If you’ve watched your windscreen being replaced, you may have noticed the fitter reaching over and across the car to apply the adhesive (PUR). Or did he lay the bead directly to the glass? Only a few installers will extrude directly to the glass. Which way is better?
Unless it’s a hand-built car, when it comes to fitting bonded window units, the overwhelming majority of car manufacturers will use an automated system. Robots don’t need tea breaks or shift changes; they work around the clock increasing productivity. They are also consistently accurate. They have to be. The computer controlling the robotic arm which applies the PUR to a windscreen can be programmed to follow an exact map of where the adhesive should be. The best way to do this is to apply directly to the glass before another arm lifts the windscreen and positions it on the vehicle. Doing this by hand, in principle, is no different. The end result is the same.
Body or Glass?
This question could be paraphrased to: which is right and which is wrong?
Glue on Glass
If the windscreen is bonded in correctly, and does not leak, creak or rattle; doesn’t allow wind noise into the cabin and generally does everything a windscreen should do, it should not matter how the PUR was applied. However, there may be some advantages and disadvantages which could help answer the question of which way is better. There are two main objectives in the application of the best possible bead:
- The gun should be perpendicular (90 degrees) when applying. This negates the risk of a ‘tunneling’ effect in the compressed bead and provided the speed of extrusion and movement is consistent, the bead height will remain uniform;
- Ideally, one join is optimal. Simply, less joins = less chance of the windscreen leaking.
Tunneling occurs when the glue gun was angled when the PUR was applied. It weakens the bond by reducing the contact made (less PUR adhering) and can also cause stress fractures due to that trapped air expanding.
The darker, shinier appearance in the image is PUR which has not made contact with the glass. The cut urethane either side of it was all that was forming the bond. When the extrusion gun is angled, the bottom of the bead circular (created by the rounded part of the nozzle). Applying directly to the pinchweld means the technician needs to be elevated (higher than the car) and will also need very long arms if he intends to extrude a bead in one start-to-end movement. Some may stand on the door-shut for this, and others may even rotate their body through 360 degrees whilst standing inside the car reaching out to the pinchweld. It’s not impossible, but it is very difficult. For this reason, fitters opt for the easy option: start extruding by standing on one side of the car (the starting point either being as far over to the opposite side of the car as possible, or the middle). The line of the previous install is then followed around the aperture. There will be more than one join as the bondline cannot be followed entirely in one start/stop movement.
Applying the urethane by hand is cumbersome, but many technicians perfect the method and will achieve very good bond lines. Some windscreens which require a push in trim to be inserted after the glass is fitted will use a damming tape. This is to keep the urethane where it needs to be so that when the glass is compressed down onto the adhesive, there will be enough product showing for the trim to push into. The added benefit is that the damming tape will act as a barrier for the inside of the car, preventing ooze on the interior side of the bond line. This will also prevent the PUR making contact with A-pillar trims (if they have not been removed for the install). Applying the PUR to the body for this type of fitment is solely reliant accuracy of a) the positioning of the PUR and damming, and b) the ‘set’ position of the windscreen. Both have to be ‘married’ by the installers eye as there are no reference points for guidance.
Applying the urethane to the glass gives better control leaving the only alignment issue to lifting the windscreen into place. This is easily referenced by at first, dry fitting the windscreen and marking out witness marks. Applying urethane to the glass in this type of fitment is not only easier, it makes sense.
Ask any fitter who swears by ‘gluing to the body’ and he will say that he does so because there is already a bond line there from the previous windscreen (hey, but what if that was wrong, or that the car has been in for a front end respray) or that applying glue to the glass can go horribly wrong if you get the lift-on wrong (suggesting a confidence problem as the same surely applies to gluing to the body). The case against gluing directly to the glass is not strong. By applying to the glass:
- You have better control of the gun; it remains perpendicular as you
manoeuvre it around the glass edge;
- The extrusion will require just one joint which can be positioned in the lower section of the glass;
- Most OE glass comes with witness marks indicating exactly where the adhesive needs to be;
- Bead height will be consistent.
- There is no overstretching to get to hard-to-reach places.
Some glass does not come with witness marks but this can easily be done by the technician.
OE glass provides witness marks according to the blueprint. It is the exact template required to replicate the factory install.
The only thing left to get right after applying the PUR to the windscreen is the lift-on. There’s only one place that piece of glass can go. For avoidance of doubt, a quick dry fit gives the installer the opportunity to mark out reference points. Removing the A-pillar trims will also help with the post installation visual to ensure adequate contact has been made and that there is no unnecessary ooze or excess product showing.
There is no right or wrong in either method. It is down to preference and experience. However, there are better advantages in applying PUR to the glass than applying to the body. The defence of applying to the body cannot be reliant on the previous bondline as the template to follow, nor can it be argued that the lift-on is risky.
All comments and feedback welcome.