SHINHOP laser

With the development of railway freight cars, heavy load, speed, and lightweight have become their main development directions. The design of railway freight car products shows diversity, and the structural characteristics are becoming increasingly complex, leading to an increasing variety and quantity of irregular sheet metal parts. From an economic perspective, cutting and blanking methods must be used for processing.
Especially in recent years, new products of railway freight cars have widely used materials such as stainless steel and aluminum alloy, which have extremely high requirements for cross-section quality and heat-affected zones. This has exacerbated the current situation of insufficient cutting capacity and low manufacturing process levels in the railway freight car manufacturing industry.
Therefore, many railway freight car manufacturing plants urgently need to purchase key equipment such as laser cutting machines to solve the current capacity bottleneck and improve quality and efficiency.

Selection of Laser Cutting Machines

Currently, there are two main types of laser cutting machines on the market: CO2 laser cutting machines and fiber laser cutting machines. CO2 laser cutting machines generate laser by passing carbon dioxide, nitrogen, and helium gas in proportion through a pair of positive and negative electrodes at several thousand times atmospheric pressure, and then use a resonant cavity to extract usable laser beams, which are transmitted to the cutting head through a flying light path. The laser is focused on the workpiece using a focusing lens, utilizing the singularity and high temperature of the laser beam to melt, vaporize, and ionize materials, and then combined with auxiliary cutting gas to blow away the melted, vaporized, and ionized materials to create a cutting seam for cutting processing. (In contrast, fiber laser cutting machines concentrate and excite a large number of semiconductor tubes to couple several laser beams, which are then transmitted to the cutting head through optical fibers.)

Currently, the power of CO2 laser cutting machines on the market in China does not exceed 6kW, and the cutting head's moving range does not exceed 2m×6m (the reason is that if the flying light path exceeds 9m, the laser beam will attenuate, affecting the focusing modulus, and thus affecting cutting quality). The mode is generally TM01 (some manufacturers promise to reach TM00), the photoelectric conversion rate is generally average, and a water cooling system is needed to dissipate excess heat. Compressed air is also required to protect and cool the flying light path and other parts. This type of equipment has a significant market share due to its early entry into the Chinese market, reliable and mature technology, a wide range of cutting thicknesses, easy procurement of consumables, a complete auxiliary gas supply chain, and numerous manufacturers. As shown in Figure 1, the commercialization of fiber laser cutting machines is relatively short, and their market share is small, mainly because their wavelength is much shorter, only 1/10 of that of CO2 laser cutting machines, which makes it easier for materials to absorb heat and produce heat-affected zones. The common thickness range of sheet metal parts for railway freight cars is between 2 and 10mm, with materials including carbon steel, stainless steel (ferritic and austenitic), aluminum alloy, etc. The dimensional accuracy requirements are high, and great attention is paid to the size of the cutting heat-affected zone. Therefore, this disadvantage is unacceptable for railway freight car manufacturing, and the procurement of railway freight cars should primarily focus on CO2 laser cutting machines.

Installation Precautions for Laser Cutting Machines

Many companies believe that once a CO2 laser cutting machine is purchased and connected to power, it can be used immediately, but this is not the case. The equipment also requires compressed air, cooling water, a stabilized power supply, pressure reducing valves, three types of laser generating gases, two types of auxiliary cutting gases, and a constant temperature room, among other supporting facilities. If using a Dewar bottle, a pressure reducing vaporization device and a filtration device are also needed.

Laser cutting machines have high environmental requirements. Vibrations from surrounding equipment can greatly affect their processing accuracy. Therefore, during civil construction, a vibration reduction trench should be set up around the outermost part of the foundation. In northern cities, winters are cold, and summers are hot, with large temperature differences between morning and evening, which greatly affects components such as lenses. Therefore, it is often necessary to install the main body in a constant temperature room, but since loading and unloading are also required, the external exchange workbench must be arranged outside the constant temperature room, passing through the wall, as shown in Figure 2.

 External Exchange Workbench

In addition, the design of the laser cutting machine body takes into account its own resonance factors to ensure accuracy. Therefore, the constant temperature room cannot have rigid contact with the machine body. There needs to be a vibration reduction interlayer and insulation layer between the main machine and the constant temperature room. The cooling device and electrical control box generate a lot of heat during cutting. To reduce the pressure on the air conditioning system in the constant temperature room, a direct exhaust port to the outside of the constant temperature room is usually set up, as shown in Figure 3. There is also a small trick: a pull-out cover can be opened here. Its function is to discharge hot air to the outside of the constant temperature room in summer and to direct hot air into the constant temperature room in winter to save energy.

Figure: Exhaust Port

Moreover, since the dust removal system's filter bags and waste drawer need to be cleaned regularly, the design of the constant temperature room should fully consider the working area and also pay attention to maintenance passages.

The deionized water required for the water cooling system is often overlooked. During the initial installation phase, 1500 liters of distilled water (industrial distilled water) with a conductivity of less than 10 µS/cm is necessary for the cooling system. Of this, 500 liters is used for the initial tank cleaning; 500 liters is allocated for the debugging and acceptance phase; and the final 500 liters is added to the water cooler after acceptance, ensuring it is replaced at least every six months. Since domestic distilled water often fails to meet standards, purified water is typically used as a substitute. Furthermore, drainage pipes should be installed at the equipment drainage outlet to facilitate the discharge of waste cooling water.

Quality Control of Laser Cutting Machines

Laser cutting machines are pre-set with reference cutting parameter values for various materials and thicknesses. Generally, after calling the cutting program, these can be directly selected for cutting. However, since railway freight cars are mostly made of medium-thick plates, the cutting quality is often poor, resulting in incomplete cuts and slag hanging. This situation is quite troublesome for operators. So how can quality be improved? The main factors can be divided into four categories:

(1) Laser Generator: output power, laser mode, laser polarization, beam parallelism, laser straightness; among these, only output power can be adjusted appropriately within the rated range, while the other influencing factors mainly rely on the equipment's factory quality and specific selection.

(2) Cutting Head: suitable cutting lens, clean cutting lens, accurate focal position, nozzle type, nozzle size, whether the beam is centered in the nozzle. Different thicknesses of plates require different cutting lenses. Manufacturers generally provide 5-inch, 7.5-inch, and 10-inch focusing lenses. Apart from Japanese manufacturers, most lens manufacturers are from the American II-VI infrared company. Before each cutting, operators should clean the lens, select the appropriate nozzle, and check whether the cutting head is perpendicular to the workpiece.

(3) Programming: reliable cutting path, pre-cutting. The cutting program should set the path as simply as possible, paying attention to functions such as micro-connection; it is best to perform a pre-cutting before cutting to confirm correctness, as shown in Figure 4.

Figure: Program Control Interface

(4) Suitable process parameters: piercing point, nozzle height, power, cutting speed, auxiliary gas purity, and outlet gas pressure.

If the piercing point is on the part contour line, obvious burn marks will still be visible, so it is better to start the arc outside the contour line.

In fact, the factors that most affect cutting quality are auxiliary gas purity, outlet gas pressure, cutting speed, and focal position. It is recommended to first use auxiliary gas with higher purity, then try to increase the outlet gas pressure, adjust the cutting speed, and finally try to change the focal position for gradual adjustment. Once the cutting quality is stable, solidify it into the processing parameters and CNC program, but be careful not to damage the original parameters; a new one should be saved.

When adjusting the outlet gas pressure and cutting speed for different plates and thicknesses, it can be based on adjusting carbon steel thin plates with "high pressure and high speed" and thick plates with "low pressure and low speed", while adjusting stainless steel and aluminum alloy thin plates with "low pressure and high speed" and thick plates with "high pressure and low speed". Additionally, generally, carbon steel cutting uses O2 piercing O2 cutting, while stainless steel and aluminum alloy use O2 piercing N2 cutting, but experiments have found that for medium-thick plates of railway freight cars, using N2 piercing N2 cutting yields better results.

There was once a user who asked why the cutting surface of medium-thick carbon steel did not show a blue dense smooth state. In fact, as long as the outlet gas pressure of the auxiliary gas is increased, it will generally appear. I hope the above suggestions are tried by more users.

Conclusion

In fact, CO2 laser cutting machines have basically developed to their limits due to constraints such as output power, while fiber laser cutting machines are gradually commercialized with the high-power laser generators from IPG in the United States, and with the continuous maturity of technology, the shortcomings of shorter wavelengths will be compensated in the future, harmful light in the coupled beam will also be controlled, and the slag hanging phenomenon will improve. Moreover, apart from needing to consume auxiliary gas and a small amount of lenses, it only requires electrical energy, and with high electro-optical conversion rates, it will inevitably become the main cutting equipment in the future, especially in the cutting of medium-thick plates for railway freight cars, completely capable of replacing plasma cutting machines and flame cutting machines. Of course, its application fields will be even broader, truly becoming a high-quality, efficient, and energy-saving main cutting equipment.