A changeover is changing the set-up of a process from one product to the next. Reducing changeover times is a common and popular way to decrease inventory or to increase available work time (see SMED). Ideally, the changeover time should be zero, allowing true one-piece flow. In reality, however, it is often not zero. This post looks in more detail at the different phases of a changeover to help you understand the process better and to reduce your changeover times.
You may think that the duration of a changeover is simple. At one point you stop the process and the changeover starts. A bit later you start the process again, and your changeover ends. While there are processes that have such changeovers, there are also many more processes where the changeover is more complicated.
The changeover process is a disruption of your normal way of working. You could also call this unevenness (or Mura). This disruption can actually be seen from two angles: First, you lose parts that could have been produced otherwise. In other words, you produce less parts than if you would have had no changeover. Please resist the temptation to do less changeovers, as per my last post What to Do with SMED. Also, realize that you may lose parts not only during the full stop, but also before and afterward during the ramp up and ramp down. Generally speaking, the changeover duration is from the last part at full quality and production speed to the first new part at full quality and production speed.
Second, you spend additional work on the changeover that your people could have used otherwise. You have to prepare, do the actual changeover, wrap up afterward, and potentially do more work on quality checks after the changeover.
Loss of Production
Let’s first look at the parts that could have been produced but were not due to the changeover. The image below shows the overview of these phases. Please note that not all processes and not all changeovers go through all of these phases. Please also note that I simplified the ramp down and ramp up as a linear change, whereas in reality the line may be more curved or even have erratic peaks and valleys.
Initially, the process is running at full speed and quality. Depending on the details of the process, the changeover could start with a ramp down of the process. In most cases this will go rather quick. However, there are also situations where it could take more time. It is possible that the production rate decreases slowly. It is also possible that the quality problems may go up during the ramp down because the process is no longer operating at full capacity.
Examples are found in processing industries, where the process may run slower while emptying material. The produced goods may also be of inferior quality compared to normal production. It depends heavily on the process if there is actually a ramp down, how long it takes, and its impact on quality.
The main part of the changeover is the time when the process is actually stopped. During this time, nothing is produced.
The ramp up is more common than the ramp down. It often will take some time before the process produces good parts at full speed again. This may be for a number of different reasons.
It is common, for example, for production lines to need time to fill up again. Hence, when the line starts working again, it will take some time until the first part exits the line again. (Note: This can sometimes be avoided with a running changeover, more on this in a subsequent post.)
Another cause is that after a changeover, the settings of the process may have to be fine-tuned. During this time, there may be more frequent quality checks, adjustments to the process, and also a higher likelihood of inferior or defective parts.
Overall, you produce fewer good parts than during normal production, hence this is part of the production losses due to changeover.
Work Spent on Changeover
You can also see the changeover from the point of view of the work needed for the changeover. As per the SMED approach, you should do as much as you can before or afterward so that the actual changeover and the actual number of non-produced parts is minimal. An overview of the different tasks is shown in the image below.
During the preparation phase, the changeover is prepared. You should have all the tools, the equipment, and the needed manpower ready before you start to ramp down.
The actual changeover is commonly measured as the time from the last part at full quality and production speed to the first new part at full quality and production speed. This includes the ramp downs and ramp ups.
Wrap Up Changeover
After the process is running good parts at full speed again, the changeover is completed, and we can now wrap up the changeover. Return tools and equipment to their storage places, maybe do the maintenance on them or set them up already for the next changeover – these are all things you can do after the machine is running again.
What many people often overlook, but what is also often necessary, is an increase in quality checks to make sure the process runs smoothly again and produces good parts. These quality efforts usually start when the machine is not yet at full speed, but when the machine just started to produce the first part. This actions often go hand in hand with the changeover to fine-tune the process settings. The increased quality checks may also extend beyond the duration of the changeover to catch quality problems caused by the changeover but happening infrequently. The details depend on your actual process, and again not all processes have this increased quality check phase.
Overall, there are quite a few steps during the changeover. To reduce waste and unevenness, you can look at all of them for improvement. The image below is the combination of the two graphs above, showing both the phases causing loss of parts and the work steps for the changeover. And again, not all changeovers go through all phases.
In any case, I hope this article helped you to understand your changeover better. Now, go out, reduce changeover times, and organize your industry!
P.S.: This post is based on a question by Agus Santoso.