Recycling Abrasive Cuts Waterjet Operating Costs

(Modern Applications News - May 1999)

Put in perspective, the savings from operating a waterjet abrasive recycling dispenser (WARD) could exceed the financing payments on a $250,000 abrasive waterjet system.

Traditionally, most metal cutting has been done with oxyfuel, plasma or laser. Recently, abrasive waterjet has increasingly been used for many of these cutting applications. Conventional methods using heat have proven effective with most common metals, but there also have been drawbacks. One disadvantage of oxyfuel, laser and plasma is that the gases used are consumables. Since it is not possible to recycle them once they are burned, the cost of gases used in heat cutting will always be an unrecoverable expense.

For abrasive waterjet cutting, the abrasive is the consumable. Since it is used only once, operators have incurred the additional expense of hauling the waste sludge to a dump site. Not surprisingly, the introduction of a WARD that enables operators to reuse the abrasive promises to revolutionize the industry by cutting operating costs and increasing profit margins. Examining the differences between the finished products of conventional cutting and abrasive waterjet cutting reveals the reasons why waterjet cutting is gaining popularity, even before the introduction of the WARD.

Cutting with heat generates a heat-affected zone that sometimes needs to be removed. Where secondary machining is required, the localized hardening of the heat-affected zone can often cause significant wear and tear to the tooling. Large start-up holes often mar the material, and sheets of metal usually cannot be stacked during cutting (particularly for plasma, where the metal must be grounded during cutting).

Since the heat in conventional cutting leaves one side of the kerf beveled and the other smooth, sufficient space must be maintained between parts cut from the same sheet. The thicker the material being cut, the wider the kerf and the greater the distance required between nested parts. The beveled side is often discarded as scrap. The operator must wear heat-resistant clothing and a welding mask while maintaining a safe distance during cutting. Most flame cutting is performed underwater to minimize gas emissions, sparking and blinding.

Abrasive waterjet cutting is a controlled, accelerated erosion process that uses an abrasive (typically garnet) to cut virtually any material, including stainless steel, inconel and titanium. Water is forced at a pressure of 20,000 - 60,000 psi, through an orifice of .003" and .016" in diameter, with an exit velocity of about 2,000 miles per hour. A controlled amount of abrasive is drawn into the stream by the vacuum generated via the venturi effect. The abrasive is then accelerated to a high speed by the water stream.

A waterjet is able to cut at speeds of 0.05 ipm (inches per minute) to 600 ipm with a tolerance of +/-0.003", depending on the material and the specific application required. The slower the speed, the fewer striations will occur on the piece being cut. There is no heat- affected zone, no large start hole, no heat distortion, or very little danger to the operator. By wearing safety goggles, the operator can observe cutting from nearby. Parts can often be nested on a common line to save material. Parts being cut can be stacked several layers thick, depending on the type of material and quality of finished edge required.

Since abrasive waterjet is able to cut virtually any material, many shops are foregoing the greater speeds of laser-cutting thin steel plate, for example, to cut with a waterjet; which opens a wide range of business opportunities. The ability to recycle the abrasive with a WARD puts even more competitive pressure on conventional heat cutting.

Recovering Garnet Yields Significant Savings

The abrasive is directly responsible for the efficiency achieved in waterjet cutting. Accelerated to a high speed by the water stream, it erodes the material and leaves a narrow kerf. As the grains of abrasive shave off particles of the material being cut, they mix to form a sludge of used abrasive, water and particles of cut material. Periodically, this sludge must be removed and disposed of at great expense. This creates downtime for the waterjet while the tank is being emptied. Then, more costs are incurred by contracting the removal and shipment of the sludge to a dumpsite. The garnet, contained in the sludge and costing $0.20 -$0.35 per lb., is deposited in a dumpsite, never to be used again.

The average waterjet company uses 1.5 - 4.5 lb. of abrasive per minute, depending on the capacity of the system purchased. In other words, a system powered by a 100 hp pump could consume 1,800 lb. of abrasive in 10 hours, at a cost of $600 - $700. Since the largest single expense of operating an abrasive waterjet system is the cost of the garnet, if the abrasive could be recycled, operating costs would drop dramatically. This is precisely where the WARD comes in: by recycling the abrasive, it significantly lowers abrasive waterjet operating expenses. Not since the development of long-life nozzles has a single item completely transformed abrasive waterjet cutting.

The first commercially available WARD has been called the WARD 24. The number following the acronym, "WARD", denotes the diameter screen used to wash, sort, and dry the abrasive (that is, the WARD 24 uses a 24" diameter screen). The diameter of the screen determines the amount of abrasive that can be recycled per minute; the larger the screen, the greater the amount recycled per minute.

However, it takes more than increasing screen diameter to recover more abrasive. The WARD 24 has been designed to balance the removal of abrasive on one side, with the drying process on the other. This ensures that all the abrasive remains completely dry once it is recycled, since moisture can cause blockages of the abrasive during cutting. Consequently, the amount of abrasive recycled per minute has to be balanced against the recycling system's drying capacity.

Recycling starts by removing the sludge from the tank with a patented nozzle that contains no moving parts. It is then pumped through a series of screens, where it is washed and separated into waste product and abrasive large enough to be used again.. The larger abrasive is then dried and collected in a hopper ready for immediate reuse.

Tests have shown that the recycled abrasive performs as well as new abrasive; and under certain conditions could perform better than new, apparently since some of the abrasives used contain impurities. Once the new abrasive has been used for cutting, the softer impurities tend to be broken down more quickly, leaving a higher percentage of garnet in the recycled abrasive.

Combination waterjet/plasma systems are now available in standard configurations, offering the best of both worlds for those who do a lot of heavy cutting. One particular manufacturer offers plasma, routers, optical tracing eyes, and remote and infrared hand-held pendants as standard operations. The improvements in waterjet systems continue to simplify the application of these technologically advanced systems. This multifunctionality is enabling contract cutting shops to offer expanded services, all with one system setup.

WARDJet is the sole distributor of the WARD 24 (Now known as WARD). Live testing of prospective clients’ sludge is available at its facility in Tallmadge, OH, provided a representative from the interested company is present to witness the results. An interactive, multimedia CD and free samples of recycled abrasive are available on request: call 330-677-9100; fax 330-677-9121; e-mail sales@wardjet.com; or www.wardjet.com.