by Bob Beranek

Last month I wrote an article on the terms used in the industry by auto glass professionals.  Today, I’m continuing it.

Plunge-cut – A plunge-cut has both a negative and positive connotation. It’s been seen negatively when it is used as a method of easing the use of a cold knife. The belief was that if you cut along the glass’s edge with a utility knife, the vertical cutting edge of the cold knife blade would be less of a drag on the tool, thus making it easier to pull. However, it would also score the floor of the pinchweld and cause oxidation to occur and undermine the urethane bead.

The positive connotation: it’s a technique to remove the flap of a “J” style moulding. Removing the extending flap makes it easier to remove the remaining portion of moulding and makes the cold knife cut smoother without hindrance. The trick is not to plunge the knife blade too deep where it contacts the pinchweld floor.

Gravity Stops – It’s the modern name for devices that support the bottom of the glass part and stops the glass from sliding off of the adhesive. Now, the OE manufacturers’ other means to support the glass while the adhesive cures that will do the job and not cause squeaking. The cowl panel is one method and additionally they use guide pins or hangers.

Guide Pins/Hangers – These are OE parts used by robotic machines that set the glass into place during assembly. You rarely see these parts used in the aftermarket, because they are usually located within about 8-12 inches from the top corner of the glass part and are held in place by double-sided tape or silicone adhesive. These guides serve two purposes: it helps the robot set the glass and keeps it in place until the urethane adhesive cures to its ultimate strength.

The above description does illustrate the use of guide pins in windshield mountings. However, there’s another use for guide pins when discussing other auto glass parts. Some side parts have a tilt-out style of mounting, like in quarter glasses and vents. These parts also use an extended “guide pin” that is threaded and used to attach the hinge to the vehicle frame. It’s also called a “T” bolt. These extended threaded bolts are usually a part of the hinge and helps the robot insert the part into the pre-drilled holes in the frame’s body. Then a nut is used to securely attach the part to the body.

“T” Bolt – “T” bolts are guide pins embedded into a plastic moulding around a glass parts’ edge. The extended bolts are used by robotic machines to set the glass part into place and to attach the glass part to the body.

Encapsulation – Encapsulation is a process where a glass part is placed into a mold and molten plastic is pumped into the mold to create a part where the glass and moulding become one.  Because the encapsulation is either PolyVinyl Chloride (PVC) or Reaction Injected Molded (RIM) plastic, the adhesives used to adhere the glass part to the body may need help to stick properly. That’s why the encapsulated parts are frequently equipped with “T” bolts embedded into the encapsulation or clips that hold the part in place until the adhesive cures.

Many modern day vehicles have eliminated or reinvented the gravity stop. What is a gravity stop? In my day we used to call them setting blocks, but the more accurate term is gravity stops. The gravity stop is a device that stops the glass from slipping off the freshly applied adhesive bead. It is needed immediately after setting the glass into the opening when the adhesive is still in its liquid state. Once the adhesive is cured to the point of holding the weight of the glass, the stop is no longer needed.

The short period of time that it is needed is one of the reasons carmakers eliminated the stop. In a carmaker’s reasoning, it is a device that costs money, must be installed and is not needed after the adhesive is cured, I believe. Because it is not needed after the adhesive is cured, it becomes obsolete. Plus, the gravity stop frequently was the cause of noise complaints by vehicle owners. Unibody vehicles have so much movement traveling down the roadway that the glass would rub against the plastic stops and cause squeaks and noise.

However, a gravity stop served another purpose for automotive glass replacement technicians. The stop aided in the act of setting the glass into the opening by providing a ledge on which the glass could rest while the tech finishes the operation. Whether the operation of setting the glass is completed by one or two men made little difference. The ledge was necessary to position the glass until the adhesive solidified.

So how do OE carmakers keep the glass from displacing while the adhesive cures? It depends on the carmaker, but most use guide pins attached to the top of the glass with a silicone adhesive or double-faced adhesive tape. The pins help guide the robot in the assembly plant to accurately set the glass into the opening. The pins insert into holes manufactured into the top of the pinchweld and hold the glass into position until the adhesive cures. Great idea for the carmakers, but it doesn’t work so well for the replacement techs.

Aftermarket glass rarely comes with the pre-applied pins. It is very difficult and time consuming to remove and perfectly position the pins on a replacement glass so the glass tech rarely does it. Most commonly technicians use adhesive tape to temporarily hold the glass in place until the bonding adhesive is cured.

Other carmakers use water soluble stops. The stops are used in the assembly plant but after exposure to rain and car washes they dissolve and disappear. Some glass technicians replace the water soluble stops with a hard plastic replacement stop but they risk the noise complaint the water soluble stops were invented to eliminate.

What about using generic stops that are fastened to the lower fire wall with screws or rivets? If the stops are going to be permanent, then consideration must be made towards the material the screws or rivets are made of. If the fasteners are aluminum or if the vehicle has an aluminum body, the fasteners must be of similar metal or galvanic corrosion could result. However, if the stops are meant to be temporary for setting purposes only, then once the stops and fasteners are removed, the holes created by the fasteners must be protected from oxidation by priming the hole with metal primer both on the top and the underside.

Hard to believe that the elimination of one little device causes this much hassle for replacement technicians but it is nothing new. All in a day’s work.