Due to the amorphous structure of glass, it does not always break the same way every time. However, as seen in the pictures below, there are some common break styles that fall into the "repairable" range. Over the years, techs have developed names that help identify them. Some of the most common are bull's eye, half moon, star, combination, daisy, bat's wing and bee's wing. Each has certain properties that an experienced repair technician understands. It is the tech's job to use this information to make the best repair possible.
All rock chips have a central "pit" area where the glass was impacted by the object. From this area, either cracks called "legs" or cavities emanate. Both of these structures are caused from the impact trauma and now contain air that has entered through the pit that must be removed in order to make room for the repair resin. This is one reason for our system's superiority over others. Any system that only forces resin into a break without first removing the gas can not fully fill the damaged area with resin and create the strongest bond. There simply is not enough room.
The reason air can be trapped in a break is that windshield glass is "laminated". This means that a windshield is actually 2 pieces of glass laminated together by a plastic interlayer. When a rock hits a windshield usually only the outer layer of glass if damaged. Therefor, air enters in through the pit and is contained by the rest of the structure. If an impact is so strong that both layers of glass break, the glass must be replaced.
With our system, we recommend that the total area of damage fit under the size of a quarter. However, this is just a general guideline. There have been times when we have had very successful repairs close to the size of a half dollar. This often depends on the assessment of the level of "crunch" in the outer surface, the position of the chip related to the driver's line of vision and the distance from the end of the longest crack to the pit area. Also, keep in mind, the level of aesthetic improvement and structural improvement after the process are not always the same.
Finally, there are some systems out there that can (or claim they can) repair longer cracks. (Up to 12 inches.) At this point in time, there is much debate over the integrity of the windshield even after the repair, as well as the cost effectiveness to the consumer between replacement and the crack repair. As of right now, we feel with the rising cost of windshields due to some of the new safety technology attached to the glass itself, there will be a rising demand for long crack repair. For this reason, we are researching the matter intensely. Once we come to a conclusion about the safety of a long crack repair and the possibilities of it interfering with the safety system's cameras and sensors we will "weigh in" on the discussion. We are, in fact, working on a safe and effective "stop crack" kit ourselves. However, we will not put anything on the market that we do not feel is 100% effective, efficient and safe. So for now, all of our literature and recommendations will assume a damaged area of around the size of a quarter or less.