# Conformal 3D Material Extrusion Additive Manufacturing for Large Moulds
## 1 Introduction  
Large-scale moulds are typically required in marine and aerospace engineering to produce structural parts in composite materials. For large-scale items, a primitive model, usually handcrafted by trained technicians or obtained through CNC operations, is used to create a unique female mould: the model is covered in wax, using a gel coat which gives a polished surface, and the lamination process starts. Layers of composite materials with increasing areal weight are deposed on the model to obtain a rigid and strong structure. Then, an external layer of gel coat resin is applied over the parts in composite material to waterproof the final product, aid in mould ejection, ensure the aesthetic appearance, and provide colouring. The part can be eventually cured in an autoclave to obtain high breaking stresses, or the composite can be polymerised at room temperature. The mould and model accuracies are constrained by the artisan worker's skill and experience, which can vary significantly across the sector. Typical raw materials machined for moulds and male models are medium density plastic and UREOL. Obtaining suitable moulds requires trained personnel and accurate polishing, and nowadays, in the marine sector, the average workforce required is around 10 h for one squared meter of mould to carry out surface smoothing [1]. When a traditional approach is followed, numerous steps are required to create a part in composite material, including the need for writing a G-code [2] program using Computer-Aided Manufacturing (CAM) software. The industrial community is therefore asking scientific research for new strategies in developing machines that can perform several tasks to reduce human interaction and speed up quality-related processes [3]. It is also worth noting that the use of epoxy or polyester resins poses a threat to the safety of the operators due to toxicity problems. Using a face mask with filters may reduce comfort during the summer or in case of prolonged use. Moreover, environmental factors push towards reducing waste of resins and fabric, which can be quite hard to recycle with current technologies and can severely harm nature if not correctly disposed of. Therefore, the automatisation of mould production assumes a high relevance for the marine and aerospace sectors: this is especially true for marine engineering, where polyester resins are used, which are more toxic than the epoxy ones used in aerospace. In this context, Additive Manufacturing (AM) technology can help to produce moulds more reliably while lowering the cost of tooling and production time [4]. Additive Manufacturing (AM) has seen a significant rise in popularity over the past 20 years due to its evident industrial benefits, becoming more popular in academia and science [5]. AM techniques represent a broad group of procedures in which new material is deposited, joined, or solidified, frequently layer by layer, under computer control. Design flexibility, a shorter design-to-manufacturing cycle, the ability to generate complicated designs without being constrained by limitations in the manufacturing process (such as undercuts, squared holes and pockets, variable density, and trabecular structures), the elimination of joints and connections, and the decrease in wasteful raw material usage are only a few of the benefits of AM. Researchers have developed methods for component production using AM technology, particularly for small batches or prototypes [6]. Among different industrial applications that have shown rising appeal in AM, the naval industry's attention to AM technology has also recently increased. In particular, it enables the creation of models and moulds to assist the manufacturing of composite material items with an excellent level of finish and accuracy for large-scale components with an exceptional level of detail, such as boats [7; 8]. For example, 3D-printed moulds have been used to create polyurethane foam components for the marine and automotive industries [9; 10].