Topology optimization for modular construction with multi-material, periodic, and demoldable units

Modular construction offers an efficient and economical fabrication route by assembling complex structures from repeatable, prefabricated units. Formwork-based casting facilitates the mass production of geometrically identical modules, while material selection allows these units to be tailored through different material properties. This paper presents a manufacturing-oriented topology optimization (TO) framework for explicit finite-size modular structures. To reconcile design flexibility with manufacturing realities, the framework integrates explicit finite-size module mapping, casting-based demolding, and module-wise material interpolation in a unified model. This enables modules with different material properties to be distributed across the design domain while ensuring compatibility with assembly and casting processes. The method yields optimized structural geometries and material layouts that satisfy manufacturing requirements. The effectiveness of this approach is validated through several 2D and 3D numerical examples. A fabrication demonstration is provided to show the physical demoldability of an optimized module. Additional generalization studies further examine the effects of finite-stiffness interfaces, module reorientation, and adaptive demolding families. This study offers a practical strategy for manufacturing-oriented structural design in the construction industry.