Pressure Ring or Trapped Air?
Part Three: Summary
A pressure ring is not a structural problem, it is a cosmetic issue only. Pressure rings will look exactly the same with the injector on the damage under pressure as it will with the injector completely removed. Once created, there is nothing that can be done to correct it. While creating the most cosmetically appealing windshield repair possible should always be a priority, a slight pressure ring on a bullseye or flowering at the end of a crack is not cause for concern and should not be considered a failed repair.
By contrast, trapped air is a structural problem as well as a cosmetic problem, particularly when the air is trapped at the edge of the damage or the end of a crack. Curing under pressure may solve the cosmetic problem, at least temporarily, but is not a substitute for properly filling the damage with resin. You would never weld up a crack in a metal plate with pressure pressed against the metal because as soon as you released the pressure there would be stress on the new weld, increasing the risk of eventual failure. The same holds true with repairing cracked glass.
Call it science or call it common sense, these are two very different issues and it is important to understand the difference. The good news is that both issues are relatively easy to control by understanding the cause, learning to identify the cause, and taking the precautions noted.
This topic was chosen in response to a windshield repair technician posting a question about the cause of pressure rings, which was answered by a misinformed technician. The misinformed technician stated that when a bridge is removed from the glass, resin leaks out of the impact point and air travels back in through the impact point, migrating all the way to the edge of the damage, leaving an air pocket. The technician stated that the only way to solve this problem was to cure under pressure. The following short video shot in real time, clearly shows the resin displacing the air in the damage while the injector is in the pressure cycle and removing the air while in the vacuum cycle. The video also shows that no air is left at the edge of the damage even after the bridge is removed.
Delta Kits, dispelling myths, one video at a time!