Minimizing surface distortion while shaping glass sheets

Abstract

This invention relates to heat treating glass sheets, and particularly relates to shaping and tempering a glass sheet comprising a step of supporting the sheet while heat softened with its upper surface against a glass sheet holder. In order to minimize surface distortion resulting from the heat-softened glass sheet replicating any surface deficiencies in the holder such as surface blemishes if the sheet is forced against a continuous surface by an upward flow of gas pressure, or those resulting from perforations through the bottom wall of the holder if the holder is of the vacuum type that holds the glass sheet thereagainst by suction, the present invention interposes a soft, fluffy, porous cover of fiber glass fabric, perferably knit fabric composed of texturized yarns between the upper surface of the glass sheet and the glass sheet holder. Other optional features of this invention comprise the use of more precise temperature control in the form of electrical resistance heaters to irradiate the upper surface of the glass sheet while a gas hearth is used to both heat and float the glass sheet, heating the upper glass sheet surface to a maximum temperature of about 10.degree. F. to 50.degree. F. less than the maximum temperature of the lower surface of the sheet so that the upper glass sheet surface that engages the soft, fluffy, porous cover is relatively hard compared to its surface hardness were the glass sheet heated to a uniform temperature throughout its thickness when the sheet is heated to an overall temperature suitable for tempering.

Dependencies are inferred from the claim text.