As-exact-as-possible repair of unprintable STL files

Purpose: The class of models that can be represented by STL files is larger than the class of models that can be printed using additive manufacturing technologies. Stated differently, there exist well-formed STL files that cannot be printed. This paper aims to formalize such a gap and describe a fully automatic procedure to turn any such file into a printable model. Design/methodology/approach: Based on well-established concepts from combinatorial topology, this paper provide an unambiguous description of all the mathematical entities involved in the modeling-printing pipeline. Specifically, this paper formally defines the conditions that an STL file must satisfy to be printable, and, based on these, an as-exact-as-possible repairing algorithm is designed. Findings: It has been found that, to cope with all the possible triangle configurations, the algorithm must distinguish between triangles that bind solid parts and triangles that constitute zero-thickness sheets. Only the former set can be fixed without distortion. Research limitations/implications: Owing to the specific approach used that tracks the so-called "outer hull," models with inner cavities cannot be treated. Practical implications: Thanks to this new method, the shift from a 3D model to a printed prototype is faster, easier and more reliable. Social implications: The availability of this easily accessible model preparation tool has the potential to foster a wider diffusion of home-made 3D printing in non-professional communities. Originality/value: Previous methods that are guaranteed to fix all the possible configurations provide only approximate solutions with an unnecessary distortion. Conversely, this procedure is as exact as possible, meaning that no visible distortion is introduced unless it is strictly imposed by limitations of the printing device. Thanks to such unprecedented flexibility and accuracy, this algorithm is expected to significantly simplify the modeling-printing process, in particular within the continuously emerging non-professional "maker" communities.