Volume 11, Issue 4 - July/August 2009

“But, My Windshield Is Distorted!”
by Russ Corsi

At the advent of bent windshields in the early 1950s, we became aware of a distracting waviness in the glass that quickly became known as distortion.

As the automobile manufacturers began to design more aerodynamic vehicles, glass manufacturers were challenged not only to bend glass to conform to the ”radical” windshield openings, but also to allow the occupants to see through the glass without getting nauseated. A good example of one of the early demanding glass opening designs was on the 1953 Corvette.

Initially, glass quality control managers developed standards that to evaluate manufactured product for acceptability prior to releasing glass to the car companies. As one might expect, the automobile manufacturers didn’t want any distortion, even in the severely bent corners of the windshield. However, the glass companies knew that having no distortion was an impossible requirement to meet.

“As one might expect, the automobile manufacturers didn’t want any distortion, even in the severely bent corners of the windshield. The glass companies knew
this was an impossible requirement to meet.”

Evaluating the Glass
OE glass manufacturers soon identified areas of the windshield that should be evaluated. Detailed quality assurance standards were written and presented to the car companies for acceptance. Once accepted by the auto companies, these standards were so specific that most trained quality control technicians would arrive at the same pass/fail conclusion after inspecting the same piece of glass.

Typically, distortion is evaluated against four basic criteria:
1. Cross car distortion;
2. Reflective distortion;
3. Transmitted distortion; and
4. Lower corner distortion.

All four of these standards require an evaluation of the subject windshield from the installation angle that is observed in the vehicle.

Cross Car Distortion—The windshield is rotated both left and right to a position that allows the observer to look through the windshield (across the car) against a blackboard with both horizontal and vertical white lines that form a grid. The windshield is rotated in both directions to a position where the angle between the grid and horizontal line is 50 degrees. The level of distortion (the grid squares observed develop wavy lines and/or become diamond shaped) observed is compared to photographs that exhibit various levels of distortion that have been approved by the car company manufacturer. It’s important to note that some level of distortion is acceptable.

Reflective Distortion—Reflective distortion is defined typically as distortion observed on the outside glass surface that is present when viewing the windshield surface, looking at the reflective image on that surface. The windshield manufacturer’s process (bending iron/frame edge support) typically causes this distortion. Windshields are placed at the appropriate installation angle. Wide dark and light strips are suspended above the windshield, parallel to the normal distortion evaluation position. The observer faces the windshield and observes the level of distortion. Acceptance criteria is very subjective. Like the cross car standard, the automobile manufacturers approve limit samples that are used by the glass companies to evaluate their products.

Transmitted Distortion—A windshield being evaluated for transmitted distortion is also mounted at the installation angle of the windshield in the car. The windshield is observed parallel to the grid board. Each individual windshield pattern is laid out in zones. The most critical zone is located 75 mm from the inner edge of the paint band (zone A). (If a windshield does not have a paint band, this zone starts at 100 mm from the edge of the glass). The balance of the glass that is not covered by the peripheral paint band is in the next zone (zone B). The paint band is zone C while the area of the glass that is covered by mouldings or body panels is zone D. (In many cases, zone C and D can overlap.)

The quality control technician employs a vertical bobbing motion while observing the degree of intensity of the oscillation of the horizontal grid lines. Acceptable levels of distortion are defined for both zones A and B.

Lower Corner Distortion—After the introduction of larger and larger windshields that are press bent (glass is sagged both top to bottom and side to side), while being installed at more and more severe installation angles, a new type of distortion appeared. It is called lower corner distortion. The technician places his chin approximately 400 mm above the bottom of the windshield. Distortion is then observed against a luminous grid board. As with the other types of distortion, the degree of acceptable distortion has been determined by the car manufacturers with concurrence by the glass companies.

All bent windshields do have some level of distortion. However, acceptable levels of distortion are clearly defined by agreement obtained between the car guys and the glass guys. The A zone of a properly manufactured windshield will appear distortion free to the average driver. However, all non-vision areas will exhibit some level of distortion. (General reference: PPG Industries Automotive Quality Assurance website).

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.

Readers Respond
I received an e-mail from Steve Harmon who read my January/February article, “How Do I Get My Glass Clean?” Mr. Harmon advised that anyone looking for alternate ways to clean glass should visit the website www.yourviewplus.com. Steve’s company, Clarity Glass and Surface Restoration, is listed as a wholesaler. I am not endorsing any of the products represented; merely offering the reader some more information to evaluate.



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