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The average mid-sized car contains 4,800 spot welds.
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Metal bonding offers a practical alternative to welding
By Jim Perritt, Contributing Editor
At the automobile manufacturing factories, welding has been the main
method of sheet metal joining for more than 80 years. However, in the
past decade, car manufacturers have been looking for alternative
methods of joining metals. Adhesive bonding technology is rapidly
gaining acceptance as an alternative to spot welding because it offers
many benefits to the automotive design engineer. These benefits
include the ability to join dissimilar materials, greater flexibility
in manufacturing and large cost savings to the automotive industry.
Today, car manufacturers often use adhesives in combination with spot
welds, or weld bonding, and for panel stiffening and vibration
dampening. The average mid-sized car contains 4,800 spot welds. The
average cost of a spot weld is about a nickel. According to a speech
given at a recent Auto Interiors Conference by George Hamilton, CEO of
Dow Automotive, it is estimated that as many as one-half of all spot
welds could be replaced in the vehicle assembly process using adhesive
bonding, resulting in savings to the OEM of more than $80 per vehicle.
Additionally, vehicles produced using adhesive bonding techniques
benefit from improved crash performance, corrosion durability, and
greater stiffness resulting in improved NVH.
With increasing use of adhesives at the factory level as well as in
aftermarket repair, OEMs are working with adhesive companies to
evaluate products and repair techniques to ensure quality and durable
repairs in the aftermarket. Since General Motors Corp. (GM) published
GM Technical Service Bulletin No. 02-08-98-001A, which provides
detailed guidelines for repair shops looking to use adhesive bonding
of exterior panels as an alternative to MIG welding, body shops have
begun to recognize the importance of using an adhesive that meets OEM
specifications for performance and durability. Recently,
DaimlerChrysler expanded its recommendations for adhesive use to
include structural weld bonding in its brochure No. 81-170-03005.
Choosing an Adhesive
The principle of adhesion starts with the concept of ?wetting,?
defined as ?the intimate contact of two panels to be joined.?
Adhesives need to achieve complete contact with both of the panels
being joined for bonding to occur. Wetting is the first essential step
for all adhesives, whether they are simple glues, cements, sealants or
structural adhesives. The importance of this concept is in the need to
choose the appropriate adhesive for each size job. The selected
adhesive must stay wet during the entire assembly process, from the
time the adhesive is dispensed until the two panels being bonded are
joined and clamped to ensure full contact of the bonded panels.
Because the size of the panels to be bonded has a direct impact on the
amount of wetting time needed to properly install the panels, it is
important to choose an adhesive that has a ?work time? greater than
the longest possible time required to complete the repair. For this
reason, most adhesive companies offer two or more adhesives for
application of small, medium and large panels.
Once good wetting takes place, an adhesive needs to become solid and
not flow at all. The technical term is setting, or cure. The ideal
adhesive will have sufficient wetting or open time to install the
panel but will set quickly and cure so that other repairs can be made
on the vehicle without disturbing the bonded panels. The faster the
set and subsequent cure, the sooner the car can be moved for frame
straightening or other procedures. Typically, acrylic adhesives set
and cure the fastest, followed by epoxies and urethanes.
Finally, a good metal bonding adhesive must be strong and durable. A
high-quality metal bonding adhesive must not only withstand
laboratory-strength tests but should also be proven through
destructive testing such as Federal Motor Vehicle Safety Standard
(FMVSS) 301, which measures fuel system integrity in a crash and FMVSS
208, which measures roof integrity, and/or offset barrier crash tests
such those performed by OEM?s to determine crashworthiness. In
addition, to ensure a lasting repair, the adhesive should be subjected
to various aging tests, heat and cold cycles, and salt-water spray
tests. These tests typically simulate the stress and durability that
an adhesive must be able to stand beyond the life of a vehicle.
Why and Where to Use Adhesives
As seen in the automotive factories, the use of adhesives for bonding
metal body panels is becoming more commonplace in the aftermarket.
Unlike welding, adhesives distribute stress uniformly, provide sealing
characteristics and sound dampening, and provide a moisture barrier,
minimizing the chance for corrosion. In fact, when using a full
bonding method for attaching secondary metal body panels, some
adhesive manufacturers provide a lifetime warranty against corrosion
as well as a standard lifetime adhesive performance guarantee.
Adhesive bonding can improve repair cycle time and, as compared to
welding, is relatively easy to learn and master.
Adhesive manufacturers are recommending the bonding of outer panels,
including door skins, quarter panels, rear body panels and roof
panels. Many adhesive manufacturers have conducted testing to validate
the bonding of these parts and secondary panels. There are some
adhesive manufacturers who are currently working with several OEMs to
develop repair procedures incorporating the use of adhesives for
repair of structural components. However, until these procedures are
finalized, it is recommended to use adhesives only for the bonding of
secondary, non-structural panels only.
To learn more about adhesive applications and repair techniques, some
adhesive manufacturers offer clinics. Many manufacturers will come to
the shop to provide instruction on the use of their products. In
addition, I-CAR offers an adhesive bonding course.
With increased use of adhesives in the factories and in aftermarket
repair, OEMs are working with adhesive companies to evaluate products
and repair techniques to ensure quality, durable and dependable
repairs in the aftermarket. Many OEMs and other manufacturers are
recommending adhesives for replacement of welding in numerous
technical service bulletins, and it?s apparent that the OEMs believe
adhesive bonding is an important repair alternative to conventional
welding techniques. Meanwhile, all of the major manufacturers continue
to investigate the use of adhesives for repair of structural
components as well as secondary body panels as they work to provide
alternative welding and varied repair procedures.
So, it sounds like that while 4,800 may be the average today, that
number is going to see a rapid decrease in the near future.
I found this article by searching for:
"spot welds average car"
It was the 4th listing.
Hope this is what you were looking for, Redhoss