Technicians can encounter a variety of replacement foams during the course of a repair, and the differences between each are not always apparent. To avoid unnecessary headaches, take the time to confirm the characteristics and applications of the replacement foam you are using.
In the case of two-part dispensable foam fillers, things aren’t always what they seem.
By two-part dispensable fillers, we mean those foams that are supplied to the body shop in cartridges that are loaded into either a manual or pneumatic applicator gun. The foam is then pumped onto the appropriate surface or into the required aperture during the course of a vehicle repair. The “two parts” referred to are a catalyst and resin (also known as a base and accelerator), and the systems are sometimes referred to as full-fill foam fillers or pumpable foam fillers.
The method of delivery just described is convenient for the technician, in that catalyst and resin are mixed automatically in the gun’s nozzle, assuring—both the proper mix ratio and the stability of the product before it leaves the gun. It’s also an efficient means of deployment; with the nozzle in place, uncombined material left over in the cartridge remains viable for the next application.
Despite those basic similarities, these products possess many differences that are not readily apparent—characteristics that, if not identified before application, can cause an inferior or unsafe repair. The OEMs and aftermarket suppliers that provide replacement foam systems make plenty of useful product information available, but challenges of use remain. Different foams look similar and are applied in similar ways, which can cause confusion. Product information is not always at hand when the technician begins working on the collision-damaged vehicle. And terminology—what manufacturers call their classes of foam—sometimes varies from supplier to supplier.
“It sounds basic, but the incorrect identification of replacement foams is one of the most common and serious problems we see in the field,” says Dr. Stuart Bingham, technical manager for 3M Corp.
The dispensable foam systems used in today’s vehicles are classified into two groups. The first are noise vibration harshness (NVH) foams. NVH foams are used, for the most part, to help dampen noise and vibration when the vehicle is being driven. This class makes up the majority of the foams in use.
The second type of foam—structural foam—helps reinforce the vehicle by stiffening and reinforcing pillars and some frame components. Compared to their NVH counterparts, structural foams are relatively new. Their use is less widespread, being confined, at this time, to a few specific applications on a few specific car lines.
Non-structural foam
NVH foams, sometimes called non-structural foams, fall into one of two categories. The first are the flexible anti-flutter or flexible non-structural foams. With a density of about 4 lbs. per cubic foot, these foams have plenty of “give”suiting them to function as a dampening agent for reducing vibration and noise during a vehicle’s ride. They are usually applied to small areas, such as gaps in roof bows and door safety beams.
The second kind of NVH foam is sometimes called acoustic foam, but is also known as rigid non-structural foam. This material takes up space in larger apertures such as those found in rocker panels, pillars and quarter panel doglegs, and is used for acoustical purposes. The density of these foams is about three times that of the flexible foams, or 12 lbs. per cubic foot.
Both classes of dispensable non-structural foam are urethane-based and go through three stages after leaving the nozzle—foam time, work time and cure time.
Foam time is the time it takes for the foam to start, well, “foaming” on the surface or back out of the cavity to which it has been applied. This marks the beginning of the material’s transition from its liquefied state in the canister to its spongier consistency after curing. Rigid non-structural foams usually take about one minute to get to this point, while it’s only a matter of 10 to 15 seconds with flexible foams.
Technicians can continue to work with the material through the duration of its second stage, the “work time,” before the product can no longer be shaped or molded. For flexible non-structural foams this is less than a minute. In contrast, rigid non-structural foams possess a generous work time of up to 75 minutes. This is advantageous because the material is generally applied to larger cavities that are somewhat difficult to access completely. Their application tends to be more time-consuming and requires more technique on the part of the technician than is the case with flexible non-structural foams, which are usually dispensed onto flatter, more open surfaces.
The third stage is the time the product takes to set and cure. For rigid, non-structural foams, this is about an hour. Flexible non-structural foams take 30 minutes to an hour depending upon the temperature and other conditions. Heat is not required for the curing of these products.
“Since the nature of flexible, non-structural foams can be extremely fluid when they come out of the nozzle,” cautions Bob Zweng, senior technical service representative for the Lord Corp., “you want to make sure you’re prepared. For example, you should cover any part of the vehicle that is not part of the application, and if it’s possible, as in the case of a door frame, lay the part flat to neutralize the effects of gravity. Gun the material slowly and carefully so as to not lose control of it.”
Steve Marks, industry support manager at the I-CAR Tech Centre, adds, “Technicians have to remember that these are dynamic products that go through chemical changes right before their eyes.” Marks continues, “A rigid, non-structural foam will travel a bit before it begins to foam up, so you have to anticipate that change in its behavior. You don’t want to be caught in a situation where the sheet metal is going to distort or become misaligned because you didn’t anticipate the replacement foam’s expansion to ten times its original volume. I-CAR recommends making practice samples so that technicians can get used to how these products behave in terms of flow, expansion and fill rate.”
Though more widely used as a technique with structural foam replacement systems, a plastic piece known as a dam can be used to control the flow and location of rigid, non-structural foam fillers. Fillers cut to fit the space into which foam is being applied and then inserted; dams help reduce the distance a foam must travel down (for instance) a pillar or rocker panel and reduce the amount of material required to fill a part. Dams are available from vehicle manufacturers and suppliers of replacement foams. Sometimes they are part of the replacement package.
Structural foam
Structural foams are designed to add rigidity and structural strength to a part without adding much vehicle weight, and are designed to improve the crash performance of a vehicle without modification to its structure.
In the aftermarket, dispensable structural foams use the same cartridge/gun method of deployment as non-structural foams, and may look similar to their NVH counterparts while in the cartridge. This causes more concern among industry professionals than any other aspect of replacement foam given the dissimilarities of their characteristics and function.
While urethane forms the chemical basis of dispensable NVH foams, structural foams are two-part epoxy compounds that are much denser, at approximately 31 lbs. per square inch. Their expansion factor is much less: while acoustical foam expands up to 1,000 percent, structural foam increases only 30 percent.
The density of the product, its limited expansion and the places on the vehicle to which it is applied, make structural foam a challenge to use. First there is that pesky identification problem, with the good news being that there is a relatively easy test a technician can conduct to find out what exactly is in that cartridge.
“When structural foam hardens, it’s almost like concrete,” says Marks, “so a useful test is to fill a soda can or other container with the product you are about to use, let it cure and test it by punching it with a screwdriver. Structural foam is extremely difficult to penetrate, while non-structural foam—even those products classified as ‘rigid’—is always somewhat spongy. In that way they are easy to tell apart.”
3M’s Bingham suggests making a sample of the foam, letting it cure and comparing it to what you know was supplied by the OEM if you still have questions after performing such a test.
“Of all the two-part dispensable foams, structural foam requires the most skill to apply, largely because of its thickness and where it is applied on the vehicle. Because it doesn’t expand much, applications require a lot of material. Often it is being pumped into long pillar or rail apertures where visibility becomes an issue,” he says.
“I’ve seen applications where up to 14 cartridges of material have been required for a D-pillar,” states Zweng. “And the technician is trying to look down this long dark space to see what he or she is doing. This problem makes damming more or less a necessity. It creates a need for less material and keeps material out of the deepest recesses of a part.”
Economic factors and complexity of application have undoubtedly contributed to the slow spread of two-component epoxy fillers as a replacement solution, Ford’s use of the material in the underbody and A- and D-pillars of Mountaineer and Explorer models being a notable exception.
The vast majority of OEMs are carefully scrutinizing such applications, with General Motors Corp. (GM) preferring to provide structural foams in other forms, such as nylon blocks—rather than structural foam fitted precisely to the part and bonded into place at the time of repair. An example of this application is the hinge pillar on the Saturn L series.
GM also provides a solution consisting of a pre-formed, molded part of structural foam bonded into place at the time of repair, an example being the hinge pillar/rocker area of the Chevrolet SSR pickup. The technician simply replaces the material when it is encountered in a collision-damaged vehicle.
GM’s third alternative to dispensing two-part structural foam at the time of repair, is providing the structural foam as part of the component part being replaced, as it does with the longitudinal rails on the Cadillac CTS. This arrangement, in effect, makes the rail an assembly. As in the case previously cited, the foam is simply replaced.
“General Motors is evaluating the use of the dispensable replacement foam,” says Brian Dotterer from GM’s New Material Technology Development, “because there are a multitude of issues to consider. For one thing, it is very difficult to come up with standardized applications because every vehicle manufacturer has its own preferences. Development costs are significant. Distribution channels aren’t yet adequately developed. Plus the pumpable stuff isn’t very forgiving when you make a mistake. This, combined with the cost and quantity of the material, makes for a potentially less than feasible solution.”
Technicians confronted with structural foams will be glad to know that the Society of Automotive Engineers (SAE) has issued a recommended practice, SAE J2621, conceived to help OEMs define the characteristics of two-component replacement structural foams. This document, which includes baseline classifications for different structural foams as well as testing methods, will undoubtedly help answer questions and clear up the muddier areas regarding these materials, including their performance in the aftermarket.
“The document should help automotive manufacturers work toward a common understanding of these products, which will, in turn benefit the collision repairer,” says Marks. “Their advance on the market has been slower than anticipated, but structural foams are definitely coming and the industry has to be ready for them.”
