Balancing of mixed-model two-sided assembly lines with multiple U-shaped layout

Two-sided assembly lines are a special type of assembly lines in which tasks are assigned to workstations placed on both sides of the assembly line. This type of assembly systems are used for manufacturing systems with large-sized product such as cars, buses, and trucks. According to the principles of just-in-time production, it is deduced that there are copious benefits associated with U-shaped assembly lines in comparison with traditional straight line system. Nevertheless, implementing U-shaped layouts in two-sided assembly lines do not significantly increase the efficiency and flexibility of the workstations placed outside of the U-shaped layout. Hence, a novel multiple U-shaped layout is proposed in this study to deal with the mixed-model two-sided assembly line balancing (MTALB) problems. A mixed integer programming formulation is developed to model such manufacturing systems in which two conflicting objectives including minimizing the cycle time and minimizing the number of workstations are considered under precedence, zoning, capacity, side, and synchronism constraints. Since MTALB problems are in NP-hard class of combinatorial optimization problems, a heuristic algorithm based on genetic algorithms is developed to solve the MTALB problem. Three test-bed problems are also used to prove the usefulness and applicability of the proposed framework.