# Applications of Digital Twin across Industries: A Review
## 3 Manufacturing
[64], a DT can provide nine types of product services which includes the service of (i) real-time state monitoring, (ii) energy consumption analysis and forecast, (iii) user management and behavior analysis, (iv) user operation guides, (v) intelligent optimization and updates, (vi) product failure analysis and prediction, (vii) product maintenance strategy, (viii) product virtual maintenance, and (ix) product virtual operation. The final phase of the product, i.e., disposal, is often neglected. Thus, the information which could improve the next generation product/system is often lost when the product is retired [67]. [PERSON] and [PERSON] [68] have proposed a novel DT-based system for the recovery of waste electrical and electronic equipment to support the manufacturing and manufacturing operations throughout the product's life cycle, from design to recovery. The product information models developed by them, from design to remanufacturing, were based on international standards. In addition to improving the manufacturing process through different phases of a product, a DT also offers benefits to the field of additive manufacturing (AM) [69, 70]. Creating a DT of a 3D printing machine can help us in achieving a product with desired properties by eliminating the need to do several iterations of trial and error tests which will, in turn, shorten the time between design and production, thus making the whole AM process time- and cost-effective [71]. The proposed model of DT for the 3D printing machine by [PERSON] and [PERSON] [71] consists of a mechanistic model, a sensing and control model, a statistical model, big data, and machine learning. [PERSON] et al. [72] used simulation, in situ sensing, and machine learning to create a DT of the AM process which could detect defects in AM parts during the process. Their DT model combined real-time data from in situ sensors with the physics-based model, along with a machine learning framework to predict flaws with higher statistical fidelity as compared to theoretical model-derived predictions or in situ sensor data alone. DT can also help in making the whole manufacturing process more sustainable and intelligent by making the whole process more autonomous and self-optimized, along with a network of manufacturing equipment, systems, and services that support each other [17, 73]. One of the most developed directions in manufacturing, especially in the scope of Industry 4.0 is the robotization and automatization of the production lines [74]. DT is playing a crucial role in this integration as industrial robots are being programmed with mainly three methods, which are closely related to the twinning of the manufacturing equipment. Those three methods are: (i) Offline, a dedicated virtual environment (usually each robot brand have their own) for programming each aspect of the robotic cell for later deployment through the network to the physical robot; (ii) Online, which is being adapted by means of sensor information, usually being twinned in the dedicated virtual environment (e.g., Robot Operation System (ROS)), and is able to directly affect the pre-programmed path and routine of the robotic systems; (iii) Manual, which is robot programming by the usage of a flex pendant, but with the introduction of Virtual Reality (VR) and Augmented Reality (AR) interfaces, it also uses a twin for manipulation near the virtual robot remotely [75]. As it is seen, combinations of those methods exploit the virtual twin of the robotic cell and are widely used across the various industrial sectors. Moreover, DT is used as a validation tool for human-robot collaboration (HRC) safety standards, to evaluate the safety level of the system first, for example, using VR human avatars, before experiments with real operators in the actual system [76]. DTs in the manufacturing industry are beneficial to future products even before the actual product exists. It helps in designing, testing, process optimization, predictive maintenance, product services, and proper disposal.