The CAD software and then

The AM process using the filament based extrusion technique of FDM illustrated in Figure 2.4 requires the material to be processed into a filament form. This filament is produced by conventional polymer processing techniques, but this filament must be extruded to a very high diametric tolerance, which can’t be achieved by conventional extruders (Song ; Masood, 2007; Comb et al., 2005). The incoming filamentacts as a piston, pushing the molten material through the liquefier.

Insufficient filament stiffness, or high viscosity can result in the buckling of the filament (Reddy et al., 2007; Bellini et al., 2004; Venkataraman et al., 2000). Gåård et al.

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, (2006); Exner et al., (2008) stated to prevent from buckling, the applied extrusion pressure must be below a critical value. Further drawbacks of this process are the potential slippage of the wire on the pinch wheel, causing an interruption of the building process (Gibson et al.

, 2010; Masood and Song; Comb et al., 2005). Figure 2.4. Extrusion AM process principle and schematic (Kruth, 1991).

2.3 AM technologies in fabrication of cores Recently 3D printing, also known as additive manufacturing has been developed rapidly, which enables the fabrication of auxetic cellular materials with precise and complex cellular geometries directly from the CAD models. Re-entrant honeycomb, conventional honeycomb, and truss cellular structures are designed using CAD software and then fabricated with 3D printing technique. McKown et al., (2008) tested a range of metallic lattice structures, all of which were built using the SLM process. Yang, et al.

, (2015) fabricated sandwich samples of various designs, including re-entrant auxetic, rhombic, hexagonal and octahedral via selective laser sintering (SLS). Yang et al., (2015) designed a sandwich structure with a 3D re-entrant auxetic core fabricated using electron beam melting and selective laser sintering. The bending behavior on these materials has been studied. Figure 2.5 shows the design of sandwich panel with auxetic core. Figure 2.5.

The sandwich panel with auxetic core usingelectron beam melting and selective laser sintering (Yang et al., 2013). Xiong et al., (2010) fabricated carbon fiber composite pyramidal truss core sandwich structures by the molding hot-press method.

Honeycombs are made primarily by different RP (Rapid Prototyping) techniques, like FDM and SLS, or general 3D printing process usually confined to the use of thermoplastic materials.Corrugated geometry cores were fabricated using SLA, FDM, and 3DP. Pollard et al., (2017) produced honeycomb cores by using FDM to a specified size as illustrated in Figure 2.6(a) and (b). Yang et al., (2013) designed various sandwich panel structures with different reticulate lattice core geometries and then fabricated in titanium via the electron beam melting (EBM) process. Yang et al.

, (2013) designed a sandwich structure with a 3D re-entrant auxetic core fabricated using electron beam melting and selective laser sintering.(Yang, 2015) demonstrated the design and verification of a 3D reticulate octahedral cellular structure using additive manufacturing, electron beam melting (EBM) process. Figure 2.

6: Cores used during testing. (a) An example core of ABS manufactured through FDM. (b) A Nomex honeycomb core of the same size; the difference in cell size due to the manufacturing limitations of the FDM process is clear (Pollard et al., 2017).2.4 Fabrication sandwich composite structure process (Chen et al.

, ) proposed sandwich composite columns were manufactured by vacuum assisted resin infusion molding (VARIM) process. The sandwich composite columns consisted of core materials and pre-impregnated fiber material as facesheets. Three 5 mm thick foam layers with different mechanical properties were bonded to each other using Biresin CR80 epoxy resin with Biresin CH80-2 hardener. Both skins were then co-cured to the layered foam cores by means of vacuum infusion process for manufacturing of the sandwich composites with six types of layered foam core arrangements (Baba, 2017). Sun et al., (2016) stated for grid reinforced honeycomb cores, as shown in Figure 2.7, a grid core was firstly assembled, and then honeycomb blocks were cut and filled into the blank of grid to form the combined core.

Figure 2.7: Geometry and sketch of the combined core sandwich specimens (Sun et al., 2016). (Hou et al.

, 2014) proposed the Kirigami manufacturing process consists in the following five steps: cutting, molding, curing, folding and bonding. Periodic distributions of slits are introduced within the plain weave woven using an Auto Prepreg Cutting machine. The horizontal walls of the honeycomb ribbons are bonded together using epoxy adhesive (Hexcel Redux810) and cured. Carbon-epoxy composite sandwich structures were fabricated from all cores using a wet-hand layup process with vacuum cure Babu.

, (2015). The sandwich composites are fabricated using a vacuum bagging technique. The facings material GFRP along with foam made of PU based material ofdifferent densities were fabricated using a vacuum layup process technique This setup were in turn kept for curing by placing them in a vacuum condition and allowing entrapped air within the laminates and foam to be removed completely as shown in Figure 2.

8. Figure 2.8: Sandwich panel fabricated using vacuum bagging technique (Babu., 2015).2.5 Several types of 3D design advanced core structured The most widely used sandwich materials in engineering applications can be distinguished by two groups of core materials, the homogeneous and the structured corematerials as in Figure 2.9. Sandwich panel made with carbon fiber reinforced composite pyramidal honeycomb grid core as shown in Figure 2.

10 has successfully made 3D lattice-core sandwich cylinder. Chen et al., (2017) the hierarchical honeycomb topology were described by replacing the cell walls of regular honeycombs with triangular lattices Figure 2.

11(a)–(d). Figure 2.9: Sandwich materials with homogeneous and structured core materials (Fan, 2006) Figure 2.10: Sandwich panel made with carbon fiber reinforced composite pyramidal honeycomb grid core (Li et al).Figure 2.11: Design and 3D printing of the hierarchical honeycomb. (a) Regular honeycomb.

(b) The cell wall of the regular honeycomb. (c) Substructure composed of a triangular lattice. (d) Hierarchical honeycomb. (e) 3D printed regular honeycomb. (f)–(g) 3D printed hierarchical honeycomb. Scale bar: 2cm (Chen et al., 2017).