Bond-line read-through (BLRT) is a distortion in the “Class “A” surface of a bonded assembly caused by
differential shrinkage between the substrates and the adhesive. When these distortions are severe enough to be
visible customers consider them to be unacceptable. In automotive applications in which a structural inner
panel is bonded to a Class “A” outer panel, the coefficient of thermal expansion of the adhesive is higher than
that of the substrate. This difference in thermal expansion between the two materials causes the outer panel to
be distorted at room temperature if the assemblies were bonded at elevated temperature. The distortions
become visible when other factors exacerbate the severity of these naturally occurring distortions.
The material and process factors that determine the severity of BLRT-induced distortions, and the experimental
and analytical work from which those relationships were determined, have been reported previously.
Understanding of the cause of and the factors that contribute to BLRT allows one to then develop design and
manufacturing strategies for minimizing the severity of adhesive-induced surface distortions in bonded
assemblies. The design and manufacturing strategies that are the final outcome of the Automotive Composite
Consortium’s BLRT project are presented.

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