Patent ID: 8210001

Claim:
A method of making a glass sheet using an overflow fusion downdraw process, comprising the steps of: (a) defining a glass sheet quality metric Q, Q being a measure related to retardation in a glass sheet; wherein Q is determined, either physically or by modeling, according to the steps of (a1) superimposing a grid of measurement nodes on a glass sheet, and (a2) measuring birefringence at each node; (b) selecting a glass sheet quality metric level Q 1 , Q 1 being an allowable level of Q for the glass sheet product; (c) identifying a glass ribbon temperature T 1 at which a which a glass ribbon will be drawn inside a drawing machine enclosure; (d) determining a relationship between the glass sheet quality metric Q and the effective width, W eff , and the absolute maximum temperature difference, ΔT max , of thermal anomalies in the glass ribbon for the glass ribbon at the glass ribbon temperature, T 1 ; (e) generating a curve of ΔT max versus W eff based on the relationship determined in step (d) to define an envelope of thermal anomalies, E1, of the glass ribbon at temperature T 1 , which are allowable for a glass sheet quality metric level Q 1 ; (f) overflowing a glass melt from an isopipe to form a glass ribbon at the root of the isopipe; (g) drawing the glass ribbon below the root of the isopipe through the drawing machine enclosure at the temperature T 1 ; (h) forming a glass sheet from the glass ribbon; (i) obtaining quality metric Q of the glass sheet obtained from step (h); (j) correlating the quality metric Q of the glass sheet obtained in step (i) with the temperature distribution in the glass ribbon inside the drawing machine enclosure to locate thermal artifacts and to determine the absolute maximum temperature difference ΔTmax and effective width Weff of each located thermal artifact; and (k) heating or cooling, or a combination of heating and cooling at the location in the drawing machine enclosure corresponding to the source of the located thermal artifact so that effective width of thermal anomaly in the glass sheet, Weff, and the absolute maximum temperature difference of said thermal anomaly in the glass ribbon, ΔT max , are maintained within the allowable envelope of thermal anomalies, E1, to thereby produce a glass sheet having a quality metric level, Q 1 .