Definition
A signal kanban is a special type of production kanban used for processes that produce in batches rather than one piece at a time — typically stamping presses, injection molding machines, or other operations where changeover time makes one-piece flow impractical. The signal kanban is usually triangular in shape (hence its Japanese name, sankaku kanban — “triangle kanban”) and is placed at a specific point within the supermarket inventory. When the inventory level drops to the point where the signal kanban is exposed, it triggers the batch process to perform a changeover and produce a replenishment lot of that part number.
Japanese Origin
Sankaku kanban (三角かんばん) literally means “triangle kanban” — 三角 (sankaku, triangle) describes the card’s distinctive triangular shape. The shape is not arbitrary: a triangle is visually distinct from the rectangular production and withdrawal kanban cards, making it immediately identifiable on the shop floor. Some Toyota literature also calls this the lot-making kanban (ロット作りかんばん) because it authorizes the production of a full lot rather than a single container.
History at Toyota
The signal kanban addresses a practical reality of manufacturing: not every process can produce one piece at a time. Stamping presses, for example, require die changes that take time even with SMED improvements. It is not economical to change dies after stamping a single part. Toyota needed a mechanism to manage batch production within a pull system — and the signal kanban was the solution.
The signal kanban works with the supermarket concept. Rather than stamping parts based on a schedule (push), the stamping process produces a batch only when the supermarket inventory of a specific part drops to a predetermined trigger point. This maintains the pull principle even in a batch environment.
How It Actually Works
Setup:
- The supermarket for a batch process contains containers of parts stacked or arranged in sequence
- A signal kanban (triangle card) is placed at a specific position within the stack — for example, at the third container from the bottom
- The position represents the reorder point, calculated based on the time needed to perform a changeover and produce a replenishment lot
The trigger sequence:
- Downstream processes withdraw parts from the supermarket, container by container
- As containers are consumed, the inventory level drops
- When the level drops to the container where the signal kanban is located, the triangle card is exposed
- The signal kanban is sent to the batch process as authorization to produce a replenishment lot
- The batch process performs a changeover to that part number and produces the specified lot quantity
- Completed parts replenish the supermarket, and the signal kanban is placed back at the trigger point
The reorder point calculation considers:
- Downstream consumption rate during the replenishment lead time
- Changeover time for the batch process
- Production time for the replenishment lot
- A safety factor for variation
Common Mistakes
Setting the reorder point too low. If the signal kanban is triggered too late, the downstream process runs out of parts before the replenishment lot is ready. The reorder point must account for the full replenishment lead time including changeover.
Not reducing lot sizes over time. Signal kanban is a compromise — it accommodates batch production within a pull system. The improvement direction is always to reduce changeover times (through SMED), which enables smaller lot sizes, which reduces the inventory needed in the supermarket. A static signal kanban system without ongoing changeover improvement is a system that has stopped improving.
Using signal kanban where one-piece flow is possible. If the process can produce one piece at a time with minimal changeover, standard production kanban should be used, not signal kanban. The signal kanban is specifically for processes where batching remains necessary.