Apparent interfacial bonding strength in hybrid overprinted bilayer tablets: influence of 3D printing parameters on...

Hybrid manufacturing by overprinting, in which 3D-printed layers are deposited directly onto injection-moulded substrates, offers a promising route to personalised oral solid dosage forms. A key challenge is achieving sufficient adhesion between the moulded substrate and the printed layer to minimise delamination during handling and downstream processing. This study investigated the influence of fused filament fabrication parameters, including printing speed, layer height, nozzle temperature, infill density, and infill pattern, on the apparent interfacial bonding strength of overprinted bilayer tablets composed of polycaprolactone and PVP-VA. Adhesion was evaluated by tensile pull-off testing, supported by fracture-surface analysis using digital microscopy and internal defect mapping by computed tomography. Lower printing speed (10 mm/s), reduced layer height (0.15 mm), elevated nozzle temperature (185 °C), and specific infill strategies, particularly high initial density and a wiggle pattern, significantly increased pull-off strength. In contrast, higher porosity and incomplete filament fusion promoted fracture within the printed region and reduced apparent bond strength. CT analysis showed that reduced defect fraction correlated with higher pull-off strength and altered failure mode. These findings establish a clear process-structure-property relationship and support optimisation of overprinting conditions for mechanically robust hybrid bilayer tablets.