Page 63 - Handbook of Adhesives and Sealants

An Introduction to Adhesives and Sealants 31

Ease of handling, waste, and reproducibility can be essential cost
factors.
Cleanup is a cost factor, especially where misapplied adhesive may
ruin the appearance of a product.
Once bonded, samples cannot easily be disassembled; if misalign-
ment occurs and the adhesive cures, usually the part must be
scrapped.

Many of these hidden costs can be minimized by the proper choice
of adhesives and processes. However, it should be remembered that
storage, cure, and waste disposal are seldom a concern in joining with
mechanical fasteners, and with welding, the joining material is essen-
tially free of charge.

1.4.10 Combining adhesive and
mechanical fastening
There are advantages in combining adhesive bonding with mechanical
fastening. The combination can provide properties that are superior
to either singular method. It is also possible to reduce the number of
mechanical fastening steps without sacrificing strength or reliability.
When combined with adhesives, mechanical joints can provide fix-
turing for the adhesive as it cures. In this way, expensive fixturing
equipment and the time required to set-up such equipment are not
necessary. The curing time is also eliminated as a potential bottleneck
to the overall assembly process. The stress distribution characteristic
of the adhesive bond also allows the designer the freedom to eliminate
thicker substrates or reinforcements that may be necessary with me-
chanical fasteners alone.
The secondary functions of the adhesive, such as sealing, vibration
damping, electrical insulation, etc., may also be used to achieve an
assembly with greater value. In the automotive industry, for example,
adhesives are used in combination with spot welding for joining trunk
assemblies. This combination provides sound deadening and sealing
in addition to a strong joint. Adhesives can also be combined with
fasteners that are designed directly into the part (e.g., snap fit con-
nectors). Here the adhesive eventually assumes the full structural
load with the mechanical fastener providing the fixturing as the ad-
hesive cures.

1.5 Functions of Sealants
Sealants are generally used as a barrier or a means of protection. In
this way, sealants are used to exclude dust, dirt, moisture, and chem-
icals or to contain a liquid or gas. They are also often used as a coating