Where to Bond in Vehicle Assembly Operations?

Posted on 6/11/2019 9:22:55 AM By Dan Daley

Let’s discuss selecting a plant location for joining two parts with structural adhesives during vehicle assembly. My example is an automotive or truck part that is a new limited sales option on existing body design and assembly. The part is a rigid plastic, molded SMC (sheet molding compound, a polyester thermoset filled with glass fiber) to be attached to the sheet metal on the body, cross body. There are no flat sections – it must conform to a compound shape positioned across the body.

Figure 1:  Example of bonded rigid plastic part on a truck– sunshade mounting bracket (sunshade is the part over the windshield with marker lights)

Source:  USA Truck

First, let’s explore why is this application is suitable for bonding. The vehicle bodyis an existing designed and tooled body assembly.  Changes will inevitably impact existing tools and must be supported on their own merits.The skin alone is not strong enough to support a fastener without reinforcement to distribute the load even if you have backside sheet metal access, which would be unlikely unless the designer preplanned the bonding. In a new body design, accommodating this type of assembly is fairly straightforward, but when the original design doesn’t call for a bonded assembly, new parts need to be designed, tooled,and assembled.

The body assembly operations that may or may not require modifications to accommodate this proposed bonding application include:

- Welding and adhesive dispensing programs
- Handling/manipulatingequipment and tooling
- Shipping and handling racks 
- Holes in the exterior sheet that are filled/sealed when not in use. Or, holes in separate parts that require it to be racked and managed.

Resulting in:
- An increase in vehicle body weight
- Increased part cost and capital
- Variation and complexity

Nevertheless, for various reasons, bonding has been specified over other joining methods. The next question circles back to the topic of this post: at which point in production do we perform the bonding operation? The body shop is an obvious option – albeit you’ll need additional fasteners for fixturing the part as the adhesive cures, which, is unrelated to reinforcement for distributing the load as the adhesive provides load distribution. Bonding to raw sheet metal with forming lubricants followed by the pretreat and the ecoat process is difficult, but materials exist that do this everyday. Bonding could likewise be performed in the paint shop  before applying top coat paint; an optimal (clean) bonding surface vis-à-vis the paint shop’s ecoating process is the key advantage. Finally, bonding can be performed in the assembly/trim shop following painting (similar to glass bonding).

In any of these plant locations, we’re introducing variation and complexity.  The basis for our decision including which adhesive chemistries are available in each shop is detailed in Table 1.

(click to enlarge)
Source:  The ChemQuest Group, Inc.

A few pertinent details, which for this blog I’ve chosen to ignore, are nevertheless critical to this decision. For instance:

Is the part designed to handle the ecoat process without causing bath transfer and contamination? 
- Finish considerations – body color or neutral pre-finished?
- Visible bond line – coverage with joint trim  (invisible bond line is ideal)
- Locating features for the partfor installation 
- Fixturing the part while curing the adhesive is critical

If you’ve deduced that the Paint Shop is the logical point at which to perform the bonding operation – you’re correct!  There is a wide selection of adhesive systems available, and the part’s surface is pristine and bond ready.  Curing heat is available.  All in all, the process quality and overall cost considerations are optimized in the Paint Shop to achieve a quality finished assembly.

Ultimately, there are many ways to assemble this part. It is necessary to optimize the cost, capital, process,and quality of the finished part. It is critical to ensure the final assembly works as required; the last thing anyone wants is parts coming off when the vehicle is in use. 


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