Developing the bounded inventory level policy as an inventory control mechanism for reentrant lines

Rhey David S. Daway

Thesis (M.S. Industrial Engineering)--University of the Philippines Diliman.-2010


Manufacturing systems involving reentrant lines have become quite common especially in the semiconductor industry. A reentrant line can be thought of as a queuing network wherein work-in-process (WIP) materials (such as water lots for example) visit one or more workstations, or service facilities, several times before eventually leaving the system. Now with the different buffers waiting to be processed at certain service facilities, and considering the fact that most of the workstations have limited or finite capacities, the complexity of the reentrant-line problem lies in the difficulty of determining the scheduling mechanism which results in the optimization of the performance of the system.

Although several studies regarding the subject matter have already been conducted in the past, the intricate structure of the problem somehow limits the researchers from being able to formulate a general model for every class of reentrant-line systems. As such, the study proposes an inventory control policy, called the Bounded Inventory Level Policy (BILP), which attempts to strike a balance between the amount of setup times incurred and the variability in the internal flow process for the purpose of reducing the long-run average total holding cost per unit of output coming out of the reentrant-line system of interest. BILP, along with the Last Buffer First Serve (LBFS) sequencing rule, constitutes the scheduling policy under study. It was initially found via mathematical analysis and simulation that there are instances wherein BILP is superior to existing inventory control policies. Therefore, the further development and usage of BILP were justified.