A method preserves shapes in a solid model when distributing material during topological optimization. A 3D geometric model of a part having a boundary shape is received. The geometric model is pre-processed to produce a variable-void mesh and to produce a frozen mesh representing the boundary shape. The geometric model is apportioned into a plurality of voxels, and a density value is adjusted for each voxel according to an optimization process. An iso-surface mesh is extracted from the voxel data, and a mesh Boolean intersection is derived between the extracted iso-surface mesh and the variable-void mesh. A mesh Boolean union between the mesh Boolean intersection and the frozen mesh.