Wood marking and engraving using Q-switched diode pumped Nd:YAG laser

While in the past, lasers were widely used in cutting and welding operations, in the recent years they have been applied to other machining like marking, scribing, selective ablations and engraving on metallic, ceramic and polymeric materials. It was possible thanks to the introduction of laser sources characterised by short and ultrashort pulses with pulse duration variable from nanosecond to femtosecond . The main advantages of these lasers are non-contact working, high repeatability, high scanning speed, worked area comparable to the laser spot, high flexibility and high automation. However, using these lasers, the material removal rates are very low if compared to mechanical processes. For these reasons they are suitable for machining metallic materials and heat sensible non metallic materials. Wood engraving and marking by laser resembles face milling to some extent, with beam 'tool' diameter smaller than that of mechanical cutters by more than two orders of magnitude. However, since the surface layers lying below those that are burnt away will remain on the object being processed, care should be exercised to avoid their damage, i.e. overheating, discoloration etc.. This places a limitation on the temperature field and heating time that are associated with the laser power and beam characteristics, thereby bounding material removal rate and confining the process to shallow and more or less uniform removal depths with low-power lasers. In this light, it is no coincidence that laser engraving technology is quite widespread in the handicraft industry to mark and engrave wooden decorative items. Moreover, it is important to choose optimised process parameters like wavelength, spot diameter, pulse repetition frequency, scanning speed and beam power according to the material absorption and the required surface quality. Aim of the present research was to study the features and the performance given by a 5 W Q-switched diode pumped Nd:YAG laser at the II harmonic generated wavelength of 532 nm, in the marking and engraving of different wood. This laser was characterised by pulse duration of about 150 ns, pulse repetition frequency up to 50 kHz, a maximum mean power of 5 W and a beam focus diameter of about 80 μm. Removed material volume and surface quality were evaluated marking and engraving wood by single and multiple laser scanning varying the diode current, the pulse frequency, the scanning speed, the distance between the single engraved lines and the number of scanning repetitions. The effectiveness of the laser and the quality of the grooves were evaluated by means of an optical microscope and a scanning electron microscope while the groove geometries and the surface morphology was estimated by a roughness measuring instrument. Experimental results showed that the laser can be used to machine wood obtaining decorative drawing and three-dimensional engraved geometries lacking in burn.