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The lightweight design of thin-walled structures depends on the spatial distribution of materials in three dimensions. Conventional plate topology optimization based on plate or shell elements, however, is restricted to a two-dimensional (2D) design space on the reference surface and lacks design freedom in the thickness direction, whereas applying a fully three-dimensional (3D) solid formulation to plate design suffers from high computational cost, locking, and a tendency to generate non-manufacturable closed internal voids. To address these limitations, this paper proposes a CUF-based 3D synergistic topology optimization framework that extends plate topology optimization with design freedom in the thickness direction. Exploiting the section expansion capability of the Carrera Unified Formulation (CUF), an implicit geometric projection strategy is introduced to describe the 3D topology through two sets of 2D design variables, which reduces the size of the optimization problem and avoids closed internal voids, thereby improving manufacturability. Benchmark and representative numerical examples show that the proposed framework reproduces the transition from membrane-dominated to bending-dominated mechanical behavior. Compared with conventional 3D topology optimization, the present approach reduces the number of design degrees of freedom while preserving numerical accuracy in the thin-plate limit (locking-free), providing an explicit 3D geometric design paradigm for plate structures.

Thin-walled structures; Topology optimization; Carrera Unified Formulation; Synergistic optimization; Through-thickness design