Development of a friction model characterizing the temperature-, relative velocity-, and contact normal stress-dependent behaviors at glass-mold interface in Precision Glass Molding (FriPGM)

• Applicant: Professor Dr.-Ing. Thomas Bergs
• Subject Area: Primary Shaping and Reshaping Technology, Additive Manufacturing
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 456107969

Precision glass molding (PGM) is a manufacturing process replacing traditional machining processes, as grinding and polishing, in the manufacturing of high precision optical components. In PGM, a glass gob is heated and subsequently pressed in tungsten carbide tools under an inert atmosphere. Thermal contraction during cooling leads to geometric deviations of the final part. These deviations are currently compensated in a trial-and-error process involving the successive design, manufacturing and testing of multiple tools. Finite Element (FE) simulation of the PGM process enables the prediction of form deviation and thereby a reduction in manufacturing costs for tools in the process design phase. A dominating factor for form deviation in PGM processes is friction. Friction is currently modelled in PGM simulations using Coulomb’s model. However, Coulomb’s model ignores the dependency of the friction coefficient on contact normal stress, relative velocity, and interfacial temperature. Calibrating and implementing a friction model that respects these parameters into the FE simulation of PGM processes will allow more precise prediction of the final shape and thereby further reduce the necessity of the trial-and-error approach. The overall objective of this proposal is the improvement of shape prediction in FE simulations of the PGM process through a comprehensive friction model for the glass-mold contact accounting for contact normal stress, relative velocity, and interfacial temperature. The objective of the first phase is the investigation of the cause-effect relations between the aforementioned parameters and the friction coefficient at the glassmold contact interface under an atmospheric environment. Under these conditions, X15CrNiSi25-21 is chosen as the mold material instead of tungsten carbide to avoid oxidation effects at high temperatures. A common glass type for the PGM process, N-BK7 borosilicate glass, is selected. Based on experimental results, a friction model for the glass-steel contact will be proposed. The objective of the second phase is the extension of the friction model to glass-tungsten carbide contact in an inert environment and validation of the friction model in an actual molding process. The objective of the first phase will be achieved by investigating friction coefficients under different contact normal stresses, relative velocities, and interfacial temperatures in a pin-on-cylinder tribometer. The cylinder is made from the tool steel and the pin is made from glass. To compensate for the forces arising from the deformation of the steel cylinder, an FE model of the tribometer test is developed. The compensated friction coefficients are then used to calibrate a friction model dependent on contact normal stress, relative velocity, and interfacial temperature. The model is finally validated against a PGM process with a stainless steel mold and an N-BK7 glass gob.

• DFG Programme: Research Grants