Polishing the surfaces of injection moulds is time-consuming and monotonous work, but it requires highest levels of concentration; any blemish in the mould can render it useless. A skilled worker may often need a whole week to polish a single mould. The dreary work has hitherto been difficult to automate due to the complicated curved shapes.
Researchers at the Fraunhofer Institute for Laser Technology (ILT) in Aachen have developed a way of automating the polishing work: 'We do not polish the surface by hand with grinding and polishing media. Instead we use a laser,' explains Dr.-Ing. Edgar Willenborg, group leader at the ILT. 'The laser beam melts the surface to a depth of about 50 to 100µm. Surface tension ensures that the liquid metal flows evenly and solidifies smoothly.' Like in conventional grinding and polishing, the process is repeated with increasing degrees of fineness. In the first stage the researchers melt the surface to a depth of about 100µm, in further steps they gradually reduce the depth. 'We can set the melting depth by means of various parameters: the laser output, the speed at which the laser beam travels along the surface and the length of the laser pulses,' states Willenborg.
Laser polishing does not achieve the same surface smoothness as perfect hand polishing; hand polishers can achieve a roughness of approximately 5nm, whereas the laser system at present can only manage 50nm. The system will be used to automate polishing to medium grades, adequate for many applications, such as the moulds used for making standard plastic parts.The high-end levels of smoothness will therefore remain the domain of skilled hand polishers.
Whereas a skilled polisher needs about 10 to 30 minutes for each square centimetre, the laser polishes the same area in about a minute. A prototype of the laser polishing machine developed by the scientists in cooperation with mechanical engineering firm Maschinenfabrik Arnold has already been built. Willenborg estimates that the system will be ready for industrial use in one to two years’ time.