Hem Flange Bonding: Part 1

Posted on 11/6/2012 9:34:23 AM By Sandy Niks

The hem flange is a joint design used when bonding the inner to outer closure panels, as in metal doors, hoods, decklids, or liftgates.

Metal bonding in the automotive industry has provided some interesting challenges. General instructions for adhesive bonding usually include steps such as “clean the mating surfaces of the substrates thoroughly”, “lightly sand the bonding surfaces”, “use an adhesion promoter”, and/or “use a primer”. These steps are to make the surface of the substrates as receptive as possible to the adhesive. Contaminants left on the surface can reduce bond strength by interfering with the adhesive’s ability to form a bond, producing a weaker chemical bond or reduced bond area. Or the contaminant may provide a fracture initiation site when the bond is stressed, again reducing the load the joint can withstand. Thus the recommendation of a cleaning step to remove possible contaminants. Sanding surfaces removes gross surface imperfections, facilitating a bondline of consistent thickness and intimate contact between the adhesive and substrate. Priming can be an actual coating of another layer that provides a more consistent, bondable surface. Or an adhesion promoter can be used that “activates” the surface, providing chemical groups ready to latch onto the adhesive when applied.  These preparations happen before paint is applied in an automotive factory, but not before applications such as hem flange bonding.

Hem flange bonding happens before painting, in the “body shop” of an assembly plant or at a metal fabrication plant and shipped to the assembly plant. The steel body panels, usually galvanized, are stamped and formed, processing which requires various lubricants to be applied, such as mill oils and drawing compounds, or prelubes. One or more may be present when the adhesive is applied – no cleaning step to remove the lubricant occurs, partly because the lubricant also functions to prevent oxidation of the metal.

After the adhesive is applied to the outer panel, the inner panel is positioned, then the outer panel is bent or crimped around the inner panel, forming the hem flange (See Figure 1). Bead size is carefully controlled to fill the bondline but not squeeze out. Adhesives that escape the bondlilne would contaminate the equipment and cause cleanliness and maintenance issues in the plant.  Ideally, the excess adhesive would help seal the cut edge of the galvanized steel, where the bare steel is exposed and subject to oxidation, but the equipment contamination issues are more critical at this stage.

Hem Flange Cross-section, transportation, automotive, adhesives, sealants 

Figure 1. Hem Flange Cross-section

A cure step may be introduced at this stage, such as induction curing. This can be a full cure or just enough to prevent any movement of inner to outer panel during subsequent processing. The body closure panels are attached to the rest of the “body-in-white”, and sent to the paint shop where it is cleaned, primed (the “ELPO” or electrodeposition step, complete with oven cure), and painted. Cure of the adhesive is completed in the paint ovens.

One of the reasons for using an adhesive in the hem flange is to reduce or eliminate the use of spot welds to hold the inner and outer panels together. Spot welding was sometimes noticeable of the outer panel, necessitating a finishing or polishing step prior to painting to make the spot weld virtually invisible. The few remaining spot welds would act as peel stoppers, reducing the susceptibility of the adhesive to any peel loads, and hold the panels together until the adhesive could be cured. This processing step brought new requirements to the adhesive related to weldability. First, the adhesive must not interfere with the integrity of the spot weld. Second, similar to the forming process, the adhesive must not escape the joint and get on the welding equipment.

We will continue exploring hem flange bonding with the next article. 

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