A tight flow control for job-shop fabrication lines with finite buffers

We propose a work-in-process (WIP) estimation flow control method which serves as a countermeasure against the throughput degradation problem caused by the redundant blocking time of conventional flow control. This method is based on a scheduling technique of which the most important features are: 1) breaking down the entire schedule into individual lot schedules; 2) lot scheduling to reduce redundant blocking time; and 3) WIP estimation for contiguous finite buffer scheduling. The method, first, schedules operational lots at each equipment unit in a fabrication line by using our scheduling procedure for contiguous finite buffers to satisfy the limit capacity of the buffers. Next, the method estimates the future WIP at each equipment group based on predetermined schedules for performing operations. Finally, the method improves the operation timings by continuously supplying WIP estimation to the scheduling procedure. In an actual liquid crystal display (LCD) fabrication line simulation, we have confirmed that the proposed WIP estimation method is a promising one from the standpoint of the line throughput which we obtained.