Selecting the right abrasive type and size for a waterjet cutting application can make a significant difference in the performance and profitability of your waterjet equipment.

Abrasive selection begins with an examination of the material and the cutting specifications. How difficult the material is to cut and the desired surface finish are two important factors in determining the proper abrasive to use. The abrasive product needs to have hardness, density, toughness, and a consistent particle shape.

Virtually every abrasive known to man—both natural and synthetic—has been considered for use as an abrasive in waterjet cutting. Almandine garnet has emerged as a mineral with the best characteristics for waterjet cutting. Other minerals may be harder, heavier, or lower in cost, but almandine garnet emerges as the one abrasive with the best combination of characteristics for waterjet cutting. The naturally occurring material is mined and processed for numerous industrial applications in addition to waterjet cutting, such as blasting media and water filtration applications. The common almandine garnet is brownish-red in colour and opaque. The much rarer, gemstone-quality almandine garnet is marked by a deep red colour and is transparent.

Other abrasives can be used in waterjet cutting. Aluminium oxide is harder than garnet and is sometimes used for cutting very hard materials such as ceramic. One drawback, however, is that using aluminium oxide dramatically shortens mixing tube life, increasing the cost of the operation.  Staurolite is a naturally occurring mineral with a density and hardness similar to garnet and is an economical general-purpose waterjet abrasive. Softer abrasives like olivine can be used for cutting less-demanding materials such as aluminium.

A closer look at abrasive characteristics

Natural attributes and mineral processing have a direct effect on how the abrasive material will perform as a waterjet abrasive. A fabricator should be aware of how these four key attributes affect the performance of the abrasive.

Hardness. Waterjet cutters need to balance cutting speed and component wear. Using a soft abrasive extends nozzle life but slows down cutting. Using an abrasive that is very hard offers fast cutting but erodes the nozzle too quickly. Nozzle erosion decreases accuracy in the cutting process, results in recurring downtime, and adds the expense of frequent nozzle replacement. Almandine garnet falls between 7 and 8 on the Mohs hardness scale, which effectively balances the need to cut quickly and provide reasonable cutting tool life.

Density. The principal cutting force of a waterjet is a function of mass multiplied by velocity. The ideal abrasive therefore has the heaviest particle that the water stream can accelerate to maximum velocity. This generates the maximum cutting force.

An abrasive that is too light won’t pack much of a punch, and an abrasive that is too heavy won’t accelerate to maximum velocity, sapping the waterjet stream of its power. As with hardness, the key is to find an abrasive that hits the sweet spot. Almandine garnet has a specific gravity of 4.0 (four times the weight of water) and falls right into the ideal range for both punch and acceleration.

Toughness. Sometimes referred to as friability, toughness plays a direct role in how well a waterjet abrasive performs. Material that is too friable breaks down in the focusing tube and ends up too fine to cut effectively. Abrasive that is too tough (think malleable like lead) rounds during the mixing process and is too dull to cut well. The ideal abrasive has a measured rate of breakdown and produces sharp, angular cutting edges. Once again, almandine garnet, with its semi-friable nature and conchoidal fracture fills the need.

Particle Shape. Abrasives are available in every particle shape imaginable, from perfect beads like steel shot, to razor-sharp, needle-like crystals found in silicon carbide, a synthetic abrasive used in high-tech applications. Recognizing that a sphere is the ideal carrier of mass projected in a high-powered water stream, a fabricator might assume that the waterjet manufacturers would look for spherical particles. However, they must keep in mind the constant balancing act involving acceleration, wear, and cutting efficiency.

Through extensive trials on numerous waterjet platforms around the world, waterjet experts have determined that the most suitable particle shape depends on two factors: the material being cut and the edge finish required. Grains with sharp, angular edges have been proven to cut more quickly and offer superior edge finishes. Subrounded grains are used in more general-purpose, standard cutting applications.

A waterjet cutting machine manufacturer or an abrasive supplier can provide more information on both types and assist in trials to determine which particle shape is best for an application.