Roller Wave and Glass Distortion
Roll distortion is defined in peak-to-valley measurement terms and related to optical distortion.
ASTM C 1048 – 04
7.4.1
“…The original flatness of the glass is slightly modified by the heat treatment, causing reflected images to be distorted….”
7.4.2
“…Fully tempered and heat-strengthened glass that has been made in a horizontal furnace may contain surface distortion (for example, picture framing, heat distortion, or roller wave distortion. Distortion will be detected when viewing images reflected from the glass surface…”
7.4.5
“Regardless of glass flatness, the degree of reflected distortion perceived is largely due to the characteristics or symmetry of
the object being reflected. Linear objects (such as building curtain walls and telephone poles) and moving objects (such as cars) may appear distorted. Irregular and free-form objects such as trees and clouds will appear to have little perceived distortion.”
7.4.6
“Specified bow and warp limits may not adequately define, or control, the distortion that may become apparent after glazing.”
A new technology has been developed to measure roll wave distortion on-line as glass conveys out of a tempering furnace. The new
system measures every lite exiting the furnace while the glass moves at normal conveyor velocity. The system mounts over existing conveyors. Roll distortion is displayed to operators as peak-tovalley measurements. The measurements are stored in a quality control database for later review and analysis. These systems are being installed today and are rapidly changing the quality paradigm for heat-treated glass.
The standard off-line method uses a flat-bottom ‘ GAR’ gauge. This tool is a flat steel bar, 300 mm (12-inches) long with a dial indicator gauge mounted in the center of the bar. With this method, a sheet of glass is brought to a flat surface, preferably a granite table, and the operator drags the gauge over the surface of the glass while recording the results. The dial indicator in units of 0.025 mm (0.001 inches) measures roll distortions as the gauge moves over peaks and valleys on the glass.
ASTM C 1048 – 04
7.4.1
“…The original flatness of the glass is slightly modified by the heat treatment, causing reflected images to be distorted….”
7.4.2
“…Fully tempered and heat-strengthened glass that has been made in a horizontal furnace may contain surface distortion (for example, picture framing, heat distortion, or roller wave distortion. Distortion will be detected when viewing images reflected from the glass surface…”
7.4.5
“Regardless of glass flatness, the degree of reflected distortion perceived is largely due to the characteristics or symmetry of
the object being reflected. Linear objects (such as building curtain walls and telephone poles) and moving objects (such as cars) may appear distorted. Irregular and free-form objects such as trees and clouds will appear to have little perceived distortion.”
7.4.6
“Specified bow and warp limits may not adequately define, or control, the distortion that may become apparent after glazing.”
A new technology has been developed to measure roll wave distortion on-line as glass conveys out of a tempering furnace. The new
system measures every lite exiting the furnace while the glass moves at normal conveyor velocity. The system mounts over existing conveyors. Roll distortion is displayed to operators as peak-tovalley measurements. The measurements are stored in a quality control database for later review and analysis. These systems are being installed today and are rapidly changing the quality paradigm for heat-treated glass.
The standard off-line method uses a flat-bottom ‘ GAR’ gauge. This tool is a flat steel bar, 300 mm (12-inches) long with a dial indicator gauge mounted in the center of the bar. With this method, a sheet of glass is brought to a flat surface, preferably a granite table, and the operator drags the gauge over the surface of the glass while recording the results. The dial indicator in units of 0.025 mm (0.001 inches) measures roll distortions as the gauge moves over peaks and valleys on the glass.