|
Highly reliable high power carbon dioxide lasers are being increasingly employed in the converting industry. New laser sources with different wavelengths are available to allow more controlled laser cutting and perforating processes. These process advantages are thought to be due to enhanced absorption of certain thermoplastics at these shorter wavelengths and this paper investigates these. Introduction Laser cutting has now been widely used for many years and almost all industries have benefited. From a materials standpoint, most engineering materials have been cut – or have been attempted. In this context, laser cutting of thin film plastics is a relatively trivial task, but as is always the case, when this process is incorporated into an industrial environment the challenge is much greater. This is especially true in the case of web processing of thin plastic films due to the very high process speeds that are typically required in the converting industry. Sealed CO2 lasers A major benefit of cutting thin film plastics is that very high process speeds with relatively low average power are possible. Sealed CO2 lasers delivering up to 500 watts have been widely used for cutting and perforating thin polymer films for some time in the converting industry. Over the past 15 years, RF-excited, sealed CO2 sources delivering between 25 and 500 W average power have become available for a diverse range of applications in light manufacturing to the point where unit sales of these lasers now far exceed those of higher power. This reflects the updating of light manufacturing production processes. Compared with more powerful gas and solid- state sources, sealed CO2 lasers offer advantages to a wide range of applications in engraving, marking, cutting and rapid prototyping markets [1]. This diversity of applications leads to a variety of requirements that, in some cases, could potentially create a large number of lasers. However, two resonator technologies, waveguide and slab construction, are becoming the leading platforms for industrial applications. These two laser architectures have been commercially available for some time, and have continued to evolve. These lasers can provide
operating lifetimes in excess of 20,000 hours with minimal maintenance.
The output power of the design scales with the area of the electrodes, allowing slab lasers to maintain cost effectiveness up to 500W. The addition of water- cooling to these designs enables production of high peak and average power for the long durations that are required by many industrial processes. Recently, new CO2 laser products have become available with significant flexibility in selecting the actual emission wavelength of the laser. The ability to select different wavelengths can be achieved in two ways: firstly, using wavelength selective optics and secondly; using a unique patented technology that uses common isotopes of the CO2 molecule [2]. Although the details of these approaches is not relevant to this paper, a number of emission wavelengths from 9 µm to 11 µm are now commercially available with the average power required by industry. Lasers operating at 9.4 µm have been developed because of the high absorption of polyimide (Kapton) polymers at this wavelength [3]. The particular wavelength under consideration here is 10.2 µm. The specification of these lasers is 10.23 +/- .05 µm, lasers run typically at the lower end of this specification. There is experiential evidence that at 10.2 µm wavelength, some thermoplastic films are more efficiently processed than at the longer 10.6 µm wavelength. Digital Converting for Labels Laser cutting and drilling processes require very highly focused lasers beams that generate power densities at the workpiece of > 106 W/cm2. In this regime, instantaneous vaporization and high speed cutting of thermoplastic films is possible. Digital converting of labels is an example of an industrial cutting process based on CO2 laser technology. Digital converting is having a direct impact on the label industry, especially as roll fed digital printing systems gain market acceptance. Laser digital converting allows label producers to make instantaneous job order changes and pattern modifications without the need for tooling - label shapes may be instantly changed and any programmable shape is possible. This enables product enhancements such as personalization. The flexibility of the laser allows automatic adjustment for micro-perforation, through-cut, kiss-cut and scoring at one laser digital converting station. Most importantly, the productivity of a roll fed digital
printing system is dramatically increased. Assuming the label substrate remains the same, the label producer can now run multiple jobs on the ...
|
