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The constant Work in process (CONWIP) method has emerged as an improvement of the Kanban and MRP, mixing the two.

Introduction

The CONWIP is an alternative to the derived stream Kanban. It was introduced in the years 90 by a team of researchers piloted by M. L. Spearman1. This system allows the problems of the specific Kanban to be fixed in a small series production context and many references.

Description of the operation

The figure below illustrates the difference in circulation of physical flux and information flux in a spécifique Kanban and in the CONWIP method.

In the approach of Specific Kanban, the flow is pulled downstream. Position P3 picks up in the in-progress P2, which itself picks up in the course P1.

In the CONWIP approach, the flow is always pulled downstream. But instead of being reassembled from Position to post Position, the Kanban is reassembled directly to Position . P1.

Interest of the CONWIP

The CONWIP allows the Specific Kanban When a production line has to produce many types of products, or when there is a strong difference in consumption between different products.

Case 1 : Many types of products

With a Specific Kanban, each Position must have the in-progress of each product reference. So the more product references you have, the more you’re going to have in-stock, which is obviously not the goal.

So, from ten different product references, it is better to set up a CONWIP.

Case 2 : Strong variability between the demand for each reference

Another example is that you have to produce 5 different product types several times a day except one, whose demand is random, about once a month.

The time limit for the information to go back to Position 1 will be long as all the Positionare consuming the other products and the information goes downstream.

The CONWIP allows the information to be traced directly to Position 1 to save time on production. It will also help to avoid having stocks of in-progress, obligatory with a specific Kanban, on a product seldom consumed.

CONWIP limit

The limit of the CONWIP occurs when the operating times are not balanced between the positions. In fact, let’s imagine that Position 2 is a bottleneck Position . With a CONWIP, the information will go directly from Position 3 to Position 1. Position 1 will produce more quickly than Position 2 and will generate an important in-progress between Position 1 and 2.

In this case, it is advisable to set up a generic Kanban.

Sources

1 -M. L. Spearman, D. L. Woodruff, W. J. Hopp (1990)-CONWIP: A pull alternative to Kanban

A. Dolgui, J. M. Proth (2010)-Supply Chain Engineering: usefuls Methods and Techniques

M. J. Schniederjans, J. R. Olson (1999)-Advanced topics in Just in Time Management

P. Poirier (1997)-Production just in time with the CONWIP method: Comparative Simulation study

M. Pillai, C. M. Bonnafous, P. Bonnafous, A. Courtois (2011)-Production management, fundamentals and good practices

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