Choosing the right additive type and grade for abrasive waterjet cutting applications can make or break your waterjet operation’s performance and profitability.
The process of selecting abrasives begins with examining the material and the cutting specifications. The difficulty of cutting the material and the desired edge quality are two critical factors in choosing the right abrasive waterjet cutting additive.
Furthermore, the waterjet machine must be configured to meet the cutting specifications for the abrasive to be delivered effectively and efficiently.
Different abrasive materials have different performance characteristics. Therefore, when matching an abrasive to a specific application, it is critical to understand these characteristics.
Various abrasives’ performance characteristics
Understand how hardness, density, toughness, and particle shape affect abrasive waterjet cutting.
Almost every known abrasive – natural and synthetic – has been considered for use in abrasive waterjet cutting. However, almandine garnet has emerged as the mineral with the best abrasive waterjet properties. Other minerals may be harder, heavier, or less expensive, but almandine garnet has the best combination of properties for waterjet cutting. In addition to abrasive waterjet cutting, this naturally occurring material is mined and processed for various industrial applications like blasting media and water filtration. The common almandine garnet is brownish-red and opaque in colour. The much rarer, gemstone-quality almandine garnet is transparent and has a deep red colour.
Abrasive waterjet cutting can also be done with other additives. For example, aluminium oxide is harder than garnet and is sometimes used to cut extremely hard materials like ceramic. However, using aluminium oxide dramatically reduces mixing tube life, increasing the cost of the operation. Staurolite is a naturally occurring mineral with the density and hardness of garnet, and it is a cost-effective general-purpose waterjet abrasive. Softer abrasives like olivine can be used to cut less-demanding materials like aluminium.
Natural characteristics and mineral processing directly impact how well a material performs in abrasive waterjet cutting. Four key attributes affect abrasive performance.
Hardness
Waterjet cutters must strike a balance between cutting speed and component wear. Using a soft abrasive increases nozzle life but decreases cutting speed. A very hard abrasive provides fast cutting but erodes the nozzle too quickly. Nozzle erosion reduces cutting accuracy, causes recurring downtime, and increases the cost of nozzle replacement.
The Mohs scale of mineral hardness describes the scratch resistance of various minerals by rating a harder material’s ability to scratch a softer material. Diamond is the hardest naturally occurring substance and is at the top of the scale.
Mohs scale of hardness
10 diamonds
9 Corundum
7.5 – 8.5 Garnet
8 Topaz
7 Quartz
6 Feldspar
5 Apatite
4 Fluorite
3 Calcite
2 Gypsum
1 Talc
Almandine garnet has a Mohs hardness of seven to eight, effectively balancing the need to cut quickly while also providing reasonable cutting tool life.
Density
A waterjet’s primary cutting force is a function of mass multiplied by velocity. As a result, the ideal abrasive has the heaviest particle that the water stream can accelerate to maximum velocity. This produces the most cutting force.
An abrasive that is too light will not have much impact, and an abrasive that is too heavy will not accelerate to maximum velocity, sapping the power from the waterjet stream. The key, as with hardness, is to find an abrasive that hits the sweet spot. For example, almandine garnet has a specific gravity of 4.0 (four times the weight of water) and is ideal for punch and acceleration.
Toughness
Toughness or friability directly impacts how well a waterjet abrasive performs. Too friable material degrades in the focusing tube and becomes too fine to cut effectively. Too-tough abrasive rounds during the mixing process and become too dull to cut well. The ideal abrasive has a controlled breakdown rate and produces sharp, angular cutting edges. Again, almandine garnet meets the needs with its semi-friable nature and conchoidal fracture.
Particle shape
Abrasives come in every particle shape imaginable, from perfect beads like steel shot to razor-sharp, needle-like crystals like silicon carbide, a synthetic abrasive commonly used in high-tech applications. Recognising that a sphere is the ideal carrier of mass projected in a high-powered water stream, a fabricator might expect waterjet manufacturers to seek out spherical particles. They must, however, keep in mind the constant balancing act of acceleration, wear, and cutting efficiency.
As a result, waterjet experts have determined that the best particle shape depends on the material being cut and the edge finish required. Grains with sharp, angular edges cut faster and have better edge finishes. Subrounded grains are used in general-purpose cutting applications.
The role of purity, particle size, and cleanliness in abrasive waterjet cutting
The abrasive waterjet cutting industry relies on additives with high purity, tight particle size distributions, and a high degree of cleanliness, which are controlled during mineral processing.
Purity
Almandine garnet is mined, milled, and processed as a natural mineral to meet the final specifications of the producer. Compared to low-purity products, high-purity materials typically involve additional processing stages and require greater attention to detail during the refining process. As a result, high-purity materials are more expensive, but they produce better-cutting results. On the other hand, low-purity products may contain materials other than garnet that impair the cutting ability of an abrasive waterjet cutting machine. Subsequently, other minerals must be processed to meet the high purity level required for abrasive waterjet cutting.
Particle size distribution
Tight, consistent particle size distribution (PSD) control is critical for maximising waterjet performance. Coarse or oversized particles risk clogging the nozzle, halting the machining process, and potentially damaging the workpiece. On the other hand, excessive fines can accumulate in the feed line or the cutting head and cause irregular feed or sputtering in the cutting stream.
Cleanliness
Abrasive cleanliness is similar to particle size distribution and the final important characteristic of a waterjet cutting abrasive. The amount of superfines present in the abrasive product is referred to as cleanliness. Because these fines are so small, they frequently cling to larger particles. Technically, the producers of high-quality garnet abrasives use a “suspended solids” measurement to quantify the product’s cleanliness. However, using an improperly processed product generates nuisance dust while loading the abrasive hopper, leading to poor feeding and cutting over time.
Obtaining the best abrasive performance
Other factors that impact cutting performance include:
· Nozzle/orifice combination
· Machine pressure
· Pump horsepower
· Configuration of machine controls
Contact Vest Profiling for details
Contact us today if you need assistance with any abrasive waterjet cutting projects. We work with our clients to ensure we balance their budget and precision requirements.