## 1 Introduction
Three-Dimensional Pre-Printed Random-Carbon-Fiber-Reinforced Plastic--3D-PPRCFRP: Traditionally, lightweight hydraulically actuated components are manufactured with carbon fiber composite material. Several researchers have worked on developing hydraulic cylinders as an example of a load-bearing part. These components must support high forces due to the exerted pressures. Generally, lightweight hydraulic cylinders are manufactured with a fabric of carbon composite fibers, and epoxy resin is placed on them with the help of a vacuum bag. This methodology only applies to parts with regular geometrical shapes, such as cylinders and plates. Adding to that, fiber-reinforced composites for load-bearing parts entail the need to investigate anisotropy. The fibers' directions in the layers of the carbon fabric define the part's capacity to withstand force. Higher strengths are achieved in the direction of the fiber, whereas lower ones are shown in the perpendicular direction. In 2018, [PERSON] et al. proposed the 3D-PPRCFRP (3D Pre-Printed Random-Carbon-Fiber-Reinforced Plastic) methodology. This method enables the production of integrated hydraulic robotic parts by combining additive manufacturing and carbon composite particles to promote a quasi-isotropic material. The process begins with printing a shell-shaped structural robotic part in plastic with internal passages for oil and electrical wires. The shell-shaped part is then infused with randomly oriented carbon composite particles to increase its ability to withstand high pressures and forces. Figure 2 shows the second generation of HYDROiD's arm, manufactured using this developed technology, with the printed shell part comprising three layers and a total of 14 internal oil passages. The process led to reducing the weight of the robotic parts by 85%, significantly reducing manufacturing time and cost [11]. The combination of additive manufacturing and composites resulted in a ground-breaking technology that provides a cost-effective way to produce hydraulically integrated components, overcoming the limitations of previous technologies. The 3D-PPRCFRP method has the most advantages compared to conventional and additive manufacturing methods. It allows the development of complex-shaped parts through plastic additive manufacturing, while the carbon composite particle material contributes to the lightweight characteristic. This process does not require post-processing, which decreases manufacturing time and cost. For these reasons, the 3D-PPRCFRP is adopted. Carbon-reinforced plastics contribute to the reduction in the weight of mechanic components. Firstly, lightweight materials improve energy efficiency by reducing the  
Figure 2: HYDROiD’s arm manufactured with 3D-PPRCFRP ([PERSON] et al. [11]). The printed shell part has three layers of tubes. It is then molded with carbon composite materials. power required to move the device. This has a positive effect on extending the battery life, which increases autonomy. Secondly, lightweight humanoids promote safety; the lighter they are, the easier they are to control, which reflects positively on risk reduction and cost optimization. Despite being lightweight, carbon-reinforced materials are also strong, which can be translated into a high strength-to-weight ratio. Consequently, carbon composite material offers high design flexibility and the capacity to develop complex shapes through molding, optimizing the device's form and weight. The 3D-PPRCFRP approach holds great promise for developing lightweight hydraulically integrated robotic parts. However, technical challenges are still associated with its implementation. One of the main limitations lies within the molding process. The curing process of the resin undergoes an exothermic reaction to harden. In this phase, voids can be formed in the mixture. Voids weaken the molded structure; therefore, they should be kept minimal. The problem can be solved in prototyping by molding small batches and performing multiple degassing processes in negative pressure chambers.