Abstract
Purpose: Few studies comparing manufacturing control systems as they relate to high-mix, low-volume applications have been reported. This paper compares two strategies, constant work in process (CONWIP) and Paired-cell Overlapping Loops of Cards with Authorization (POLCA), for controlling work in process (WIP) in such a manufacturing environment. Characteristics of each control method are explained in regards to lead time impact and thus, why one may be advantageous over the other.Design/methodology/approach: An industrial system in the Photonics industry is studied. Discrete event simulation is used as the primary tool to compare performance of CONWIP and POLCA controls for the same WIP level with respect to lead time. Model verification and validation are accomplished by comparing historic data to simulation generated data including utilizations. Both deterministic and Poisson distributed order arrivals are considered. Findings: For the system considered in this case study, including order arrival patterns, a POLCA control can outperform a CONWIP parameter in terms of average lead time for a given level of WIP. At higher levels of WIP, the performance of POLCA and CONWIP is equivalent. Practical Implications: The POLCA control helps limit WIP in specific áreas of the system where the CONWIP control only limits the overall WIP in the system. Thus, POLCA can generate acceptably low lead times at lower levels of WIP for conditions equivalent to the HMLV manufacturing systems studied.Originality/value: The study compliments and extends previous studies of CONWIP and POLCA performance to a HMLV manufacturing environment. It demonstrates the utility of discrete event simulation in that regard. It shows that proper inventory controls in bottleneck áreas of a system can reduce average lead time.
Highlights
Two strategies for controlling work in process (WIP) are Constant Work in Process (CONWIP) and Paired-cell Overlapping Loops of Cards with Authorization (POLCA)
The number of orders processed becomes equivalent at a CONWIP parameter value of 57 to those processed when the work-in-process is unconstrained with lead time equivalent to that achieved when the work-in-process in not constrained
Recall that the maximum WIP level is less than the number of POLCA cards due to the overlapping loops
Summary
Two strategies for controlling work in process (WIP) are Constant Work in Process (CONWIP) and Paired-cell Overlapping Loops of Cards with Authorization (POLCA). It is suggested that POLCA can have advantages over a KANBAN control system in a high demand variability [high-mix] environment (Kabadurmus, 2009) This tends to lend itself toward the HMLV manufacturing model where orders can be of varying size in varying frequency. One primary study comparing CONWIP and POLCA is by Germs and Riezebos (2010) who studied pull systems in make-to-order (MTO) production. They state that improvements in average total throughput time are due to the workload balancing capability of a pull system, but that many systems lack this capability. Germs and Riezebos (2010) suggest that a POLCA system could face longer shop floor lead times when compared to a similar CONWIP system. Kabadurmus (2009) had similar results when comparing POLCA and CONWIP in a hypothetical system
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