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Automotive Body Panels

Posted on 9/19/2012 7:59:04 AM By Sandy Nicks
  

Today’s topic is bonding automotive body panels.

This includes bonding inner to outer panels, as in fiber reinforced panel (FRP) hoods or doors; hem flange bonding of metal hoods or doors; or bonding body panels to the frame. We will concentrate on the latter for today.

Let’s first start with understanding the substrates that will be bonded together.

The primary structure is welded steel that has been through the initial stages of the painting process, i.e., the “ELPO” or electrodeposition step has been executed, complete with oven cure. For those not familiar with ELPO, it is an epoxy based primer that is not only an excellent surface for bonding, for both adhesives and paints, but also protects against corrosion.

The secondary structure is comprised of the fiber-reinforced plastic (FRP) body panels. The product(s) in this example used a polyester resin, glass fiber reinforcement, and a sheet molding compound process. The panels, as delivered to the assembly plant, had the exterior side primed for painting and some hardware already attached. The interior surface was specified to be free of any dirt, paint overspray, mold release, or other impediments to successful bonding.

The adhesive selected is a two-part, ambient cure urethane. The mixed adhesive is a low viscosity, highly thixotropic material that is applied robotically to the FRP body panels. The body panels are married to the primary structure, held in place by a few mechanical fasteners while the adhesive cures. The closures (hood, doors, etc.) are added and the body is returned to the paint shop for the final steps of color and bake. The adhesive has been designed to be sufficiently cured so that it does not “run” out of the bondline, nor does the heat of the paint ovens degrade the adhesive.

Sound simple? That’s what plant trials prove. As much as prior experience, planning, and potential issue resolution was done in the design, there always seems to be something no one anticipated. But issues are found, the causes determined, solutions found, and testing run to confirm the issues have been resolved. Typically testing for application such as this involved several batches of single overlap shear samples per shift plus fully bonded and painted vehicle bodies being torn apart to inspect the bondline. Once assured the issue was resolved, destructive testing frequency decreased to industry accepted levels. Lap shear testing checks the adhesive batch; a bead of adhesive would be applied to a panel to check for striations, indicating improper mixing if present.

Destructive testing, whether tearing body panels apart with a crowbar or pulling lap shear test specimens apart with a universal testing machine, is just that – destructive. The samples are tested until they “fail”. The parts cannot be reused. In these tests, the adhesive is stronger than the FRP and the “failure” is driven into the FRP substrate. This is what is wanted. If the adhesive released from either substrate (FRP or ELPO coated steel) or the fracture was fully within the adhesive, the adhesive would be unacceptable for the application and another candidate would have to found that would meet this basic requirement. Laboratory tests used to evaluate the strength of the adhesive were SAE J1525 and ASTM D5041, as well as some in-house tests.

The material specification identifies the requirements (with test methods), including basic material properties such as density, tensile strength, and flexural modulus; adhesion properties such as shear strength; and handling properties, such as viscosity, thixotropy, and open time. Environmental exposure tests (hot, cold, humidity, water soak, salt spray, etc.) are also run on test specimens to ensure the adhesive will continue to perform during the service life of the vehicle. This is independent of all of the vehicle testing that is also run to simulate customer usage. Adhesives must also meet all of the safety and hazardous material requirements, such as supplying the Material Safety Data Sheet (MSDS) and meeting any restricted material requirements.

I found the videos linked below for a demonstration of bonding by Lord and Nordson. Although neither quite shows the method used at the vehicle assembly plant, there are similarities. Please note that this is not an endorsement of any company.

Aluminium trailer bonding with LORD adhesive - Scabro

Nordson Unity Dispensing & 3M Plastic Bonding Adhesives: Total Consumer Electronics Assembly



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