Mixed Model Design: What is it Good For?

What is it good for

So what is Mixed Model Line Design good for? If your instinctive response is “Absolutely Nothing!”, then you’re probably thinking about the Edwin Starr 1969 Vietnam War protest song. And you are seriously dating yourself. And you’d be wrong.

Let me start with a quick definition of what I’ll be talking about. Mixed Model Line Design is a methodology for designing Value Streams that can efficiently produce a variety of different products in the same Value Stream, with high quality, high productivity, and in the minimum time. In manufacturing a “Value Stream” is often a production line, but as you’ll see, the method can be applied to non-manufacturing environments as well. “Methodology” means that there is a proven step-by-step process that you can follow to achieve great results, with a minimum of confusion and risk.

A misconception that you might have about Mixed Model Line Design is that it can only be truly useful for companies like Toyota or John Deere, for designing labor-intensive assembly lines building products with wheels. Or that the method mainly applies to new line design opportunities, which admittedly are relatively rare. In this post I’ll explore some of the many opportunities to use these Lean skills, disavow the confusion mentioned above, and convince you that this domain is definitely something you’ll want to have in your toolbox of Lean skills.

This is very different from a more general Lean process improvement approach, of process mapping, error-proofing, eliminating waste, standardizing work, and so on. Sure, all of these things are good, valuable, and necessary, but without a comprehensive and formal methodology, you can easily be wasting time and focusing on the wrong things, while declaring victory after every Kaizen event! So let’s jump right in, and go through the opportunities for you to formalize and accelerate your improvement opportunities with the Mixed Model Line Design method.


Since Toyota has been so instrumental in the evolution of Lean, it should come as no surprise that the Classic opportunity looks very much like an assembly line environment. The products can vary… automobiles are not required. Industries that fit well into this model (and that we’ve consulted with in the past) include industrial equipment (like industrial pumps), air conditioners, agricultural equipment and implements, medical equipment products, and many others. The main point is that these lines are characterized by having many different models, and/or many different options, all built efficiently on the same production line. Product mix and volume can vary daily, and both the work flow and the material delivery system can respond well to these changes.

Much of our manufacturing line design work over the past 20 years has been in these kind of environments, since the Mixed Model Line Design method fits so well. But even big and successful companies that have been applying these methods for years can “lose the recipe”, especially as older engineers move on or retire, and new ones have not been trained.


Similar to the Classic example is the smaller Production Cell opportunity. A cell is similar to a line, but smaller and with fewer machines and workstations. Light assembly, products with low work content, specialized electronics, and aerospace components are examples that I’ve seen of Production Cell work. Even larger factories may produce sub-assemblies in a Production Cell style.

One important characteristic of a production cell is that you may be building products in batches, as opposed to trying to mix them. The cell would be set up to run a particular product, for a small quantity. When the batch is completed, the cell would then be set up for the next product, hopefully very quickly. The line design would be done for each individual product, and the line balancing would be done for each product, based on a fixed number of workstations.


Here’s where the application of the Mixed Model Line Design method starts to deviate from the norm, and I’ll start with a simple example. If you only have one product, with no options, this is not a “mixed model” environment, but can the line design methodology still be used? Answer: yes, certainly. It’s still a perfect fit, but simpler than a mixed-model line. You will still go through all of the line design steps and follow the methodology, as before, but since you are only dealing with one model, it’s a lot easier. For example, product sequencing is critically important in a mixed-model line, but a non-issue if you only have one product.

Single product lines are rare these days, except for very high-volume consumer products, which tend to be highly automated. So let’s now take a look at that option.


For a highly automated production line, we need to ask the same question: does the Mixed Model Line Design methodology apply in this environment? And the answer, as you might guess, is yes again.

All of the line design steps will need to be followed, and the difference will be that most of the work will be performed by machines instead of people. Machine cycle times, process control, and product movement will need to be considered in the creation of the design, just as with labor-intensive work. In fact, you can make a strong case that an automated line will be less variable than a labor-intensive one, with more predictable throughput, and therefore easier to balance. The importance of a good design up-front is critical, since it is expensive and difficult to change an automated line once it is physically in place. Hence the additional importance of a solid line design method.


We have applied the Mixed Model Line Design method in non-manufacturing places like hospitals. The method makes sense for Value Streams that deal with discrete items, that have the potential to flow, and that are more amenable to standardization. The example that you see here in this photo is the Value Stream called “Sterile Processing”, where instruments that have been used in surgical procedures are cleaned, sterilized, and repacked for the next use. The Mixed Model Line Design method was a great fit, and you can read about an example of this type of effort in an article in OR Magazine.


The key to applying the Mixed Model Line Design method to office or administrative processes is to look for opportunities to “flow” work across various disciplines or departments. Without good flow, the elapsed time for many administrative processes is measured in days, in order to accomplish minutes worth of work. Sure, you will be dealing with more variability in the work time, compared to working on discrete products, and even the definition of a “product” may be a challenge, but the Mixed Model Line Design approach can work well.

Case History: an aerospace supplier was averaging more than 5 days to simply get an invoice in the mail after shipping the product, delaying the receipt of hundreds of thousands of dollars in payments. The invoice needed to be routed to several departments, including accounting, customer service, and engineering, before it could go out the door.

Solution: design an invoice-processing cell, using the Mixed Model Line Design method, to complete a typical invoice in less than 30 minutes, by having all of the necessary partners work together in the same physical area for a part of the day. Cross-train the participants, in order to overcome imbalances and delays. Result: average time to complete an invoice consistently dropped below 1 day.


Here is possibly the biggest opportunity of all, the application of the Mixed Model Line Design method to continuous improvement efforts. Instead of a shotgun approach to improvement, based on a list of opportunities derived from a Value Stream Mapping effort, use the line design approach to improve a manufacturing or administrative Value Stream that already exists. The method works like this: with an existing Value Stream, and existing products, go through the Mixed Model Line Design method, step-by-step, within the framework of a Kaizen event. This will create a high-performing “Future State” design, which you can implement in phases or immediately, with the confidence that the Value Stream has been designed correctly. This is how many (but obviously not all) Kaizen events should be conducted.


When it comes to improving or designing Value Streams, you need to have a consistent and proven method, and that’s what we are calling Mixed Model Line Design. Can you think of any other opportunities to formalize your Value Stream Design efforts, using this approach? I’m sure they exist, and let us know by adding your comments below.


We hold quarterly worshops on this subject, hosted by Toyota Material Handling. Click here to find out more about it. This is the single best way to learn this method quickly, and to see it in action in one of the best factories in North America!

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