## 2 Manufacturing Processes of Musical Instruments Made of Composites and Alternative Materials
### Composites in Musical Instrument Manufacturing
The manufacturing process for musical instruments made from fiber-reinforced thermoset polymer matrix materials typically initiates with mold preparation [27; 28; 29; 30]. A mold is commonly made from aluminum alloys, and frequently manufactured using automated CNC machines [3; 23; 28]. Another common approach used in instrument making is the use of composite molds [17]. Basic types of manufacturing processes for musical instruments are open and closed molding. Open molding produces objects with an exposed surface, such as the soundboard of a stringed instrument [3]. On the other hand, closed molding produces objects with a closed surface in at least one direction (of the surface), as seen in the manufacturing of instruments like drums [16; 29]. The means of manufacturing and the lay-up process determine the design of the mold. Lay-up [31] is the manufacturing process of a composite product on a mold, typically performed layer by layer on the mold's surface by reinforcement fabrics. Several crucial parameters influence the lay-up process, including the type, the direction and inclination of fabrics and the chosen types, the area and the sequence covered by each layer. The choice of these parameters determines the mechanical properties of the lay-up. For instance, a specimen with a lay-up of UD0/twill/twill/UD0 may exhibit a significantly different bending behavior in the zero-degree direction compared to a lay-up of twill/UD0/UD0/twill. In the former, the UD is in the tensioning-compressing sides, while in the latter, it is closer to the bending neutral line. Hand lay-up is a common method in polyester parts, which lacks a debulking process but involves a hand roller to press the layers onto the mold surface, and ultimately yields suboptimal mechanical properties in the final product. The matrix impregnation is a pivotal factor in the manufacturing process of musical instruments, which achieves high quality results when prepreg materials are facilitated. Prepregs consist of reinforcement fabrics pre-impregnated with the matrix, remaining in a non-cured state. They find extensive usage in instruments manufacturing [3; 4; 16; 28; 32]. The vacuum-assisted resin transfer molding (infusion or VARTM, a subcategory of RTM) [12; 17; 23], stands out as the second-best process for producing high-quality composite objects. Both prepreg and VARTM processes necessitate the use of vacuum bags for pressing the lamination onto the mold surfaces. The use of a vacuum bag in the lay-up process consistently results in parts with a favorable reinforcement-to-fabric ratio, while the trapped air in the layers is removed. In thermoplastic matrices the most common way of matrix infusion is the injection of the matrix on a closed mold. The easiest process of resin infusion is the manual deposition of it in the lamination, while the curing method for thermoset matrices is crucial. Some resins cure at room temperature, but high-quality resins often require specific cure cycles in ovens with specific rates of temperature rise and cooling [23; 28]. Autoclaves [13; 16; 32] represent the most advanced version of ovens, providing controlled temperature and pressure conditions for an optimized curing process. Machining, joining, and shearing-forming follow for the manufacturing of every part and the assemble of the final instrument is completed. Figure 1 demonstrates the steps for the manufacturing of a violin top plate via a detailed lay-up procedure, while Figure 2 shows the lamination and VARTM processes during the manufacturing of a Greek bouzouki. Representative research works in the literature highlight the variety, the advantages, and the machinability of composites, especially when used in musical instrument manufacturing. [PERSON] et al. [15] proposed that carbon fiber-reinforced polyurethane foam (Uf) could serve as a viable replacement for spruce soundboards, contingent upon factors such as the direction of the UD carbon fiber layers, their lay-up stacking sequence, and the volume fraction of fibers to matrix.