Since the 1970s, lasers have provided the manufacturing industry with a new tool. Due to the nature of the laser, which provides not only large amounts of energy but also precision and speed, the manufacturing possibilities of lasers have expanded exponentially. One of the mature industrial lasers is the carbon dioxide (CO2) laser, which has had a long and successful history in applications such as cutting, welding, drilling, and marking. However, despite the myriad of applications, new innovative applications continue to be discovered.
Most CO2 lasers produce a stable TEMoo beam in either continuous wave (cw) or pulsed modes with a wavelength varying between 9 and 11µm. This long wavelength is especially suited for machining most organic materials, due to the fact that it is strongly absorbed by these materials. Perforations are required in several materials
The largest market for which CO2 lasers are best suited is perforated plastics and other organic materials. Perforated plastic is being used increasingly in food packaging industry to increase the shelf life of fresh produce. CO2 lasers are ideally suited to perform this task due to the infrared absorption spectrum of most hydrocarbon molecules of which the plastics are made.
Various of our TEA CO2 laser systems have shown themselves capable of successfully perforating paper and plastic films of various thicknesses and with hole sizes varying between 50 and 500um. The type of laser system required is determined by the speed at which the perforations have to be made and the size of the perforations.
The process used to drill holes in a material by means of laser pulses is known as percussion drilling, and involves the use of multiple pulses to remove small amounts of material with each pulse. Due to price considerations, lasers are used to drill holes with diameters of less than 500mm in diameter. For larger holes, mechanical methods are more cost effective. To enable a typical TEA CO2 laser to drill such small holes, the laser beam generated by the laser must be focused down with the aid of an appropriate lens. A gas nozzle, incorporating a focusing lens, can be used to ensure delivery of as much of the laser energy as possible to the material sample. The role of the nozzle is twofold:
- Reduce the risk of a plasma formation at the surface of the material
- Reduce the risk of damage to the focusing optics.
More is not necessarily better when it comes to laser materials processing. Depending on whether the material is a strong or weak absorber, so the peak and average intensities needed for a particular application will differ.
But at some intensity, vaporization of the target will greatly increase the chance of plasma formation at the surface. Sometimes operating at lower energies, and thus lower intensities, increases the efficiency of the process. It is also not always true that one can drill proportionally faster with higher repetition rates. The coupling of laser energy into the material depends on the temperature of the material, which is influenced by all the previous pulses. Higher repetition rates are better as far as speed is concerned, but maybe not always so with efficiency.
The operating wavelength also needs careful attention. Depending on the material being processed, certain wavelengths may be absorbed preferentially. For example, in drilling certain plastics, the absorption spectrum of plays an important role. Materials
Plastics absorption at 9.3µm is significantly higher than it is at 10.6µm. However, the energy difference from a typical TEA CO2 laser at these two wavelengths is not significant. Thus, the operating energy can be orders lower if the laser is operated closer to 9.3µm.
In trying to remove material through intense local heating, a heat-affected zone (HAZ) is created. In most applications it is essential to reduce the size of the HAZ by minimising the amount of laser energy distributed around the area being processed. This can be accomplished by structuring the beam profile with. The best results are achieved with a top hat profile that can be achieved by creating a super Gaussian as shown below
In the past the use of pulsed TEA CO2 lasers in these types of applications has been limited by the low repetition rate of the lasers, typically 100 to 200Hz. The lifetime of the gas has also been a limiting factor, with external gas loop systems taking up lots of space. At PaR Systems (Pty) Ltd we have designed power circuitry for our TEA CO2 lasers that allows the achievement of repetition rates in the region of 750Hz with pulse energies in the range 0.01 to 0.2J and pulse lengths between 40ns and 5μs depending on the application.
Excellent beam quality means that these beams can be focused down to micron size diameters. Through the use of room temperature catalysts, the TEA CO2 lasers developed by PaR Systems (Pty) Ltd can run for extended periods of time at high repetition rates without flowing the gas or replacing the gas load.
The innovations discussed above show that a high repetition rate TEA CO2 laser can be a viable option when considering a laser to be used for the cutting and drilling of materials, such as ceramics, plastics, and fiber-glass composites, that have up to now been extremely difficult to process.
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