CONWIP

CONWIP, short for Constant Work In Process, is a pull-based production control method used in manufacturing to regulate the total amount of work in the system. Unlike per-station signaling in traditional Kanban, CONWIP enforces a fixed global WIP level by circulating a limited set of CONWIP cards that authorize new work entry into the shop floor.

At the start, a fixed number of CONWIP cards are kept in a pool. When a job is released into the production line, a card is taken from the pool and attached to the job. As work progresses, the card remains with the job; when the job completes and leaves the system (typically after shipping to the customer), the card is returned to the pool, freeing space for another job to enter and maintaining the target WIP level.

Implementation choices vary. In a simple CONWIP system, there may be a single entry point for new jobs controlled by the card pool. Some deployments tie card release to the line’s bottleneck or choose different WIP targets for different product families to accommodate mixed-model production.

Advantages of CONWIP include simplicity of implementation, reduced work in progress, and applicability to mixed-model lines. It can be robust to demand variability and reduces lead times when well-tuned. Limitations include dependence on an accurate WIP target and line balance; if the bottleneck shifts or if throughput measurement is poor, performance can degrade. It also provides less granular control at individual stations than per-station Kanban.

CONWIP is a key member of pull-based manufacturing systems and is often contrasted with Kanban. It is used in just-in-time manufacturing settings where a global WIP limit and straightforward card-based release are advantageous.