What are Dams?

I am interrupting my series on the ANSI/AGSC/AGRSS™ Standard 003-2015 to discuss the use and the slow obsolescence of (some types) of glass dams. A glass dam is a device that holds back a material from going where it is not supposed to go. Dams in our industry are used in three different ways: as a positioning device, an aesthetic feature to hide unappealing material or as a sound barrier between the engine noise and the quiet interior.

Most of the dams used in automobiles are made of an open-celled foam extrusion and are placed between the urethane bead and the interior of the vehicle. Vehicle manufacturers use dams to solve a dual problem. They can help to position the glass a distance away from the pinchweld metal to allow movement to the glass in the opening. They can also keep the glass from flattening out the low viscosity urethane used in the manufacture and keep it from oozing into the passenger compartment. Carmakers decided on open-celled foam to allow good air flow for curing purposes.

The third use of a dam is acoustical. The acoustical dam is always found at the bottom of the windshield mounting area. It can be attached to the glass, pinchweld, dashboard or firewall. Many times this dam is damaged upon removal of the glass by inside cutting tools such as long-handled utility knifes, power tools and wire out tools and/or the lack of care taken by the technician. Unfortunately, these dams are rarely replaced when damaged. This is due to the unique style, cost, availability and the lack of reimbursement by insurance companies.

So, what happens if the technician fails to replace the dam? Usually very little. The positioning dam is not needed as long as the technician uses a high-viscosity urethane. The aesthetic dams are rarely used anymore because of decorative interior trims and mouldings. However, the removal of an acoustical dam is one area where the end result could be complaints about air and engine noise. It is recommended that acoustical dams be salvaged or replaced to save the expense of a customer calling you back to fix a noise problem.

Here is a little hint that I have used for many years that may help you with this quality issue. The biggest problem with replacing acoustical dams is the uniqueness of design and the cost. I have a solution that may help. It is called backer rod or glazing rod. You can find it in most hardware stores or online.

A backer rod is an open or closed cell foam extrusion that is used in the building trades to fill the gaps between the millwork and the walls of a building. It allows for the expansion of joints, but seals from weather and noise. Usually it is inserted in the joint and then covered with a caulk or sealing compound. Our use is to provide a sound barrier from the engine compartment.

NOMACO Engineering Foam Solutions.
NOMACO Engineering Foam Solutions.

I use a round open-celled foam of about ¾ or 1 inch in diameter. I try to find a black or dark grey color to provide a neutral palate. Once the glass is placed in the opening, I push the rod between the glass and the dashboard. The rod will compress and then expand when it reaches a gap or cavity. It is very easy to install, it is inexpensive and it forms to the unique shape of the vehicle.

Even though positioning and aesthetic dams are going by the wayside, acoustical dams are becoming more important due to the increasing number of voice activated systems in the interior of today’s vehicles. It is important for us to restore the acoustical integrity of the vehicle’s interior. Backer rods are a way to do this easily and economically.