The science of the repair.
An auto windshield is comprised of three separate layers of glass that have been laminated together. When a rock from the road hits an auto windshield, it breeches the top layer of glass causing an air gap between the plastic middle layer and the top or outer layer. Debris and glass shards from the top layer of glass are propelled inward. Windshield repair attempts to clear away as much of the debris as is possible, and then fill the air gaps by injecting a clear acrylic resin into the damaged area. Alternating vacuum and pressure cycles are used to continuously remove air and debris trapped in the damaged area so that the resin can flow further and further into the break, eventually filling in. Magnification of rock chips on your windshield would show many micro cracks that extend out in all directions. The legs that extend outward are unstable and can easily spread resulting in a costly windshield replacement. One significant bump or extreme change in temperature can easily cause a rock chip to crack. If the damage is repaired soon after it occurs, the resin injected into the damage area will totally fill the break structurally restoring the windshield to pre-damaged condition. The appearance of the damage will be enhanced between 75 to 95%. The sooner the repair is made after the damage has occured the better the results.