A new method for a selective laser ablation.

Bliedtner, Jens ; Schoele, Holger ; Baumann, Robert 等

1. INTRODUCTION

Textile fibres can be applied to almost all materials as a high-quality surface refinement. The application process is very efficient and economical in most cases. That is why this surface technology increasingly grabs the attention of designers and developers. Precondition for a series production are high process stability and reproducibility as well as a high flexibility (Beck & Rossig, 2007), (Abele, 2007). The application of laser ablation can increase the flexibility.

One essential aim of the project is to develop and test such a selective, process-controlled laser ablation method. Applying this method can lead to a better and more efficient manufacturing of high-quality components e.g. in the field of car interior. Another aim is to automatise the process and to guarantee a high quality of the components.

Currently this process of surface refinement with textile fibres is characterised by a high finishing effort in order to provide a high quality of the components. With the introduction of the new laser ablation method costly finishing steps such as e.g. manual cleansing can be reduced. Furthermore the selective laser ablation creates new design possibilities. New material developments in the layer structure shall enable colourisation to increase contrast as well as structuring and branding.

2. EXPERIMENTAL SETUP

The application of fast laser scanning systems is advantageous for achieving high ablation rates with the new selective laser method (Carr, 2008). A preferred laser source is a 50 W sealed off C[O.sub.2]-laser. The wavelengths of the Nd:YAG and Excimer laser can also be used for certain material combinations. Table 1 shows the selected laser parameter.

The emitted laser beam is widened by 50% by means of a telescope and afterwards circularly polarised by means of a quarter-lambda mirror. Two fast-moving galvano mirrors guide the laser beam on a meandering course over the sample surface. F-Theta lenses focus the laser beam with a corrected focal length on a focus diameter of 0.25 mm (0.1inch). A special spectral photometer is used for the process diagnosis. The measured curves help to define the ablation depth. Figure 1 illustrates the setup of the process components.

[FIGURE 1 OMITTED]

It is, however, difficult to achieve high ablation selectivity. This means that the plastic carrier should not be damaged during the ablation process. This problem is due to the similar absorption qualities of the fibre layer and the plastic carrier. Figures 3 and 4 illustrate such a material joint.

A further important task is the qualitative assessment of laser-structured surfaces. The selected measuring procedure as well as the assessment of the measured results has to enable a reliable classification of the created laser structures. Figure 2 illustrates the selected test setup.

The qualitative assessment of the laser ablation was carried out based on industrial image processing (machine vision). Different camera and lighting setups were applied to gain the image data. A black-and-white camera with 16 bit resolution and adjusted fibre optical and LED-based lighting systems was used. The laser-structured surfaces were systematically photographed in series and saved for the subsequent image processing. Additionally a light-section with a line laser was used for the ascertainment of profile information.

[FIGURE 2 OMITTED]

3. EXPERIMENTAL RESULTS

Significant process parameters, which directly influence the ablation result, were listed in a test schedule. These are the laser power, the scanning speed and the overlapping degree of the hatching. A square of 1[cm.sup.2] served as a test contour. In the first tests the hatching density and the focal length were kept constant and only the speed and the laser power were changed. After this the influence of the hatching density was examined at constant scanning speed. Figure 3 shows a SEM image of a laser processed edge of the test square.

[FIGURE 3 OMITTED]

The overlapping degree of the hatching had a significant influence on the ablation result. The closer the lines are the higher is the heat input into the material. Already with low laser powers and small distances between the hatching lines intense interactions occur, which can lead to the destruction of the carrier material. The distance between the lines, however, should not be greater than the focus width of the laser beam because otherwise the heat distribution becomes inhomogeneous. Furthermore, single hatching lines are visible when working with low laser powers.

The correct scanning speed and laser power also have a significant influence on achieving a homogeneous ablation result. Both parameters show a linear dependence on each other. They have a direct impact on ablation depth and structural effects such as shininess and colour. However, a separate optimisation is necessary for different textile fibres and carrier materials. Table 2 shows the optimised ablation parameters for a black polyamide flock on a plastic carrier.

As a result of the ablation tests first car components could successfully be laser-structured. Figure 4 show photographs of laser-processed interior components.

The examinations also showed that during the processing a number of influencing factors can lead to faulty ablation results. These must be ascertainable and assessable in the industrial application of the new ablation method.

[FIGURE 4 OMITTED]

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The above described measuring procedure enables a fast ascertainment of the laser-structured surfaces by means of digital image processing. Subsequently so-called operator pipelines were created by means of the rapid prototyping software "VIP-Toolkit". These enable the data processing as well as the computation of characteristics and the classification of error parameters. Besides the grey scale value statistics also features from the correlation/covariance matrixes or the corresponding FFT power spectrum constitute a powerful feature group (figure 5) (Lucht, 2008).

The assessment of the measurements can be considered as a classification with supervised learning, i.e. for the given attributes the respective class is always known. The ML classifier and the hyper cuboid classifier were used in the examinations in order to test the feature quality and the separability of laser-structured surfaces.

4. CONCLUSION

The results of the examinations show that high-quality surfaces can efficiently and selectively be ablated by means of the laser technique. The wavelength of the C[O.sub.2] laser can be utilised advantageously for a multitude of different textile fibres and carrier materials. An ablation speed of 0.5 m/s can be reached reproducibly with relatively low laser powers through the application of fast scanning systems.

The examinations have also shown that the selected measuring procedure of the digital image processing successfully enables a separation and classification of laser-structured surfaces as well as the detection of errors. The introduced new laser ablation method provides the opportunity for an economical industrial application in the field of selective ablation of high-quality decor surfaces.

5. REFERENCES

Abele, H.; Planck, H. & Stegmaier, T. (2007). Simulation des elektrostatischen Beflockens (engl. Simulation of electrostatic flock coating), Melliand Textilberichte

Beck & Rossig--European Patent Attorneys (2007). Flockmaterialien zum Beschichten von Teilen (engl.: Flock material to coat components). Patent DE202005021362U1

Carr, K. (2008). Optical Materials: Silicon carbide mirrors benefit high-speed laser scanning, Laser Focus World, Vol. 45, No. 8

Lucht, C. (2008). Optische Inspektion von laserbearbeiteten Flockoberflachen (engl. Optical survey of laser machined flock surfaces), scientific report, Gesellschaft fur Bild- und Signalverarbeitung mbH. Ilmenau

Schlobach, M. (2009). Erprobung einer Bildverarbeitungssoftware zur Bewertung von Dekoroberflachen (engl. Testing a image processing software to evaluate decor surfaces), Bachelor Thesis, University of Applied Sciences Jena

Tab. 1. Selected laser and process parameters

 C[O.sub.2]-Laser Nd:YAG-Laser Excimer-Laser

power 50 W 65 W 7 W
pulse frequency cw Max. 30 kHz
wavelength 10640 nm 1064 nm 193 nm

Tab. 2. Ablation parameters BMW branding

wavelength 10640 nm
power 17.5 W
scanning speed 500 mm/s
process time 5 s