Withdrawal Kanban
Kanban is a system that serves the purpose of connecting parts of the value stream where one piece flow cannot take place. It is in fact an information that flows from the customer (internal or not) to the supply process about what parts are needed and where. This creates a pull, making sure no waste of overproduction is generated while leveling demand to the downstream operations.
This system empowers management at the gemba, providing a standard process of communicating demand and managing work in process inventory making any planning unnecessary, except for the most upstream process.
There are two basic types of Kanban: the Withdrawal Kanban provides information of parts that need to be moved, while the Production Kanban provides the instruction to produce what has been consumed.
Withdrawal kanban authorizes the movement of material from storage to the point-of-use. As materials get consumed, the pull system (ideally in the form of a Kanban card) is released and gives instructions to the material handler to replace it from its store location.
It doesn't really matter weather the supplier-customer are two consecutive processes in a manufacturing location, a warehouse feeding parts to production or a supplier delivering to a customer at a different location, the functioning of Kanban card is identical.
There is often a strong temptation to use containers instead of cards as pull systems. While Kanban cards have a strong argument in their favor: they can be detached from the container and that may become critical as one tries to improve productivity of the material handler, allowing an easy adjustment of the withdrawal sequence at the store location; containers may be harder to lose. One final argument is how easy it becomes to make adjustments to the system by simply replacing cards.
Withdrawal Process
Card Pull is a system that uses production pull cards to authorize movement of parts. As a container of parts is used, the pull card is removed and placed in a holding post/box where it can be reached by a material handler. As the material handler shows up at the use location(s), he removes empty containers and cards at the holding post(s). While he moves to the storage location(s) he disposes the boxes at its predetermined location. At the supermarket, he sequences cards to match the optimum retrieval of containers from storage.
Using the location indicated at the cards he removes each corresponding container. If containers are stored with their production Kanban, those are placed at a holding post at the supermarket and the withdrawal Kanban is placed on the box. Containers with their corresponding withdrawal Kanban cards are placed in their designated place on the transportation cart.
The material handler moves back to the delivery location(s) and places containers at the locations indicated by each Kanban card, completing the cycle.
In case the pull system is a container, the process is exactly the same, where information for the handler is fixed to the container. There is obviously no need to detach cards from containers. Empty containers have to be disposed after being replaced by full containers as the required information is fixed to the box, therefore the sequencing of retrieval is severely jeopardized with wasteful motions.
Container pull are then used when storage lot size is too big to be placed at the work (user) location. In this case, the container is filled at the storage and returns to the point of use without being disposed of.
Setting up the System
It is fundamental that a fixed location storage system (supermarket) is installed to support the pull system. In association, a clear “address system” is expected to be placed so the exact locations can be easily found by the material handler, both at the point of use and storage.
Finally, Kanban systems are tied to the concept of a standard material handler routine that guaranties frequent, systematic routes that deliver materials in a consist pace throughout the working shift.
Number of Cards
The number of cards (or containers, in case of a container pull) is determined by the time of the complete cycle of activities described above (t); the consumption at the point of use at that time (U); the capacity of the container (Q); any safety margin one decides to add for imbalances in the processes involved(S):
Cards = (Ut)/Q + 1+ s
An extra card is needed to assure the point of use is never left with no parts.
Card “design”
The layout and the size of the cards are not relevant, it’s the information for the user, in this case, the material handler, that is critical so as a minimum the card would state:
- Exact storage location;
- Exact point of use location;
- Part ID;
- Quantity of units or type of container.
Often it will also state the card number and the total number of cards for that particular part as a way to audit cards.
Summing Up
As technology evolved, Kanban systems are now often electronic, with cards being replaced by electronic signals. In any case, the fundamentals remain the same: to pull from customer demand to its supplier, based on real consumption.
As companies start to use Kanban to drive pull throughout their value stream, physical forms of Kanban are most beneficial as a learning process for the whole organization. As maturity evolves, and flow between processes improve, demand levels out and commitment to improvement becomes unquestionable, other less visual Kanban systems may become a viable option.
There is often a strong temptation to use containers in exchange for cards as pull systems, but arguments are also often that "cards get lost easily”, and one would question if the problem is really being addressed of "is waste generating more waste".
The fact is that withdrawal Kanban is quite easy to implement in itself but it's part of a more sophisticated system as it relies on supermarkets and delivery routes for it's set up.
It is important not to lose sight of the great benefit on managing flow at gemba, shortening lead time and leveling demand for optimum productivity and service levels.