[Part 3] Limits to Mixing

In this video I want to talk about some of the limits to mixing, and offer some concrete examples applying the three rules that we discussed earlier: commonality of processes, commonality of work content time, and commonality of materials.


Let’s start with an example of a lack of common processes. A pure job shop environment is an example of this, and will be a Mixed Model challenge. This is an environment where you don’t have good process flow. The routings for every product could be different, and typically you have a lot of shared equipment. There are things you can do to improve the work flow. Take a look at the technique called POLCA: paired overlapping cards with authorization. Little’s Law states that the more stuff you have in process, the longer it will take to get through the system. The POLCA method will help you to reduce the amount of stuff in process, or WIP, in a job shop environment.

A common example of the lack of common work content time might be a high mix, low volume environment, even if you have pretty good process commonality. You will almost certainly be building in order quantities or batches, and you may have a fairly wide range of work content times to deal with from product to product. Operators may not be up the learning curve for most products, if they haven’t built them in a while, so you’ll need to deal with that as well. That said, the high-mix low-volume environment is less of a stretch for the Mixed Model method than the job shop example, and can be successfully applied.

Lack of commonality of materials does not need to be a show-stopper, but the proliferation of different parts and materials across many different products is something that many companies are struggling with today. There is a slow but clear trend that I’ve seen, away from pure Kanban-based material delivery methods to a much higher percentage of sequenced material and material kits. These methods may have an offsetting benefit: while the material handling work-load increases, the productivity for the operators using the material may also go up, since they no longer need to spend non-value-adding time on part selection.

I’m not going over these challenges to discourage you in pursuing a Mixed Model line design strategy, on the contrary. You just need to make sure that you’re going in with your eyes wide open, and take the appropriate counter-measures when necessary.

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