Experimental investigation of squeeze flow mechanics of structural adhesives in dissimilar polymer–metal joints

Control of adhesive flow during squeeze processes is critical for regulating bond-line thickness and ensuring reliable joint formation in structural assemblies. This work presents a mechanics-based experimental study of adhesive squeeze flow using a custom-designed Hele-Shaw cell integrated with a universal testing machine. The setup enables precise force control together with synchronized top-view imaging of the spreading front, allowing coupled mechanical and kinematic analysis of the flow. The influence of substrate surface conditions was assessed using polypropylene (PP) and aluminum (Al) substrates, each subjected to representative industrial treatments: untreated, flame-treated, and plasma-treated for PP; untreated, laser-treated, and anodized for Al. Furthermore, the effect of compression speed was investigated by conducting tests at two distinct loading rates. Throughout compression, force–thickness curves were obtained, and image-based tracking provided quantitative characterization of spreading dynamics and final adhesive geometries. Results indicate that surface treatments had a negligible influence on squeeze flow mechanics under the tested conditions, whereas compression speed significantly altered the force–thickness response. These findings demonstrate that adhesive rheology and loading conditions govern squeeze flow behavior, with surface properties playing a secondary role when interfacial failure mechanisms are absent.