Most vehicles have a windshield with a unique shape and set of features. However, there are common traits and principles that pertain to all windshields. We will address these commonalities and take a look at how they pertain to rock chip repair.
Most modern windshields are curved in three dimensions. Because of the bending process needed to create these curves, a windshield has some areas that are weaker than others. Usually near the edges where bends are more prominent, it will take less of an impact to create a break. To further this issue, the black frit on most windshields adds to the higher chance of breakage due to the baking process with which most frits are applied as well as the higher level of heat due to the black color. These factors, along with stress, which we will address next, contribute to windshields breaking in different ways depending on where they are impacted.
STRESS plays a huge role in many aspects of windshield repair. Glass is stressed in different ways for different reasons. Just the fact that a windshield is glued to the vehicle creates a form of stress. In fact, the induced stress from a proper installation will cause a glass to break easier than the same glass sitting on a stand! There is also good stress caused by the bond of the PVB layer that will actually pull a break closed and keep it from spreading. It is for these reasons that a crack emanating from the edge will be wider by the frit and begin to close as it reaches the center of the glass. This leads us to the subject of wicking.
WICKING is a process that allows liquid resin to flow into the small capillaries of a break. Whether we are dealing with a small rock chip or a long crack there may be parts of the break that are held closed by stress. To perform a repair properly, this stress must be removed or reversed by various methods depending on the situation. These methods are discussed elsewhere, but some examples are applying pressure from the inside with your thumb or a crack expander, or "popping a bull's-eye" on a tight star break. Wicking is primarily a passive process that we enhance by changing stress factors and applying vacuum and pressure to the liquid resin. This will help us achieve the strongest bonds which leads us to wetting.
WETTING is the process of adhesion of a liquid to a solid surface at an intermolecular level. Our resins are designed to have a high wettability factor with glass. They have a lower wettability with the PVB layer between the two layers of glass. It is for this reason one should do everything they can as a tech to avoid unnecessary exposure of the PVB layer. Never drill through the entire first layer of glass to the PVB and don't allow thin resin to seep between the glass and PVB.
Other things that could cause potential problems with a windshield repair may include, discoloration of the shaded PVB area due to heat being released during the curing process or poor adhesion to a large exposed portion of PVB, interference with accessories mounted to the glass that need a totally clear area for them to function properly, and other issues as windshield design continues to change over time.