Machining Injection Molds with Ultrashort Laser Pulses

Systems for homogeneous LED lighting have in the past used a large number of LEDs and optical diffuser panels for distributing the high spot-luminance. As an alternative, ever compacter optical waveguides, adjusted to the lighting requirements, are increasingly being used, in which the light is diverted and distributed with refractive outcoupling by roughening or structuring. Refractive light outcoupling is performed with microlens arrays. Such an array may consist of several hundred thousand microlenses with geometries in the micrometer range. Producing such structures by injection molding requires a high, uniform quality. It is determined by the precision of the molds.

At the Fraunhofer-Institute of Laser Technology (ILT) Aachen, Germany, injection molds were machined with ultrashort laser pulses. This could significantly improve the process. New, industrial ultrashort pulse lasers reduce machining times to a few hours, compared with conventional methods, which take at least a week. Moreover, the ultrashort pulse lasers permit high precision for machining steel molds. The picosecond laser pulses are about a thousand times shorter than a conventional quality-switched solid-state laser, like that used, e.g., for marking. While melt expulsion still occurs with conventional lasers, the ultrashort pulse laser evaporates the material with high precision and no residues. Subsequent laser polishing permits a smooth and uniform surface on the molds down to the submicrometer range (Ra > 100 nm).

For the tests, commercial systems with a focus diameter of 20 µm, several 10 µJ of pulse energy and a repetition rate of >100 kHz were used. The mold insert microstructure generated by laser texturing has depressions with a diameter of 50 µm and more. While commercial systems have so far only been available in the range below 100 W, prototypes of over 1,000 W average output have already been developed at the Fraunhofer ILT.