The kanban system - 看板 (from Japanese, kan = "visual," ban = "tag") is a decoupled pull flow control tool that provides for materials management directly from the shopfloor-that is, from the area where production takes place. In theory, it does not require any software for its management, although today it is almost always associated with a computer program in order to achieve greater control, flexibility and reduction of non-value activities.
Concretely, the kanban takes the form of a tag attached to a container that communicates material demand to suppliers, internal or external. The tag typically carries the most important information about the material and the container with which it is associated: for example, the product code, description, the supplier from which it arrives, the customer for whom it is intended, quantity, and lead time.
It is usually used in three areas:
- purchase kanban: used to request code purchase from the supplier;
- production kanban: used to request production of the code from the upstream stage to which it is linked;
- handling kanban: used to pick up and move material from a centralized warehouse to the place of use.
Preliminary concepts: decoupled flow
One of the key metrics when redesigning a process is lead time, which is the time interval between two events. It is especially important to compare the lead time required by the customer with the process lead time:
- when the lead time demanded by the customer is greater than the process lead time the two can be synchronized and the flow of transformation of goods take place in the same sequence as the demand (coupled flow);
- when the lead time demanded by the customer is less than the process lead time the latter must be decoupled from customer demand and the management of process outputs occur through accumulation and inventory (decoupled flow).
The kanban is a tool for decoupled flow control.
Size the kanban
For the kanban to be effective, it must be sized by determining for each code under analysis the number of containers that must rotate (empty, fill, empty, ...) in the system.
In the literature there are multiple formulas for doing this. The following one is probably the simplest and most practically applicable:
Where:
- #KBis an integer number of containers, hence the need to round up the first part of the formula;
- Cis the maximum consumption during the process lead time-in pieces/units of time (typically days)-that the system will incur. Overconsumption will result in overstocking, while underconsumption will lead to stock-outs. Assuming that no kanban will shelter against extreme spikes in demand, to determine C a good practice is to start with the historical consumption series - possibly corrected with appropriate up- or downward coefficients and "trimmed" to a certain level of service - and derive its moving average over the LT lead time of replenishment, so as to absorb changes in demand over the period;
- LTis the replenishment lead time, which is how many days it takes the internal or external supplier to restore the component to supermarket;
- LTSis the security lead time, which is how many additional days of coverage you want to have to protect yourself from any supply problems;
- Qis the quantity of pieces per container. In practice, in fact, a kanban stockpile is nothing more than a reorder point - ROP=CxLT - divided into many containers small enough to facilitate stock rotation and large enough to make handling convenient;
- The "+1" represents an extra container to be added to the sizing according to the fact that the first kanban will be "detached" when the first container is emptied. It is used to keep the process fed before the first consumed container is restored.
Kanban types
Kanban batch - batch of tags
The tag batch, also known as kanban batch, is a system used when material reordering cannot be done according to the quantity indicated by the individual tag. This may depend on multiple factors:
- material requirement during the restoration lead time too low compared to the supplier's economic production batch;
- excessive number of orders that would come off to the supplier with a release at each tag (order cost, EOQ);
- need to create smaller and more numerous containers (e.g. better subservience of codes shared among multiple lines) without revising supply parameters such as minimum reorder quantity.
Signal kanban
The signal kanban involves a single reset tag, which is placed at a certain stock level called the reorder point - reorder point. When the stock of material falls below the reorder point, a request for material restoration is made.
The signal kanban can be used when:
- the production/purchase batch is very large compared to the consumption and it is not possible to take action to reduce it;
- if #KB'is less than 2, where #KB' is the number of kanban not rounded up to the next integer.
Double bin
It is the "simplest" form of kanban. It consists of two containers and is characterized by the absence of any real information flow--the empty container constitutes restoration order.
This system is typically used for low-value, high-turnover codes, where the stock value does not justify frequent resizing and is usually oversized.
Kanban system advantages
The advantages of using the kanban system are many:
- makes stock breaks less likely;
- significantly reduces stocks compared to push logic;
- enables quick responses to changes in demand;
- improves escort accuracy;
- simplifies scheduling, reducing the use of MRP.