1. Influence of cutting speed on cutting effect
For a given laser power density and material, the cutting speed conforms to an empirical formula. As long as it is above the pass threshold, the cutting speed of the material is proportional to the laser power density, that is, increasing the power density can improve the cutting speed. The power density referred to here is not only related to the laser output power, but also to the beam quality mode. In addition, the characteristics of the beam focusing system, that is, the size of the focused spot, also have a great impact on laser cutting. The cutting speed is inversely proportional to the density (specific gravity) and thickness of the material to be cut.
When other parameters remain unchanged, the factors to increase the cutting speed are: increase the power (within a certain range, such as 500~2000W); Improve beam mode; Reduce the size of the focus spot (for example, focus with a short focus lens); Cutting materials with low initial evaporation energy (such as plastic, plexiglass, etc.); Cutting low-density materials (such as white pine); Cut thin material.
Especially for metal materials, when other process variables are kept constant, the laser cutting speed can have a relative adjustment range and still maintain a satisfactory cutting quality. This adjustment range is slightly wider than that of thick pieces when cutting thin metals. Sometimes, the cutting speed is too slow, which will lead to the ablation surface of the discharged hot melt material, making the cut surface very rough.
2. Influence of focus position adjustment on cutting quality
Because the laser power density has a great influence on the cutting speed, the selection of lens focal length is an important issue. After the laser beam is focused, the spot size is proportional to the focal length of the lens. After the beam is focused by the short focal length lens, the spot size is very small, and the power density at the focal point is very high, which is favorable for material cutting; However, its disadvantages are that the focal depth is very short and the adjustment margin is small, which is generally suitable for high-speed cutting of thin materials. As long focal length lens has wide focal depth, it is more suitable for cutting thick workpieces as long as it has sufficient power density.
After determining which focal length lens to use, the relative position of the focus and the workpiece surface is particularly important to ensure the cutting quality. Because the power density at the focus is the highest, in most cases, the focus position is just on the workpiece surface or slightly below the surface when cutting. In the whole cutting process, it is an important condition to ensure that the relative position between the focus and the workpiece is constant to obtain stable cutting quality. Sometimes, the lens is heated due to poor cooling during operation, resulting in a change in focal length, which requires timely adjustment of the focus position.
When the focus is at the best position, the kerf is the smallest and the efficiency is the highest. The best cutting speed can obtain the best cutting result. In most applications, the beam focus is adjusted just below the nozzle. The distance between the nozzle and the workpiece surface is generally about 1.5mm.
3. Influence of auxiliary gas pressure on cutting effect
Generally, auxiliary gas is required for material cutting, and the problem mainly involves the type and pressure of auxiliary gas. Generally, the auxiliary gas and laser beam are ejected coaxially to protect the lens from contamination and blow away the slag at the bottom of the cutting area. For non-metallic materials and some metal materials, use compressed air or inert gas to remove melted and evaporated materials, while inhibiting excessive combustion in the cutting area.
For most metal laser cutting, active gas (as long as it is O2) is used to form oxidation exothermic reaction with hot metal. This additional heat can increase the cutting speed by 1/3~1/2.
On the premise of ensuring the auxiliary gas, the gas pressure is a very important factor. When cutting thin materials at high speed, high gas pressure is required to prevent slag sticking on the back of the cut (hot slag will damage the cut edge when it hits the workpiece). When the material thickness increases or the cutting speed is slow, the gas pressure should be reduced appropriately. In order to prevent the plastic cutting edge from frosting, it is better to cut at a lower gas pressure.
The practice of laser cutting shows that when the auxiliary gas is O2, its purity has a significant impact on the cutting quality. A 2% reduction in O2 purity will reduce the cutting speed by 50% and lead to a significant deterioration in the quality of the incision.
4. Reflectivity of material surface
For 10.6 mm far-infrared beam emitted by CO2 laser cutting machine, non-metallic materials can absorb it well, that is, they have high absorptivity; Metal materials have poor absorption of 10.6mm beam, especially gold, silver, copper and aluminum metals with high reflectivity. Generally, CO2 laser beam, especially continuous wave beam, is not suitable for cutting such materials. For aluminum and copper metals, it generally requires more than 3kW to form sufficient initial power to obtain the initial holes required for penetration effect. Ferrous steel materials, nickel, titanium, etc. have a certain absorption rate for 10.6 mm CO2 beam, especially when the material surface is heated to a certain temperature or oxide film, its absorption rate will be greatly improved, so as to obtain better cutting effect. For opaque materials, the absorptivity=(1 - reflectivity) is related to the surface state, temperature and wavelength of the material.
The absorbance of the material to the beam plays an important role in the initial stage of heating, but once the holes in the workpiece are formed, the blackbody effect of the holes makes the absorbance of the material to the beam close to 100%.
The surface state of materials directly affects the absorption of light beams, especially the surface roughness and surface oxide layer will cause obvious changes in the surface absorptivity. In the practice of laser cutting, sometimes the effect of material surface state on beam absorption can be used to improve the cutting performance of materials.
5. Influence of cutting torch and nozzle
The design and manufacture of cutting torch have an important influence on obtaining good cutting quality, especially the nozzle. If the nozzle is not properly selected or maintained, it is easy to cause pollution or damage, or the roundness of the nozzle mouth is not good or the hot metal splashes cause local blockage, which will form eddy currents in the nozzle, resulting in significant deterioration of cutting performance. Sometimes, the nozzle mouth is different from the axis of the focused beam, forming a beam to cut the nozzle edge, which will also affect the trimming quality, increase the slit width and make the cutting size misaligned. For the nozzle, two problems should be paid special attention to, namely, the nozzle diameter and the distance between the nozzle and the work-piece surface.
6. Influence of external optical system
The original beam emitted by the laser is transmitted through the external optical path system (including reflection and transmission), and accurately shines on the surface of the workpiece with extremely high power density.
The optical elements of the external optical path system shall be checked regularly and adjusted in time to ensure that when the cutting torch is running above the workpiece, the beam is correctly transmitted to the center of the lens and focused into a small light point to cut the workpiece with high quality. Once the position of any optical element changes or is polluted, the cutting quality will be affected, or even the cutting cannot be carried out.
The external optical path lens is polluted by impurities in the air flow, splashed particles in the cutting area are bonded, or the lens is not sufficiently cooled, which will overheat the lens and affect the beam energy transmission. It will cause the drift of the collimation of the optical path and lead to serious consequences. Overheating of the lens will also cause focal distortion, and even endanger the lens itself.
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Six Factors Affecting Processing Effect of Laser Cutting Machine
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