Aspherical optics and freeform surfaces are being used increasingly in the lighting sector as well as for imaging optics. As a result, the demand for adaptable and quick polishing processes for optics with nonspherical surfaces is growing. For this application, the laser polishing process developed at the Fraunhofer Institute for Laser Technology ILT provides a flexible and economical alter native to conventional polishing methods.
Polishing glass and plastic materials with CO2 laser radiation is based on the absorption of the radiation in a thin border layer of the work piece such that temperatures just below the evaporation temperature are attained on the surface. As a result, the viscosity of the material is reduced so that the roughness flows and the surface is smoothened due to surface tension. In comparison to conventional polishing methods, the innovation of laser polishing lies in a fundamentally different active principle: surface smoothing through remelting rather than material removal. This way, laser polishing achieves, among other things, a favorable smaller micro roughness.
By choosing the corresponding process parameters, such as preheating temperature, feed speed and intensity distribution, a user can flexibly adapt laser polishing to nearly any surface form. Due to the high process speeds of up to 1 cm2/s for quartz glass and up to several 10 cm2/s for plastics, laser polishing is faster than conventional polishing processes by up to two magnitudes.
The conventional polishing process for glass materials uses disc or point tools and a polishing liquid, which is applied to the work piece. In this process, large amounts of waste arise. By means of laser polishing, glass surfaces can be polished without creating waste, independent of the surface form and with the same tool. In addition, the processing time of laser polishing is smaller by a factor of up to 100 times. It can attain a surface roughness of quartz glass down to Rms < 5 nm (1 x 1 mm2 measuring field) and micro roughness down to Rms < 0.4 nm (50 x 70 μm2 measuring field). Applications for laser polishing of glass surfaces are, among others, lighting optics, for which the values currently achieved are sufficient. By increasing the form accuracy and reducing the waviness, the Fraunhofer ILT aims to make the process suitable to polish imaging optics which need to fulfill higher demands. The process can be applied to nearly all kinds of glass, whereas higher process speeds are reached for low-melting glasses.
Since thermoplastics such as polycarbonate can be formed when heat is applied, laser polishing is also suitable for this kind of plastic. On account of the smaller temperatures required, the process speeds for laser polishing of plastic are – at several 10 cm2/s – significantly faster than those for glass. The overall surface roughness attainable is already sufficient for lighting optics (Rms < 25 nm, 1 x 1 mm2 measuring field), whereas the micro roughness, at Rms < 1 nm (50 x 70 μm2 measuring field), exhibits comparable values to those achieved using conventional polishing methods. The process development aims at reducing the resulting surface roughness even further such that the process will be suitable for imaging optics as well. A possible application is for polycarbonate eyeglass lenses.
Laser polishing can process non-accessible areas such as the inner walls of drill holes, a great advantage in specific applications. By means of a deflector unit, the laser radiation is directed onto the wall inside the drill hole. The processing times do not change and the process can attain a micro roughness on the inner walls of drill holes in quartz glass of Rms < 5 nm at an initial roughness of approx. Rms ≈ 500 nm (measured length = 5 mm).
Laser polishing can also be used to remove burrs and to round off edges on glass as well as on plastics. For example, burrs that occur between the molds on injection molded parts can be removed using this process. This can significantly improve the visual appearance of products. In addition, when glass edges are rounded off, the stability of the entire component increases.
Due to remaining form deviations and waviness, laser polishing cannot currently be used to process imaging optics. One solution to this may be the expansion of the process by a subsequent laser-based step to correct form deviations. In this processing step, material is ablated locally in very small amounts at the desired location, which thereby reduces waviness. In addition, laser radiation can also be applied to generate the surface form itself – this process step can be placed upstream of the laser polishing. The resulting laser based process chain for manufacturing optics, existing of these three steps, is currently under development at the ILT and shall be used to manufacture freeform optics, since the production times for this kind of optics can be reduced dramatically.
Fraunhofer Institute for Laser Technology ILT
