Tempering glass sheets by cooling comprising sublimation quenching

Abstract

In tempering a glass sheet to a high temper by heating the sheet to approximately its softening point followed by rapid chilling using a quenching composition that comprises a tempering medium having high heat transfer characterisitcs, the heat-softened glass sheets are protected from breakage during fabrication and resulting optical distortion by applying the tempering medium through exit orifices of a delivery system toward the heated glass in a novel cycle. The initially applied tempering medium has a low heat transfer rate and is mixed with a second tempering medium having a higher heat transfer rate to gradually increase the heat transfer rate of the mixed tempering medium. In a specific embodiment, dry air is the first tempering medium and a sublimable material, such as carbon dioxide, is the second tempering medium. The delivery system has an uninsulated portion that assures that initially the sublimable material is applied as a gas. Dry air which has a lesser heat transfer coefficient than the sublimable material in its gaseous state, may be supplied to the uninsulated portion if humidity conditions require so that dry air having a lesser heat transfer coefficient is initially directed through the exit orifices followed by mixtures of dry air with increasing concentrations of said sublimable material in the gaseous state followed by increasing concentrations of solid particles of said sublimable material. Movement of the glass sheet is coordinated with movement of the delivery system for said quenching composition to avoid 'batter'.