Adhesive Bond-line Read-through: Theoretical and Experimental Investigations

Adhesive bond-line read-through is a visible distortion of the substrate over a cured adhesive bond-line. Bond-line read-through deformations are primarily the result of a difference in the thermal expansion coefficients between the substrate and the adhesive. Substrate and adhesive thermo-mechanical properties play a large role in determining the severity of the distortions.

This work describes the approaches taken to understand, predict, and minimize bond-line read-through. It presents a simple model which relates physical deflections in SMC (Sheet Molding Compound) sheets to the thermal stresses that arise as part of the adhesive cure cycle. Model predictions of both the shape and magnitude of the deflection over the bond-line will then be experimentally verified for two extreme types of bond designs. General approaches for reducing bond-line read-through by way of process, part design, and adhesive formulation modifications will be discussed.