Workload Balancing Capability of Pull Systems in MTO Production

Pull systems focusing on throughput time control and applicable in situations with high variety and customization are scarce. This paper compares pull systems that can cope with such situations: POLCA and CONWIP. Both systems constrain the maximum amount of work in progress (WIP) within a loop, but their effectiveness in terms of reducing total throughput time is questioned. They mainly differ in the way routing variety in the shop is being handled. POLCA uses an authorization mechanism that uses route-specific capacity signals (POLCA cards). These signals level the maximum amount of WIP within a loop, where each loop connects two route segments. A CONWIP loop covers two or more operations. Several CONWIP loops can operate simultaneously, resulting in an m-CONWIP system with controlled release based on the actual progress of jobs on the shop floor. Theory states that an improvement of the average total throughput time will be due to the workload balancing capability of a release mechanism, but that many release mechanisms lack this capability. This paper shows this workload balancing capability to exist for POLCA and m-CONWIP, but not for CONWIP. The magnitude of the effect strongly differs, depending on the number of cards, utilization level, order arrival patterns and processing time variety of the orders.