Rationalising the onset of bistability in polylactic acid (PLA) bilayer laminates using Fused Filament Fabrication (FFF)

Fused Filament Fabrication (FFF)-based 4D printing offers a versatile route for creating shape-morphing structures. However, programming advanced mechanical functionalities such as bistability into standard thermoplastics remains a significant challenge. The fundamental mechanism governing the onset of such bistability, specifically how process parameters translate into the necessary internal energetic landscape, has yet to be systematically rationalised. To address this gap, this study investigates the effects of in situ printing parameters on polylactic acid (PLA) bilayer laminates by combining experimental characterisation with thermal process simulations. The findings demonstrate that bistability is governed by the energetic balance between induced and stored pre-strains, arising from the entropic elasticity of amorphous polymer chains stretched during extrusion. Printing speed is identified as the primary factor for tuning the curvature, defining a specific bistability window, whilst an inherent process asymmetry between layers necessitates differential speeds to achieve equilibrium. Furthermore, cyclic testing confirms that this process-induced bistability is transient, limited by viscoelastic stress relaxation upon repeated actuation. This work provides a comprehensive framework for programming bistability via FFF, establishing the physical boundaries for its application in cost-effective, customisable deployable structures where manufacturing simplicity is prioritised over long fatigue life.