Modeling Of Porous Structure Of Implants And Direct Manufacturing By Selective Laser Melting

The metallic implant manufactured via traditional method easily causes patients’ discomfort due to its overweight and excessive stiffness; besides, due to its solid construction, there is no mutual meshing and fixation between implant and host’s bone, thus it is easy to cause let-go, which shortens the service life of implants. It is able to design the surface or whole model of implant as porous structure in order to obtain the implant with better biomechanical properties. Although there are many design and manufacturing/preparing methods for porous structures, most of those methods are established on the basis of traditional manufacturing method; with the development of additive manufacturing (AM) technology, the porous structure with bigger design freedom can be manufactured, including some refined and tiny geometrical features. Therefore, more consideration on bionic structural and mechanical parameter is a significant research for porous design and manufacturing of implants.This paper researches design rules for porous structures based on selective laser melting (SLM) technology, proposes a method that the implicit surface represented by implict functions is used to construct porous unit and then carries out modeling transformation for porous unit via shape function control method to construct porous structure in spatial domain and conducts representation of structures and mechanical parameters, develops the modeling system of porous structure, and finally directly manufactures porous implants by SLM technology to confirm the feasibility of methods and technologies established in this paper. The main research contents and results include following aspects:(1) This paper analyzes the geometric constraints of porous structure processed via SLM, such as dimensional accuracy, outline accuracy and geometric feature molding resolution; gives a discussion on technological constraints such as laser deep penetration and spot size; studies the method that the contour offset is used to control porosity; analyzes the manufacturability of strut which is the basic element of porous structures; establishes the design rules for SLM technology based on porous structure. Under the guidance of these rules, it is able to control the structure parameters of porous structure at design stage to make it meet requirements of SLM technology, and also improve the manufacturability of porous structure through controlling technological parameters.(2) This paper proposes a method that the implicit surface represented by implict functions is used to construct porous unit, and defines porous unit with different shapes and porosities via defining different spatial real-valued functions; researches key technologies of implicit function in modeling of porous unit, including the generation of surface boundary representation, surface intersection, surface triangularization and extraction of STL model, etc. With high efficiency and controllability, this modeling method can be combined with existing design method to provide a new method for design of porous implants.(3) This paper makes use of hexahedral element to carry out mesh generation for partial and whole model of implant, and then utilizes information about8-node hexahedral elements in spatial domain to make mappings for standard unit in parameter domain via shape function control method, and then obtains irregular hexahedral sub-unit in spatial domain through interpolation. This paper also merges STL models of all sub-unit in the spatial domain, and deletes the overlapping triangles to make between adjacent sub-unit linked as a connecting entity, thus constructing continuous porous implant geometry.(4) This paper also researches the method used to represent structural and mechanical parameters of porous structures, and utilizes direct representation method and mesh-based methods to estimate the structural parameter; makes use of Micro-CT to carry out structural performance tests for porous titanium manufactured by SLM technology, and calculates the elastic modulus of porous titanium via compression tests; utilizes the value obtained via representation methods, finite element analysis and tests to carry out calculation sampling respectively on the structural and mechanical parameter, establishes the coefficient of implicit function of P unit and corresponding response surface model of relationship between structural parameter and mechanical parameter of porous structure, and then makes use of this model to estimate its structural and mechanical parameters.(5) On the basis of the above mentioned theories and methods, under VC++environment, this paper combines with OpenGL to develop modeling system of porous structure, and establishes the data interface with Ansys, through which the system can read hexahedron and tetrahedron elements and nodes files derived from Ansys. Based on design rules, this paper designs and directly manufacturers two kinds of typical porous implant by use of modeling system of porous structure and SLM technology; the implants have good forming quality with an aperture of600-1000μm, and its elastic modulus is equivalent to that of bone. All of those confirm that the methods and technologiesy established in this paper is feasible.