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Knowledge Base/SMED Changeover Time Analysis: Internal vs. External Operations
Lean Production6 min read

SMED Changeover Time Analysis: Internal vs. External Operations

This article addresses the challenges faced by production line engineers regarding equipment utilization and lead times, particularly when mold changes cause significant delays. It explains why changeovers often get stuck due to mixing internal and external operations and introduces the core concept of SMED (Single-Minute Exchange of Die) as a method to significantly reduce changeover times by performing tasks while the machine is running.

The CPK Report Came Out, The Room Fell Silent For Three Seconds, And The Production Line's Mold Change Got Stuck Again!

Damn, you know that feeling? You've been incessantly lectured by your boss about improving equipment utilization and shortening lead times, only for a machine changeover to get stuck for another 3 hours! Just last week, the boss mentioned in the weekly meeting that our machine changeover time was 40% longer than competitors, urging us to find a solution. And what happened? That afternoon, the production PM called, frantically saying, "Engineer, the machine is stuck again! We've been trying to change the mold for a new product for almost two hours, and it's still not done. The client is pushing us, come take a look quickly!" At that moment, only one thought was in my head: "What the hell is SMED even for?!"

What's The Problem?

In plain terms, you're doing everything all at once! SMED (Single-Minute Exchange of Die) might sound trendy, but its core concept is incredibly simple: shorten changeover time as much as possible. In the past, when we changed lines, we would dismantle the old mold, install the new one, and then start adjusting parameters and test running. The problem is, many actions can actually be completed while the machine is running, without needing to wait for the machine to stop before frantically scrambling.

For example, when you usually drive out, you'd fill up the gas, check tire pressure, and set your navigation route before starting the engine and heading off, right? You wouldn't wait until you're halfway through your drive to say, "Hold on, I think I'm running low on gas, I'll pull over to refuel." Right? The same logic applies to changeovers. So the key is to differentiate between "internal operations" and "external operations."

How To Actually Do It?

"Internal operations" are tasks that "can only be performed when the machine is stopped," such as dismantling old molds and installing new ones. "External operations" are tasks that "can be performed while the machine is running," such as preparing new tools, checking the condition of new molds, and material preparation, etc. Our goal is to move all tasks that can be converted into external operations to be completed before the machine stops.

How to differentiate? It's simple.

  1. Record the process: Record the entire changeover process from start to finish. Don't think it's troublesome; this step is the most crucial.
  2. List out actions: Break down the recorded content into individual small actions, the more detailed the better.
  3. Determine internal/external: For each action, ask yourself one question: "Can this action be performed while the machine is running?"

* If the answer is "Yes," then it's an external operation. For example: cleaning spare molds, preheating new molds (if possible), confirming the SOP for the new product, preparing the tool cart.

* If the answer is "No," then it's an internal operation. For example: tightening mold screws, adjusting mold alignment, trial running samples.

  1. Convert internal to external: This is the most critical step. Can a fixture be designed to allow the mold to be pre-assembled outside the machine? Can tool design be changed to reduce screw tightening time from 5 minutes to 1 minute? For instance, we previously had a machine where just adjusting parameters during changeover would take half an hour. Later, we found that parameters for frequently used products could be pre-saved as Recipes and loaded directly during changeover, instantly saving 20 minutes. This is an example of converting the internal operation of "parameter adjustment" into the external operation of "loading Recipe."

Most Common Pitfalls

Let me tell you, the most common frustrating thing is when everyone thinks they've clearly differentiated the tasks, but the actual operation still gets stuck. Once, our production line engineer confidently declared that he had completed all external operations in advance. But when I went to check, the tool cart was still missing a special wrench. He ended up running back to the tool room to find it, which took almost 10 minutes round trip! This is a typical case of "insufficient preparation."

Another pitfall is the lack of standardization. Every time a changeover occurs, different people have different approaches, and the time taken naturally varies wildly. Some are satisfied with a Cpk of 1.08, while others insist on 1.67 before release, leading to DPMO differences of dozens of times. SMED not only aims for speed but also demands "consistency." Therefore, clearly document the changeover SOP, ensure everyone follows it, and regularly review the SOP for potential optimizations.

One Thing You Can Do Today

Record your changeover process. Find three consecutive changeover videos, then break them down into the smallest actions and mark them as internal or external operations.

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