Precision molding

Principle and process


Precision molding primarily differs from related technologies due to its isothermal molding conditions. Alternative processes such as standard blank molding process externally heated, usually rod-formed raw glass. The hot glass is pressed between two colder molding tools and immediately thereafter removed in order to cool down externally. This generally generates only a finished functional surface, since only after subsequent machining can the central thickness of the lens be configured. Due to the significant shrinkage of the glass during cooling, moreover, it is difficult to achieve form precision.

In precision molding, the generation of optical components with functional surfaces on both sides and with high precision without subsequent machining is standard. Precisely preformed or preprocessed glass blanks with at least fire-polished top surface quality are heated directly in the mold in contact with the molding tools, and brought to their final shape at temperatures just above the transformation temperature Tg (400 to 800 °C for optical glass, 1500 °C for quartz glass). The cooling phase initially maintains a precisely regulated pressure within the machine, that is, the mold and the glass remain in contact with one another and must be optimally adapted to the specific application case by means of precise pressure/temperature control.

Quelle: Fraunhofer IPT


In the first step, the glass blank is inserted into the lower molding tool. Then an inert gas atmosphere is generated in the molding chamber. In step 3, the glass blank and molding tool are heated with infrared radiation. When the molding temperature is reached, a temperature slightly above the transformation temperature Tg of 400 to 800 °C according to the glass type, and after the homogenization time has elapsed, the two halves of the tool are moved together, with a pressure force of up to 30 kN in contact with the glass, depending on the application and tool size. The glass blank and tool characteristically have the same temperature during the molding process. A distance-controlled molding phase can directly influence the lens thickness during the process. At the start of the cooling phase, the gas volume flow is increased in the molding chamber. Initially, this cooling process is very slow and still under load. This process prevents uncontrolled shrinking of the glass. Only after the temperature falls below the transformation temperature does the fast cooling process begin, and the tools move apart. In the last step, the molded lens is removed after opening the molding chamber. The loading and unloading processes can of course be carried out by appropriate automation systems.

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