BlastTalk: Getting to know your blasting medium (Part II)
In the last edition, we touched on size, shape and hardness of the blasting medium. We will now look into why abrasives need to be both hard as well as tough to be effective and efficient for blast cleaning.
Abrasives that are both hard and tough (as opposed to friable) provide the best means of transferring energy to the surface during blast cleaning. Tough grains are resistant to breakdown upon surface impact and, hence, there is better conversion of energy from the blast stream into surface cleaning and profile formation. Minimal breakdown also means lower dusting. This improves operator visibility and reduces possible occupational health and environmental impacts.
The higher the density (specific gravity) of an abrasive particle, the more energy it will carry to the surface, compared to a less dense particle of the same size. This is illustrated by the equation e = mv2 - whereby energy equals mass (determined by density) multiplied by the velocity (provided by air stream) squared — and explains why the finer grades of higher-density abrasives can achieve the same surface profile as coarser grades of lower-density abrasives. The advantage to using finer abrasives is they have exponentially more particle impacts per unit area per second, cleaning faster than coarser abrasives.
In addition, higher-density particles are more likely to fall to the ground after blasting, rather than becoming airborne, so there is less dust and abrasive dispersion, making it is easier to clean up.
Another factor to consider is the inertness of the blasting medium.
Most natural mineral abrasives are inert and pose little if any OHS risk. A notable exception is silica sand (quartz). Free silica dust, including that from shattered silica sand abrasive, is a known human carcinogen which can cause lung silicosis when inhaled. Heavy metals released from some smelter slag abrasives are also a known environmental hazard, particularly in waterways and marine environments. There are several human health risks associated with over-exposure to heavy metals. Such abrasives should be avoided where possible.
Therefore, it is important to choose the right abrasive for your blast cleaning project as it helps maximise productivity and ensures good surface quality to achieve a coating application that lasts.
By Martin Taylor, Technical Advisor