Ask The Doctor
pros who know
The Weak Spot
by Rich Campfield
The forbidden topic that windshield manufacturers often will not discuss
or speak of is the “weak spot.”
When we first came out with one of our products years ago, we offered
use of the system to windshield manufacturers, including PPG (prior to
the sale of the company’s auto glass business). After we made the offer,
an engineer from there called me to discuss it. He mentioned the phrase
“weak spot”—I’d never heard this term before, and when he realized I did
not know what it was, he said, “I am not allowed to talk about it,” and
hung up. I knew from repairing edge cracks and surveying parking lots
that there was something going on at the first couple of inches around
the perimeter of a windshield, but this was the first time I heard a name
A few weeks later at a trade show at which we had a booth, a Ford vice
president came up to the booth and I asked him, “How do like the way we
cured the weak spot?” He began to talk and then set up meetings with Ford
engineers during which we had many discussions about the “weak spot.”
We have since discussed this spot, including the product liability issues
that it entails, with many manufacturers.
In my discussions with these manufacturers, I’ve discovered that the most
feasible alternative to curing this defect is to coat the area, which
would cost manufacturers about $1 per windshield, using the same silkscreening
machine that puts the frit on the inner-lite or the numerous glass coatings
they already make for other glass products. What are they waiting for?
Probably a consumer class action, like with the rapid acceleration issue
(Toyota), and Explorer tires (Ford). There was a class action suit filed
against Honda for an issue with the Element’s windshield cracking at the
edge from the bottom, but it was settled before trial.
How Does It Happen?
What makes the “weak spot?” During the molding process, the cut flat glass
for the windshield is placed on a metal mold and placed into an oven.
The glass softens and then sags into the mold. When it comes out of the
oven, temperatures clash at the edge area; at the extreme edge, part of
the glass is near the still-hot frame, while the glass cools from the
outer surface inward. The outer layers contract faster than the interior,
creating a pulling effect on the interior, resulting in residual stress—
and creating the weakest area of the windshield.
I’ve found that the weakest area of the windshield also is the area in
which it is glued to the vehicle, increasing the residual stress and adding
its own installation stress. The residual stress causes this area to fracture
two to two and a half times more easily than the rest of the windshield,
and the installation stress will cause the fracture to crack immediately
(resulting in an edge crack) to relieve the installation stress, which
is usually at about 8 to 12 inches.
OEM versus Aftermarket Glass
So how does the weak spot differ in OEM and aftermarket auto replacement
glass (ARG)? The major difference in the costs to manufacture
OEM glass versus ARG is time and heat; time is money and so is
heat. The OEM is annealed more slowly (100 degrees Fahrenheit
per minute), taking more time and using more BTUs. Under the contract
with the vehicle manufacturer, the specs and tolerance required
for an OEM must keep the “weak spot” around the perimeter edge
under 1,000 psi with an average not over 750 psi. Vehicle manufacturers
do not want the non-OE windshield to crack at the edge before
the vehicle is sold. There is no contract to keep the weak spot
(residual stress) down, so they are annealed faster using less
time and BTUs, and the residual stress can be much higher and
sporadic around the entire perimeter. It can be 1,000 psi on a
spot on the side and 1,500 at the lower corner; some have more
than others. —RC
Richard Campfield is the founder and president of Ultra Bond Inc.
in Grand Junction, Colo. Mr. Campfield’s opinions are solely his own and
not necessarily those of this magazine.
© Copyright 2011 Key Communications Inc. All rights reserved.
No reproduction of any type without expressed written permission.