Multi beam microprocessing for printing and embossing applications with high power ultrashort pulsed lasers

Abstract

Abstract Micro structuring of surfaces is of great interest for various applications, e.g. for the tooling industry, the printing industry and for consumer goods. In suitable mass production applications, such as injection molding or roll-to-roll processing for various markets, the final product could be equipped with new properties, such as hydrophilic behavior, adjustable gloss level, soft-touch behavior, light management properties etc. To generate functionalities at reasonable cost, embossing dies can be augmented with additional micro/nano-scale structure using laser ablation technologies. Despite the availability of ultrashort pulsed (USP) high power lasers (up to several hundred watts), it is still a challenge to structure large areas, as required on embossing rolls, in an acceptable processing time for industrial production. In terms of industrial implementation, direct digital transfer is a limiting factor for ultrahigh resolution. Shorter machining times by further increasing spot or workpiece motion are limited. Enlarging the ablation diameter, and thus the tool diameter, delivers a higher ablation rate with the comparable ablation quality, but entails a reduction in resolution. While maintaining the achieved state-of-the-art performance, upscaling of single modulated lasers provides a less demanding way to increase productivity. In the processing of steel surfaces, an increase in material removal can also be achieved by using pulse burst. In this work, the parallel process of single modulated multi laser sources is compared with a laser source split by diffractive optical elements (DOE) for applications in a cylinder micro patterning system. A newly developed highly compact ps laser with repetition rates up to 8 MHz and an average power of 300 or 500 W was divided into 8 or 16 parallel beamlets by a DOE. The ablation rate of each approach was investigated by typical microstructures on copper surfaces. At surface speeds of 10 m/s and a resolution of 5080 dpi, an ablation rate of up to 27 mm³/min was achieved. Different functional surface geometries were realized on an embossing roll as master, which is used for replication of the structures in roll-to-roll processes. Functional structures, such as friction reduction, improved soft touch or light guiding elements on large surfaces are demonstrated.