1. Influence of cutting speed on cutting quality
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 here is related not only to the laser output power, but also to the beam quality mode. In addition, the characteristics of the beam focusing system, that is, the spot size after focusing, also has a great impact on laser cutting.
The cutting speed is inversely proportional to the density (specific gravity) and thickness of the material being cut. When other parameters remain unchanged, the factors to improve the cutting speed are: increase the power (within a certain range, such as 500 ~ 2000 W); Improve the beam mode (such as from high-order mode to low-order mode to TEM00); Reduce the size of the focusing spot (such as focusing with a short focal length lens); Cutting materials with low initial evaporation energy (such as plastic, plexiglass, etc.); Cutting low-density materials (such as white pine, etc.); Cut thin materials.
Especially for metal materials, when other process variables remain constant, the speed of laser cutting machine can have a relative adjustment range and still maintain a satisfactory cutting quality. This adjustment range is slightly wider than that of thick parts when cutting thin metals. Sometimes, the slow cutting speed will also lead to the discharge of hot melt materials to erode the surface of the mouth, making the cutting 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 choice of lens focal length is an important problem. 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 focus is very high, which is very beneficial to material cutting; However, its disadvantage is that the focal depth is very short and the adjustment allowance is small. It is generally suitable for high-speed cutting of thin materials. Because the long focal length lens has a wide focal depth, as long as it has sufficient power density, it is more suitable for cutting thick workpieces.
After determining which focal length lens to use, the relative position between the focus and the workpiece surface is particularly important to ensure the cutting quality. Due to the high power density at the focus, in most cases, the focus position is just on the workpiece surface or slightly below the surface when cutting. In the whole cutting process, ensuring that the relative position between the focus and the workpiece is constant is an important condition to obtain stable cutting quality. Sometimes, the lens is heated due to poor cooling during operation, resulting in changes in focal length, which requires timely adjustment of focus position.
When the focus is in a better position, the slit is smaller, the efficiency is higher, and a better cutting speed can obtain a better cutting result.
In most applications of laser cutting machines, the beam focus is adjusted to just under the nozzle. The distance between nozzle and workpiece surface is generally about 1.5mm.
3. Influence of auxiliary gas pressure on cutting quality
Generally, auxiliary gas is required for material cutting, and the problem mainly involves the type and pressure of auxiliary gas. Usually, the auxiliary gas is ejected coaxially with the laser beam to protect the lens from pollution and blow away the slag at the bottom of the cutting area. For non-metallic materials and some metallic materials, use compressed air or inert gas to treat the melted and evaporated materials, while restraining the excessive combustion in the cutting area.
For most metal laser cutting, active gas (as long as O2) is used to form an oxidation exothermic reaction with hot metal. This additional heat can increase the cutting speed by 1 / 3 ~ 1 / 2.
Under the premise of ensuring the auxiliary gas, the gas pressure is a very important factor. When cutting thin materials at high speed, higher gas pressure is required to prevent slag sticking on the back of the incision (hot slag sticking to the workpiece will also damage the cutting edge). When the material thickness increases or the cutting speed is slow, the gas pressure should be appropriately reduced. In order to prevent the plastic cutting edge from frosting, it is also better to cut with a lower gas pressure.
The practice of laser cutting shows that when the auxiliary gas is oxygen, its purity has a significant impact on the cutting quality. A 2% reduction in oxygen purity will reduce the cutting speed by 50% and lead to a significant deterioration in the quality of the incision.