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Lean Production6 min read

Kanban System Design: Implementing Pull Production

This article offers a compelling account of a production line's near-halt due to an empty material staging area, despite MES showing sufficient stock. The author discovered materials were moved by a previous shift, revealing critical flaws in their 'push production' model where products are pushed to the next station regardless of demand. This experience underscores the hidden problems of such a system and introduces the profound concept of 'replenishing only when shelves are empty' in lean manufacturing.

Replenish Only When Shelves Are Empty? That Day, Staring at the Empty Shelf, I Had an Epiphany.

Do you know? Early last month, our production line almost had a major incident. At that time, the Mura machine suddenly ran out of wafers, and the entire line came to a halt. Engineers, OPs, and the section chief all looked at each other. I rushed over to check, and damn it, I found that the material staging area for the preceding process was empty, yet the MES still showed sufficient quantities. Upon investigation, it turned out that the night before, an OP from the previous shift, to rush a shipment, had diverted materials originally intended for Mura to another machine. As a result, Mura couldn't get the materials, running idle for nearly half an hour. At that moment, I thought to myself, "Damn, our production model is fundamentally flawed!"

Where's the Problem? The Daily Annoyances of Push Production

To put it bluntly, our usual production model is often "push production." What does that mean? It means that once I finish making something, I directly "push" it to the next station, regardless of whether the next station needs it or has space for it. Like the situation I just described, the preceding process produces materials, stacks them in the staging area, and then the next process "assumes" there are materials available. Only when the machine actually needs them, it's discovered that the materials have been moved.

What does this type of push production often lead to?

  1. Inventory Accumulation: You make a bunch, I make a bunch, resulting in a pile of work-in-progress (WIP) on the floor, taking up space and increasing management difficulty.
  2. Outdated Information: The quantities displayed on the MES often don't match the actual quantities on the shop floor. By the time you discover a material shortage, it's too late.
  3. Lack of Flexibility: If just one step goes wrong, it creates a domino effect, bringing the entire line to a halt.

So the key is, we need a smarter production method that allows demand to guide production, instead of production accumulating inventory.

How to Actually Do It? The Pull Magic of a Kanban System

This is where Pull System comes into play. The most classic implementation is the "Kanban System." What is Kanban? You can imagine it as a "signal" that tells the preceding process: "Hey, I'm short on materials here, hurry and replenish me!"

How is it done?

  1. Define Your 'Standard Container': Suppose a material box can hold 25 wafers. We stipulate that each replenishment means replenishing one material box.
  2. Set the 'Kanban Quantity': This is crucial. You cannot replenish indefinitely. Suppose we find that the Mura machine consumes an average of 50 wafers per shift. To be safe, we set "2 Kanbans." This means that the material staging area for the preceding process can hold a maximum of 2 material boxes filled with wafers.
  3. Activate the 'Pull' Mechanism: When the Mura machine takes a material box, the OP will then place this empty material box, along with its "Kanban card," into the designated "empty material box collection area." The OP from the preceding process sees a Kanban card in the collection area and knows: "Okay, Mura needs one material box; I need to produce one material box's worth to replenish it now."

See, isn't the whole process similar to buying a drink at 7-Eleven? The staff only restocks when the shelf is empty, rather than continually piling items onto the shelf just because the warehouse is full. This is the core spirit of "pull" production. Through this mechanism, we've been able to reduce our work-in-progress inventory on the floor from 5 days' worth to 1.5 days' worth, a very significant effect.

The Most Common Pitfalls: Inaccurate Numbers and Human Intervention

Speaking of Kanban, the most common pitfall I've encountered is "inaccurate Kanban quantity setting." Some, fearing material shortages, directly set 5 Kanbans—how is that any different from push production? Inventory still piles up. Others, fearing inventory, set only 1 Kanban, resulting in material shortages with even slight production fluctuations. So, how should this "Kanban quantity" be determined? You need to consider several factors:

  1. Average Consumption Rate: How much material does this station consume on average per day?
  2. Preceding Process Lead Time: How long does the preceding process take to produce one unit?
  3. Safety Stock: How much do you need to reserve to cope with occasional abnormalities?

These all require data support. For example, if our Mura machine's Cpk is only 1.08 and DPMO is as high as 6210, it indicates that its stability is insufficient, and the Kanban quantity cannot be set too tightly.

Another pitfall is "human intervention." Just as I mentioned at the beginning, an OP moved materials to rush a shipment. Such "bypassing the rules" behavior directly disrupts the balance of the Kanban system. Therefore, after the system is established, discipline and training in its execution are even more indispensable.

One Thing You Can Do Today

Break down your process, identify the bottleneck point that most frequently runs out of materials, and try to define its "standard container" and "maximum inventory level."

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