Mixed Model Value Streams
Does your value stream produce more than one product? Does it produce many products in any given month, or week or hour?
By James Myers
Senior Advisor
Lean Advisors Inc.
What is Mixed Model?
Mixed Model flow is a somewhat common occurrence in Lean. They are parts of your value stream that produce multiple products - not one product - within a given time period.
Lean implementation can be more straightforward in dedicated parts of your value stream - those places where only one product is made and where one-piece flow cells are appropriate.
But what if your value stream produces more than one product? What if it produces many products in any given month, or week or hour? Mixed Model flow is the solution you may have been looking for.
What is Mixed-Model flow? Mixed-Model flow is making value flow by taking out the waste in your value stream so that multiple products are made in each time period.
This is accomplished by making the Mixed-Model flow part perform as if it were a dedicated asset. Each product "flows" at the rate of customer demand, even though multiple products are made there.
Business Definition
First, you must understand what kind of business you're in. The basic question here is are you in a MTS (Make To Stock) or a MTO (Make to Order) business? Make To Stock means you will have some sort of Finished Goods Inventory (FGI) at the end of your Mixed-Model flow. Make To Order means your product will dovetail directly into a "customer process" at the end of your Mixed-Model flow line.
Product Families
Second, you need to spend time understanding (really understanding) your product families. A product family is a combination of products that share similar processes. In your Mixed-Model flow value stream there are most likely a number of products. For example, some Circuit Card Assembly operations have as many as 250-1500 different products. But this doesn't mean they will have that many product families. Generally speaking, when you look closely at your products and the common processes they travel through, you'll end up with a small set of product families.
Takt Time
Third, you start getting a bit more technical. You have to figure out what the takt time is for the product family. What is the average rate of customer demand for the family of products? This takt time becomes important in setting up and in staffing the Mixed Model flow line.
The Customer
Fourth, you will need to begin defining value for each product family from the customers' perspective. What are their expectations? Where is the value being created in your current flow? Where is the non-value added cost being incurred (the waste)? Also included here is the work of creating a current state Value Stream Map of your Mixed-Model flow. What does it look like? How does it work? What are the outcomes in terms of VAT (Value Added Time) and Lead Time?
One Piece Flow
Fifth, where in your Mixed-Model flow can you achieve one-piece flow for each product family? This is where you can create cells in your Mixed-Model flow.
Pull Systems
Sixth, where you can't flow, where will you install pull systems? Where will you put supermarkets and FIFO lanes? These are essentially inventory buffers between the shared resource part and the 1-piece flow parts of your Mixed Model flow value stream. There's quite a bit of hard work required here, since supermarkets will have to be sized appropriately, located in the right spots, kanban cards (withdrawal and production) created, procedures changes, etc. in order to make this happen. The same for FIFO Lanes.
At this point you're ready to tie this thinking together on your future state value stream map. You can "flow where you can" and "pull where you can't flow" on a future state map. Of course, this doesn't mean you've physically accomplished "flow and pull" yet, but at least you have the concept on paper of where and how it'll be achieved in the future.
Heijunka Box
Seventh, you need to determine how often you want to make each product family. What is the "mix" of products going to be in each product family? How frequently will you make this product family? This will be your production interval. This is important because it determines how you will produce your product family mix, your lot sizes and the frequency of changeovers in your Mixed Model flow value stream. Normally, we use a concept called the Heijunka box. The Heijunka box is a visual scheduling system and allows the product family team to see how well they are doing against takt time (customer demand) as the day progresses. It also indicates the work to be done by each team in the selected time frames. The Heijunka box becomes the production controller for the operation both in manufacturing and in administrative environments.
Quick Changeover
Eighth, you will want to be working on SMED / Quick Changeover in your Mixed Model flow (almost always necessary. This is where you build the capability to rapidly and efficiently switch between one running product and the next running product to achieve true Mixed Model flow.
Standard Work
Ninth, you need to balance the operator work in the Mixed Model flow. When completed, this becomes the Standard Work based on actual customer demand for the products in each Family. You also need to check machine capacity compared to this customer demand to be sure you have adequate capability to meet the demand in the interval you've chosen.
Scheduling
Tenth, you will have to figure out where to schedule the Mixed Model flow. The key is one point and only one point in the whole Value Stream is to be scheduled. Once that point is selected, you "pull" up to that point and "fast flow" from that point to the customer. Typically this schedule point is found immediately after the shared resource asset in your Mixed Model Value Stream.
Fine Tuning
Eleventh, you are now ready to "go live" with your Mixed Model Value Stream and basically test and debug it's operation. There will be a number of fine tuning elements to make it work, but this is normal.
Continuous Improvement
Twelfth, once up and running in your Mixed Model flow, you now have the opportunity (continuous opportunities) to improve its operation.
The only advice we can give you is to be bold and get started with your Mixed Model flow. Following the process described above will insure you are successful. Virtually all of the Lean Tools are available for use in Mixed Model flows. And the results beat the old "batch & queue, push scheduling system" that's in place today for your shared resources.
Good luck!