FAQ > Windshields > Windshield Repair 101 Knoxville's Finest Auto Glass Installation Center Windshield Construction - Windshields are Laminated, usually in three layers.    The two layers of glass are  separated by a layer of tough, flexible plastic.  A clear windshield has no coloring in the laminate at all.  A  tinted windshield on the other hand has a uniform color to it.  The shaded windshield is tinted and also has  a darker, visor band across the top.  The tint or shading is usually done by applying dye to the plastic inner  layer.  Glass repair is only done only on windshields and a very few back glasses because, as a norm all the  other glass in the typical vehicle is made of tempered glass.  Tempered glass is hardened by heating and  cooling which makes it extremely brittle so that it shatters into small pieces when damaged, so it cannot be  repaired. Why The Windshield Cracks - Understanding windshield repair begins with understanding how a windshield is damaged. The  severity of damage that can be repaired is more than a surface pit but less than shattered glass.  When an object, such as a  stone, hits the windshield at high speed a pit is created on the outer glass surface.  Part of the surface glass is basically  pulverized by the object.  The ground glass appears white on the surface of the windshield.  If the pit is just white, with no  visible cracks coming from it, it's only surface damage, and usually poses no threat to the windshield, but if the pit appears  black or silver, there is a crack with an air cavity beneath it. The impact may have also caused a small cone of glass to  separate from beneath the surface of the outer glass.  There may be more air cavities underneath the separated cone.  The concern with air cavities is that the air will eventually expand and separate or crack the glass.  Air, like any gas, expands with heat.  Most people link cold weather to cracking, but cold actually causes  the air in the damage to contract.  Then when the heater or defroster is turned on it is like blowing up a  balloon.  Air expands to a point where the glass can no longer contain it.  The air separates or cracks  the glass to make room for itself as it continues to expand.  The primary objective of windshield repair is to restore the structural integrity of the windshield.  While the windshield repair process usually results in a much better appearance, restoring structural integrity is the main  reason for doing a repair.  There will still be some evidence of damage after the repair.  If you want the damage to totally  disappear, you must replace the windshield.  And of course, if the damage is in the driver's view of the road, the windshield  must be replaced.  One example of a post-repair blemish occurs when the shaded area of the windshield is damaged.  Air leaks  in and bleaches the dye in the laminate at the damage site.  When the repair is finished, the outline of the damage can be seen  easily from inside the vehicle, because of this bleaching process.  But if the repair is done right, the integrity of the glass  structure will be sound.  Windshield repair prevents damage from spreading to the point where glass replacement becomes  necessary.  To stop the damage from spreading, ALL the air must be taken out of the damage.  If air remains in the tips of a  star break damage for example, it is not a good repair.  Getting all the air out may require drilling extra holes in the tips of the  star.  This would not be done if appearance was the main objective.  Finding the air spaces - The first step in making a windshield repair is to determine where the air spaces are.  The air  spaces can be in one or more locations.  •  Air In The Separated Cone - Sometimes the size or force of the object hitting the windshield  separates a cone of glass and then cracks or even crushes the cone as it is pressed into the  plastic layer.  This leaves an air space in the cracks of the cone.  The pit on the surface of the  windshield is usually much larger in this type of damage (see figure 7).  •  The Need For Drilling - It is often necessary to drill the glass to release trapped air.  If you don't  have access to the air space, or if you have only limited access, trying to replace the air with resin is like trying to fill a  bottle without removing the cap.  Windshield Repair Equipment - All windshield repair systems work on the same principle - the air is replaced with clear  liquid resin.  Windshield repair equipment comes in kits often contained in a case for taking to remote locations.  There are  several different systems available.  Most of these systems use some kind of pressure or vacuum method to draw out the air  before injecting clear resin into the air cavity. Another method that requires drilling into the air space and allowing the resin to  simply flow into the cavity. This method would require the break to be completely open to easily accept the resin through  capillary action.  It can work but if the resin doesn’t flow into the break your one chance at making a good repair is gone.  It's  very important to use the right equipment the first time around because you only get one shot at repairing a break in a  windshield.  Once it's been done and the resign hardens it's over and cannot be repaired again.  Here at Phoenix Glass we use AEGIS windshield repair equipment because we feel is the best equipment in today’s market. Pressure injector tool Repair resin Drill with carbide drill bits Single-edge razor blades Ultraviolet (UV) light Pressure Injection Type Tools - Pressure injection tools usually use a combination of pressure and vacuum to help the resin flow into the air spaces.  The repair fixture tool holds a chamber with an O-ring seal to the glass surface during the repair.  The process usually creates a vacuum by unscrewing the injector piston followed by screwing the injector piston down to create pressure to the point where windshield repair fluid is just short of beginning to seep from the perimeter of the O-ring seal on the glass while carefully observing the crack legs.  These types of units typically do not have any type of vacuum or pressure indicator to go by other than sight so it's possible (in our opinion) to not obtain enough vacuum and to unknowingly inject to much pressure and cause the laminate to separate and or make the breaks crack legs run out farther. •  Air Beneath the outer layer of glass - One location for an air space is  between the separated cone of glass and the outer glass (see figure 4).  This air  space appears black when viewed from directly above the damage (see figure 3). •  Air Between the Plastic And Outer Glass Layer - The separated cone of glass may also  press into the plastic layer, which begins to break down the bond between the plastic and the  outer glass around the perimeter of the damage.  This separated area becomes another air space  in the damage (see figure 5).  This air space appears milky when viewed from directly above the  damage. •  Air In The Outer Glass - If the object hitting the windshield creates any cracks in the outer  glass, there's also an air space in the outer glass.  This air space may be located at the surface of  the outer glass or on the bottom side of the outer glass, or both (see figure 6). These air spaces will  look black or silver. Windshield Repair 101  Questions regarding Windshield Repair are a common place for our Customer Service Representatives on a daily basis.  This  page will explain some of the need-to-know basics about windshield repair.  It is not a how-to article and it's intent is not to list  procedural steps, or to focus on any one repair system, but to describe what windshield repair is designed to do and what the  focus should be.  These basics can be applied to any system of windshield repair.  There are a Myths about windshield repair  which can lead to a great deal of confusion.  Glass "repair" and "replace" are often used interchangeably.  A lot of people are  not aware that glass can be repaired.  Some people who have never seen a windshield repaired compare it to placing a  bandage on the glass.  Others expect the result to be flawless with no evidence of damage remaining. The  truth is somewhere between these two extremes.  As more people begin to understand that a windshield  is a structural part of the vehicle there has been a bit of a controversy on the size or type of windshield  damage that can be repaired, and when replacement is the only choice.  Technicians have been repairing  spot damage in windshields for over 30 years, and those who have been in the business that long don't  see why there should be any controversy.  Windshield repair is not only about whether a repair can be  made or how it will look after the repair, but also routine windshield maintenance.  Figur Figur Some of the more common pieces of equipment in windshield repair kits include: Syringes or eye droppers Glass cleaner Magnifying glass Suction Cup Mirror Probe