Laser Cutting Theory
LASER CUTTING PROCESS
Laser cutting systems combine the heat of the focused beam with assist gas which is introduced through a nozzle coaxial to the focused beam. The high velocity gas jet serves to:
- Aid in material removal by blowing out excess material through the backside of the workpiece
- Protect the lens from spatter ejected from the cut zone
- Assist in the burning process.
The best example of the chemical effect of the assist gas is the use of oxygen for the cutting of steels where performances are increased by the exothermic reaction of combustion of iron in oxygen. Another example is clean cutting stainless steel with high pressure nitrogen. As the stainless steel is cut by the laser beam, the high pressure nitrogen blows the melted material away.
While carbon dioxide lasers are capable of generating tremendous heat intensity, it is an incorrect assumption that they are capable of vaporising and cutting all known materials. Rather, each material has its own unique response, some of which are not suitable, to the effects of C02 lasers. Therefore, the question of suitability of using a laser for cutting that material hinges on how well it handles the added energy input. That interaction is dependent upon three key factors of the material.
- Surface condition – how well it initially absorbs the energy
- Heat flow properties – its coefficients of thermal diffusivity and conductivity
- Heat phase-change requirements – the amount of excess heat required to induce a change as a function of the materials density, specific heat, and latent heat of vaporisation.
The following information is intended to provide general inputs on the major categories of materials, keeping in mind these factors.
In general, non-metallic materials are good absorbers of infrared energy as produced by a C02 laser. Likewise, they are generally poor conductors of heat and have relatively low boiling temperatures. As such, the energy intensity of a focused beam is almost totally transmitted into the material at the spot and will instantly vaporise a hole.
Most manufacturers of high power laser cutting machines strongly discourage cutting non-metal materials due to the fumes emitted, some of which are quite dangerous. Most high power laser machines are supplied with dust collectors, not fume extractors.
Although at room temperature, almost all metals are highly reflective of infrared energy, the C02 laser with its 10.6 micron wavelength (far infrared) is successfully employed on many metal cutting applications. The initial absorptivity can range from only 10% to as little as 0.5% of the incident energy. However, the focusing of a beam to provide power densities in excess of 1 million watts per square cm can quickly (in a matter of microseconds) initiate surface melting. The absorption characteristics of most metals in their molten states increase dramatically, raising the absorptivity of energy to as much as 60% – 80%.