Understanding Laser Cutting

Understanding Laser Cutting

Understanding Laser Cutting

    Laser cutting is a fabrication process which employs a focused, high-powered laser beam to cut material into custom shapes and designs. This process is suitable for a wide range of materials, including metal, plastic, wood, gemstone, glass, and paper, and can produce precise, intricate, and complex parts without the need for custom-designed tooling.

    There are several different types of laser cutting available, including fusion cutting, oxidation cutting, and scribing. Each laser cutting process can produce parts with precision, accuracy, and high-quality edge finishes, and with generally less material contamination, physical damage, and waste than with other conventional cutting processes, such as mechanical cutting and waterjet cutting. However, while laser cutting demonstrates certain advantages over more conventional cutting processes, some manufacturing applications can be problematic, such as cutting reflective material or material requiring secondary machining and finishing work. The requirements and specifications demanded by a particular cutting application—e.g., materials and their properties, energy and power consumption limits, secondary finishing, etc.—help determine the type of cutting process most suitable for use.

    While each cutting process has its advantages and disadvantages, this article focuses on laser cutting, outlining the basics of the laser cutting process and the necessary components and mechanics of the CNC laser cutting machine. Additionally, the article explores various laser cutting methods and applications, the benefits and limitations of the process, and comparisons between laser cutting and other types of cutting processes.  

    The Laser Cutting Machine and Process

    Laser cutting is a non-contact, thermal-based fabrication process suitable for metal and non-metal materials. For the laser cutting process to run smoothly and at optimum capacity, several factors should be taken into consideration, such as the flatbed CNC laser cutting machine’s configuration and settings, the material being cut and its properties, and the type of laser and assist gas employed.

    Stimulated Emission: The photons that are produced by spontaneous emission travel within the medium, which is contained in a cavity of the laser resonator between two mirrors. One mirror is reflective to keep photons traveling within the medium, so they continue to propagate stimulated emissions, and the other mirror is partially transmissive to allow some photons to escape. Stimulated emission is the process in which a photon (i.e., the incident photon) stimulates an atom that is already at a higher energy level. This interaction forces the stimulated atom to drop to its ground state by emitting a second photon of the same fixed wavelength or coherent with the incident photon.

    The process of one photon propagating the emission of another photon amplifies the strength and intensity of the light beam. Thus the stimulated emission of light photons (i.e., a type of electromagnetic radiation) causes the amplification of light; in other words, light amplification by stimulated emission of radiation. Improperly aligned photons within the resonator pass through the partially transmissive mirror without being reflected into the medium, generating the initial laser beam. Once generated, the beam enters the laser cutting head and is directed by mirrors into the focusing lens.

    Beam Focusing

    The focusing lens focuses the laser beam through the center of the nozzle at the end of the laser cutting head incident to the workpiece’s surface. By focusing the beam, the lens concentrates the beam’s energy into a smaller spot, which increases the beam’s intensity (I). 

    Where P represents the power of the initial laser beam, and πr2 represents the cross-sectional area of the beam. As the lens focuses the laser beam, the radius (r) of the beam decreases; this decrease in radius reduces the cross-sectional area of the beam, which in turn increases its intensity since its power is now distributed across a smaller area.

    Localized Heating and Melting, and Material Ejection 

    As the beam strikes the material’s surface, the material absorbs the radiation, increasing the internal energy and generating heat. The high intensity of the laser beam allows it to heat, melt, and partially or completely vaporize a localized area of the workpiece’s surface. The weakening and removal of the affected area of the material forms the desired cuts. Siphoned into the laser cutting head and flowing coaxially to the focused beam, the assist gas—also referred to as the cutting gas—is used to protect and cool the focusing lens, and may be used to expel melted material out of the kerf—the width of the material removed and of the cut produced—and support the cutting process. Laser cutting employs several different types of material cutting and removal mechanisms, including fusion cutting, chemical degradation cutting, evaporation cutting, scribing, and oxidation cutting.

    Fusion Cutting: Also referred to as inert gas melt shearing or inert gas cutting, fusion cutting is employed by CO2 and Nd:YAG laser cutting machines. The laser beam produced by the cutting machine melts the workpiece, and melted material is expelled through the bottom of the kerf by a jet of the assist gas employed. The assist gas and the assist gas pressure employed are dependent on the type of material being cut, but the inert gas is always chosen based on its lack of chemical reactivity in regards to the material. This mechanism is suitable for laser cutting most metals and thermoplastics.

    Chemical Degradation: Chemical degradation is employed by high end laser cutting machine and is suitable for laser cutting thermoset polymers and organic material, such as wood. As thermoset and organic materials do not melt when heat is applied, the laser beam burns the material instead, reducing it to carbon and smoke.

    Evaporation Cutting: Evaporation cutting is employed by CO2 laser cutting machines and is suitable for materials such as laser cutting acrylic and polyacetal due to the closeness of their melting and boiling points. Since the laser evaporates material evaporates along the cut, the edge produced is generally glossy and polished.

    Scribing: Scribing is employed by CO2 and Nd:YAG laser cutting machines to produce partial or fully penetrating grooves or perforations, usually on ceramics or silicon chips. These grooves and perforations allow for mechanical breaking along the weakened structural lines.

    Oxidation Cutting: Also referred to as flame oxygen cutting, oxidation cutting is employed by CO2 and Nd:YAG laser cutting machines and is suitable for laser cutting of mild and carbon steel. Oxidation cutting is one example of the reactive gas melt shearing cutting mechanism, which specifically employs chemically reactive assist gases. As with inertness, the reactivity of an assist gas is relative to the material being cut. Oxidation cutting, as the name implies, employs oxygen as the assist gas, which exothermically reacts with the material. The heat generated accelerates the cutting process and produces an oxidized melted edge which can be easily removed by a gas jet to allow for a cleaner, laser-cut edge.

    Beam Movement

    Once the localized heating, melting, or vaporizing has started, the machine moves the area of material removal across the workpiece to produce the full cut. The machine achieves the movement either by adjusting the reflective mirrors, controlling the laser cutting head, or manipulating the workpiece. There are three different configurations for low power laser cutting machine, defined by the way in which the laser beam moves or is moved over the material: moving material, flying optics, and hybrid laser cutting systems.

    Moving Material: Moving material laser cutting machines feature a stationary laser beam and a movable cutting surface to which the material is affixed. The workpiece is mechanically moved around the stationary beam to produce the necessary cuts. This configuration allows for a uniform and consistent standoff distance and requires fewer optical components.

    Flying Optics: Flying optics laser cutting machines feature a movable laser cutter head and a stationary workpiece. The cutting head moves the beam across the stationary workpiece in the X- and Y-axes to produce the necessary cuts. The flexibility of flying optics machines is suitable for cutting materials with variable thickness and sizes, as well as allowing for faster processing times. However, since the beam is continually moving, the changing beam length has to be taken into consideration throughout the process. The changing beam length can be controlled by collimation (alignment of the optics), using a constant beam length axis, or employing an adaptive optics or capacitive height control system capable of making the necessary adjustments in real time.

    Hybrid: Hybrid high power laser cutting machine offer a combination of the attributes found on moving material and flying optics machines. These machines feature a material handling table that moves on one axis (usually the X-axis) and a laser head that moves on another (usually the Y-axis). Hybrid systems allow for more consistent beam delivery, and reduced power loss and greater capacity per watt compared to flying optics systems.

    Lasers are produced as either pulsed beams or continuous wave beams. The suitability of each depends on the properties of the material being cut and the requirements of the laser cutting applications. Pulsed beams are produced as short bursts of power output, while continuous wave beams are produced as continuous, high power output. The former is typically employed for scribing or evaporation cutting applications and is suitable for cutting delicate designs or piercing through thick materials, while the latter is suitable for high-efficiency and high-speed cutting applications.

    Types of Assist Gases

    Laser cutting employs a variety of assist gases to aid the cutting process. The cutting process employed and the material being cut determine the type of assist gas—either inert or active—that is most suitable for use.

    Inert gas cutting (i.e., fusion cutting or inert gas melt shearing), as indicated by the name, employs chemically inert assist gases. The particular assist gas employed depends on the material’s reactive properties. For example, since molten thermoplastics do not react with nitrogen and oxygen, compressed air can be used as the assist gas when laser cutting such materials. On the other hand, since molten titanium does react with nitrogen and oxygen, argon—or another similarly chemically inert gas—must be used as the assist gas in laser cutting applications involving this material. When laser cutting stainless steel via the inert gas cutting process, nitrogen is typically used as the assist gas; this is because molten stainless steel chemically reacts with oxygen.

    When laser cutting material via the reactive melt shearing process, an active (i.e., chemically reactive) assist gas—typically oxygen—is employed to accelerate the cutting process. While in inert gas cutting the material is heated, melted, and vaporized solely by the power of the laser, in reactive gas cutting the reaction between the assist gas and the material creates additional heat which aids the cutting process. Because of this exothermic reaction, reactive gas cutting typically requires lower laser power levels to cut through a material compared to the power level necessary when cutting the same material via the inert gas cutting process.

    The cutting pressure of the assist gas employed is also determined by the cutting process employed and the properties and thickness of the material being cut. For example, polymers typically require gas jet pressures of 2–6 bar during the inert gas cutting process, while stainless steel requires gas jet pressures of 8–14 bar. Accordingly, thinner materials also generally require lower pressures, and thicker materials generally require greater pressures. In oxidation cutting, the opposite is true: the thicker the material, the lower the pressure required and the thinner the material, the higher the pressure required.

    Types of Laser Cutting Machines

    There are several types of laser cutting machines available which are categorized into gas, liquid, and solid state lasers. The types are differentiated based on the state of the active laser medium—i.e., whether the medium is a gas, liquid, or solid material—and what the active laser medium consists of (e.g., CO2, Nd:YAG, etc.). The main two types of lasers employed are CO2 and solid-state lasers.

    One of the most commonly employed gas state lasers, a CO2 laser employs a carbon dioxide mixture as the active laser medium. CO2 lasers are typically used to cut non-metal materials since early models were not powerful enough to cut through metals. Laser technology has since evolved to enable CO2 lasers to cut through metals, but CO2 lasers are still better suited for cutting through non-metals and organic materials (such as rubber, leather, or wood) and simply engraving metals or other hard materials. Pure nitrogen lasers are another commonly used gas state laser. These lasers are used for applications that require the material not oxidize as it is cut.

    There are several varieties of solid-state lasers available, including crystal and fiber lasers. Crystal lasers employ a variety of crystal mediums—e.g., neodymium-doped yttrium aluminum garnet (Nd:YAG) or neodymium-doped yttrium orthovanadate (Nd:YVO4)—which allow for high-powered metal and non-metal laser cutting. Although versatile in regards to their material cutting capabilities, crystal lasers are typically more expensive and have shorter lifespans than other types of lasers. Fiber lasers offer a cheaper and longer lasting alternative to crystal lasers. This type of laser first generates a beam through a series of laser diodes which is then transmitted through optical fibers, amplified, and focused on the workpiece to perform the necessary cuts.

    Laser Cutting Machine Considerations

    As described in the previous section, the type of laser suitable for a laser cutting application is largely determined by the material being cut. However, other considerations may be taken into account when choosing and setting up a laser cutting machine for a specific application, such as the machine configuration, laser power, wavelength, temporal mode, spatial mode, and focal spot size.