## 1 Introduction
Various applications of six-bar linkages include a hand rehabilitation robot based on the Watt II linkage [7], leg mechanism based on the Stephenson III linkage [8], body guidance mechanism based on the Watt I linkage [9], and a furniture hinge mechanism based on the Watt I configuration [10]. Synthesis is typically the starting point of any mechanism design. For any given problem, the only thing known is the desired output or motion for some input. The design process usually involves the iteration between synthesis and analysis. So, to start the process, a potential solution is required to analyze, and then the solution is optimized based on the results. There are various available techniques for the synthesis of linkages to create potential solutions for mechanisms [11]. After the tentative mechanism is synthesized, the design is then analyzed to validate the linkage. The linkage is analyzed for the position of links and joints, at different inputs, their velocities, and accelerations. Linkages are also analyzed for dynamic forces to study the force propagation in the linkage, which can then also be used for stress analysis in the mechanism. Again, various analysis methods are available that can be used to analyze the linkage. A graphical approach can be used to draw linkages at various positions to determine position, velocity, and acceleration of output. Tools like CAD can be used to assist, but this approach involves a lot of work and can be very time consuming for complex mechanisms. Linkages can also be analyzed analytically by deriving general equations. Different approaches can be used to derive and solve these analytical expressions. Either algebraic representation or vector loop representation can be used to for the position analysis of a linkage [11]. Conventional/traditional manufacturing processes like forging and casting produce the highest quality parts in terms of surface finish and dimensional and geometrical accuracy. However, Additive Manufacturing (AM) technologies present tremendous opportunities and advantages over conventional processes [12]. AM provides the possibility to manufacture any design regardless of design complexity and allows for the production of integrated components. This is revolutionary towards wide open-mind designs without considering manufacturing constraints related to molding and machining. An improved globe valve design can be manufactured using additive manufacturing due to its capabilities to produce complex designs. From the study in [13], the additive manufacturing design of a globe valve showed the highest flow coefficient. This improved design can be used to manufacture valves with better performance. Pressure drop can be reduced with improved flow as a larger pressure drop creates the need for more mechanical strength in the valve body. Optimized pressure through better design such as optimized flow channels that can be reliably manufactured by AM can also reduce the overall weight of the valve. Layer by layer manufacturing in AM can enable us to manufacture a valve with structures like honeycomb, that are lightweight but still give the same mechanical strength as a solid part [14]. Valves can be tested on different internal structures to reduce the overall weight of the valve while maintaining its structural integrity. Additive manufacturing is potentially a motivating force in numerous industries towards advanced manufacturing initiatives [15]. The goal of this evolution is to produce better performing products with reduced costs and efficiency. Valves can be optimized by AM after improving their designs, which can result in better operating pressure ranges and improved flow rates through valves. Figure 1: Bellow valve working principle [4] (Reproduced with permission from [PERSON], Seals and Sealing Handbook; published by ELSEVIER, 2014).