by Jim Kerr
Automobile glass makes up a large portion of any automobile. The glass may look the same as it has for decades, but there have been subtle changes. Most of the changes involve the windshield.
Automobiles produced before the 1950’s used flat glass for the windshields and two piece windshields were common. The first major change occurred in the 1950’s with the use of curved glass. Flat sheets of glass were heated as they moved on a conveyor belt and the glass was allowed to sag into shaped over molds. Curved automobile glass is still manufactured by this method today.
Safety glass windshields have been used for decades by the manufacturers. These windshields have a tough plastic sheet bonded between two layers of glass. Today’s automobiles use this type of windshield but many use a coloured sheet of plastic to give the windshield a “tinted” appearance. The plastic layer holds the glass together in an accident and makes it difficult for an object to be thrown through the windshield.
Heated windshields are available as an option on a few luxury automobiles. The glass is heated to quickly melt ice and snow by passing an electric current through a conductive sheet bonded between the windshield layers. The plastic sheet is similar to a regular “safety glass” one but it is coated with a thin film of gold or has fine heating lines. The thin layer of gold may give the windshield a slight pink or copper tint but clarity is as good as with other windshields. When the heated windshield is turned on, a safety module measures the resistance of the windshield grid and turns off the power if there are any cracks that could cause electricity to pass to the outside of the glass. Heated windshields may use close to 100 volts to heat the glass and a cracked and wet windshield could give a person a nasty shock if the built-in safety module was bypassed.
Head up Display (HUD) also requires a special windshield. This type of dash display projects information such as vehicle speed, signal lights, and warning lights onto a special area of the windshield. To the driver, the projected display appears to float in the air outside the car, but it is actually just projected on the windshield. HUD allows the driver to monitor important vehicle information without having to take their eyes off the road. Early windshields built for HUD had a small transparent projection screen in the lower drivers side of the glass. The latest HUD windshields look like regular windshields, but the glass in the area of the projections has been tested for optical clarity. As can be expected, these windshields cost more than the standard unit.
Side and rear windows are also a form of safety glass, but they are not laminated like the windshield. Instead, the manufacturer tempers (surface hardens) the glass after it is formed. When tempered glass breaks, it shatters into a million tiny pieces. The small pieces are sharp but not big enough to cause extremely serious injuries
There are few options when it comes to repairing windshields. Injecting an epoxy resin into the crack can repair short cracks and large chips. The repair and the crack become almost invisible, but it can be seen if inspected carefully. Many glass shops can perform this repair, but the windshield may require replacement if the cracks are large.
Today, glass has become a structural part of the car body. All fixed or non-opening glass is glued in place to add strength to the upper body. In the past, fixed windows were installed in rubber mouldings or held in place with clips and sealed with a ribbon of sticky butyl rubber. Now, urethane adhesive is used to bond the glass in place. This strong glue takes several hours to cure, so often the vehicle must sit overnight when a new windshield is installed. To prevent the urethane adhesive from breaking down under sunlight, the glass is coloured black around the edges.
The glass has also become thinner. Compare a windshield from twenty years ago to a new one today and the new one will be about half as thick and weight. In the quest to lighten the automobiles and reduce vehicle weight for improved fuel economy, glass has been changed too, but the method of bonding the glass in place has made the assembly stronger than ever.
Finally, windshields are expensive. Most automotive glass comes from only a few manufacturers. The windshields with the best optical clarity are used for HUD systems. The next best quality ones are used by the automobile manufacturers for new car production and replacement glass sold at dealerships. The rest of the glass goes to the aftermarket glass shops. There may be slight flaws so the glass is cheaper, but if the flaws are not obvious, then the glass is a good deal.
Last, come the rejects. This is glass with poor optical clarity or many flaws. If you find a real deal on a new windshield, be sure to check the quality. It may have been a reject, or it may be just a case of having too much inventory on hand and the glass installer is trying to reduce inventory!