Discrete Event Simulation for Optimization of Codependent Assembly Lines

Abstract: This study is conducted at Scania, a globally leading company that manufactures trucks and delivers transport solutions to customers around the globe. In this study, the end flow of the chassi manufacturing was mapped in order to identify and eliminate wastes and bottlenecks, with the help of lean principles and discrete event simulation (DES) software. The assembly line consists of a make-to-order production head line that sends out semi-finished trucks to the end flow. The end flow is thereby codependent on the output from the head line. A synchronized tacted flow would minimize the wait in the stations, which is considered a waste in production, and enhance quality of work as deviations become more visible. The complexity of the production system has however made the department disregard the usage and implementation of simulations tools for improvement and decision making. The aim of this study was to evaluate the applicability and feasibility of DES as a tool for waste elimination, to increase throughput in a complex system and for investigating a better ratio between the takt times of the codependent assembly lines. This was done by developing future scenarios with different configurations of the model, studying how the parameters' behavior change in DES. Recommendations for reaching more effective codependent assembly lines with the help of DES were developed, analyzing the benefits and challenges of using simulation solutions for future decision making. The study offered useful insights and practical guidelines for companies with complex codependent assembly lines, wanting to use DES to implement lean principles. Five different improvement scenarios were tested and it was concluded that an increased number of buffer spots would have the largest impact on the throughput. An optimized takt time for a maximized throughput was also possible to identify with an enumeration method with scenarios. The drawback of DES for complex codependent lines was found to be the inability to consider all surrounding factors such as safety, ergonomics, quality and communication, which makes it a less applicable tool for waste elimination. The benefit of the DES simulation was the ability to manage complexities such as; different takt times between lines; gaps that occur in the production because of trucks removed from the head line; the interplay of the various inflows merging; the additional time to receive trucks from the reparation department; and stop times that cause waiting time at the production line. It was therefore considered an applicable tool for deciding the takt time for codependent lines.