CWIPL II, a mechanism for improving throughput and lead time in unbalanced flow line

Abstract

Critical WIP loops II (CWIPL II) is a proposed material flow control mechanism for an unbalanced flow line environment. CWIPL II is based on CWIPL and it determines critical loops in unbalanced lines. The WIP of critical loops identifies the time of releasing raw material to the line. CWIPL II proposed a new classification for unbalanced flow line which is ‘near unbalanced flow line’ and ‘perfect unbalanced flow line’. In near unbalanced line, there is one bottleneck and a raw material release to the line if ‘WIP of the bottleneck’ or ‘WIP upstream the bottleneck’ is less than defined level. In perfect unbalanced line there are multiple bottleneck and a raw material release to the line if ‘WIP upstream the slowest machine’ or ‘WIP between two primary bottleneck’ is less than defined level. Like CWIPL, the necessary condition for releasing the raw material is ‘idleness of the first machine’. CWIPL II is compared with CONWIP and TOC by simulation. Different scenarios are employed in the comparison analysis. The scenarios address variables such as number of machines, processing time distribution, WIP target level. Location of slowest machine and location of two primary bottlenecks are considered in examples. Simulation results and statistical tests of 141 numerical examples show that CWIPL II improves lead time in near unbalanced line and throughput in perfect unbalanced line compared with TOC. Because of the trade off between line throughput and lead time, the mechanism that improves one of them while maintaining the other at previous level is valuable. It is shown that CWIPL II has improved TOC in the cases that TOC hasn’t improved CONWIP.