Putting the Metal to the Metal

Jan. 1, 2020
Adhesives have been used in the automotive industry for a long time, and metal-to-rubber bonding has been used for vibration control devices in motor mounts for more than 100 years.

Continued testing, materials and repair procedure development brings respectability to new procedures.

Adhesives have been used in the automotive industry for a long time, and metal-to-rubber bonding has been used for vibration control devices in motor mounts for more than 100 years. Although adhesives have been used for many years in construction, the collision repair industry has only just begun to address panel bonding with structural adhesives.

During the mid 1990s, panel bonding was becoming a viable alternative to welding. Fusor Repair Systems, together with Tech-Cor, crash tested three cars: Manufactured, MIG-welded and bonded, each at a speed of 8 mph while moving backward into a 30 degree barrier.

The challenge was to compare these test results to two standards: The OEM standard and the MIG-welded industry repair standard. Prior to performing the crash test, the bonded car and MIG-welded car had their right quarter panels and rear body panels replaced. The three cars were crash tested and all three showed exactly the same damage. These results were promising for the structural adhesives industry.

However, it was soon realized that it was possible for a quarter panel to have been replaced with structural adhesive in a shop six months prior to a second accident. Would the panel withstand the required pulling on a frame machine? They discovered they could use the damaged right quarter panel to pull the unibody back into dimension without any problems.

But how well would structural adhesives hold up in a serious accident?

The next set of tests took place at MGA Research for three more vehicle crash tests: Manufactured, MIG welded and bonded. As in the previous crash test, each car had the right quarter panel and rear bond panels replaced and each car was subjected to Federal Motor Vehicle Safety Standard 301. While in neutral, a 4,000-lb. sled hit the vehicles from the rear at 30 mph. All three cars handled the test with excellent results.

The next round of testing was conducted by members of the industry. Manitoba Public Insurance of Manitoba, Canada, took several Ford Escorts and bonded on roof panels using adhesive from several manufacturers. The cars were then run into a collapsible barrier at 30 mph. All cars met the test criteria. Another test was conducted using a vehicle with the same body style as the Escort that was bonded using the same adhesives used in the previous Escort crash test. These cars were run into a solid barrier at 40 mph and once again, the adhesive surpassed all expectation and the cars met the test criteria.

OEM/Insurance

DaimlerChrysler Corp. has issued a weld-bonding publication, bulletin No. 81-699-99097, which addresses the use of a combination of adhesives and compression spot welds. The publication calls for precise placement of the compression welds and the proper use of adhesives. General Motors Corp. (GM) has issued "Material Specification GM6446G Automotive (metallic) Panel Bonding Performance Guidelines For After Market Service Use." This publication lists the guidelines an adhesive must meet to be accepted under GM's specification. Now that GM has an aftermarket materials specification, it can begin the development of specific recommended procedures and guidelines for use of the material. At this time, there are no specific GM-recommended procedures or guidelines for adhesive bonding of metal panels, in reference to GM6449G.

Ford also uses adhesives in several of its technical service bulletins, and considers adhesives to provide excellent anti-corrosion benefits to repairs. Some companies have taken a neutral stance, while others have conducted their own testing. Insurance companies have varying opinions; some say only use adhesives where adhesives were used at the factory, while others do not.

Body Shop Usage

Having traveled extensively around the country and having visited shops from coast to coast, I am privy to the comments about bonding that I have heard vary from, "I only bond panels" to, "I only weld panels." While speaking with shop owners and technicians, I began to realize that each shop has its own comfort zone.

If you are using adhesives regularly to bond panels, you are probably comfortable with the testing and training you have received from the adhesive manufacturers. If, however, you are not comfortable with panel bonding, begin using adhesive on panels where welding would be difficult.

There are many places where using adhesive bonding instead of welding is preferred. One of the most obvious is the wheel opening on the quarter panel. Welding around the wheel opening has never been easy: You're welding overhead, the fit isn't necessarily the best, and there is hot, dripping undercoat to contend with. Other suitable adhesive bonding areas are on the lower mating flanges of the quarter panel. This area can benefit from anti-corrosion inhibitors, which are included in many adhesives, as well as providing an excellent adhesive seal.

Skill Level and Training

From the first time a technician picks up an MIG welding gun, how much practice is necessary before the technician is qualified to produce welds that will be safe on a consumer's car for travel at highway speed? The skill requirements to become an accomplished bonder are much simpler than those required to become an accomplished welder. Adhesive companies have detailed instructions that explain how to set up a panel for repair, as

well as how to prepare cartridges for use.

Most adhesive manufacturers recommend purging the tube and performing a run-out as long as the length of the mixing tip. Adhesive manufacturers will provide their own specific methods of preparing the panels and mating the car flanges. Some recommend removing the E-coat, then galvanizing and bonding to clean steel, while others recommend cleaning the E-Coat and using a scratch pad to prepare the bond area. Afterward, the panel is ready to have the adhesive applied and the panel to be installed on the car. Even the task of adhesive measuring and mixing has been removed. Cartridges are available in mix ratios of 1:1, 2:1, 4:1, etc. The static mixers installed on cartridges will guarantee a complete mix. It doesn't get much easier.

The I-CAR Adhesive Bonding Course is a good place to gain basic bonding knowledge. Adhesive manufacturer's clinics, offered through local BPE jobbers, are an excellent way to become knowledgeable about adhesive bonding procedures. Some adhesive manufacturers offer lunchtime clinics and most manufacturers will come to the shop to provide individual instruction about their products.

What to Bond and What Not To

Adhesive manufacturers are recommending the bonding of outer panels, including door skins, quarter panels, rear body panels and roof panels. Adhesive manufacturers have conducted extensive testing to validate the bonding of these parts and secondary panels. At this time, I am not aware of any manufacturer suggesting the installation of a primary structural part with adhesive.

A common question I'm asked is, "Can I bond the trunk floor pan or spare tire well?" My answer is that this is not a candidate for bonding because the rear floor pan establishes the closure for the rear frame rails, a primary structural part. I don't know of any adhesive manufacturer promoting this practice. The reason is that by bonding the mating joints, the spacing between the welds would be eliminated. The energy traveling through the rail would not be able to produce an accordion effect between the welds, and the energy would travel farther and cause more damage.

Whichever adhesive you choose, be sure to follow the directions closely. Each adhesive manufacturer uses different chemistries for their products with its own characteristics that are dangerous to modify or vary.

Accept change. There is much change occurring in the autobody repair industry. The way a car was built 30 years ago is quite different than today's cars. Years ago, we were told to have a tune up every 10,000 to 15,000 miles (I even remember a carbon and valve job being done on my Dad's car at 35,000 miles) However, cars today typically run over 100,000 miles without any major problems. Now that's progress!

Likewise, more structural adhesives are being engineered into today's vehicles, and that type of progress will be beneficial to everyone.

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