Hot Embossing Rivals Molding

Hot embossing, a recently optimized technology in which glass is heated to high temperatures and molded on both sides, has been found to be up to 10 times faster and 70 percent cheaper than precision molding, with the lenses produced of comparable quality.

Precision molding is the current standard for producing aspherical glass lenses, which can be found in cameras, fiber optics, auto headlights, infrared cameras and optical sensors for machine control. It is also used to form the focusing optics for items such as telescopes and scientific research instruments and can be found in high-power diode lasers used in the automotive and medical instruments industries.

In 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 10 minutes for a batch to finish.

Now hot embossing, developed and optimized by researchers at the Fraunhofer Institute for Mechanics of Materials (IWM) in Freiburg, is seeking to rival precision molding by producing comparable lenses faster and cheaper.

“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,” said Peter Manns, PhD, who leads the IWM research group. The technique's innovation is that the lenses are molded on both sides, with both surfaces being of high optical quality, and with no post-processing required.

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 10 degrees lower than the glass -- enabling controlled heat exchange during the embossing process and saving expensive processing time.

The speed of the process depends on the temperature of the tool (the colder the tool, the faster it cools the glass), the volume (the larger the tool, the more heat it can absorb) and the type of mold material (some materials conduct heat better than others).

“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,” said Manns. The researchers had to find the right balance between all three 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,” Manns said.

The hot-embossing method has only been tested in the laboratory, with the next step being to implement it in industrial-scale pilot plants, Manns said.

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