Foam Fundamentals

Jan. 1, 2020
The term "structural foam" has been around the industry from as early as the late 1980s but often had different meanings based on the product supplier and vehicle manufacturer. There does not seem to be a consensus as to why the term was used initial

Foams are being used throughout vehicles for a number of purposes. Get to know these critical substances.

The term "structural foam" has been around the industry from as early as the late 1980s but often had different meanings based on the product supplier and vehicle manufacturer. There does not seem to be a consensus as to why the term was used initially since the original foams basically did not add any structural enhancements. Their primary purpose was to control noise, vibration and harshness (NVH). Today, these foams do reflect the structural term since they are used widely to add strength and crash protection to the vehicle structure. They deserve a closer look.

'Non-structural' structural foams

Several applications within vehicles require the replacement of any foam that has been removed or damaged. These foams are designed to absorb sound, reduce wind noises in open cavities and control vibration and flutter of panels. These foams always should be replaced with materials recommended by the vehicle manufacturer.

Aftermarket foams may not be the same color as those used on the assembly line, although they match the flexibility and density of the original. Always examine the original foam being replaced if there are no particular foams specified within the repair procedure to match the original's density and flexibility. You may find that softer foams most often are used in areas that require the best possible absorption of sound. Whatever product line you select, you can compare your choices by creating small samples with excess material dispensed into paper or plastic cups.

Often replacement parts may have the foam already applied. This is great as long as the foam is not going to interfere with the replacement of the part, primarily during welding. Note that these foams are flammable and must be removed and then replaced afterwards if they interfere with the installation.


Many manufacturers allow foam to be removed intact then reattached with additional new foam. This saves on the use of multiple cartridges and can be performed as long as there is access to reinstall the original foam block that was removed. Depending on the design, you may be able to slide the detached foam within the panel to weld and apply epoxy primer to the inside section. You then slide the foam into a position to add additional foam. Often however, the removed foam cannot be reinstalled since there is no access large enough to feed it into the installed part.

If a part such as an A-Pillar is being sectioned and is filled with foam, you must plan your sectioning joint to ensure proper corrosion protection and proper reinstallation of the foam. Reinstallation requires some type of access opening. A new access hole cannot be drilled for this. Drilling additional holes in a structural part such as an A-Pillar typically is not an acceptable procedure. You may be able to remove the foam from within a larger portion of the part to get to the original access opening or hole. This is difficult but usually can be achieved.

Removal of the foam in an A-Pillar usually is performed with a screwdriver or a pick tool that can dig the material out. Another method uses a burr bit on a pneumatic drill or die grinder. For this method, note that you will most likely expose bare metal on the inside of the panel, which will require application of epoxy primer. The epoxy must be allowed to cure properly before the foam is replaced. Note too that it is best to use a burr bit with larger flutes like those used in woodworking in place of the typical bits used on metal. Metal bits tend to have smaller and closer flutes that clog more easily.

Proper cartridge installation

When installed, twin cartridge tubes require the cartridges be "leveled" like all packaging of this type. During storage, the materials can settle at different levels. Leveling ensures the material coming out of the mixing tubes properly mixes at the correct ratio. To level the tubes, simply remove the end while pointing the installed end vertically. Dispense a small amount until both sides of the cartridge are at the top. Be careful to wipe off any materials dispensed since any mixed materials will begin chemically expanding.

Once the cartridges are leveled, install the correct mixing tip. Cut the end to the proper tip opening, if required. Caution: It is possible to install a wrong cartridge tip designed for a different product line. Make sure you are using a tip designed for foams and not one for adhesives or plastic repair materials.

Application variables

Installing foam requires planning and precisely determining the necessary amount foam to be applied. Just as you would not add more welds than the original number when replacing a panel, don't fill an entire cavity of a panel with foam just because you didn't know the proper amount and location. Many manufacturers provide exact entry locations, amounts and additional instructions to properly replace foam. Adhere to these instructions.

Chrysler, for one example, provides the exact entry and amount of foam in grams to be dispensed into a specific cavity. The company also often provides the sequence and position of the mixing tube if multiple "shots" are required.

If the manufacturer doesn't supply this information, you can determine it from other sources. For the specific application amount note that typically aftermarket foam is sold in 6.7 or 10 fl. oz. cartridges. One ounce of dispensed foam is very close to 33 grams. A 6.7-fl.-oz. cartridge would then contain approximately 221 grams. Often manufacturers will call for shots in increments of 25 grams. If you divide the cartridge into equal parts (221 divided by 25), each increment will be very close to .5 inch. (13 mm). The 10-fl. oz. cartridge would then contain 13 equal increments (10 x 33 divided by 25) that also will translate to .5 inch. (13 mm).

In the I-CAR program DCX01 – The Collision Repair Overview for the Chrysler Group vehicles, the class exercise employs a wooden paint stick marked with .5-in. (13 mm) increments as the measuring tool for the plunger rods used on the cartridge guns. A measuring tape can be used as well. Once the material has been properly installed on the dispensing gun, you can then use a marker to mark .5 in. lines from a fixed point where the plunger rods move through on the plunger rod.

You also can make other measured markings if and when the required dispensed material is not a factor of 25 grams. This method allows you to dispense a very accurate amount of material as per the manufacturer's repair procedures.

If there are not amounts listed in the repair procedures and you need to fill the cavity to a defined area, you will need to determine the volume required and relate that to the total volume the cartridge can fill. This will also allow you to determine how many tubes you may need. Take measurements of the area you wish to fill. Multiply the length by the width and height to obtain the required volume in cubic inches or millimeters. The product sheet should provide the total volume dispensed available.

Dam it

The biggest problem with foam often is getting it where you want it to begin and keeping it there. Once mixed, foam reacts and expands based on the characteristics of the product. Some foams are very quick to expand while others feature a delayed reaction (foam time). Some foams employ a lower viscosity and run quicker into the cavity area (flow rate).

The amount of volume the product will fill (expansion rate) may differ by product design as well and be affected by temperature and shelf life. Generally these types of foams feature an expansion rate approximately 10 times their liquid volume.

It takes some experience to get used to these variables. If you aren't familiar with a product, practice on something other than the vehicle before installing.

Vertical type panels such as an A-Pillar or the center of the B-Pillar can make it difficult to gauge the level of the foam run down before it begins expanding and fills the cavity. By dispensing the product slowly, the material will run down for a shorter time before expanding than if it was dispensed quickly and at a high volume. Product manufacturers may provide different products for different needs for circumstances like this. Be sure you select the right one.

It takes some experience and practice to be able to judge this process and allow the product to build its own dam. You can assist this process with other methods depending on the access available. If access is good, you can build a cardboard or paper dam or foam blocks to the level you need the foam to begin. This makes it easier to fill the cavity to the proper level.

Another method involves using an inflated balloon to act as the dam and then simply popping the balloon afterwards. Be sure to tie a string to the balloon so it can be pulled out and is not left in the cavity. Make sure you also don't leave any other cardboard or paper dam materials.

Structural 'structural' foams

Foams are becoming more popular today because they add strength and crash protection to specific vehicle areas while saving weight. With the increased need to improve side intrusion into the vehicle, don't be surprised to see these foams in more vehicles in the future. These foams are different from traditional foams and should not be substituted with other products.

You can apply them using the general procedures discussed here, but note that their expansion rates are much lower than the other foam varieties. Once they expand to their level and cure, these foams become extremely hard and strong.

Manufacturers offer widely differing repair procedures should these foams become loose inside the cavity as a result of an accident or any other cause. Ford Motor Company, for example, permits its foams to be reattached by adding additional foam. However, General Motors stipulates that on the Cadillac DTS if its foam becomes damaged or loose, the frame rail requires replacement. Always check the manufacturer's recommendations.

Other foams

Foams also are used in a range of other applications. Not all are liquid applications. Some are placed in panels and other cavities. These include bumper absorber pads, door impact pads, anti-flutter and solid blocks. Repair of these foams frequently is misunderstood.

Bumper absorber pads generally are "replace only" when compressed or deformed from a collision, but often are not replaced because they remain hidden in the structure. There are only a few possible repair procedures with these foams for a select range of vehicles. If they are damaged, they should be replaced.

The inner door impact foam pads placed for occupant protection, generally for side impacts, are like one-time fasteners for many manufacturers. Once they are compressed, they should be replaced. Indeed, if they are removed for any reason, many manufacturers require replacement.

Anti-flutter foam is used in many areas to eliminate the contact between close panels such as the inner door beam to the outer door panel and the inner braces of the hood, roof or deck lid to outer panels. Use foam when appropriate, but also be certain to use the correct product. These products typically do not expand very much and may actually resemble adhesive more than foam. They also may be very flexible. Using the wrong foam here can damage the outer larger panels due to excessive expansion.

Many manufacturers use foam blocks attached with adhesive or attached to a carrier that may be bolted into place. Even though these foams may seem unimportant they should be re-installed and identified before a stud welder is used to pull cosmetic damages.


While we've covered a lot of ground on foam here, there is still a lot more about foams we all need to understand to ensure a proper application. We also need to keep learning because the use of foams will only increase. You can count on that.

About the Author

Tony Passwater

Tony Passwater, president of AEII, has been in the collision industry since 1972. AEII is an international consulting, training and system development organization founded in 1986. Tony has worked with collision shop owners worldwide and developed computer solution software programs, training seminars, and on-site consulting services for many of the top organizations. He is also a founding partner in Quality Assurance Systems International, QASI, the leading organization for process improvement in the collision industry through ISO international standards and certification.

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