Method of manufacturing sheets of glass with reduced total thickness variation

A method of manufacturing a sheet of glass comprising:

• (a) forming, from molten glass, a vertically oriented ribbon of glass that moves downward as a function of time, the ribbon of glass having a first primary surface and a second primary surface that face in generally opposite directions and a core disposed between the first and second primary surfaces;
• (b) as the ribbon of glass moves downward, passing the ribbon of glass adjacent to (i) a first raised temperature zone that applies a heat flux to the first primary surface that lowers a viscosity of the first primary surface sufficiently so that surface tension at the first primary surface reduces a total thickness variation of the ribbon of glass while a temperature of the core remains sufficiently low to prevent attenuation of the ribbon of glass and (ii) a second raised temperature zone that lowers a viscosity of the second primary surface so that surface tension at the second primary surface further reduces the total thickness variation of the ribbon of glass while the temperature of the core remains sufficiently low to prevent attenuation of the ribbon of glass;
• and
• (c) after the ribbon of glass moves below the first raised temperature zone and the second raised temperature zone, separating a sheet of glass from the ribbon of glass, wherein;
• between (b) and (c) and after the total thickness variation of the ribbon of glass has decreased, the temperatures of the first primary surface, the second primary surface, and the core move closer to equilibrium, an effective viscosity of the ribbon of glass decreases such that the ribbon of glass then attenuates.

A method of manufacturing a sheet of glass comprising:

• (a) delivering molten glass as a stream to a nip between a pair of forming rollers, the molten glass delivered forming a puddle of the molten glass on the pair of forming rollers;
• (b) rotating the pair of forming rollers to roll the molten glass delivered to the nip into a vertically oriented ribbon of glass that moves downward as a function of time, the ribbon of glass having a first primary surface and a second primary surface that face in generally opposite directions and a core disposed between the first and second primary surfaces;
• (c) as the ribbon of glass moves downward, passing the ribbon of glass adjacent to a first raised temperature zone that raises a temperature of the first primary surface, without physically contacting the ribbon of glass, sufficiently so that surface tension at the first primary surface reduces a total thickness variation of the ribbon of glass while a temperature of the core is sufficiently low to prevent attenuation of the ribbon of glass;
• (d) after the total thickness variation has decreased, the temperature of the first primary surface and the temperature of the core move closer to equilibrium and an effective viscosity of the ribbon of glass decreases such that the ribbon of glass attenuates;
• (e) a pulling step comprising pulling the ribbon of glass downward with pulling rollers, the pulling rollers disposed lower than the first raised temperature zone;
• and
• (f) after the pulling step, separating a sheet of glass from the ribbon of glass.