Modelling of Deformations and Stresses in Glass Tempering

Abstract

Glass tempering is a heat treatment process which increases the strength of plate glass. In the past the process has been developed mainly on the basis of experimental results, but numerical modelling has now improved the ways of studying the process. The problems are most common with thin glass. A thin glass sheet requires high temperatures and rapid heat transfer to produce a good quality product. Furthermore the traditional support arrangements have to be modified. This thesis considers the most common quality faults and presents the reasons for their occurrence. With thinner glass more effective heat transfer than is currently available is required to temper the glass. The increased heat transfer coefficient changes the mechanical behaviour so that a higher glass temperature is needed. However, the temperature increase makes the glass less viscous before cooling and the lower viscosity increases deformations. The influences of temperature, heat transfer coefficient and support arrangements for glass plates have been investigated in this thesis using numerical methods. In the numerical modelling state-of-the-art models are presented and used. The cost effective one-dimensional dedicated program and a more general, three-dimensional commercial FEM program, are used in the simulations. The simulations explain and give more detailed information about the behaviour of glass. The deformations are dependent on several parameters and the entire thermal and mechanical history has a bearing on the results. The results shown in this thesis can be used to design a new support system for a tempering furnace. In the glass market the use of coated glass is increasing. The coating changes the thermal and mechanical behaviour of glass and the effect of coating on the glass behaviour has been demonstrated. The tempering of coated glass is more challenging than that of pure glass. Although, the cases considered deal with the tempering process, the results can also be utilized in glass annealing.