Volume 10, Issue 4 - July/August 2008

Off the Line
oem glass manufacturing

A Few Technical FAQs About Glass
by Russ Corsi

As I was searching the “Old Memory Bank” looking for subjects for this article, I came across two technical papers that addressed a couple of areas that have raised numerous questions over the years.

Stress Breaks
There isn’t an auto glass installer who hasn’t had to deal with a stress crack (break) in a windshield. The stress crack is probably the most often cited reason for a windshield cracking shortly after replacement.

In most instances, the crack was either caused by: an out-of-bend windshield that was forced to comply with the contour of the pinchweld; a non-uniform urethane bead (too thin in areas along the pinchweld); or an edge chip on the glass that wasn’t detected before installation.

There are instances, however, where the manufacturer’s process can accelerate the development of a stress break. Manufacturer-induced breakage falls into two categories: low-stress single-line breakage and high-stress single-line breakage.

A low-stress failure occurs when the glass edge tension is below 1,500 pounds per square inch (psi). The low-stress break develops at an 85- to 90-degree angle to the glass edge. This type of break would not have occurred if there were no edge damage (i.e., chip or light fracture at the glass edge).

A high-stress failure occurs when the glass edge tension exceeds 1,500 psi. The most significant characteristic of this break type is that the breakage runs at an angle less than 85 degrees as measured to the edge of the glass. This high-edge tension break (too much stress in the glass) is the only true stress break that can be attributed to the manufacturing process.

Seeing Strange Patterns
A phenomenon known as “strain pattern” is often observed in tempered glass. This observation is magnified if the observer is wearing polarized sunglasses.

Strain patterns appear as an iridescent effect, which might appear as a checkerboard, link chain, or most commonly, multi-circular patterns that are typically related to the multi-nozzle tubes found on many air quench sections in the horizontal roller hearth and automotive tempering furnaces. (The quench blows cold air on the glass that causes the rapid temperature drop that tempers (i.e., hardens) the glass surface.)

Although observation of strain patterns may be offensive, it is not considered to be a defect, either at the OEM or AGR level. In reality, observation of any form of a stress pattern is confirmation that the glass is thermally strengthened.

The optical phenomenon that we have just discussed is known as birefringence. When visible light interacts with glass that has been thermally strengthened (tempered or heat-strengthened), the light waves will travel at different velocities in the X (width) and Y (length) directional axes of a lite of glass. Birefringence occurs because of the surface compression and center tension stresses that are introduced in glass by the tempering or heat-strengthening processes. 

I hope you can use this knowledge not only to satisfy your own curiosities, but also to share this information with your customers when appropriate. 

Russ Corsi retired as manager of technical services from PPG Industries’ Automotive Replacement Glass business unit after 31 years in the glass industry. He now serves as a consultant to the industry. Mr. Corsi’s opinions are solely his own and not necessarily those of this magazine.

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