PANEL REPLACEMENT - New Technologies Challenge Old Habits

Adhesive bonding and resistance spot welding are finding their way into collision repair facilities across the country. Despite controversy surrounding these emerging technologies, the efficiency and uniformity of both are attracting attention.

Little has changed in panel replacement since the standardization of the MIG welder in body shops. It has taken more than 20 years to wean technicians off their oxy-acetylene and brass diets and get everyone reading from the same page with regard to safe and acceptable collision repairs. But recent years have brought us some alternatives to the traditional methods with which car and truck bodies can be efficiently rebuilt.

One such method is the revolutionary adhesive bonding systems. The other is squeeze-type resistance spot welding (STRSW). Both offer prep time, speed, appearance and durability advantages compared to the accepted MIG welding process. In fact, the OEM factories use both processes in construction--bonding to a much lesser extent. While steel bodies have traditionally been of spot-welded, steel construction, the use of structural adhesives has increased steadily. Structural adhesives hold together the Corvette, the early Lumina minivan and portions of the Olds Aurora body. On a much smaller scale, body technicians have used structural adhesives for years to successfully glue door skins on cars.

Bonded Bodies

While adhesive bonding has been used in industrial applications, including aircraft, the aerospace industry and manufacturing fields, North American automakers have produced only a select few designs with bonded panels. The leap of faith it would take to glue an entire car together is a bit much for them. However, some more exotic carmakers have taken a serious crack at designs using structural adhesives.

The Lotus Elise, for example, is a bonded assembly of anodized extruded aluminum with an integral roll bar and composite outer body panels. In an effort to recapture the excitement of the legendary Super Seven, Lotus built a 190-hp, high-performance vehicle with emphasis on weight savings. "Its bonded aluminum chassis structure was developed in conjunction with Hydro Aluminium in Denmark," said British auto enthusiast writer Art Markus of Car and Car Conversions. "The bonding process excludes the problems inherent in welded aluminum structures. Chassis are supplied complete from Hydro Aluminium. The chassis is massively over-engineered, according to Elise Project manager Tony Shute, and some significant weight savings I suspect may remain to be found as the process is refined and developed. As it is, the Elise chassis looks and feels immensely strong, and like the backbone chassis of the original Elan, the concept looks like it will be adopted for a number of different vehicles in future."

While the purpose for the ultra-lightweight aluminum space frame on the Elise is purely performance, it gives us a glimpse at what sorts of fuel-efficient designs we may see in the not-too-distant future. Despite today's relatively stable oil market and fuel prices, our supplies of oil and gas could be interupted rather suddenly, at which point the demand for increased fuel efficiency would resume. Aluminum-intensive vehicles would surely accompany such a resumption, and collision repairers could then be faced with bonded vehicles on a large scale.

Glue Instead

Bonding systems rely on adhesive applied to the flanges of a panel to function in place of the factory spot welds. While today's vehicles were never intended to be rebuilt with structural adhesives, the collision repair industry and its suppliers are establishing that bonding is truly a viable alternative to welding in body reconstruction. And through some intensive crash testing, adhesives manufacturers are actually proving their case.

An independent research company conducted a series of crash tests in March 1996 to evaluate the integrity of structural adhesives. Three cars were tested against Federal Motor Vehicle Safety Standard (FMVSS) 301, which measures the soundness of motor vehicle fuel systems. The following crash-test results were provided by Lord Corporation, makers of Fusor structural adhesives: "Each car was placed in a stationary position and then impacted from the rear by a moving barrier--a 4,000-pound sled with a flat frontplate--traveling at 30 mph. Prior to the tests, the rear driver-side quarter panel on two of the cars was removed and reinstalled, using welding for one vehicle and Fusor Metal Bonding Adhesive for the other. The third car was left in original factory condition.

"On all three cars, the fuel tank systems remained intact after impact, successfully meeting the requirements of FMVSS 301. This marked the first time an automobile having rear quarter and rear body panels replaced using an adhesive passed this critical safety test."

In another independent crash test that was completed in January 1997, an automobile repaired with an adhesive successfully withstood FMVSS 208, which measures roof integrity. "For the crash test, the roof panel of a Nissan Sentra was removed, and a new panel was installed using Fusor Metal Bonding Adhesive and [a] novel roof skin bonding technique (patent pending) developed by Fusor Repair Systems," Lord Corporation's test results explain. "The vehicle was mounted at a 23-degree angle on a rollover cart with the passenger side toward the ground. Then the cart and vehicle were towed down the test track at 32.4 mph until the cart struck a set of decelerator shock absorbers. After contacting the shock absorbers, the car was catapulted laterally, landing on its right wheels. The vehicle rolled once, slid on its roof, then came to rest on its wheels. Results prove that roofs repaired with Fusor Metal Bonding Adhesive maintain their integrity during crash conditions. The test also confirms the effectiveness of the roof skin bonding technique."

Another major player in repair adhesives is Polymer Engineering, makers of the Duramix line of structural adhesives. They also underwrote tests of their adhesive bonding system, and one assessment actually proved their adhesives to be superior to MIG welds in terms of lap shear strength. (Note: Like all manufacturers of this type of adhesive, Polymer Engineering recommends completely removal of metal coatings--including galvanization--prior to bonding. In fact, the company's data showed that Duramix adhesive strength exceeds that of galvanizing.)

The Polymer Engineering tests duplicated FMVSS 301 standards, which are intended to test fuel cell and fuel delivery systems at the OEM level. While a repair that used bonding or MIG welding to replace quarters does not comply with the letter of the standard due to the fact they were testing a repaired vehicle, the test results do provide data useful for evaluating the bonding process. In that sense, Polymer Engineering does not claim to have passed FMVSS 301. Instead, the company claims that its product was tested under the standard's general test conditions.

After a 30-mph impact from a 4,000-lb. ram, a quarter panel installed on a 1995 Chevrolet Lumina with Polymer Engineering's urethane adhesive stayed intact. But the OEM welds on the control quarter panel showed minor breakage over a 4-in. span. "Duramix meets or exceeds factory spot welding in tests of overlap shear strength," said Richard L. Jacobs, president of Polymer Engineering. "Testing took into consideration a range of temperatures from -40degreesF to 140degreesF. Crash testing indicates that [cars repaired with] Duramix perform similarly to [welded cars] at the crush zones."

What Do They Say?

Many technicians and owners look to industry sources such as I-CAR and car manufacturers for a position on these developing technologies. Steve Marks, research coordinator at the I-CAR Tech Center, says I-CAR has developed a course on bonding with structural adhesives. This new course--scheduled for release shortly--also marks I-CAR's entry into a new delivery format for its training curriculum. The course is available on a CD and will operate on a program similar to Microsoft's PowerPoint.

I-CAR's official position on adhesive bonding is that "during collision repairs, replacement panels should be attached to the vehicle using procedures recommended by the vehicle maker. If a repair facility chooses to use adhesive bonding for panel installation, tested procedures should be used that have been validated by a reliable product maker or research or testing facility."

Despite I-CAR's deference to the OEMs, the fact that the organization developed the course and devoted the resources to the technology speaks volumes about the future of adhesives in collision repair. I-CAR Master Instructor Ed Staquet, who is also a senior market specialist for Lord Corporation, spoke with ABRN about the technology. Staquet comes from the body shop floor (with the knees to prove it), and lends a technician's critical perspective to the research and marketing of the products. Staquet builds the cars Lord Corporation crash tests.

"We're testing against the real-world conditions of OE construction and MIG welds," Staquet said. "Our tests are the only way this technology will gain the acceptance of the repair industry. We not only test the bonded assemblies under collision; we also do re-repairs and replacements of the same panels. We actually set the cars up on frame machines so we can see how the installation reacts--we want to know if we can we tug on it. Our company doesn't want techs to go beyond our testing."

Given today's vehicle construction, in which nearly every body part plays a role in body strength, there is the question of what is structural and what isn't. If use of the bonding adhesive is limited to non-structural parts, who decides where it can go? "Everything on a car has some structural value," Staquet said. "Just ask yourself, 'Does it control energy?' At this point in our research, we limit the application of Fusor to door skins, quarter panels, rear body panels and roofs--the secondary structural body parts. We do not recommend the application for the primary structural parts, such as frame rails, rockers, aprons, core supports and wheelhouses."

Asked when and if research and testing would be done on applications for the primary structural parts, Staquet smiled and said, "We'll keep you informed."

Your Mission, Should You
Decide To Accept It ...

Many repairers are hesitant to perform a procedure that has not been endorsed or recommended by a vehicle's manufacturer. Their goal is to replicate, as much as possible, the original construction--or pre-accident condition. But in reality, it is quite impossible to recreate the factory welds and coatings. Even the the most advanced technology is a compromise at best. Technicians are left to use their best instincts and experience to repair cars and to return them as close as possible to their original structural strength and appearance. And don't be lulled into believing that adherence to factory-recommended repair procedures will in any way reduce the repairer's liability when things go wrong. It won't.

The two competing--but equally controversial--technologies of adhesive bonding and squeeze-type resistance spot welding (STRSW) are fighting for acceptance by the North American automakers. Ford, Chrysler and GM have yet to accept either one. But European and Asian manufacturers have moved effortlessly into the next century with both, albeit not at the same time.

Toyota, maker of the best-selling car in the United States for two years running, has signed on to STRSW. Roger Larsen, Toyota's course developer, instructor and body service administrator, told ABRN his company does recognize and, in fact, train their technicians in the use of STRSW. "The Japanese have endorsed [STRSW] a little earlier than the rest of the manufacturers," he said. "It really beats out the plug welding in every way in terms of appearance and durability. We have guides for collision repair that specifically point out where it can be done and where [these welds] can not be done. It's generally sheet metal replacement, as opposed to structural assemblies."

But Larsen declined to endorse adhesive bonding. "We were in Japan last summer and this particular issue was covered," he explained. "They didn't feel there had been enough study on the technology. We are watching to see how the research develops, but no policy has been established at this point."

Nissan, on the other hand, recommends structural adhesives to repair specific parts and assemblies of their cars when damaged in collisions. Because a large number of Nissan's vehicles are manufactured using the company's "weld-bond" technology, repairers have little choice but to use the same process when rebuilding the cars. The 1990­96 Infiniti Q45, the 1993­97 Infiniti J30 and the 1993­97 Nissan Altima feature structural adhesives in their OE construction.

Steve Przybylo, manager of serviceability for Nissan North America, confirmed the manufacturer's stance. "Some of our models use weld bonding in joining certain sheet metal seams of the A- and B-pillar, rocker panels, roof panels and rear quarter panels," he said. "This technique combines a two-component epoxy adhesive with spot welding to achieve a superior joint. Weld bonding is used to increase torsional rigidity and strength of the body. It improves resistance to penetration of corrosive forces in weld seams. When a vehicle is damaged in a collision, weld-bonded seams must be repaired with a combination of adhesive and MIG welding to duplicate the factory specification [as outlined in the replacement operations of the model-specific body repair manual]." Interestingly, Przybylo said Nissan declined to sign on with STRSW to replace weld-bonded panels.

STRSW: What It Is

STRSW is the joining of overlapping pieces of metal by applying pressure and electrical current, explains Paul Wilcox, inventor of Tite-Spot welders and president of Tite-Spot Welders, Inc. These joints created by spot-welding form a "button" or "fused nugget." Resistance spot welds are typically found on flanges staggered in a single row of consecutive welds. Vehicle manufacturers use resistance spot welding at the factory because it allows them to produce high-quality welds at a minimal cost.

Spot welds are formed when a high amount of current is passed through the panels for a measured amount of time and with a measured amount of pressure. In a typical spot-welding application, two electrodes located opposite each another squeeze the metal pieces together. Several thousand amperes of current are applied to the metal for a specified period of time. This heats the pieces being welded. As the temperature increases, the metal is heated to a plastic state, the force of the welding tip deforms the metal and a small dent is formed. The temperature continues to rise, and a small liquid pool of metal is formed at the interface, which is typically the size of the welding tip face. When the welding temperature is reached, the timer should shut off the current. The weld zone cools very quickly because the copper welding tips pull heat out of the weld zone, and because heat escapes as it flows into the surrounding metal.

There are four variables to consider with resistance spot welding: pressure, weld time, current and tip diameter.

Pressure is the amount of force applied to the weld. Too little pressure creates a small and weak joining area. Too much pressure can cause cracking in the weld due to the welding tips' quenching effect. It can also thin the metal and cause a weakness. The welding electrode's thickness causes a depression on sheet surfaces--the depth of which should never exceed 25 percent of the thickness of the sheet metal. A body shop typically welds 16- to 24-gauge steel.

If a spot welder has adjustable-length tongs, use a pressure gauge to ensure the proper pressure setting. (Note: The pressure of Tite-Spot pliers is set to the middle of this range and is not adjustable.)

A standard weld timer controls the amount of time the current flows into the welding transformer. The inherent problem is that the timer runs even when no welding is being done. Therefore, if welding current is only flowing for part of the cycle, the timer may run out before the weld is formed. Generally, technicians increase the length of time the clock is running, which overheats the welding tools and transformer. Double cycling on the weld zone is a second technique, but this also causes overheating.

Sometimes the operator can bypass the timer, allowing manual timing of the welds. With this method, good welds can be produced in one-half to 1.75 seconds. This procedure is likely to put less thermal stress on the welding tools and the transformer than would the standard weld timer.

A digital timer control verifies welding is taking place. This type of timer checks all positive and negative cycles of a 60-cycle second and will not actuate the timer unless welding current is flowing. This timer has a precise interface for selecting and adjusting the power and timer settings and puts the least amount of thermal stress on the welding tools and transformer.

Welding current and time are used to bring the metal to welding temperature (2,550degreesF). The two factors are inversely proportional. In the formula "weld temperature = i2 x t x R,"welding current is represented by "i," weld time is "t" and resistance is "R." Welding current in a body shop environment has a range of 3,000 to 5,000 amps, and the technician controls current and time. The gauge of the parts being welded determines resistance. Because welding current is squared, changes in weld current are much more dramatic than changes in weld time.

Proper current settings are essential when welding today's vehicles, and welders that don't control current are more difficult to use. If weld current is at the low end of the range, weld time must be increased. Using low current can cause overheating of the welding tools and the transformer. Conversely, if weld current is high then weld time is decreased. Using high weld current increases the problem of expulsion, which is seen as the sparks escaping from between the layers of steel. The galvanized coatings found on today's steel aggravate the problem.

There are two types of weld current controls. The analog type uses a knob set like a radio, and the digital control uses an LED display to tell the technician the exact power setting. The typical digital interface is a push button. The ideal welding controller is digital with a preheat timer and weld current verification. The digital interface is so precise the operator can set the machine easily, so one can make adjustments in power or time to make perfect welds. Timer verification allows the timer to "tick" only if the correct amount of current is flowing into the welding transformer. A verified preheat timer is the best way to minimize expulsion.

Preheating allows the primers between the steel layers to be slowly burned out of the way. Galvanized coatings can be vaporized at 1,350degreesF, thus eliminating the zinc coating from the weld zone before welding takes place. Temperature is determined by the length of time the zone is preheated. Preheating also allows the steel to bend a little and fit perfectly before the welding power is turned on. These things happen only if there is preheat current verification, which is a function of the welder's system that enhances its efficiency.

The ideal welding controller verifies the welding current, thereby eliminating the problem of over-welding. Using this type of control, the technician is able to produce good welds every time without over-welding the work area. This also reduces the heat stress on welding tools and transformer.

Welding tip diameter is another critical factor in the spot-welding process. Once the tips become oversized, their efficiency drops. The tips can be allowed to fatten to 1/4-in. diameter before they need to be sharpened.

Welding tips have a flat face when new. This face crowns quickly with use, and this crowning effect is desirable. The crowning radius should be about 1.5 to 2 inches. Note that the closed height of the welding tips is 1.5-in. when new. Welding tips should not be shimmed, and should be discarded when their closed height exceeds 13/8-in.

Weld Inspection

There are three forms of weld inspections, the first being a visual inspection. The welds should look uniform, have a small dent from the welding tip and have very little expulsion when formed. The remaining inspections are destructive testing techniques: the "peel" test and the "chisel" test. These tests should be done on scrap metal from the work pieces before welding on a vehicle begins.

The peel test consists of peeling apart a spot weld. A body shop typically uses frame equipment and sheet metal clamps for this purpose. After pulling apart the weld, the button should be measured, and the average diameter calculated. The pull-out size should be two times the gauge of the material in thousandths, plus 0.10 in. For example, a weld on an 18-gauge piece of metal should have a weld nugget size of at least 0.2 in., and 22-gauge material should have a nugget of at least 0.16 in.

The chisel test consists of forcing a tapered chisel into the lap on each side of the weld until the weld or base metal fails. The edges of the chisel must not touch the weld being tested. This test is to be used when the peel test is not feasible. The button size is determined the same way it is for the peel test.

Final Word

Tite-Spot's Wilcox remains curious about the industry's resistance to this technology--especially when it comes from the automakers. "You know, it is ironic that GM will not sign on to STRSW when you consider they are using my machines in their own body shops at the plant. In fact, I just sold them another one recently," he said.

His statement is indicative of where the industry is--and where it's going-- with STRSW and bonding technology. Despite the politics and legal wrangling that tend to suppress innovation, there are those determined to press on regardless. Collision repairers tend to be a breed of innovators and fabricators themselves. If a part or a tool isn't available, they invent one. In that spirit, it is the same for their attitudes towards STRSW and adhesive bonding. Because it works, the industry has taken the ball and is running
with it.