The importance of the stiffness of a 3D printed object has only been realized gradually over

the years. 3D printed objects are always hollow with interior structure to make the fabrication

process cost-effective while maintaining stiffness. State-of-the-art techniques are either

redundant (using much more material than necessary to ensure the stiffness in any case) oroptimize the structure under one of the most-probable load distributions, which may fall short

in other distribution cases. Furthermore, unlike industrial products using brittle materials

such as metal, cement, etc., where stress plays a very important role in structural analysis,

materials used in 3D printing such as ABS, Nylon, resin, etc. are rather elastic (flexible).

When considering structural problems of 3D printed objects, large-scale deformation always

comes before reaching stress limits when loads are applied. We propose a novel approach

for designing the interior of an object by optimizing the global stiffness — minimizing the

maximum deformation under any possible load distribution. The basic idea is to maximize

the smallest eigenvalue of the stiffness matrix. More specifically, we first simulate the object

as a lightweight frame structure, and optimize both the size and the geometry using an

eigen-mode-like formulation, interleaved with a topology clean. A postprocess is applied to

generate the final object based on the optimized frame structure. The proposed method does

not require specific load cases and material strengths as inputs, which the existing methods

require. The results obtained from our experiments show that optimizing the interior structure

under unknown loads automatically keeps strength where the structure is the weakest and is

proven to be a powerful and reasonable design framework.