Is Mixed Model Line Design a good fit? There are two areas that the Mixed Model methodology needs to fit: your company, and yourself as a process improvement specialist. I’ll talk about whether or not Mixed Modeling is a good fit for you personally in Part 7 of this series, but for now let’s focus on whether this is a fit for your company, and your company’s processes and products.
Here’s the good news: most companies are a good fit for the Mixed Model Line Design method, as long as you apply it intelligently. In fact many companies are doing Mixed Modeling already, whether they call it that or not. Here are the main criteria you’ll want to assess:
Point 1: Commonality of Processes
This means that the grouping of products that you wish to mix share the same processes. The match does not need to be 100% common, but the more variation in the process or process flow, the more complex your line design will need to be. Imagine two products with completely different processes, with no overlap. Your line design would end up being two independent designs, since they don’t share any processes. Now imagine two products that share 50% of their processes. The work flow would come together in some processes, and diverge in others, like a railroad that shares some track with another railroad. The signaling system would need to be very carefully managed to avoid a train crash! The message is this: the more variation in processes, the harder it will be to design and manage a smooth work flow, value stream, or production line. Our rule of thumb is 80% common processes, but this is not a hard rule. You’ll need to apply your own product and process knowledge to determine if this makes sense.
Point 2: Commonality of Work Content Time
Even if you share a high percentage of common processes, that’s not enough. You also need to keep your work content time (standard times) within a reasonable range if you want good work flow. Too much variation from one model to another will result in one of two things: either the downstream worker will not be able to move a finished product out of his/her station (because the next station is full), or the downstream worker will be waiting because the previous station is still busy. This combination of starving a station of work and/or blocking a station from starting a new unit will seriously degrade throughput, productivity, product cycle time, and WIP. The more variation in work content time, the worse this condition will be. There are some remedial measures that you can take: correct sequencing rules, the use of buffers (In-Process Kanbans), and flexible workers, but these remedies will never completely overcome process time variation. Our rule of thumb is to keep work content times within a +30% range.
Point 3: Commonality of Materials
Not sharing common materials does not need to be a total show-stopper, but the more unique each product’s material is, the more challenging it will be to deliver it. A material Kanban system is ideally suited to an environment where a high percentage of the material is shared, i.e. used on more than one product or model. As the different types of material proliferate, the ability to store all of them at the Points of Use becomes hard, and you’ll need to revert to kitting and sequential delivery methods. That will work, but this will also drive up material handling costs.
Good Mixed Model product family definition, based on commonality of processes, commonality of work content time, and commonality of Materials is a great start on the road to designing a high-performing Value Stream. There is more to consider, however, and I’ll discuss other important requirements in the next video.
