The Role of Work-in-Process Inventory in Serial Production Lines

In serial production systems, storage may be provided between processes to avoid interference due to lack of synchronization. This paper investigates the behavior of lines buffered in this way and explores the distribution and quantity of work-in-process (WIP) inventory that accumulates. We study simple, generic production systems to gain insight into the behavior of more complex systems. The authors are surprised by the sometimes counterintuitive results, but are joined in this surprise by both academics and practitioners with whom the study has been discussed. Results are presented for: identical workstations with and without buffers; balanced lines in which variability of processing times differs between stations; unbalanced lines; and lines with unreliable workstations. In general, buffers between workstations increase system capacity but with sharply diminishing returns. Position as well as capacity of the buffers are important. These results are preliminary, to be confirmed and extended by further study—indeed, a primary purpose of this paper is to stimulate such study. However, even these preliminary results yield design guidelines that should be useful in industrial practice.