Basic knowledge that you must master in laser cutting

        As early as the 1970s, lasers were first used for cutting. In modern industrial production, laser cutting is more widely used in the processing of sheet metal, plastics, glass, ceramics, semiconductors, as well as materials such as textiles, wood, and paper. In the coming years, the application of laser cutting in precision machining and micro machining fields will also see substantial growth.

laser cutting

     When the focused laser beam shines on the workpiece, the irradiation area will rapidly heat up to melt or vaporize the material. Once the laser beam penetrates the workpiece, the cutting process begins: the laser beam moves along the contour line while melting the material. Usually, a jet of air is used to blow the molten material away from the incision, leaving a narrow gap between the cutting section and the plate frame, which is almost as wide as the focused laser beam.

flame cutting

    Flame cutting is a standard process used for cutting low-carbon steel, using oxygen as the cutting gas. Oxygen is pressurized up to 6 bar and blown into the incision. There, the heated metal reacts with oxygen: it begins to burn and oxidize. Chemical reactions release a large amount of energy (up to five times the laser energy) to assist the laser beam in cutting.

Figure 1     The laser beam melts the workpiece, 

                                                      and the cutting gas blows away the molten material and slag in the incision

Melting cutting

     Melting cutting is another standard process used in cutting metal. It can also be used to cut other fusible materials, such as ceramics. Use nitrogen or argon gas as the cutting gas, and blow a gas with a pressure of 2-20 bar through the incision. Argon and nitrogen are inert gases, which means they do not react with the molten metal in the incision and only blow them away towards the bottom. Meanwhile, inert gas can protect the cutting edge from air oxidation.

Compressed air cutting

    Compressed air can also be used to cut thin plates. Air pressure of 5-6 bar is sufficient to blow away the molten metal in the incision. Due to nearly 80% of the air being nitrogen gas, compressed air cutting is basically a melting cutting.

Plasma assisted cutting

    If the parameters are selected appropriately, plasma clouds will appear in the plasma assisted melting cutting incision. Plasma clouds are composed of ionized metal vapor and ionized cutting gas. The plasma cloud absorbs the energy of CO2 laser and converts it into the workpiece, coupling more energy to the workpiece, resulting in faster material melting and faster cutting speed. Therefore, this cutting process is also called high-speed plasma cutting.

    Plasma clouds are actually transparent compared to solid lasers, so plasma assisted melting and cutting can only use CO2 lasers

Figure 2

Gasification cutting

     Gasification cutting evaporates materials and minimizes the thermal effect on surrounding materials as much as possible. The above effect can be achieved by using continuous CO2 laser processing to evaporate materials with low heat and high absorption, such as thin plastic film, wood, paper, foam and other non melting materials. Ultra short pulse laser enables this technology to be applied to other materials. Free electrons in metals absorb laser and heat up violently. The laser pulse does not react with molten particles and plasma, and the material sublimates directly without time to transfer energy to the surrounding material in the form of heat. There is no significant thermal effect, melting, or burr formation during picosecond pulse ablation of materials。

Figure 3   Gasification cutting: Laser causes material to evaporate and burn. 

                                                          The pressure of steam causes the slag to be discharged from the incision

Parameter adjustment machining process

F

Polarization degree

 Polarization degree indicates what percentage of laser is converted. The typical polarization degree is generally around 90%. This is sufficient for high-quality cutting.

Focus diameter

The focal diameter affects the width of the incision, and can be changed by changing the focal length of the focusing lens. A smaller focal diameter means a narrower cut

Focus position

The focus position determines the beam diameter and power density on the surface of the workpiece, as well as the shape of the incision.

Figure  4   Focus position: inside the workpiece, 

                                                                      on the surface of the workpiece, and above the workpiece

laser power

The laser power should match the processing type, material type, and thickness. The power must be high enough that the power density on the workpiece exceeds the processing threshold.

Figure  5 Higher laser power can cut thicker materials

Working mode

Continuous mode is mainly used to cut standard contours of metal and plastic in millimeter to centimeter sizes. To melt perforations or produce precise contours, low-frequency pulsed lasers are used.

Cutting speed

The laser power and cutting speed must match each other. Too fast or too slow cutting speed can lead to an increase in roughness and the formation of burrs.

Figure  6 The cutting speed decreases as the thickness of the board increases

Nozzle diameter

The diameter of the nozzle determines the gas flow rate and airflow shape ejected from the nozzle. The thicker the material, the larger the diameter of the gas jet, and correspondingly, the diameter of the nozzle mouth also needs to increase.

Gas purity and pressure

    Oxygen and nitrogen are often used as cutting gases. The purity and pressure of the gas affect the cutting effect. When using oxygen flame cutting, the gas purity needs to reach 99.95%. The thicker the steel plate, the lower the gas pressure used. When using nitrogen for melting and cutting, the gas purity needs to reach 99.995% (ideally 99.999%), and higher air pressure is required for melting and cutting thick steel plates.

Technical parameter table

     In the early stages of laser cutting, users had to determine the setting of processing parameters through trial operation. Now, mature processing parameters are stored in the control device of the cutting system. For each material type and thickness, there is corresponding data. The technical parameter table enables even those unfamiliar with this technology to operate laser cutting equipment smoothly.

Factors for evaluating the quality of laser cutting

     There are many standards for judging the quality of laser cutting edges. Standards such as burrs, indentations, and patterns can be determined with the naked eye; Verticality, roughness, and incision width need to be measured using specialized instruments. Material deposition, corrosion, heat affected areas, and deformation are also important factors in measuring the quality of laser cutting.

Figure  7 Good cutting, bad cutting. Standards for evaluating the quality of cut edges

Broad prospects

The sustained success of laser cutting is beyond the reach of most other processing methods. This trend continues today. In the future, the application prospects of laser cutting will also become increasingly broad.