Due to the various demands from global customers, job shop production systems play a more and more important role in the industry. However, in a job shop production system, the product type may be diverse, and the manufacturing process and operation time are quite different. It would make it difficult for management and increase work-in-process (WIP). Kanban is a tool that is used in the pull system to implement lean production. It can translate customer requirements into production signals to all workstations and then control the number of WIPs. However, it is commonly seen in flow shop production systems. CONWIP (Constant Work-In-Process) is a pull system that controls WIP levels through Kanban. However, different from the typical pull systems, CONWIP emphasizes controlling the total WIP level of the entire system, rather than the number of work-in-process items at each workstation/process within the system. Although CONWIP can effectively control WIP levels, when there are significant differences in production processes and operation times across products, bottleneck shifts may occur during production, thereby reducing the efficiency of the production system. This study aims to optimize production scheduling within a CONWIP system to minimize makespan. This involves controlling the WIP level through the CONWIP mechanism and maintaining production efficiency by minimizing makespan. To this end, this study proposes a mathematical model for scheduling optimization within a job shop production environment under the proposed CONWIP system, aiming to minimize makespan. Based on a simple case study, the mathematical programming method proposed in this study can provide optimal scheduling within a reasonable computation time by using the commercial software LINGO 17. Furthermore, compared to the results without considering CONWIP, the schedule generated by the proposed mathematical programming, while having a higher makespan, effectively controls the WIP level. Therefore, the method presented in this study can indeed simultaneously maintain production efficiency and control WIP levels for job shop production industries.

Keywords

Cellular Production; Operator assignment; Mathematical Programming; Cell Loading; Operator sharing