Ask The Doctor
pros who know
Repairing the Edge Crack
by Richard Campfield
Editor’s Note: This is the first part in a series.
Crack repair has been a part of the windshield repair industry for more
than two decades. The most important and critical component of crack repair
is the resin used. You must know a resin’s properties and use the right
resins. The resins have to be able to deal with four stresses, excluding
the thermal stress from exposure.
The Edge Crack
Ninety percent of cracks repairs are edge cracks due to both residual
stress and induced stress at the edge of the windshield. As with any adhesive
application you must understand the stresses you are dealing with in order
to obtain a proper bond.
Residual stress is created during the annealing or cooling process after
the glass has been bent and molded and comes out of the oven. The level
or magnitude of residual stress can be limited by controlling the cooling
rate through the annealing range. This prevents molecules from splitting
apart by the varying rates of cooling occurring at the edge area. This
phenomenon causes a perimeter band of residual tensile stress and is hence
the weakest area of the windshield. Very quick cooling results in high
stress levels while slow cooling reduces the levels and hence increases
the time and cost to manufacture the windshield. A band of tension exists
just in from the edge. This tensile stress will taper to zero usually
1.5 inches from the edge. This area is called the “weak spot” by windshield
Induced stress comes from the installation and is also referred to as
“installation stress.” This is the stress that causes the fracture in
the “weak spot” to crack and is also why nine out of ten cracks will be
longer than 6 inches. Because the windshield is being glued to the vehicle
at its weakest area, the residual stress is both increased in magnitude
and expands to about 2 inches in from the edge. You can feel the induced
stress by knocking with your knuckle at the edge and going inward until
you feel and hear it change. This usually occurs 4 to 5 inches inward.
This is the stress your resin must be able to handle. This stress causes
the fracture to split apart (outward or perpendicular to the crack), creating
a gap. The stress upon cracking is relieved for the time being and then
the opposing lamination stress pulls the crack back together, usually
after it has cracked 8-12 inches and the crack stops. This is why an edge
crack has a wide gap at the edge and it has to be pried open at the point
to inject resin.
So, remember an edge crack has well more than 1,000 psi of stress and
a gap at the edge and no stress and no gap at the point. The resin used
at the edge area, therefore, needs to have the high-viscosity properties
of a structural adhesive.
If the resin does not have the strength to hold the stress at the edge
or it loses strength after exposure, it will look like Figure 1 after
a few months.
The lamination stress is why stone breaks out of the “weak spot” do not
crack without an outside force, such as temperature change (see Figure
2). This is an inward stress from the bonding of the PVB to the glass.
This is the stress that stops the crack when it becomes greater than the
induced stress, which is usually at 8-12 inches.
There are mechanical bonds and a chemical bond to be achieved. The strongest
and most durable bond to glass is achieved with acrylic adhesives. Surface
tension is created with acrylics because they are attracted to the glass
and not the PVB, so when they are cured they will pull off of the PVB
as the molecules crosslink/chemically bond with the glass. Since there
is no chemical bond to the PVB, you must bond mechanically to the PVB
or the repair will have a line of refraction, which is a separation between
the resin and the PBV. The resins viscosity controls and determines this
bond to the PVB. The viscosity has to be such that it is mechanical enough
to stay put while curing (see the field tests in (my) Patent No. 5,425,827).
Working with different temperatures affects the windshield repair resin
and can call for alternative resin applications, which we will cover later
in this series. In the next issue, I will cover the tools needed to repair
edge cracks and their functions.
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 2010 Key Communications Inc. All rights reserved.
No reproduction of any type without expressed written permission.