Can material bins themselves serve as kanban signals?
A process A is consuming part X which is produced by Process B. Process A will have bins containing the kanban number of part X and there will be a kanban card attached to the bin. This kanban card will have various details such as part name and description, the making process, the consuming process, the kanban quantity etc.. So once the quantities have been consumed from that bin at process A (depending on replenishment lead time, they might wait for all quantities to be consumed or a certain number from the bin), the kanban card (say card#1) attached to that bin will be taken and placed at the upstream process, and another bin of part X will be brought along with the kanban card attached to that bin (card#2). At the upstream process, the operator will see that card#1 is lying and this will signal production for the number of quantities as mentioned in the card.
Now instead of the physical kanban cards, it could be quite possible that the empty bins or trays that carry the material can themselves serve the signal for replenishment or production. For example, whenever process A has consumed all of part X from its bin, it can take the empty tray and place it at process B and can get a tray full of part X back for consumption. Now when the operator at process B sees an empty tray, this can signal for production to happen for part X at process B.
In the above case, the two processes can just use empty trays to signal further production and material movement. Of course, for this to work, process A and process B need to plan it out and adhere to the plan. For example, they need to agree on what is the kanban quantity and that process B will produce only to the kamnban quantity, they also need to agree that process B will produce only if there is an empty cart lying in its area, they also need to agree that process A will pull material from process B only when it has emptied its current kanban tray of X parts and that will will leave the empty tray at process B whenever it is pulling a new tray.
So while the above mechanism might not work at a large plant level, in localized processes, such empty trays or bins can serve as kanbans themselves and not need a physical kanban card to be attached the bins. Let me know what you think.