Editor Randolph Rapple posted a great article on the fabricator.com, where factors into selecting the right waterjet abrasive is discussed. We’ve summarized the article below, but you can read the full post here: http://www.thefabricator.com/article/waterjetcutting/selecting-the-right-waterjet-abrasive
One way to significantly affect the performance and profitability of your waterjet cutting is selecting the right abrasive type and size. Abrasive selection begins with an examination of the material and the cutting specifications. The difficulty involved in cutting the material and the desired finish can help you determine the proper abrasive to use. The abrasive product needs to have hardness, toughness, and a particular 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 material with the best combination of characteristics for abrasive waterjet cutting.
Natural attributes and mineral processing have a direct effect on how the abrasive material will perform as a waterjet abrasive. We will now go over each of these characteristics in further detail.
Hardness: waterjet cutters need to balance cutting speed and component wear. Using a soft abrasive extends nozzle life, but slows down cutting. Using a hard abrasive offers fast cutting but erodes the nozzle quickly. Almandine garnet falls between 7 and 8 on the Mohs 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. Almandine garnet has a specific gravity of 4.0 and falls right into the ideal range for both punch and acceleration.
Toughness: 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 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.
Particle Shape: Abrasives are available in many different shapes, from perfect beads. To razor sharp, needle like
crystals. Recognizing that a sphere is the ideal carrier of mass projected in a high powered water stream, a fabricator must keep in mind the constant balancing act involving acceleration, wear and cutting. After numerous extensive
trials, experts have determined that the most suitable particle shape depends on the material being cut and edge finis requirements. Share, angular grains cut more quickly and offer superior edge finishes while sub rounded grains are used in more general purpose, standard cutting applications.