Additive manufacturing enables the nearly uncompromised production of optimized topologies. However, due to the overhang limitation, some designs require a large number of supporting structures to enable manufacturing. Because these supports are costly to build and difficult to remove, it is desirable to find alternative designs that do not require support. In this work, a filter is presented that suppresses non-manufacturable regions within the topology optimization loop, resulting in designs that can be manufactured without the need for supports. The filter is based on front propagation, can be evaluated efficiently, and adjoint sensitivities are calculated with almost no additional computational cost. The filter can be applied also to unstructured meshes and the permissible degree of overhang can be freely chosen. The method is demonstrated on several compliance minimization problems in which its computational efficiency and flexibility are shown. The current applications are in 2D, and the proposed method is readily extensible to 3D.

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Additive manufacturing (AM) and topology optimization (TO) have a synergetic relation, as AM can produce complex TO designs, and TO provides high-performance parts that utilize the form freedom provided by AM. Recently, TO has been tailored more toward AM with the inclusion of the minimum allowable overhang angle as a design constraint: resulting designs can be built without any support structures. This work is an extension thereof, by allowing support structures only if they are accessible, such that they can be removed after manufacturing. This is achieved by applying a conventional overhang filter twice, combined with basic operations such as geometry inversion, union, and intersection. The result is an accessibility-aware overhang filter that can be incorporated in TO. Compared with conventional overhang filtered designs, the accessibility filter results in increased part performance and better convergence behavior. Furthermore, a modular filter structure is presented to easily construct the accessibility filter, and its effectiveness is demonstrated on several numerical cases. K E Y W O R D S accessibility of supports, additive manufacturing, overhang, topology design 1 INTRODUCTION The most attractive feature of additive manufacturing (AM) is the unprecedented degree of permitted form freedom. No longer bounded by the design limitations of conventional manufacturing techniques, such as casting or milling, the challenge becomes to find the geometrical layout of the component that truly gives the best performance. 1,2 This is exactly what is provided by topology optimization (TO). Compared with other structural optimization methods, such as size or shape optimization, it exploits maximum design freedom, and is widely recognized as a design approach perfectly suitable for AM. 3,4 Although AM can produce geometrically complex parts created by TO, there are design restrictions that have to be taken into account to prevent an increase in production cost. For example, a design might require a large amount of supports if the angle between a down-facing surface and the base plate is below a material/process specific value, the so called critical overhang angle oh. 5-7 Support structures not only increase the printing cost due to increased material and postprocessing cost, they can even become impossible to remove once the printing is completed. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.