# Design and Optimization of Lattice Structures: A Review
## 2 Lattice Structures
### Manufacturing Methods of Lattice Structures  
At present, there are two kinds of methods of fabricating lattice structures, i.e., traditional machining processes and additive manufacturing. [PERSON] and [PERSON] [74] summarized the traditional processing and manufacturing methods, such as water jet cutting, investment casting, electroless plating, electrodeposition, and various processing methods in additive manufacturing including fused deposition modeling (FDM), selective laser melting (SLM), selective laser sintering (SLS), electron beam melting (EBM), binder jetting (BJ), etc. There are advantages and disadvantages in the fabrication of lattice structure by traditional processing methods. In terms of advantages, the lattice structures with large-scale structure can be fabricated by traditional processing method. Adversely, using traditional processing methods to fabricate the lattice structures will be time-consuming and cumbersome post processing and cause more energy consumption and produce waste material. For instance, the investment casting method proposed by [PERSON] [21; 24] has high requirements for the fluidity of melt-liquid metal, resulting in being limited to nonferrous casting alloys with high fluidity. Additionally, the process of investment casting is very complex and easy to produce defects. Figure 10 is principle of 3D lattice structure of metal prepared by investment casting. The cell size of octahedral lattice materials fabricated by this process can only reach millimeter scale, which is difficult to meet the requirements of micro-nano-scale lattice structure. In addition, the extrusion and electro-discharge machining method (as shown in Figure 11) proposed by [PERSON] [122] still needs special mold, and the fabricating process is particularly complex and increases the generation cost. It is difficult to fabricate lattice structures with thin ribs and panels. Besides, stretching net folding method and punched hole mesh drawing method are proposed by [PERSON] [123; 124]. The biggest advantage of stretching net folding is that the plate mesh is made of the original sheet metal, which wastes less raw materials in the production process and saves the raw materials greatly. However, it still needs severe deformation in the manufacturing process and can only be used for materials with high ductility, resulting in low structural strength and complex processing technology [125; 126]. Additionally, stamping forming method caused most of the material wastes. The splicing technology proposed by [PERSON] [126] still has the defects such as unstable lattice structures and limited performance. Figure 10: Investment casting fabricating lattice structure [21; 24]. Additive manufacturing (AM) technology, as a new processing method, adopting the method of adding materials layer by layer, provides promising opportunities to automatically and flexibly fabricate structures with complicated shapes and architectures that could not be fabricated by conventional manufacturing processes. The combination of lattice structure and additive manufacturing technology is an unprecedented breakthrough in the field of industrial design. The fabrication of lattice structures by AM technology has the following advantages: (1) design flexibility; (2) fabricate lattice structure with wide range of sizes; (3) many kinds of materials (rubber, metal, alloy, ceramic, fiber, etc. ); (4) set up program for automatic processing; (5) save energy and reduce cost, etc. There are also some shortcomings in AM technology, i.e., when designing lattice structures with suspended geometry, support materials need to be added, which also causes certain material waste and increases post-processing time. However, based on AM technology, the lattice structures have high design flexibility and can be fabricated into structural materials with complex internal geometry and small geometric size.