On–Line Measurement of Stresses and Optical Distortion for QC of Tempered Glass

Alex S. Redner

Modern tempering furnaces are equipped with controls to adjust temperature and air pressure settings to achieve the desired pre-stress levels. To accomplish this objective, operators use their experience and a trial and error approach. The adjustments are verified using OFF-LINE surface-stress measurements or fragmentation testing, both time consuming and costly.

A newly developed sensor installed at the exit of the tempering furnace yields ON-LINE, real-time measurement of pre-stress levels as each lite emerges from the furnace. The system is based on Spectral Contents Analysis (SCA) technology, broadly used for float line stress scanning. Newly developed hardware and software permits higher data acquisition speed to obtain realtime measurements of pre-stress in tempered glass.

In addition to the new stress measuring system, a new noncontacting sensor measuring optical distortion caused by roll wave was developed for installation and operation in conjunction with the stress sensor. Roll waves are surface waves introduced in the tempering process which introduces optical distortion of reflected images in architectural glass.

This new measuring system is PC-based, non-contacting and can be installed ON- or OFF-LINE. The combination of stress and distortion measurements permits an optimum production control level, eliminating roller wave while maintaining stress levels compatible with specifications.

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