Importance of Edge Finishing on Thermal Tempering

Suresh Gulati and Timothy Roe, Corning Incorporated and Jorma Vitkala, Tamglass Ltd. Oy

The tempering process induces transient tensile stresses during early portion of tempering both on the surface and edges of float glass. Depending on the temperature and viscosity of glass these stresses may or may not be relieved by viscous relaxation before the permanent beneficial stresses begin to build in. In view of good quality of float glass the temporary tensile stresses can be sustained by the surfaces but not necessarily by the edges. Indeed, the edge quality which depends on the type of edge finish is inferior to that of tin and air side surfaces. Consequently, premature fracture may initiate at these edges if the combination of temporary tension and flaw severity is unbearable. For example, such a premature breakage occurs when the glass temperature is not high enough and the quench rate is too high! Glass breakage during tempering cuts down productivity and at the same time reduces glass quality.
In this paper we describe a novel technique for measuring edge strength and present data for different edge finishes. The effect of seaming and beveling on the strength of scored edge is presented. Fractographic examination of edge flaws is used to compare their severity as function of finishing process. Potential healing of these flaws during the heating portion of tempering process is discussed. The paper provides an estimate of temporary tension, based on tempering parameters, and assesses the probability of fracture from Weibull distribution of edge strength.
Sag measurements during flaw healing were also carried out for 3 mm thick glass specimens in the beam bending viscometer. The data show that such deformation can be minimized by optimizing heating rate and duration of tempering cycle. Indeed, certain trade-offs may become necessary for improving edge strength and minimizing glass deformation simultaneously during the heating portion of tempering cycle thereby preserving glass quality.

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The Authors

Mr. Suresh Gulati Research Fellow/Consultant Corning Incorporated Dr. Gulati obtained his B.S. degree in mechanical engineering from University of Bombay (India), M.S. degree from Illinois Institute of Technology (Chicago, USA), and Ph.D degree in Applied Mechanics... Read more