Laser cutting

Laser cutting, and laser cutting is a thermal separation process for plate-shaped material (usually metal sheets, and wood panels and organic materials ) and 3- dimensional body ( eg tubes or profiles) using a laser.

The method is used where complex contours (two - or three-dimensional), a precise, fast processing ( typically 10 m / min, but also to over 100 m / min), the production of three breakthroughs (including hard to reach areas ) and / or a non-contact, almost force-free processing are required. Compared to alternative methods such as punching, laser cutting can be used economically even at very low quantities.

To combine the advantages of laser cutting with those of nibbling and punching, the manufacturers also offer combined machines that allow both operations to the punch head and the laser machining any contours.

For use are focused high-power laser, most of the CO2 laser ( gas lasers) or increasingly Nd: YAG lasers ( solid-state lasers ) as well as the efficient, well- focusable fiber laser.

• 3.1 Pollutants and Safety

Components and design

Important elements of a laser cutting machine, the laser beam source, the laser beam guide and the machining head ( focusing optics ) including cutting nozzle. The laser beam source beam leaving may in the near infrared (Nd: YAG lasers, fiber lasers, disk lasers ) over fiber optic cable, be guided during CO2 laser on deflection mirror for focusing optics at the machining point. The focusing optical system focuses the laser beam into a focus, thus producing the required cutting depth.

Plants with CO2 lasers usually consist of a stationary laser beam source and a so-called flying optics. A reflecting telescope ensured over the entire working space a constant raw beam diameter on the focusing lens. This is necessary, since the beam exiting from the laser has a fixed divergence. Different barrel lengths of radiation for different processing positions would change without compensation by the raw beam diameter mirror telescope on the lens. Different F- numbers and intensities would result.

The beamline between resonator ( laser source) and focusing is realized by optionally water-cooled mirror. The mirrors are gold - or molybdenum- coated and made of monocrystalline silicon or pure copper. Laser radiation in a wavelength range of about 1 micron (Nd: YAG lasers, fiber lasers, disk lasers ), however, can also be conducted over long distances with fiber optic cable.

For a directional cut quality phase- rotating mirrors are arranged in linearly polarized laser beams between resonator and telescope. Of a linearly polarized beam a circularly polarized beam is usually generated. The mirror used have a multilayer coating whose function of a lambda / 4 plate equals. The often folded resonator of CO2 lasers causes a linear polarization and makes the use of a phase shifter required. The polarization-dependent absorption of laser radiation in the kerf would lead to a directional edge quality and cutting efficiency.

The focusing optics, also called processing head consists of Nd: YAG lasers, and other lasers in the near infrared range of a particular glass, in carbon dioxide lasers of single crystal of zinc selenide or an off-axis parabolic mirror made ​​of copper. The beam is focused by the so-called cutting nozzle, which consists mostly of copper and also the blow - bzw.Prozessgas redirected to the processing site.

Method

Laser cutting is composed of two concurrent subtasks. First, it is because the focused laser beam is absorbed at the cutting front, and so introduces the energy required for cutting. On the other hand, the concentrically arranged for laser cutting nozzle, the process gas or blowing gas ready, which protects the focusing optics from vapors and splashes and continues to drive the ablated material from the kerf. Depending on the achieved in the active region temperature and supplied Prozessgasart zones differing physical states of the joint material. It is depending on whether the material is a liquid, or steam oxidation product is removed from the kerf, divided into the three variants laser beam fusion cutting, laser cutting and burning Laserstrahlsublimierschneiden.

Currently, the maximum workable thicknesses for steel at about 40 mm for stainless steel at 50 mm above; Aluminum is cut to about 25 mm with laser. It is to cut than steel technically more complicated, such as aluminum or copper, as the majority of the applied radiation is first reflected and therefore a much larger power or power density during penetration is required. Even if a major proportion of power to be absorbed in the kerf during cutting, the cutting efficiency is very much less than the case of iron materials, as the thermal conductivity of aluminum and copper is much higher and does not support oxidation will prevail.

Copper and other metals conduct heat well are difficult or can not be cut with the CO2 laser. However, this does not depend only on the thermal conductivity, but the fact that a very large proportion of the introduced radiation is reflected, and the material is thus hardly heated. For thin plates, however pulsed Nd can: YAG lasers are used - with all these materials can be cut.

The most critical process during laser firing and fusion cutting is piercing. It is time consuming because often must be worked at a reduced medium pulsed laser output, in order to prevent back-reflection and the focusing hazardous metal splashes. Modern laser machines have sensors with which the puncture was made can be detected in order to save on operating time and to ensure that the start of cut does not take place before the complete piercing of the material.

In laser cutting of steel hardening takes place at the cutting edges due to the high temperature differences. This may in subsequent processing lead to problems.

Flat material is cutting on a support ( teeth, tips, edges) must meet several conditions:

• Smallest possible contact area - waste or small parts must fall through
• Low back reflection - otherwise possible damage to the workpieces from the bottom or the laser beam source
• High resistance to Laserstrahlabtrag - long maintenance intervals

The procedures are divided as follows:

Laser beam fusion cutting

The formation of the kerf happens when fusion cutting by continuously melting and blowing out of the cutting material with an inert or inert gas. Of the gas jet in addition prevents oxidation of the surface. For cost reasons, mainly nitrogen, argon or helium rarely used. The gas pressures reached in this case up to 22 bar (high pressure inert gas cutting). Due to the low absorption coefficient of the material, the cutting speeds for fusion cutting include depending on the available laser power. So a typical cutting speed of 1.1 m / min is achieved with a 5 kW CO2 laser cutting machine with 10 mm thick stainless steel 1.4301. The procedure usually is used when oxide-free cut joints are required on stainless steels. Aluminum alloys and high-melting non-ferrous alloys represent another application dar. also normal structural steel is sometimes cut in thicknesses up to about 6 to 10 mm with nitrogen, since the cut edges for subsequent painting or powder coating no longer need to be reworked.

A high cutting quality is characterized by a low scoring on the cut edges and the absence of formation of burrs at the bottom of the cut. Of the laser beam, the material is liquefied in this case not only at the cutting front, but a semi-circle on the cutting edges. By the continuous feeding, and the resultant melting of the material, the material can solidify at the cut edges. The solidification occurs this wavy, which accompanied defines the characteristic ridge structure and the roughness of the cut. A beard or burrs caused by insufficient driving forces of the gas flow, so that the melt can not be completely expelled. Melt drops on the cut bottom edge can solidify and form a more or less strongly adhering beard / ridge. The parameters affecting the quality of the cut, are, inter alia, the focus position, the feed speed, the laser power or the intensity distribution of the cutting gas.

Laser flame cutting

The most common variant for cutting of ferrous metals is flame cutting. Similar to the oxy-fuel cutting, the material is heated to ignition temperature and burned by the addition of oxygen ( blow gas ). The energy liberated during combustion supports the cutting process significantly, which compared to the fusion cutting about 1.5 to 3 times higher cutting speeds are possible. The resulting iron oxide ( slag) is blown out by the oxygen jet. In some non-ferrous metals introduced by the exothermic heat of reaction is not sufficient to support the cutting operation significantly. Accordingly, only materials can be processed, the ignition temperature is below the melting temperature. During laser flame cutting on the cut edges remaining oxide layers further processing can affect (eg, welding) or powder coating or painting.

The main application area is the processing of low - and unalloyed steels, and individual cases of special steels. As the beam source here find most CO2 laser. The average speeds of up to 250 m / min for sheets less than 1 mm in thickness and, for example, with a 4- kW CO2 laser at about 0.8 m / min for 20 mm thick mild steel.

Ridge and Zunderanhaftungen and Schnittflächenrauhigkeit are the main technological and qualitative parameters. The kerf width is slightly larger than the other two methods, and, depending on sheet thickness about 0.1 to 0.8 mm. The burrs can be virtually avoided in laser gas cutting through the appropriate parameters. To avoid burning sharp contours and dirty puncture holes, the laser power must be turned down.

The Blasgasdruck (oxygen) is a few bar.

Laserstrahlsublimierschneiden

Characteristic for the sublimation or the evaporation of the pyrolysis of the heated material and the immediate blowout of the vapors. Materials in non- molten state are the essential scope of the Sublimierschneidens; which can be both inorganic and organic substances. The transition of the material from the solid to the gaseous state is happening here directly ( sublimation ), ie without intervening to be liquid. The process gas blows not only the steam out of the cut, but also prevents condensation thereof in the kerf. Typical materials include wood, leather, textiles, homogeneous and fiber-reinforced plastics.

Sublimierschnitte are in principle free of burrs. Resulting gases are often flammable. As the blowing gas, air or nitrogen is usually used. The darkening of the edges in wood, by pulses, use of oxygen- blowing gas (air), good focus or fast cutting with sufficient power can be reduced.

PMMA can be edited with burr-free transparent, smooth cut edges.

Pulsed lasers with high peak power and high power density, materials can also be removed almost free from a melting or heat-affected zone, which normally do not sublime. The material is largely displaced immediately into the plasma state.

Other methods

More laser -induced separation processes are the cracks and the so-called thermal laser separation (TLS).

When scoring, one of the earliest laser procedures in brittle materials, a scratch track is introduced ( notch or series of blind holes ), along which subsequently can be broken mechanically. Typical materials are semiconductor wafers, thick-film resistor and ceramic substrates and glass.

When TLS thermal stresses along a line are generated, leading to a continuous thermally induced fracture. Requirement is that at the beginning of an incipient crack is present. Not all forms are to be made so. There will be no melting and no material removal. Suitable brittle materials such as semiconductor wafers, ceramics, and glass.

Pros and Cons

• Low minimum quantities starting from 1 piece, high flexibility
• Depending on the system, all materials can be cut
• High material utilization, economic
• Clean depending on the material, narrow, often rework-free cut edges
• Engraving / marking and cutting is often possible with the same beam source and in the same operation

• High investment costs
• Occupational health and safety (see below)
• Gas consumption ( Blas and process gas or gases for gas lasers, especially the expensive helium)

Pollutants and Safety

In laser cutting, invisible laser radiation is employed. The power is so high that even scattered and reflected beam components can cause skin and eye damage. Laser machines usually have therefore a closed cabin, which can be opened only when the laser beam is switched off. The risk of (mostly undetected ) damage to the eyes, in particular Nd: YAG lasers. The resistance of the beam enclosure is with increasing laser power, and more particularly with the large focal distances, the fiber laser ( remote cutting) growing problem. Even thick concrete slabs are often steeped in a few seconds. Therefore people to so-called active enclosures that recognize the impact of a laser beam or the incipient destruction and turn off the laser.

The material of the kerf falls in metals as an aerosol. The cutting of structural steel is usually viewed as less problematic, on the other hand enter the alloying elements ( cobalt, nickel, chromium, etc. ) in high-alloy steels in appearance. However, Extremely dangerous cutting beryllium copper.

Organic materials are decomposed with laser cutting through pyrolysis in health often questionable chemicals. Particularly problematic is the cutting of halogenated organic materials such as PVC or PTFE, or even provided with flame retardants materials, this produces highly toxic dioxins and furans, and also highly corrosive gases ( chlorine, hydrogen fluoride). Any flammable gases as well as the laser beam itself constitute a fire hazard. The exhaust from laser cutting machines by running against fire -proof filter systems ( particulate filter, carbon filter).

Work Preparation

For the offline programming of two - or three-dimensional (2D or 3D ) cutting contours CAD / CAM systems are mainly used. The treatment ( contour detection, cutting sequence, material-saving arrangement by nesting, cutting gap correction, short post called processing) of the created with a CAD system geometric data is also often directly to the machine control. Complex three-dimensional sectional contours are often caused by Teach -In ( short teaching) created, corrected or completed on the machine. The software used for planning operations sometimes also permits the determination of the average length of the processing time and the required material and media volumes.