# Functionally Graded Additive Manufacturing: Bridging the Gap between Design and Material Extrusion
## 1 Introduction
### Current FGAM Software Limitations  
At present, the standard file formats used in the field of AM are the STL and OBJ file formats, which are triangular facet models represented by polygonal meshes. These file formats describe only the surface geometry and contain no material or volumetric property information. In addition, various data exchange formats may be suitable for the production of FGAM components, including AMF (additive manufacturing format), FAV (fabricatable voxel), SVX (simple voxels), and 3 MF (3D manufacturing format). These file formats, beyond a defined geometric description, can include information about the material gradient and micro-scale physical properties [1; 15; 17]. However, at present, slicing software tools, also known as computer-aided manufacturing (CAM) software, do not support the description of graded materials, and therefore, it is not possible to process the information stored in those files into the computer numerical control programming language (G-code) required for manufacturing FGAM parts.  
For this reason, new methodologies for slicing, evaluating, and manufacturing FGAM are required. New software for FGAM processes should include the capability to control the density and the allocation of material fractions based on the functional requirements for the generation of FGAM geometric models. Some novel approaches for handling those new features, required to produce functionally graded components, were proposed by researchers over the last years. For example, [PERSON] and [PERSON] [18] proposed a computational approach that uses CPPN (compositional pattern producing network) encoding a scalable algorithm using NEAT (neuroevolution of augmented topologies) to embed functional morphologies and macro-properties of physical features using multi-material FGAM through voxel-by-voxel descriptions according to its cartesian (x, y, and z) coordinates [19]. The conversion to a voxel model from a common geometric format (i.e., an STL file) for FGAM is computationally demanding and difficult to achieve, especially when dealing with highly refined details [20]. To overcome this issue, [PERSON] and [PERSON] [18] introduced an alternate design-supporting system to represent material-geometry-topology, with a volumetric texture map using voxels models that are algorithmically generated. Modifications can be performed manually on voxels, and then compiled back into the texture description.  
Still, these methods are not straightforward and require a lot of time and knowledge to be implemented. Moreover, implementations on commercial design software are not available for design or manufacturing, which are indispensable in industrial applications.  
This work aims to provide an original methodology for the fabrication of FGAM with limited computational effort, avoiding sophisticated and proprietary file formats for data exchange of FGM geometric models. Starting from a surface model and a material distribution, the paper is focused on handling the G-code in FDM coextrusion according to a volumetric model. The methodology is then tested on headphones, in which a hard case and a soft earpad are consolidated in a single part with graded stiffness.