CONWIP: a simple method to improve planning

By Pascal Pollet – Sirris, Belgium

In a previous article, we discussed the many advantages of limiting Work In Progress (WIP). Reducing WIP helps shorten and stabilise lead times in production. Stable lead times make production more predictable and planning simpler. In short, they increase overall efficiency. Less Work In Progress is achieved first of all by releasing fewer orders.

In this article we present two more advanced, card-based methods for limiting WIP: Kanban and CONWIP.

Kanban: one card per item in production

CONWIP stands for CONstant Work In Progress and is inspired by the Japanese Kanban system. Kanban comes from the Japanese ‘kan’ (visual) and ‘ban’ (card or board). The system was developed by Taiichi Ohno at Toyota to increase efficiency.

The Kanban system controls the material flow by attaching cards to the materials. These cards contain at a minimum the necessary information about the material:

• Item number
• Quantity
• Internal or external supplier
• Consumption site
• etc.

The accompanying card is detached as soon as a material unit enters production. The card is then returned to the supplier or to the previous work centre. The arrival of a Kanban card is the signal for production or delivery of a new quantity of the item in question.

A big drawback is that the Kanban system is less suitable for production processes with more different products. As the product variety rises, the number of cards will also rise proportionally. That makes the system less efficient.

CONWIP: one card per amount of work

In a CONWIP system there is a limit on the number of products that are in the entire production process at the same time. This limit is enforced with CONWIP cards. Unlike Kanban cards, these cards are not product-specific, but represent an amount of work, corresponding to for example a number of items, a gitter box, a cart or a number of working hours. That means that the number of cards does not increase as more different products are produced.

When a product reaches the end of the production process, the accompanying CONWIP card is released and goes back to the first process step. Only then may a new order go into production. The card itself does not determine which product will be produced; the planning list with orders is consulted for this. The CONWIP card released is allocated to the next order.

In other words, CONWIP is a ‘pull system’. Work orders are only started when there is demand and WIP is also available. The orders are ‘pulled’ on the basis of availability of production capacity, and not ‘pushed’ by the process. That ensures a constant and processable workflow. Furthermore it helps against waste, overproduction, waiting times and a needlessly high inventory.

The biggest difference between the two systems is the function of the cards. In Kanban the cards stand for the stock of materials and products. In CONWIP the cards follow the products through the process. If no CONWIP cards are available, no new orders can be started.

Figure 1: Production line with 4 stations with a CONWIP system and 8 blue CONWIP cards. Four units of work are at the third station, indicating that this station is the bottleneck.