Shaw Resources is a supplier of blasting abrasives to many different industry segments with over 50 years of experience. We’re proud to work closely with our customers to help them do their jobs quickly and effectively, all while helping them maximize their profits. The key component of this equation is making sure our customers are choosing the right abrasive.
There are many sandblasting media from which to choose and each performs differently depending upon the task at hand. This is due to a number of key abrasive properties of the medium: shape, size, hardness, and density.
One important result of sandblasting is what is called the “anchor pattern.”
As a general rule, the blaster should use the smallest abrasive necessary to attain the required surface characteristics. A fine abrasive will give you more impacts per volume. The more particles in the stream, the more work is accomplished at the same time. Soft substrates such as concrete or wood do not require a hard abrasive or a coarse particle.
However, when preparing an iron or steel surface substrate for painting, there are additional considerations. Coatings adhere poorly to hard, flat surfaces, so it is important to prepare a pattern of indentations for which the paint to adhere – otherwise known as the anchor pattern.
When a sufficiently hard abrasive particle strikes steel, it deforms the surface into a valley and pushes up peaks. The distance between the top of the peak and the bottom of the valley is known as the depth profile.
The depth profile is commonly referred to as mils – thousandths of an inch. In the metric system, the micron (one-millionth of a meter) is used.
1 mil = 25.4 microns.
When a particle impacts the surface of a metal, it forms a small crater. This tiny crater will be lined by ridges, which are pressed up on all sides of it. The depth of this crater is typically measured in thousandths of an inch and affects how well the coating will bond to the surface of the metal. The deeper the craters, the better the bond.
However, if the craters are too deep, the ridges may poke out through the surface of the coating or paint causing pin-point rust. This can cause premature corrosion and weakness in the coating which may result in premature coating failure. Generally, the correct profile depth will be 25-30% of the dry film thickness of the total coating system. For most industrial coatings, the typical steel profile is between 2-3 mils, not normally exceeding 5 mils.
Knowing the surface characteristics and the profile depth you need to achieve, the choice of abrasive should be selected upon its: size, shape, hardness, and density.
Often, but not always, we measure particles according to their “mesh” size. This indicates what size mesh screen the particles will fit through. A mesh size of 16 has 16 holes per square inch, which would fit relatively large particles, whereas a mesh size of 120 would indicate much smaller particles. If the size is given in a split format such as 30/60, this indicates that 95% of the mix will fall through a 30 mesh but not pass through the 60.
Quick logic might have us conclude that bigger particles do more work than small ones, but in reality, while a bigger particle may carry more momentum, small particles clean faster, provide better coverage, and result in a more uniform profile. The most efficient approach is to use the smallest particle necessary to achieve the desired profile.
The sharpness of angles of a particle is a key influence in how deeply it will cut into the underlying substrate. A sharp particle will dig farther into a metal surface than a round particle of the same size.
Shapes are classified according to angularity: angular, sub-angular, sub-rounded, and round. Angular particles cut through soft coatings and rust, cleaning faster, and producing sharper anchor patterns. Rounded particles produce a more even, peened surface, good for breaking away hard brittle coatings and mill scale.
How hard a medium also plays a role in how deep the particles will penetrate into a surface. Softer abrasives are ideal for removing dirt, grease, or paint without creating an anchor pattern or damaging underlying substrate such as removing grime on sandstone or graffiti on concrete.
The Mohs Hardness Scale was created by German mineralogist and geologist Freidrich Mohs.
The higher the number, the harder the mineral; talc is a 1, diamond is a 10. Higher numbered materials can scratch or etch lower number materials.
Abrasives by Hardness (MOHS)
|Plastic||3.0 – 4.0|
|Silica Sand||5 – 6|
|Crushed Glass||5.5 – 7|
|Olivine||6.5 – 7.5|
|Staurolite||7.0 – 7.5|
|Garnet||7.0 – 7.5|
|Steel Shot/ Steel Grit||8.0|
Denser particles transfer more of their kinetic energy into impacts resulting in deeper profiles. Softer, less-dense particles will compress when they collide with the object, absorbing more energy in the collision and transferring their momentum less efficiently. More pressure upon impact results in deeper profiles (assuming the shape and hardness of the media being compared are equivalents). Both hard particles and softer particles have uses depending upon the project requirements.
Density is a property of matter and can be defined as the ratio of mass (weight) to a unit volume of matter (such as gms / Cm3). Specific gravity is a measure of a material’s density relative to the density of a reference substance, typically water at 4 degrees Celsius. Using the table below as an example, a solid mL of Copper Slag has three times the mass (weight) of a mL of water.
Abrasives by Density (Specific Gravity (g/mL))
|Garnet||3.5 – 4.3|
Velocity or speed of the abrasive is the only factor that can be easily adjusted on the job site. Once you’ve selected the abrasive that is required to provide you with the desired anchor pattern, the depth profile can be fine-tuned by adjusting your blast pressure and flow rate by adjusting the metering valve.
For optimal results, make sure your equipment is in good repair including hoses, valves, and nozzles. Also, make sure you are using the properly sized equipment for the task at hand as sandblasting can be hazardous. Please follow all health and safety protocols to protect those doing the work as well as those surrounding the job site. Consult with your equipment provider, abrasives supplier, and follow the appropriate provincial department of labour guidelines if you have any questions.