AskTheDoctor_pros who know, AGRR, November/December 2010

Volume 12, Issue 6 - November/December 2010

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 manufacturing engineers.

Induced Stress
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.

Lamination Stress
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.

Surface Tension
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.