## 2 The application of Additive Manufacturing (AM) in industry 4.0  
### An Introduction to AM  
AM processes are basically the processes that add some materials to the previous surface via different deposition techniques that lead to different part quality, density, and geometrical accuracy [22; 23]. The conventional processes are usually subtractive or a combination of several processes in case of complicated parts [24]. The major drawback of conventional processes is the high amount of material waste and lack of control systems to continuously modify the processes based on the current conditions. With the rise of computer-controlled machines, the latter problem is solved to some extent, but the material waste is still a challenge [25]. In the current era, which is also known as the fourth revolution of industry, Industry 4.0, it was decided to utilize the physical facilities with modern information technology [26; 27]. The goal of this integration is that the control over different manufacturing processes will reduce while it is possible to make the fabrication in fewer steps with less time and material waste leading to a higher benefit-cost ratio [28].  
All the AM processes are computer-controlled and it is possible to control an unlimited number of machines from a computer at once. The general procedure of all AM processes is that a layer of material is deposited, and this cycle continues to the point that the final 3D object is completed [29]. Some of these processes need post-processing and some of them make parts in net shape with the minimal processes needed to be done. Based on the materials used in a specific process, the source of deposition varies. The most common materials used in AM processes are polymers, engineering plastics, ceramics, metals, metallic oxides, and metallic alloys [30]. The feedstock is also available in different forms of solids and liquids, such as liquid polymers/resins, rods, wires, sheets, powders, etc. Depending on the used feedstock and its state of the material, different sources of energy are used, such as resistance heating coils, hot tubes, laser/ion/electron beams, ultrasonic vibration, and Ultraviolet (UV) light [31; 32; 33; 34; 35; 36].  
As mentioned, Industry 4.0 is a combination of information technology and highly controllable computer-driven machines [4]. AM machines of different types are such devices controlled by computers and the processes can be modified online with a single control unit [37]. As a result, this technology gives the opportunity to integrate many machines in a factory and control them online. The outcome of this combination is that a user-specific product can be produced within each machine [5]. This flexibility in the manufacturing of different products at the same time with the almost unlimited level of complexity provides the opportunity to utilize AM machines as an inevitable part of the modern manufacturing era. There are some terms used in this category of which rapid prototyping, rapid manufacturing, three-dimensional (3D) printing, smart manufacturing, and cloud manufacturing are the most used [38]. Cloud manufacturing refers to the processes that are highly service-oriented and can be modified online [6]. In order to clarify this process, a customer orders the desired geometry to be purchased. After accessing the design tools provided by a factory, they can change the materials, colors, and other aesthetic features of their desired product and at the same time, they can check the availability of the materials, machines, and the transportation systems. By checking all the items, the customers can easily upload their designs and receive their specific and unique product [39]. Figure 2 represents the collaborating segments in a cloud manufacturing scheme.