Used for years to quiet cars, foam now plays a role in strengthening cars. Structural foam is entirely different in purpose and application.

The use of foam as a sound control mechanism in cars dates back to the 1970s. Little plastic bags filled with fiberglass were once tossed into various areas of a vehicle, such as the inside of quarter panels, to absorb sound. Today, instead of loose sound-deadening bags, two-part flexible or rigid foam is pumped into place at the factory to help control noise.

However, there is now a new use for foam: reinforcing a vehicle’s structure. In this process, rigid structural foam with very high compressive strength is pumped in place, which contributes to the structural integrity of the vehicle. Each of these applications is specifically engineered and must be restored during any repair process that disturbs the foam.

Foam Evolution
Foam is a term that has been around since the 12th century. It originally referred to the “light, frothy mass of fine bubbles,” such as what appears on the top of a glass of beer. In 1943, the term was applied to “expanded plastic,” or foam rubber. The common characteristic all foams share is the air bubbles, or voids, which explain why foams are lightweight. The materials they are made of determine the mechanical properties of each type of foam. 

Those mechanical properties are measured in several ways. The density of the foam, expressed as weight per cubic foot, is one measurement. Denser foam of the same material will support more weight. For instance, a denser foam pillow will not compress as much. 

Flexibility, or elongation, is another measurement. This is simply how easily the material bends, stretches and compresses. Some foams are soft and spongy, like pillows. Pipe insulating foam is flexible but relatively stiff. There are plastic, ceramic and metal foams that have a solid feeling to them. Depending on the mechanical properties needed by the engineers, different materials are selected for use in manufacturing. Foamed aluminum also has very good energy absorption ability. 

Repair Technicians’ Responsibilities
Collision repair specialists are responsible for returning vehicles to pre-accident condition. When it comes to foam, this means products specifically intended to replace the damaged material should be used. This is the only way to ensure that the function of the foam will be restored. Steve Marks, research coordinator for I-CAR, says that technicians regularly call I-CAR with questions about foam, some which include: What does it do? How does one decide what to use to replace it? and How is it removed? Marks says this is why it’s important that technicians understand how to distinguish between acoustical foam and structural foam.

Noise, Vibration and Harshness
Noise, vibration and harshness (NVH) is a major concern for automotive engineers. They know that how a car sounds and feels makes a big impact on the owner’s perception of quality. Economy cars that have little sound-deadening material are often referred to as “tin cans” because they are obviously noisier than other vehicles. Luxury car makers advertise how quiet the interiors of their best models are. Engineers, however, must be concerned with the additional weight required to make them quiet. 

Full-frame vehicles had rubber bushings to isolate the body from noise and vibration in the frame. The early generations of unibody structures were made of heavy, mild steel, which did not produce as much noise. Modern lightweight materials and manufacturing techniques result in structures that tend to amplify sound very well. Think of the structure as similar to that of a bell or drum. To control this effect, manufacturers may use sound- dampening pads glued to floor, firewall and door panels. Additionally, undercoating may be sprayed under the floor and wheel wells.

Sound deadening materials add weight to the vehicle. Stopping sound from being generated can greatly reduce the need for heavy dampening pads. A small amount of lightweight foam inside the pillars and rocker panel may reduce the structure’s ability to produce and transmit sound. This makes acoustical foam literally a sound engineering decision.

To control NVH, two types of foam are used: flexible and rigid. The two materials are vastly different in their mechanical properties. If the vehicle manufacturer uses flexible foam, flexible material must be used for replacing any foam removed or damaged in the collision or repair process. Likewise, rigid foam must be replaced with rigid foam. All the major adhesives suppliers have foam to replace what the vehicle manufacturers use for NVH. 

These foams are two-part materials that form a foam when mixed together in an intermix nozzle. Determining the proper amount of foam to install may require a small test sample. A few inches of material, pumped onto a flat surface and allowed to cure, will demonstrate how much the foam expands. This test will allow the technician to judge how much foam to place inside the panel. This is important because too little will not adequately control NVH, while too much may result in moisture accumulating. 

Replacing these NVH control foams should be a part of every estimate and repair plan when the material is present in a damaged area. Determining whether the foam is rigid or flexible is a necessary step in the process and is usually done by the technician during the teardown process. Flexible foam springs back when pushed on, while the rigid foams break apart. Removing either of these may be done with simple cutting tools and a steel brush. On many repairs, deciding where to section a panel may be influenced by the need to have access to replace any foam. Some panels are serviced with the foam already in place. This will require keeping the heat of welding away from this area during the repair process. 

The Wrong Stuff
The single-container urethane foam sold by hardware stores and lumberyards for caulking is a very different product. It is not suitable for replacing the engineered foams specified by the OEMs. It has different mechanical properties and will not perform identically. It also uses a different curing process. Inexpensive foam that sprays out of a can cures by hydration when exposed to air. It requires a large surface area exposed to the air to cure all the way through. The one-part foam will form a skin on the end that is exposed to air and not cure inside the tube formed by the A-pillar. An old I-CAR prop works well as a demonstration. Fill a beverage can with the one-part foam and let it sit for a day or two. If you cut the can in half, the exposed foam inside the can will still not be hard. 

Structural Foam
Using foam as a structural element is not a new idea. This process is common in other industries, particularly with composite structures. Wind surfing boards and snowboards are made of fiberglass and carbon skins with foam cores. The foam supports the skin so it doesn’t have to be as thick. This method results in the lightest, strongest product. 

Ford Motor Co. is bringing the practice to steel unibody structures on the Explorer, Mountaineer as well as on future models. Sources say other manufacturers are looking at the process but have not announced any immediate plans to introduce it on new models.

Ford Technical Service Bulletin (TSB) titled, “Body Joint Reinforcement Material (Structural Foam)—Repair Procedures,” explains the process. There are two types of structural foam applications. The first is Carrier Application, in which a steel or nylon-backed foam is placed in a cavity. The heat used in the OEM E-Coat baking cycle causes the foam to expand, filling the cavity. According to the bulletin, this results in “a joint that is reinforced from the inside where traditional welding would not reach.” 

The Full-Fill Application involves pumping two-part epoxy foam into place. This foam is described in the bulletin as “cement-like material that adheres to the metal and creates a solid joint.” The replacement method for both applications is to pump in structural epoxy to contain any foam that is left in place and to replace any missing foam. Only one specific epoxy foam is approved in the TSB. The kit comes with cartridges of foam and a small piece of foam that can be used as a dam to keep it contained during placement. The cartridges should be preheated to 115ºF to 165ºF to allow the material to flow easily. This can be done in the paint booth, under lamps or in hot water. There are very specific directions for each location, including placement of foam dams, aluminum tape over holes and how many cartridges should be used in each location.

Depending on the area, as many as 13 cartridges may be needed. This foam does not expand very much. Structural foam is a unique product and must be replaced with an equivalent product. John Hughes, paint specialist for Ford, stressed that failure to do so may compromise the structural integrity of the vehicle.

The Future of Foam
Flexible and rigid foams have been on the market for several years to help with NVH. Identifying and replacing them is just a matter of judging if they are flexible or rigid and then selecting an appropriate replacement material. With the introduction of structural foams, the bar has been raised for collision repair facilities. Specific vehicle repair information is increasingly required to make proper repairs. 

Identifying structural foams can be done in two ways. The new material is very hard and dense, has a smooth surface, and it cannot be penetrated with a probe. Ford recommends a simple test to determine if a foam is structural or not. A screwdriver will penetrate NVH rigid foam relatively easily, and cannot easily be pushed into structural foam. When structural foam is identified in a vehicle, OEM repair procedures should be obtained to ensure the structural integrity of the vehicle is restored during the repair process. ABRN