Page 208 - Handbook of Adhesives and Sealants

Quality Control and Nondestructive Tests 179

5.2.1 Pre-manufacturing processes
The human element enters the adhesive-bonding process probably
more than in any other fabrication technique. An extremely high per-
centage of defects can be traced to poor workmanship or lack of un-
derstanding regarding adhesion. This generally appears first in the
surface-preparation steps, but may also arise in any of the other steps
necessary to achieve a bonded assembly. This problem can be largely
overcome by proper motivation and education. All employees from de-
sign engineer to laborer to quality-control inspector should be familiar
with adhesive bonding technology and be aware of the circumstances
that can lead to poor joints.
A great many defects can also be traced to poor design engineering.
Table 5.1 lists the polled replies of experts to a question: ‘‘Where do
design engineers most often err in designing bonded joints?’’ The most
common errors relate to training, design considerations, and produc-
tion line problems. The probability of many of these problems occuring
can be minimized or eliminated through proper training and educa-
tion.
Specifications are a necessary part of a quality control program. A
specification simply is a statement of the requirements that the ad-
hesive, sealant, or process must meet in order to be accepted for use.
A product specification is an agreement between supplier and user.

TABLE 5.1 Where Design Engineers Commonly Make Errors 2
Adhesive technology Design consideration Production-line problems
Low peel strength Using a butt joint when Lack of careful surface
lap joint would be preparation
Overlooking such factors
as pot life, curing time, stronger Expecting prototype
operating temperatures Loads causing performance from bonds
unsuspected cleavage made on assembly line
Failure to get technical
help from supplier in forces Failure to keep surfaces
selecting an adhesive Overlooking effect of clean until adhesive is
increased service temp in applied
Assuming that strongest
decreasing resistance to
adhesive is always the Failure to consider the
best without considering chemicals application method and
cost or processing Failure to check equipment when
coefficients of expansion designing joint
Lack of care in test
when unlike materials
procedures
are bonded
Calling for heat-curing
adhesive on a part that
will not stand the heat
Overdesigning by asking
for more strength or heat
resistance than is needed