Glass lenses can be found in numerous objects: Cameras and automobile headlamps, infrared cameras and optical sensors for machine control. The state of the art for producing aspherical lenses is by precision molding: The glass is heated together with the forming die and then pressed into shape, after which it gradually cools while still under pressure in the forming die. It takes about ten minutes until one batch is finished and the next pieces of glass can be molded to shape. Now, precision molding is getting a rival: hot embossing, which has been developed and optimized by researchers at the Fraunhofer Institute for Mechanics of Materials IWM in Freiburg. “We were able to cut down the cycle time to less than one minute. This saves production time and is cheaper than precision molding. The cost of the lenses drops to less than 30 percent – at comparable quality,” states Dr. Peter Manns, who leads the IWM research group. The essential innovation is that the lenses are molded on both sides – both surfaces are of high optical quality, and there is no need to post-process the lenses. In hot embossing, the researchers heat the glass to higher temperatures than for precision molding. The dies are heated too, but to a temperature some ten degrees lower than the glass – enabling controlled heat exchange during the embossing process. That saves expensive process time. The speed at which it happens is affected by a number of parameters: One is the temperature of the tool – the colder the tool, the faster it cools the glass. Another is the volume – the larger the tool, the more heat it can absorb. The type of mold material makes a difference, too: Depending on its thermal conductivity, some materials conduct heat better, others do not so well. “The speed at which the lenses cool is crucial for their quality. If the glass is cooled too quickly, thermal stresses occur and the quality is diminished. If the glass cools too slowly – because the die is too hot– the molding process takes too long, which pushes up the costs,” says Manns. The researchers therefore had to find the right balance between conflicting requirements: The challenge was to adapt the material and the design of the tools accordingly, as well as their temperature. “The machine aligns the molding tool with micrometer accuracy, so we can precisely adjust the optical axes of the lenses,” explains the researcher. The scientists have tested the hot embossing method on a laboratory scale. Their next step will be to implement the method in industrial-scale pilot plants.
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