Spiral kinematics: A biomimetic approach to enhancing demolding efficiency in 3D-printed polymeric formworks for customized hollow concrete structures

The customization of hollow concrete components has gained significant attention for enhancing multi-functional performance, including structural efficiency, thermal and acoustic properties; however, it also poses challenges in fabricating complex geometries. Conventional concrete formwork often faces demolding difficulties, which can damage both the formwork and the concrete and lead to increased costs and environmental impact. This study introduces a novel approach where polymeric formworks with biomimetic spiral designs are fabricated by 3D-priniting. Such customized 3D-printed formwork designs introduce a kinematic mechanism to enhance demolding efficiency while maintaining structural integrity. Polylactic acid (PLA) and thermoplastic polyurethane (TPU) were used to fabricate 3D-printed polymer bars with varying spiral gap lengths (0.2 mm and 0.6 mm), which were tested under monotonic pull-out conditions, mimicking formwork extraction from hollow concrete components. The spiral designs significantly reduce pull-out resistance, demolding difficulty, and associated damage. The kinematic benefits from spirals can be further amplified by adopting wider spiral gaps or by selecting TPU as the 3D printing filament, due to its greater toughness and flexibility, which resemble those of elastomeric materials. This work advances concrete demolding through innovative design optimization and offers practical solutions for greater customization and fabrication efficiency for intricate concrete structures.