Hem Flange Bonding: Part 2

Posted on 11/26/2012 9:30:24 AM By Sandy Niks

In the last blog we introduced the challenge of hem flange bonding in the automotive industry. Now let’s look at some of the approaches to meeting the challenge.

One of the earliest solutions was to use vinyl plastisol – a material that was already being used for some sealant applications on the vehicle, as well as for an “anti-flutter” adhesive. (“Anti-flutter” adhesives will be reviewed in a future post.) The adhesive was functioning more as sealer in the hem flange, less as a structural adhesive.

Another adhesive used was a two-part epoxy. Cure started under ambient conditions and was completed in the paint ovens. The use of this adhesive shifted the focus to a higher strength, more structural adhesive.

The most widely used adhesive was a one-part epoxy. It had sufficient thixotropy so it stayed within the bondline through application, mating of the inner and outer panels, attachment to the vehicle, movement through the plant, electrodeposition primer dips, and cure in the paint ovens. An alternative processing method, introduced in Part 1, was induction curing. Again, this was used primarily when the component (door, hood, deck lid, or similar construction) was assembled at a “fabrication” plant and then shipped to the assembly plant, where it was attached to the vehicle. The induction process is a rapid (generally only a few seconds) cure technique that begins the chemical cross-linking process within the adhesive so it is “set” or “gelled”. The cure is not usually complete, but will finish in the paint ovens. The induction cure fixture is part specific – the induction coils are designed to follow the bondline so only that limited area is heated. When the part design changes, a new induction cure fixture must be designed.

For some limited applications, there has been a return to the concept to use an adhesive/sealer that could function as both the hem flange adhesive and the anti-flutter adhesive on deck lids. Adhesive technology had continued to improve and a stronger, eleastomeric adhesive was selected instead of the vinyl plastisol.

 The material specification for adhesives used in hem flange bonding changed over time, with new requirements being added as issues were discovered and addressed. Also, some ingredients were restricted, necessitating reformulation (and the subsequent testing and qualification) when found. Also, new computerized techniques in designing joints required tests to supply their data needs – such as tensile strength, modulus, and Poisson’s ratio. However, many of the tests used for quality control, strength, durability, and handling properties remained essentially unchanged over the years. A future article will review these tests in more detail.

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