There are many reasons for the rapid acceptance of waterjet technology. It is a cold-cutting process that can cut virtually any material without adding heat or stress, and is extremely easy to set up and operate. These advantages allow manufacturers to produce small or large batches of parts quickly for even their toughest projects.
HyperPressure vs. Normal Pressure
ในช่วงแรกที่มีการนำระบบวอเตอร์เจ็ทแรงดันระดับอัลตราไฮ (Ultra High Pressure หรือ UHP) มาใช้นั้น ค่าแรงดันมักจะอยู่ในช่วง 36,000 พีเอสไอ แต่ค่าแรงดันนี้เพิ่มสูงขึ้นเรื่อยๆ ในทุกทศวรรษ จนเป็น 55,000 พีเอสไอในช่วงปลายทศวรรษ 1980 จนกระทั่งเพิ่มขึ้นเป็น 60,000 พีเอสไอในช่วงกลางทศวรรษ 1990 ซึ่งเป็นค่ามาตรฐานที่ใช้มาจนถึงปัจจุบัน ต่อมาในปี 2004 บริษัท Flow International ได้เปิดตัวปั๊ม HyperJet ที่ให้แรงดันพิกัดอยู่ที่ 94,000 พีเอสไอ สำหรับใช้กับสภาพแวดล้อมการผลิตมาตรฐาน ความก้าวหน้าดังกล่าวถือเป็นการเริ่มยุคใหม่ของระบบตัดเฉือนวอเตอร์เจ็ทแบบ HyperPressure
When ultrahigh-pressure (UHP) waterjets were first introduced, pressures were in the 36,000 psi range. Every decade since, pressure has increased, moving to 55,000 psi by the end of the 1980’s, and reaching the current standard of 60,000 psi in the mid 1990’s. In 2004, Flow International introduced the HyperJet pump, rated at 94,000 psi, into standard manufacturing environments. This breakthrough began the era of HyperPressure cutting with waterjets.
HyperPressure is defined as pressure at or greater than 75,000 psi. With abrasive waterjet cutting, it is actually the abrasive particles within the waterjet stream that erode the material and make the separation cut. The water is the abrasive accelerator. Higher pressure increases the kinetic energy of the abrasive particles contained within it. Water and abrasive particles move faster, the jet diameter becomes smaller, and the jet’s power density and efficiency increases.
Manufacturers quickly discovered that when compared to other pumps that operate at approximately 60,000 psi,the HyperPressure pump dramatically improved productivity. Just as increasing wattage increases CO2 laser cutting productivity, increasing pressure significantly improves waterjet productivity.
However, beyond the approach of raising pressure, many other alternatives to improve productivity were attempted: increasing the horsepower, running multiple heads, using very aggressive abrasives, and optimizing tool paths, to name a few. Of these, the only improvement that has held true is the optimizing of tool paths. Today, advanced waterjet machine tools have tool path optimization that speed up on straight lines and slow down on tight geometry to control finished part anomalies caused by stream lag and also to shorten part cycle times. Even more advanced systems have taper compensation where an articulated wrist tilts the head over slightly to compensate for the naturally occurring V-shaped taper produced by waterjet cutting. The other attempts did not produce efficiency gains for a number of reasons. Increasing horsepower cuts faster but demands a proportional amount of additional abrasive, driving costs up. Adding heads splits the power between the heads doing little for throughput and requiring the operator to ensure both are cutting at precisely the same level. Using more aggressive abrasive drives up operating costs by virtue of the high abrasive cost and also the rapid erosion of the mixing tube nozzle (5 to 10x faster).
Pressure = Productivity
Raising the pressure improves efficiency. Increasing pressure speeds up cutting and reduces cost per inch. At 60,000 psi,the garnet abrasive accounts for over half of the machine operating cost. Running continuously at 87,000 psi, the abrasive cost falls to less than half. Pierce time – the amount of time to drill a start hole – is dramatically reduced as well.
Shorter cycle times mean more parts produced per hour and more jobs completed per day. Fixed cost such as building space, overhead, and equipment depreciation are covered faster which adds to bottom line profitability.
Higher pressure also enables greater cutting detail due to the smaller stream diameter. Maintenance is easier since the new pump is designed
More energy from the waterjet stream is focused on a smaller area, making the abrasive particles more efficient. In other words, each abrasive particle performs more erosion. That power density increases in relation to operating pressure to the power of 1.5 is expressed in this formula | Eda = KP1.5where Eda is power density |
HyperPressure waterjets are ideal for high production environments and where fast turnaround is required. It is also perfect for applications where precision parts are required, since the 25% smaller diameter stream enables more intricate inside corner cutting – commonly down to a 0.015 inch radius.
Velocity Matters
Pressure equals productivity and efficiency because of jet velocity. Why is that so? As pressure goes up, the speed of the stream increases. Once the stream exits the orifice, it’s all about velocity. There is no pressure in the waterjet stream once it exits the cutting head; pressure in the water has been converted to velocity as the water exits the waterjet orifice. A faster and smaller waterjet stream means the abrasive particles move faster, carry more momentum, and remove more material, more aggressively. Less abrasive is used per length of cut because each grain can erode more material. The goal is to make the abrasive go as fast as possible. Stream velocity is the key to efficiency.
The only way to make a waterjet stream go faster is to raise pressure – not through increasing horsepower. This might seem counter-intuitive, but it is true. Every pump has a maximum operating pressure so to gain the benefits of a higher velocity stream you must have a pump designed to operate at higher pressure.
Article by FLOW & MEGA Tech Magazine