# Conformal 3D Material Extrusion Additive Manufacturing for Large Moulds
## 2 Materials and Method  
The methodology proposed in this study is based on dividing the different phases of manufacturing a typical large-scale mould into different materials with which a different machining process is associated. After the CNC milling of the supporting structure, the additive phase can be seen as a virtual CNC machining with the extruder's size equal to a virtual tool used for surface finish machining. Thus, a single G-code and a single machine can be used to manufacture the structure, increasing the efficiency of the overall manufacturing workflow. The milled core of PVC (PolyVinyl Chloride) or polystyrene acts as a support for the additive deposition of the external coating. Therefore, the use of support material, typical of the FFF process, can be avoided entirely, minimising post-process actions of operators to remove unneeded material and optimising material deposition in a conformal way, meaning that fused material continuity is assured. Different parts and moulds could be manufactured directly with a single hybrid machine. For example, a core of expanded PVC, milled with a traditional process, can support the additively manufactured coating deposition with PLA, PETG, or clays for large structures. Expanded PVC foam board is a lightweight, rigid PVC sheet used for various applications, including marine structures [25], interior design, thermoforming, prototypes, model making, and much more. It can be easily machined with the low power needed. A schematic representation of the structures obtained with the proposed methodology can be seen in Figure 1.  
Figure 1: Schematic representation of hybrid structures that can be manufactured using the proposed methodology: (**a**) Standard mould with a core and external clay; (**b**) Sandwich structure with external coatings and a dense core; (**c**) Standard mould with CF (Carbon Fiber) or FG (Fiber Glass) deposition over the mould. Furthermore, the machine manages the component's surface polish, thus reducing human intervention. Before starting with the lamination of the layers of composite material which will build up the parts, skilled operators are only required to examine the mould's dimensions and quality in the final step. Indeed, a typical hybrid manufacturing machine is equipped with the two following tools: a spindle with interchangeable milling tools for roughing, finishing, and polishing; an extruder nozzle for extruding clay or filament. The hybrid processing method described in this research is based on milling a slab of raw material during the early stage after the 3D digital model is available. The same machine then facilitates the additive deposit of the coating on the support's exterior surface along 3D paths computed by the proposed methodology. For the sake of clarity, a graphical representation of the general workflow proposed in this research is shown in Figure 2.  
The G-code is an example of a machine-readable language based on a series of tasks to be carried out. It includes the path's definition and the settings required for the machine: how quickly to move, along what trajectory to take in the available space, and many other settings. The G-code file can be automatically compiled using CAM software for the traditional processes and slicing software for the AM machines. While Computer-Aided Design (CAD) software is used to design products, CAM software packages enable users to obtain processing cycles that begin with raw materials slabs and end with the manufactured CAD-designed object. The CNC machine's different settings, such as the feed rates, cutting speeds, and other cutting parameters, are converted by CAMs into G-codes before the machine starts running. In the design of the hybrid manufacturing machine, particular attention should be given to the geometry restrictions related to potential collisions between the extruder head and walls of machined mould in concave parts. In this research, an optimal design of endstop positions and relative distance between the additive and subtractive tools avoid dangerous impacts.