METHODOLOGY
Kanban Method & Conwip Method - Operations and Benefits
11/05/2023
Reading time: 5min
The Kanban method
The Kanban system was theorized by Taiichi Ohno for Toyota. The goal was to enable the Japanese company to compete with American automakers.
The engineer starts from an observation of American supermarkets. He notes that each product has a specific place in the store and is restocked only when the product is purchased. He extends this model to the industry by imagining the kanban system which allows limiting the space allocated to each specific component in the factory.
One considers a factory consisting of production lines where the workstations are arranged one after the other so that the production flow is linear.
The specific Kanban method allows overlaying the production flow with a kanban flow, thus an information flow, which goes in the opposite direction of the physical production flow (figure below).
If we zoom in between two workstations, one must imagine that in front of each station there is a kanban production schedule (figure below). Each time station 2 consumes parts manufactured by station 1, it detaches the corresponding kanban tag and sends it back to station 1.
This tag is a production order for a container of parts corresponding to the kanban tag. Once station 1 has completed the production order, it attaches the corresponding Kanban tag to the container and places it in front of workstation number 2.
As long as there are kanban tags in front of the loading station, I produce. Otherwise, I do not produce. With the Kanban method, production is driven by downstream needs.
This means we reason in pulled flows.
How to determine which OF to complete first?
For this, a total Kanban index and a warning zone index are used.
1) The first indicates the total of each type of Kanban that can be absorbed.
2) The second indicates the warning zone beyond which production must be launched.
In the context of the figure below, the priority production is that of product C because it is the one closest to the warning zone and the total Kanban index.
The information found on a Kanban
Reference of the manufactured part
Container capacity (therefore quantity to be produced)
Address or reference of the upstream supplier station (here station 1)
Address or reference of the downstream customer station (here station 2)
Limits of the specific Kanban
Does not allow managing a large number of products and components.
Alternative to Kanban
The CONWIP method
The CONWIP method
If we take the specific Kanban method, station 3 pulls from the work in progress of station 2, which in turn pulls from that of station 1. The flow is pulled by the downstream.
However, this method does not adapt to sudden changes in production. Suppose we produce product A for a while. Suddenly, station 4 is informed that it must produce product B. It will take some time for the information to reach station 1.
The goal of the CONWIP method is to bring this information, which is available downstream, as quickly as possible to the beginning of the production chain. For example, once we receive information at station 3 that we need to produce product X, a kanban is sent to station 1 informing that we need to produce product E. This production will then be pushed to station 3.
To be more precise, rather than talking about a Kanban label, we talk about a CONWIP ticket. Each ticket is generic, meaning they are assigned to a specific OF.
Once this OF is completed, the ticket becomes blank again. If we go back to our example, once product X is completed at station 3, the CONWIP ticket becomes blank again and is ready to accept a new type of product to be produced (in our case product E). The ticket is then returned to the beginning of the line or to the previous buffer. This ticket then constitutes an OF.
As long as there is no blank ticket on the CONWIP board, the entry of new OFs on the line is not allowed. And we can segment the production line based on the identified buffers. This allows for multiple conwip flows on the chain.
The advantages of Kanban and CONWIP methods
These methods allow issues to be raised within the workshop. Instead of accumulating stocks, the Kanban solution suggests reducing them to reveal the areas in the workshop that are sources of disruption.
Information flows quickly within the workshop and between workstations (machine issues, breakdowns, defective parts...) and the dependency between workstations fosters cohesion within the factory.
Lead times and customer satisfaction are improved. These methods allow for delivering a bit of everything every day, thus adapting to customer demand, which is becoming increasingly volatile.
They promote the reduction of stocks, which leads to a decrease in cash flow and a gain of space in the factory and in warehouses, making stock management easier.
Reactivity is higher throughout the production chain, making it more flexible.
