4D injection molding.

Injection molding is a foundational manufacturing process for the large-scale production of plastic parts and is also applied to rubber, metal, ceramic, glass, wood, and composites. Regular injection molding focuses on minimizing warpage and ensuring dimensional uniformity and high repeatability, as non-uniform material shrinkage in complex mold cavities causes undesirable deformations. In contrast, we reverse this traditional paradigm by leveraging controlled solidification shrinkage to induce purposeful, programmable deformation. We propose 4D injection molding, utilizing localized thermal activation and selective in-mold bonding to create non-uniform temperature and pressure distributions in spatiotemporal dimensions during the molding cycle. This enables the generation of complex, customized geometries from a single mold. Integrated with predictive modeling and multi-objective response optimization, our approach transforms warpage from a defect into a design feature, yielding functional parts with tunable shapes and performance characteristics. This method supports rapid, scalable, and cost-efficient mass customization across various materials. By reimagining shrinkage not as a limitation but as a design opportunity, the method represents a transformative shift in manufacturing science, with promising applications in biomedical, aerospace, and responsive consumer products.