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Body Repair

Auto Foam: Overlooked and Misunderstood

Although foams might seem like a minor part of the repair, they are not. There can be serious ramifications for improperly using them, improperly placing them or not replacing them at all.

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Mitch Becker has been a collision industry trainer for 30 years and an I-CAR instructor for more than 25 years. Contact him at (763) 585-6411 or [email protected]

here is an example of foam sheeting used to seal out water from entering a particular area of a vehicle.

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Here’s a quiz for you and your technicians:

1. What does NVH stand for?
2. Where is pillar foam found?
3. What is the ratio of expansion of foam?
4. What primer is most often recommended under foam?

The answers to this quiz may, or may not, surprise you. If you read through this article and then answer all correctly, then congratulations! You’re keeping up on technology. If you miss a couple, then you, like myself, could probably use a reminder on the use and need for automotive foams.


soft foam used at the bottom of a quarter panel to block moisture and reduce noise, vibration and harshness.
A Nuisance

Technicians often look at foams in vehicles as almost a nuisance in the repair process. The need for caution while welding near any foams due to their flammability looms in the mind of a technician whenever they pick up the torch. Sometimes, they wonder, “Why do they use foam here?”
When it comes time to replace the foam used in today’s vehicles, technicians may not understand the ramifications of improperly using or placing them, or not replacing them at all. This is where shops need to understand that even what may seem insignificant may be much more.

anti-flutter foam used at the top of a quarter panel to reduce wind noise and wind vibration.Location
First, identifying where foams are located is necessary to avoid causing problems during repairs.

Many automobile manufacturers have sectioning procedures designed in areas to prevent foam damage or fires. Others require foam removal before welding or sectioning to prevent fires. Once repairs are complete, all foams must be replaced. Procedures for locating foams can be found on manufacturer websites, OEM repair procedure providers or the I-CAR FOM-01 course. Another source for information is the State Farm tool tech site, which has some short films on the proper use of many products.

foam used between panels to reduce vibration.Function
Foam in today’s vehicles has many functions, some obvious and some not so obvious.

One function is noise vibration and harshness (NVH). Foams used for this purpose help to keep wind noise levels down in areas where air flow becomes strong. The foams are also used in places where vibration of parts or panels could create unwanted road noise.

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Foams also protect parts from the harshness of roadways, environment and use. Simply put, foams are used to keep water and roadway chemicals from entering where they should not. Foams also can be used to keep metal from fatiguing prematurely, such as in door hinge areas.

As you read this article, it’s important to note that foams do not always require replacement. There is some reparability on some foams to be reused, but where and how is up to the car manufacturer. Many foam manufacturers will also have instructions as to when and where repair and reuse is possible. Some will require a new product or part. The same is true of rails and quarter panels. In rocker panels, we may find plastic carrier foams that activate during the manufacturing process to control air and stiffen panels. If removed for repairs, they may be trimmed and reused if not damaged. Some carriers may need to be added, such as in the Camaro quarter panel.

here, the automaker filled the area where the fender meets the door to seal out wind noise.Forms
The foams come in many forms, from the soft foam blocks to urethane two-part products. Soft foam blocks are low-density products, whereas sound dampening materials and structural foams are strong. Each type has one or more qualities that serve as reasons why and where it’s used.

  • Soft foam or foam fillers. This foam is found throughout the vehicle in areas where wind or howling may be an issue. Most times, we find it sort of jammed in places. An example is in rearview mirror pockets on doors. Because of its flexible nature, many times it’s removed and reinstalled. It’s most likely used in dry areas where moisture is not a concern. The foam is open celled in nature, and water can and will wick in or saturate. Although simplistic in nature, it may be overlooked and not replaced. This may bring a vehicle back to the shop after repairs due to the wind noise that often occurs from not reinstalling the foam or improperly replacing it.
  • Flexible foams or flutter foams. These types of foams are used in between panels and reinforcements. They’re usually used to prevent the fluttering of the metal as the vehicle travels down the highway. It’s soft, but not too soft. When compressed, it will return to its original shape. The foam may also adhere to both surfaces, securing the panels. This is evident on door skins and intrusion beams. Another application is for the blocking of water or moisture. When undamaged, it will be a closed cell structure, forming a skin that can block water from entering. If the closed cell foam is damaged or cut, it should be sealed with two-part urethane sound deadening material or urethane adhesive, which will prevent moisture issues and reduce the chance of corrosion. This is evident on the new Ford trucks on the upper rail under the fender where it comes into the A-pillar. The foam prevents water from getting to interior welds and areas that are less corrosion-protected. It may not look pretty, but it’s an important area to apply foams if damaged or removed.
  • Semi-rigid foams, pillar foams and rigid foams. Depending on which product manufacturer you use, these foams are used in areas that may need to have a stiffener to prevent flex or vibration. This helps keep the vehicle quieter while traveling down the road. The stiffness may prevent metal panels from moving and causing metal fatigue, which is why we see these foams near hinges. As the door opens and closes, metal movement may cause fatiguing of the steel, which may cause cracking and failure of steels further down the road. Semi-rigid and rigid foams can also be used to prevent water from entering the body cavities of a vehicle. These two-part products fill a void and may adhere to all surfaces. Semi-rigid foam, when compressed, will not return to its original shape. It offers limited reinforcement to help maintain shape and manage energy. It is not to be confused with structural foam and is not in any way interchangeable. If the foam is damaged or cut, seal the cells with a sound dampening material or urethane adhesive to prevent water from entering. The stiffness and placement of these foams is critical to some safety system sensors. Side impact airbags may rely on the foam stiffness to deploy the curtain airbags faster in a collision, so failure to replace or improper placement could change reaction times.
  • Structural foam. This is a two-part product that, when cured, is very strong. It’s used to stiffen engineering changes and reinforce energy areas of the vehicle and should never be substituted for any other foam product. Structural foam can normally be found in torque boxes on frames, but it may also be found in pillars and lower rails. Because of its strength, structural foam can be difficult to remove. Find ways to remove all existing foams, but limit the damage to metals.
  • Pre-formed foam blocks. These play a critical role in energy movement in doors and under panels. As mentioned with the semi-rigid foams, pre-formed blocks may play a role in airbag activation during side impact and rollover situations. They may be held in by clips or glued in place. When damaged, they must be replaced. With some vehicle manufacturers, if the glued-in-place blocks must be removed or have come loose, they must be replaced – even if they are not physically damaged. The fact that they came loose or the glue seal broke is enough to be concerned as they may be contaminated to the point where they can’t be reattached. I cannot express enough the importance of them being in their proper locations and staying in there proper locations during the crash. The pre-formed blocks also are behind trim panels. They protect in body strike zones to prevent the human body from hitting steel components. They also can help steel components from hitting the human body, which can be seen in door panels and A-pillar interior trim panels.

WARNING: The use of foams that are not specifically engineered or approved for automotive use can cause damage to vehicles. Many consumer foams may be available at stores and should not be used. These foams can be acidic in nature and expand at uncontrollable rates, causing major problems for technicians. Once the foam goes in, getting it out is very difficult.


The triangular foam (1) attached by adhesive blocks holes where air passes through, acting as a filter so no dust enters the occupant compartment. The other foam (2) is a two-part soft foam designed to reduce noise, vibration and harshness.Application
The application of foams can be as simple as shoving them into a void, or applying them into a trapezoid-shaped area. How to apply enough to be effective and not overdo it and waste money and time is a fairly simple approach.

Follow the manufacturer’s instructions in preparing the vehicle for foams. I emphasize this because I continually see corrosion issues on vehicles that have been repaired. But also understand that there are shops out there that do an incredible job on all aspects of repairs. If I were their customer, I would be ecstatic to get my car back better than it ever was. The number of these types of shops is growing as instructions become more available.

This is a semi-rigid foam on a 2014 Range Rover designed to keep moisture out of the rocker panel.Removal
Find a way of removing foam that’s the least intrusive and/ or least damaging to metal. I find using new, hard plastic chisels causes less damage to metals. I also like mild abrasive wheels or bristle disks as they will remove material and not thin the metal. The lack of grinding will also keep the heat from damaging the metal. Also, there are file belt sanders on the market that are incredible tools for removing existing foams where required.

Remember that any bare metal exposed must be primed according to directions. Many recommend using an epoxy primer in repair areas as it is an excellent barrier coat for moisture and chemicals. The foams will also adhere to the epoxy primer very well.

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The application of foams can be as simple as cutting to size and smushing in place. Two-part epoxy and urethane foams, however, will be different in that they will require some thought: how much foam? Where does it need to be when cured? Where is access to apply?

Establish access points. Figure out where foams need to be and how to get there. In most cases, there is a hole to work with. Drilling or creating access is not an option.

As in some plastic carriers, they may be glued in place when panels are put together for welding. Squeeze type resistance spot welding is great for not affecting or burning materials used. When access is found, how much travel to the destination needs to be established. For example, the access may be 30 inches from where the foam needs to be. Thus, you would need to pump it in as liquid very quickly to get it to travel that distance before expanding.

Pre-cut placement foam designed to reduce noise, vibration and harshness. How Much?
We need to find out how much foam is needed. If it’s in a long rail, an oddly-shaped hollow part or a gap, simple math equations will help to establish how much volume is necessary.

Once cured volume is established, we need to know the expansion rate of the foam to be used. Many are a 1:10 ratio. This means for every one ounce of product dispensed, it will expand to 10 ounces of cured material.  We want to be sure of how much material to use because if too much is used, it will expand and block areas not necessary or flow out of holes and joints, making a mess. If too little foam is used, we may not have enough required to keep the vehicle quiet, among other things.

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The expansion is related to the temperature being used. If the temperature of the product is low, it will react slower and expand less in volume. If warmer than recommended, it will expand fast with more volume. Be sure to check temperature and expansion rates before use. Many suppliers offer a chart on these.

Structural foam will react differently. This foam has a low expansion rate. Some manufacturers’ product must be heated before dispensing. This foam must be used (as all others) according to product and vehicle manufacturers’ specifications.

Mixing Tips and Speed
Once we learn our expansion ratio, we need to be sure the proper mixing tip is used. They are specific for different products used, and they may fit the package but will mix incorrectly. The wrong tip could create a major mess. Adhesive mixing tips and foam mixing tips may be totally different.

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We also need to figure in trigger speed, which will affect the viscosity of the product. If we need foam to travel a distance down or into a panel, we need a lower viscosity product to achieve this. By the technician triggering fast, the liquid foam will travel to the desired area. If the foam is needed right at the access point, we need a higher viscosity or damming material. By gunning slower, the technician will create a foam that begins expanding immediately out of the mixing tip, limiting travel.

Damming materials will be used to hold foams in place when the traveling of liquid is not desired past a certain point. Dams can be made of different materials and cut to size. Be sure to use materials not affected by moisture such as paper and cardboard. A painter’s glove or balloon inflated in the area works well as a damming material and can be removed once the foam has cured. Mentioned before are plastic clips or carriers, which may be trimmed down and reused in most cases.

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After the foam is applied, follow vehicle manufacturers’ guidelines as to corrosion protection. This important step will help to seal out moisture and prevent corrosion.

Bumper Foams
As far as pre-formed foams or foams used in bumpers go, it’s best to replaced damaged ones. Vehicle manufacturers have guidelines to follow regarding the use of these blocks or pieces of foam. Also, these foams or foams attached to the top of wheel skirts may not be structural but they do keep a lot of road noise to a minimum. A good rule of thumb is if the vehicle manufacturer did not need it, the part or foam or product would not be there.

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One step often overlooked in this procedure is to follow directions. Product makers have wall charts to identify which foam to use and its various uses. The next is application. All instructions take just a little time to read for the best results.

Verify
Always verify to see that the shop is using the right foams at the right places. Although they may seem like a minor part of the repair, they are not. Happy customers are the results of things going right the first time. If technicians are having problems, look on YouTube for manufacturer videos. These videos can become priceless for training technicians.

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Mitch Becker is a technical instructor for ABRA Auto Body & Glass. Contact him at (763) 585-6411 or [email protected].

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