Study On Several Major Issues In Precision Glass Compression Molding

Precision glass compression molding is an emerging innovative net-shape approach for optics fabrication. During this process, polished glass preform is heated well above its transition temperature range, and then the precision profiles of molds are duplicated onto the softened glass by enclosing the two half molds. Finally, the molded glass lens is cooled down to room temperature by purging nitrogen, and can be directly used for optical application without any subsequent processing. It is suitable for fabricating numerous kinds of optical components, especially for aspherical lens, micro-lens array and diffractive optics elements. It has attracted arising interests from both industrial and academic communities, due to its advantages of high volume, low cost and environmental friendliness.However, because of the thermal shrinkage of lens and temperature distribution non-uniformity within lens during cooling, the molded lens could have some defects, such as profile deviation, residual stress and refractive index change, which would degrade the optical performance of the molded lens. Therefore, studies have to be made to thoroughly investigate the molding process and the influences of the processing parameters by using finite element analysis, in order to enhance the quality of the molded lens and improve the productivity efficiency.In consideration of the diversity of glass, mold and coating materials being studied these years, several types of materials were included, such as common moldable glass G-11, BK7 and low transition temperature chalcogenide glass AS2S3 as glass preform, tungsten carbide and silicon as molds, and graphene as coating. Efforts were made to simulate the whole molding process, analyze the curve profile deviation of the molded lens, measure the friction behavior between glass and mold, and deduce the stress relaxation parameters, therefore providing scientific and reliable references for researches on precision glass compression molding process.In order to predict the quality of the molded lens, taking G-11 glass as example, the whole molding process of a plane-convex lens was simulated with full consideration of viscoelastic stress relaxation and structural relaxation behaviors. These results were obtained, such as the deformation and flow shapes of the molded lens during pressing, the temperature and stress distribution within lens during cooling, and the final profile curve deviation of the molded lens. Furthermore, by taking the profile curve deviation of the molded lens as the compensation reference of the designed mold, with two iterative mold compensation procedures, the maximum value for the profile curve deviation of the molded lens was successfully decreased from 10 microns to 0.035 microns, so the form accuracy criterion for precision optics was finally met.Then the sensitivity study was made to investigate the influence of the processing parameters on the profile deviation in the cooling stage. The evolution curves with time for the three variables was obtained and the relationship among them was discussed, i.e. the profile deviation of the molded lens, the profile deviation of the molds due to thermal expansion, and the gap between them. Finally, the influences of the processing parameters on the profile deviation were discussed, such as the releasing temperature, the cooling rate and the magnitude of the hold-up force, and the predefined goals were achieved.Barrel compression tests were conducted at different molding temperatures, in order to investigate the interface friction behavior between the graphene-coated molds and the softened BK7 glass. The friction coefficients were calculated first by using empirical equations, and then by comparing the height change curves and the final dimensions of the specimens from simulations with the experimental data, the best matching friction coefficient was obtained. What’s more, the results of different approaches well coincided, all showing that the friction coefficient was 0.20-0.25 in the temperature range of 660℃-700℃.Lastly, the creep tests of AS2S3 cylinder specimens were performed at three different temperatures, in order to deduce the creep compliance functions. Then by using the relationship between creep compliance and relaxation modulus, the shear stress relaxation function for As2S3 was achieved and modified with consideration of the friction influence on the height change. The calculated shear relaxation function was then adopted as the glass material inputs for finite element analysis, and the predicted height change results were well consistent with the experimental ones. In addition, the refractive index changes of As2S3 after molding process at different cooling rates were calculated by using the Tool-Narayanaswamy-Moynihan model for structural relaxation behavior. The results can serve as crucial references for the viscoelastic behavior of As2S3 in compression molding or other similar applications.