Comparing Chips & Cracks
Repairing a chip is relatively simple, and most technicians are qualified to do chip repairs on windshields. However, crack repairs are more difficult and require different tools and resin. If a crack has been present for a long time or spread, it is possible that it can't be repaired in a way that will ensure the safety of the windshield in the event of a new impact.
The strength of the repair resin is related to its viscosity (or ability to flow). For chip repair, the viscosity is lower (allowing the resin to flow more easily) so that it can be injected uniformly into the small, irregular nooks and crannies of the damaged glass. This lower viscosity results in less binding strength, but in a chipped windshield the structural integrity of the glass is still good.
However, in a cracked windshield, the structural integrity of the auto glass is more likely to be compromised, therefore requiring more binding strength from the resin. A higher viscosity resin ('thicker') is needed to rebind the compromised glass so that it will be safe and protected from further damage (due to moisture, temperature changes, rough roads, etc.). Luckily, cracks are more uniform and accommodate the thicker resin well. To repair a crack, the technician must first drill a 'tip' or anchor for the crack. Once the tip is filled, the crack can be filled with the resin – starting at the impact point and moving along the damaged lines in the glass until the 'tip' has been reached. The resin must then be cured (as in chip repair) before the vehicle can be driven.