Key Considerations when Selecting a Laser for Cutting Applications

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- GoPhotonics

Jan 18, 2024

Laser cutting parameters rely on factors such as beam characteristics, the needed cutting rate, material composition and thickness, and the desired quality of the cut edge. The selection of appropriate laser and workpiece parameters is critical for the laser cutting process and achieving desired cut quality. Issues with cutting quality may arise from gradual process variations and disruptions caused by fluctuations in velocity, variations in power, spatial intensity distribution, and disturbances in optical integrity.

Parameters for Laser Cutting

Laser Power: Laser power measured in Watts (W), represents the average energy stored in the laser pulses. Higher power fiber lasers excel at cutting more challenging materials. The required laser power is contingent upon the material type to be cut with the fiber laser cutter. It's crucial to note that laser power is the average output power; for instance, a continuous laser of 100W power emits pulses at 100W, while a pulse laser of 100W power can emit pulses as high as 10,000W.

In continuous-wave laser generators, the cutting process is significantly influenced by the power levels they maintain. Despite the presumed advantages of higher power, the most effective choice might not always be the maximum power setting. Increased cutting power can alter the laser mode, affecting the beam's focal point. To optimize efficiency and quality, a power setting below the maximum often ensures the highest power concentration at the focal point.

Wavelength: Wavelength, measured in nanometers (nm), denotes the color of the laser-emitted light. Specifically, it correlates with the frequency and energy of the electromagnetic radiation produced by the laser. Shorter wavelengths, like ultraviolet, have higher frequencies and, consequently, higher energy compared to longer wavelengths, such as infrared. Reflectivity of metallic materials to laser light is a function of laser wavelength, where metals exhibit higher reflectivity to long infrared wavelengths (CO2 laser wavelength) compared to shorter infrared wavelengths (Nd:YAG laser wavelength). An Nd:YAG beam can be focused to a smaller diameter than a CO2 laser beam, providing increased accuracy, a narrower kerf width, and lower surface roughness.

Laser Energy: Laser energy is a fundamental parameter that determines the intensity of the laser beam used in cutting is expressed in Joules (J). It directly influences the depth and efficiency of material removal. The laser energy level needs to be carefully controlled to ensure optimal cutting results without causing undesirable effects, such as excessive melting or burning. Balancing laser energy with other parameters is crucial for achieving precise and efficient laser cutting across various materials and applications.

Cutting Speed: Cutting speed is the linear length of material that the laser cutter can cut per unit time. It is expressed in inches per minute (IPM) or millimeters per minute (mm/min). Generally, thin materials or high-power fiber lasers result in higher cutting speeds.

Maintaining an appropriate cutting speed is pivotal for superior cutting quality. Incorrect speeds can lead to surface overheating, enlarging heat-affected zones, wider kerfs, and burnt edges, resulting in rough surfaces. Modulating speed controls the average kerf width, especially pronounced with smaller tube diameters. As speed rises, the interaction time lessens, affecting energy absorption, reducing temperature, and narrowing kerf width. Excessive speed may cause incomplete cuts or breaks, impairing overall cutting quality.

Assist Gas Type and Pressure Role: The selection of assist gas - whether compressed air, inert gases like nitrogen for non-metallic and some metallic tubes, or active gases like oxygen for most metallic tubes-plays a pivotal role. Furthermore, determining assist gas pressure is of significant importance: higher pressure is vital for cutting thin-walled tubes at high speeds to prevent slag adherence, while lower pressure is preferred for thicker walls or slower speeds to prevent incomplete cuts. Additionally, maintaining the optimal position of the laser beam's focal point during tube cutting is imperative, ensuring minimized kerf, enhanced cutting efficiency, and superior cutting outcomes.

Pulse Frequency (Pulse Repetition Rate): Pulse frequency, or pulse repetition rate, is a pivotal parameter in laser cutting, specifying the number of laser pulses released per unit of time. Measured in hertz (Hz), it indicates the rate at which the laser beam emits pulses while cutting. This frequency is instrumental in shaping the overall cutting speed, efficiency, and the resulting quality of the cut.

Material Thickness: Laser cutting is significantly influenced by the properties of the material being processed. Its thickness and the required cutting contours define the basic conditions. The material's thickness influences cutting speed, required laser power, and cutting quality. A higher thickness may reduce cut quality in laser cutting.

Specific laser types, such as fiber lasers, demonstrate heightened efficiency in cutting metals such as stainless steel or aluminum, whereas CO2 lasers showcase exceptional performance in cutting non-metals like wood, acrylic, and paper. It is crucial to comprehend the material's thickness; for instance, thicker materials may necessitate lasers with a higher power range.

Type of Material: Laser cutting performance is influenced by different materials with varying physical characteristics, including material reflectivity and thermal conductivity. Reflectivity is often assessed on a scale from 0 to 1, where 0 signifies perfect absorption (no reflection), and 1 denotes perfect reflection (no absorption). Metals, particularly at certain laser wavelengths, exhibit high reflectivity. For instance, materials like aluminum and copper can effectively reflect a significant portion of the laser energy. In contrast, non-metallic materials, such as plastics and wood, typically demonstrate lower reflectivity.

In terms of thermal conductivity, it is commonly measured in watts per meter-kelvin (W/(m·K)). Metals, such as copper and aluminum, are known for their high thermal conductivity. On the other hand, non-metallic materials like ceramics and certain polymers may exhibit lower thermal conductivity.

Cutting Path: A linear cutting path is fast and easy, while complex paths reduce cutting speed and require higher control over fiber lasers. The slowest laser cutting occurs at sharp corners.

Materials Not Suitable for Laser Cutting

  • Certain plastics are unsuitable for laser cutting due to safety concerns and material properties:
    • Polyvinyl Chloride (PVC): Laser cutting produces high concentrations of acids and toxic fumes, posing danger and potential damage to the laser cutter.
    • Polycarbonate: Laser cutting can cause severe discoloration and burning in thicker sections of this material.
    • High-Density Polyethylene (HDPE) and Acrylonitrile Butadiene Styrene (ABS): These plastics melt easily during laser cutting, resulting in poor quality cuts. ABS also emits toxic fumes, including cyanide and various derivatives.
  • Fiberglass, composed of glass and epoxy resin, is challenging to laser cut due to glass reflectivity and the emission of toxic fumes such as hydrogen cyanide and various hydrocarbons from epoxy resin.

Polystyrene and Polypropylene Foam are generally considered too flammable for laser cutting, leading to burn, misshaping, and discoloration at the cutting edge. While traditionally ruled out, advancements in lasers and methods have allowed some facilities to laser cut thinner sections of these foams with appropriate safety measures.

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