In the past, the automobile assembly process solely relied on attaching metal parts through mechanically fastening using rivets or welding. Mechanical fastening has its shortcomings, including introducing stress points where the substrates being fastened. To eliminate this and other potential problems, using structural adhesives can allow for the reduction and distribution of stresses along an entire area, instead of singularly placing the stress in the weld or hole that would hold the mechanical fastener.
Why use Structural Adhesives?
Cosmetics can play into the decision to use adhesives instead of welding or mechanical fasteners. When using an adhesive, it’s not necessary to refinish or rework a surface as seen when a weld or mechanical fastener is used.
Welding typically destroys most anti-corrosive coatings that were previously applied. Assembly using galvanized or hot dip galvanized coatings could then experience corrosion at a later point. A structural adhesive allows different types of composites to be joined and acts as an insulator between the different metals. For example, if aluminum and steel come in contact with each other, it could result in galvanic corrosion, whereas when a structural adhesive is used between the two different metals, it insulates them from galvanic corrosion. With mechanical fasteners, sometimes a sealer may be placed between the panels, but even the rivets will be a dissimilar metal and may cause galvanic corrosion in those areas. Another aspect to consider is that welding typically does not work with dissimilar metals.
Using mechanical fasteners may cause warranty issues in the long run. Screws can loosen up over time due to the stress concentration and then the rivets or screws become leak paths. Once they loosen up, the rivets or screws can also break.
Labor and Capital Savings
Another difference that becomes apparent between using welding versus adhesives is the cost savings on labor. Welding, for example, requires highly skilled workers to perform the task. Adhesive bonding can be done robotically resulting in a significant cost savings. It’s also much less expensive to use dispense equipment for adhesives versus having several welding stations on an assembly line. Adhesives typically have a smaller capital outlay and allows for greater design flexibility in the assembly line. Another reduction in labor time is surface preparation since the substrates to be bond or adhered by adhesives don’t typically require any surface preparation.
Another advantage is that adhesives have excellent environmental resistance to various the settings that you need them to withstand.
Designing for Your Needs
When designing a bonding joint, you typically want to have shear forces on your joint instead of peel. Compression is acceptable on peel or tension-type joints, so it is necessary to design for what type of adhesives are needed, consider what it does, and allow for the distribution of stresses in the joints.
There are times where you may want to use a mechanical fastener in conjunction with the adhesive to prevent peel forces on the adhesive joints. In those situations, you would reduce the amount of mechanical fasteners used – and perhaps only use the mechanical fasteners at the beginning and end of the linear bond joint.
Where are These Adhesives Used?
In general, structural adhesive joints are used predominately on closure panels, such as hoods, lift gates, tail gates and doors. They are also used in reinforcement areas, such as in the front of an automobile, in particular the car’s grill – where many of the complex parts are made of composites. Any type of automobile joint that can be designed into a shear mode is capable of using an adhesive.
In particular,our Versilok® adhesives are recommended for the automotive industry as they enable lower-cost assembly of automotive closure panels, such as doors, tail and liftgates. Versilok adhesives achieve high dimensional stability of closure panels through a low temperature cure, obtain increased corrosion protection through excellent adhesion properties, and simplify the manufacturing process by eliminating or reducing process steps. Substantial cost savings are realized due to process optimization and reduced energy consumption and labor.