BlastTalk: Getting to know your blasting medium
In the last edition, we talked about corrosion, the importance of protective coatings and the benefits of abrasive blasting. The next key factor to look at is the blasting medium. A wide variety of options are available in the market, ranging from soft abrasives like baking soda to hard abrasives such as garnet.
Selecting the right abrasive for the job should not be based on price per tonne or habitual practices. In addition to achieving the highest productivity rates, the main criteria should include achieving the required anchor profile and high quality surface cleanliness to ensure effective adhesion and long-lasting coating life.
In addition to achieving high quality technical results, the overall cost per square meter cleaned should also be a prime consideration — not the price per tonne of abrasive. Cheap, inferior abrasives generally require more tonnes per unit area cleaned, more clean-up and disposal handling, and they often yield more dust. Other considerations that are equally important are safety risks to the workers on site and the environment.
In order to compare different blasting abrasives, it is important to understand their physical properties and how they impact blasting performance.
Larger particles will produce a bigger indentation; however, they produce less impact per square meter than the same volume of smaller particles, which work faster. Smaller particles also produce a more uniform surface profile and a cleaner surface. In addition, smaller particles can increase the number of peaks produced per square centimetre of surface, which can create more surface area to adhere to than a deeper surface profile. Therefore, the most efficient approach is to use the smallest particle necessary to achieve the required anchor profile.
Abrasives are classified in three different shape types: rounded, subangular and angular. Rounder particles without cutting edges will pound or ‘peen’ a surface, while sharp particles with points and edges remove surface material on impact. Both angular and subangular abrasives will create angular profiles.
As for subangular particles, they present a larger surface area for contact while still maintaining sufficient angularity for cutting. Moreover, subangular particles are more resilient to breaking down than angular particles and less likely to cause embedment (abrasive splinters stuck in the metal surface).
A harder particle will generally be more aggressive in cleaning the surface and imparting a deeper profile; however, if the particle is friable (the tendency of a solid substance to break into smaller pieces) and shatters on impact, the force on the surface will be reduced. A good example is diamond, which is extremely hard but also brittle.
In the next edition, we will look into why abrasives need to be both hard as well as tough to be effective and efficient for blast cleaning. We will also talk about density and inertness.
By Martin Taylor, Technical Advisor