When oxygen is used as an auxiliary gas, an exothermic reaction occurs, especially when cutting carbon steel. This phenomenon is documented and widely recognized. In this process, oxygen obviously acts on the laser beam projection point on the metal surface to heat the steel, which brings about an exothermic reaction, causing the temperature of the area to rise to the melting point, and the molten liquid metal flows down the cutting wall and interacts with the beam. The area of action is separated. As the metal melts and peels off, fresh metal is exposed to the surface, and the end result is a fairly smooth cut surface with an oxide layer.
On the other hand, stainless steel has a higher melting point and its characteristics are different. When the quality and thickness of the cut are factors that need to be considered, oxygen assist is usually an unacceptable method because it will generate on the metal surface. Residual oxide. In order to solve this problem, in the early 1990s, people usually used low-pressure nitrogen auxiliary gas. With the emergence of high-pressure optical components in the later period, higher gas transmission pressures have become possible. This is an endothermic process, essentially an evaporation/dissipation process, which does not produce any residual oxides on the cut surface, thereby improving The quality of the cut. Because the auxiliary gas plays a role in removing molten metal from the cutting area in this process, gas pressure and purity become important factors. Higher pressure means greater gas consumption, which means higher operating costs. The nozzle diameter has a greater impact on gas consumption, because cutting thicker materials requires larger nozzles.
The history of people starting to use nitrogen instead of oxygen as an auxiliary gas is not very long, and a business that supplies nitrogen to laser cutting plants has begun to flourish. The use of small and large gas tanks (cylindrical tanks and Dewars) to supply nitrogen has become an industry standard. Nowadays, there are very few tall liquid nitrogen storage devices connected to the production workshop. In the past, it was easy to judge through this device that this company must cut a large amount of stainless steel and other easily oxidized metals, which had to consume a lot of Nitrogen.
The most important component of air is nitrogen (78%), and air is free. A feasible and inexpensive way to obtain nitrogen is to use a nitrogen generator instead of buying large quantities of liquid or gaseous nitrogen. A nitrogen generator can take in air, separate the nitrogen from it and use it in the processing of laser cutting equipment. Companies such as On Site Nitrogen have such equipment that can produce nitrogen with an oxygen content of only 0.0005% or 5PPM. The cost of using the nitrogen generator, in addition to the equipment cost, is the electricity consumed to run the air compressor. Compared with the traditional way of buying gas from local suppliers, most companies believe that the use of nitrogen generators saves a lot of money. Most factories already store compressed air on site, so you only need to pump it into the dryer, then into the nitrogen generator, and then into the storage cylinder and pressurize it. In this way, a cheaper auxiliary gas for stainless steel cutting is obtained.
Some companies, such as O2N2 Site Gas Systems, recognized the need for a nitrogen generation system for laser cutting, so they started to provide customized systems to the laser cutting industry 10 years ago. O2N2 Site also recognizes that it is very easy for the processing workshop to produce nitrogen by itself, which can save up to 80% of the cost compared to purchasing nitrogen. Coupled with the lower installation and operating costs of the nitrogen generator, as well as the always available air source (compressed air) and other factors, it has become its main advantage when used to cut large quantities of stainless steel and other easily oxidized metals.
The company uses pressure swing adsorption (PSA) or membrane technology to separate nitrogen molecules from the air. The compressed air used by PSA is fed into a pressure chamber filled with carbon molecular sieve (CMS). Under pressure, CMS separates molecules from the air, traps nitrogen and discharges oxygen into the air. After that, the nitrogen is pumped out and sent to the storage tank. These customized generators can produce nitrogen with a flow rate of 75,000 CFH (cubic feet per hour) and a purity of 99.9995%. The purity flow rate of nitrogen can be changed, and the user can adjust different nitrogen purity by pressing a button.
Alco Metal Fab, located in Santa Ana, California, first started business in 1948, but was merged into Alco in 1962. The company runs four laser systems: three of them Amada (2~4kW) systems and one Bystronic (3kW) system. The company mainly cuts mild steel and stainless steel up to 1 1/4 inches in thickness.
The company's main original intention for installing laser equipment was to minimize the setup time, which was determined by its large number of short-time processing orders. In the process of using the first Bystronic laser machine, they found that the monthly gas cost was about US$8,000, with two shifts a day. When they bought the second 2kW laser machine (Amada), the monthly gas cost jumped to 10,000~12,000 US dollars, mainly because of the high pressure gas used in cutting. Alco realized that the use of purchased Dewar gas is not economical when a large amount of nitrogen is needed. The company sees the possibility of using large-capacity cryogenic gas storage tanks, which can be used to replace multiple Dewars, but the Santa Ana region's regulations hinder the realization of this idea.
In 2005, Alco decided to find a way to replace canned gas, so it found On-Site Gas Systems. Both parties designed and installed a system at Alco's site to enable it to directly generate nitrogen on site. Alco calculated that the investment in these two equipment can be recovered within 30 months. So far, the company's investment in nitrogen generators is not worth mentioning compared to traditional outsourcing transportation and storage costs. When the company purchased the nitrogen generator, it also purchased the third laser machine, which further reduced the return on investment cycle. When the unit was installed in 2006, Alco added an Amada laser cutting machine.
When Ed Hare, a partner of Alco, was asked whether the nitrogen generator could meet the needs of four laser cutting machines, he replied that the company deliberately set aside a margin in the design stage before purchasing the nitrogen generator Therefore, the incremental demand for laser machines in the later period can also be met. Mr. Hare believes that the only problem the company currently encounters is the need to replace the gasket on the high-pressure pump after 30,000 hours, but this has minimized maintenance costs. The company chose a 400psi high-pressure system to cut thick stainless steel plates.