Balancing a robotic spot welding manufacturing line: An industrial case study

Resistance spot welding is an important procedure used in the automobile manufacturing industry. After stamping, sheets of metal are mostly welded together to build the car's body. Many of the industrial robots found in assembly lines are spot welders. Process characteristics include accessibility limitations, the need to move between welding points and (when there are multiple robots per station) potential interferences between robots. The combination of such characteristics is not present in classical models for the assembly line balancing problem. In this paper, the balancing problem of robotic spot welding manufacturing lines is presented and modeled based on a real-world car factory on the outskirts of Curitiba, Brazil. The studied line has 42 robots that perform over 700 welding points on the later stages of the body-shop. The model was developed with Mixed Integer Linear Programming (MILP) techniques, validated with empirical data, and solved with a universal solver. The optimized balancing achieved a cycle time reduction of up to 6.6% compared to the as-is configurations. The total movement time was observed not to be necessarily minimized during the optimization process, implying that trade-offs exist between movement times and the number of welding points performed.