by Jim Kerr

I just had a new windshield put in my truck. Last summer a big stone had a disagreement with the windshield and now cracks were starting to spread across my field of vision. Not only were the cracks annoying, they can be downright dangerous. The sun hits the cracks and blinds everyone inside, so it was time for a change.

I called up a long time friend who has been in the glass business for many years to install the windshield. In fact, he was installing windshields for the dealer when I first started turning wrenches. As junior person on staff, I got the job of helping him install them. I learned a lot. Decades have passed and automobile glass has changed; I’m still learning!

Decades ago, most car windshields were set into rubber mouldings that slipped over a metal frame on the vehicle’s body. Slide the rubber into place on the body, squirt in a little rubber lubricant, place a strong cord or string inside the rubber channel, and while one person pushed, another would pull the cord out, forcing the rubber over the edge of the glass. Anyone working on antique automobiles still goes through this process.

Some of the newer vehicles used butyl rubber tape to seal the windshield. Butyl did a good job of sealing, but installing the glass was a little trickier. The old windshield was “cut out” using sharp knife blades and saws to cut through the butyl. Then all the remaining butyl in the car’s frame was cleaned out with a scraper and a new strip of butyl was rolled into place on the frame. Rubber blocks at the base of the glass positioned the glass and the new windshield was set in place. Set it in the wrong place and the butyl stuck to the glass firmly. Then we had to try to lift it off again, install new butyl and try again. Making a mistake wasted time, so we were careful setting them in place. Finally, the mouldings were reinstalled around the windshield, as these were what really kept the glass in place.

My truck’s windshield is typical of what we find on all modern cars. Instead of soft butyl rubber to seal the windshield, now a Urethane adhesive is used to bond the glass to the body of the vehicle. Glass has now become a structural member of the vehicle. Occupant safety, vehicle handling, and even the reduction of noise and vibrations all depend upon the correct bonding of the glass.

Bonded glass is still cut out of the vehicle’s body just as it was when butyl was used, but now the job is much more difficult; Urethane adhesive is very tough. After the glass is removed, the window frame is cleaned up and a primer applied to help the adhesive bond better to the body. The adhesive is applied with a caulking gun, just as you might when resealing a bathtub. Then the glass is set in place. Better not make a mistake here – touch the glass to adhesive and it is stuck. You will probably need another windshield if you have positioned it incorrectly. There are few second chances!

New glass often comes complete with mouldings and edgings already bonded to the glass. The vehicle now needs to sit in a warm area for a few hours before it is driven because urethane adhesive takes a few hours to cure. Sometimes the installer will use duct tape to hold the glass in position while the adhesive cures.

Have you noticed that all the vehicles now come with glass that has fancy black outlines around the border of the glass? This isn’t just for show, although some manufacturers take advantage of the blacked out effect to enhance the vehicle’s looks. The black border is actually there to protect the urethane adhesive. Ultraviolet light (sunlight) will damage the adhesive over time, so the solution was to simply block out the sunlight with black.

Finally, windshields seem to pit more quickly that they did a few decades ago. This happens because the glass is softer, making it less likely to shatter but more prone to pitting. If it becomes difficult to see when driving into the sun, it is time to think about installing a
new windshield, but this is one job best left to someone with the experience and tools to do it right.

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