ra by up to 0.5 µm. However, these values are usually experimentally found. There is no universal Ra conversion factor for abrasive blasting.

How does bead blasting change surface roughness differently?

Unlike sandblasting, which uses sharp cutting grains, bead blasting uses spherical glass beads that impact and deform the component surface. The resulting surface is more uniform, matte in appearance, and less directional than machined surfaces. Bead blasting is best used to reduce the visual contrast of machining marks during part finishing. While sandblasting is suitable for more aggressive surface treatment, bead blasting is suitable for delicate surfaces.

Which blasting parameters have the greatest effect on surface roughness?

The final Ra values of the surface depend on the combination of abrasive media, not on a single parameter.

Abrasive particle size

Particle size affects the size and spacing of surface indentation. When a large or coarse angular particle strikes a component surface, it creates a large indentation, producing a more pronounced surface profile. This strike produces deeper valleys on the metallic substrate and peaks. Deeper valleys and peaks are normally undesirable as they can lead to corrosion and coating failure.

On the contrary, smaller abrasives produce a finer surface texture with more closely spaced impact features. However, the number of individual impacts exceeds that generated by coarse abrasive materials. These smaller abrasives are usually used where engineers seek to clean the surface while keeping the surface texture. However, it is not clear that smaller particles result in lower roughness. Other parameters like pressure, particle shape, and particle velocity must be kept constant, as illustrated in the equation below.

E ∝ ρD³v²

Wo E is kinetic energy of the particle, D is particle diameter, ρ is abrasive density and v is the particle velocity. From the equation, even a smaller particle can have greater impact if the velocity is increased. This can be achieved by increasing the pressure of abrasives flow.

Blasting pressure

Blasting pressure influences the kinetic energy and velocity of the abrasive particles striking the component. At high pressure, the abrasive particles leave the nozzle at high velocity. This increases the energy available for surface deformation, thereby cutting and removing the material. If blasting pressure is not controlled, it can have several effects on the component. The following table summarizes some of the effects of different pressure levels.

Stand-off distance

Stand-off distance is the distance between the blasting nozzle and the workpiece surface. Keeping all abrasion factors constant, the stand-off distance directly affects the energy delivered to the material’s surface. It further determines the distribution areas for applying the abrasive particles. Thus, when positioning the abrasive particle nozzle, it is important to treat the stand-off distance as a controlled process parameter.

The abrasive particle stream tends to expand as the abrasive particles leave the circle. The circular concentration at the nozzle is smaller than at the surface of the part. The air drag as the particles flow changes the particle velocity and impact intensity. At the same ejection pressure, the particles’ velocity decreases as the stand-off distance increases. This consequently leads to smoother surface roughness. However, this is not always the case. Other parameters, such as nozzle design, pressure, abrasive size, density and blast angle, dictate the intensity of abrasion, thereby defining the abrasion roughness.

What Is the Difference Between Sandblasting and Bead Blasting for Surface Roughness?

The most notable difference between sandblasting and bead blasting is the abrasive materials used. The abrasive materials used in sandblasting, like sand and aluminum oxide, are sharp and angular. These materials cut, etch and aggressively strip the component’s surface. Sandblasting works fast, removing base material. It can remove thick rust and stubborn paint. It is ideal for heavy industrial cleaning, rust removal and stripping old paint from industrial structures.

On the other hand, abrasive materials used in bead blasting, such as glass and ceramic, are tiny and spherical. They gently peen, clean and polish the surface. Unlike sandblasting, bead blasting does not remove material from the component surface as heavily. It is slower and gentler, and it does not remove the part’s underlying dimensions. Common applications for bead blasting are cosmetic finishing, soft cleaning of delicate components and polishing engine parts.

Choosing Between Sandblasting and Bead Blasting

There are several considerations that engineers take into account when deciding whether to adopt sandblasting or bead blasting.  First, engineers look at the desired surface finish. Sandblasting is chosen for rough, textured surfaces. This is common for painting, coating and bonding. Bead blasting is chosen for smooth surfaces, especially in surface polishing. Second, manufacturers look at operator skill and experience. Sandblasting requires more skilled operators and experience to manage nozzle angle, stand-off distance and abrasion pressure. Bead blasting requires less-experienced operators with basic knowledge of a few factors, such as bead size.

The other parameter is equipment availability. It is easier to access sandblasting equipment, which can accommodate any other abrasive material. Bead blasting requires specialized equipment; thus, it is not widely available. Further, manufacturers weigh in on project scale and timelines. On this basis, sandblasting is characterized by a higher material removal rate and is thus used in large-scale projects. This technique can help engineers meet deadlines or shorten manufacturing time. Bead blasting is slow and is therefore used in small-scale projects. However, it leads to high-quality production.