Lean manufacturing is intended to create continuous flow, keep material moving, and reduce work-in-process and cycle time. Smaller lot or batch sizes are a necessary part of that goal, with a lot size of one as the ideal. But what does one-piece flow mean to a company manufacturing potato chips — one chip ... one bag ... one case ... one pallet? Process manufacturers including food and beverage, chemical, pharmaceutical, and many other companies can’t really go all the way to one-piece flow. But that doesn’t mean it shouldn’t be a target to aim for.
Understanding one-piece flow
The ASCM Supply Chain Dictionary tells us that one-piece flow involves processing items directly from one step to the next, one unit at a time. This method is a cornerstone of lean manufacturing, designed to eliminate wasteful queues and unnecessary waiting between production stages. By minimizing work-in-process and reducing inventory buffers, one-piece flow helps advance operations by shortening lead times and streamlining communication, making it quicker and easier to pinpoint any problems as they arise.
Achieving one-piece flow may require some process redesign, but the benefits are well worth it:
- Increased flexibility and responsiveness: If each item being processed can be different from the prior or following item, there will be much more flexibility in planning and scheduling.
- Reduced inventory: Smaller lot or batch sizes lead to more frequent production of each material, reducing inventory and, thus, the carrying cost of that inventory and the floor space it consumes.
- Improved safety: Smaller lot sizes can be moved in smaller containers or carts, easing forklift traffic.
- Improved quality: If individual parts are made and used one at a time rather than in large batches, any out-of-tolerance parts will be quickly recognized.
One-piece flow in process manufacturing
Manufacturing refrigerators, cars or cell phones is a distinctly different process from manufacturing automotive fluids, house paint or disposable diapers (more on that later). As previously noted, the need for chemical or mechanical processing makes one-piece flow unrealistic.
Let’s return to our potato chip plant for an example. Picture a typical chip plant. Its output includes chips in various shapes (flat, wavy, rippled) and flavors (salted, barbecue, sour cream and onion) in bags with many different sizes and packaging labels. A number of bags go into each case that then goes onto a pallet. Single chip, bag or case flow is ridiculously impractical. Even with single-pallet flow, workers would spend an hour building a pallet, then incur a 30-minute changeover, which would severely hinder manufacturing throughput.
How to apply one-piece flow manufacturing
The good news is that it’s possible to move closer to this desired state using the following four strategies:
1. Implement elements of cellular flow. Group products into families with similar characteristics and dedicate each family to a specific machine or line. This will reduce complexity and perhaps the number of changeovers. For instance, a furniture manufacturer might dedicate one assembly line to standard dining chairs and another to more complex, custom office chairs, optimizing the specific tooling for each family.
2. Where the equipment will successfully process smaller batches, explore the practicality of going in that direction. For example, on a machine producing rolls of weather-proofing house wrap, the standard may be to produce 2,000-pound rolls. But if there’s enough capacity to double the changeovers, then running 1,000-pound rolls will cut the batch size in half, reducing inventory and cycle time.
3. Aggressively pursue reductions in changeover time, using techniques such as single-minute exchange of dies (SMED). And understand that SMED is not a one-and-done process: After the benefits of a successful SMED event have stabilized, go back and do it again. New opportunities for reduction may be revealed. Toyota invented SMED, and it still took the company 20 years to get from several hours to several minutes for stamping die changes.
4. In cases where the equipment doesn’t require large lots or batches, use economic production quantity (EPQ) calculations to determine if moving to smaller batch sizes will be more cost effective. EPQ is a classic way to calculate the most economical lot or campaign size by approximating the best balance between total changeover costs and inventory carrying cost. Larger campaigns require more inventory to supply customers or downstream operations while specific products are not being made; smaller campaigns require more changeovers.
One-piece flow in action
In the 1980s, Luvs launched a highly successful market innovation for boy- and girl-specific diapers. The ad campaign highlighted customized absorbency features for each gender, emphasizing a better fit and improved leakage protection. Volume increased by 30% almost overnight, and the business soon began running out of manufacturing capacity.
Luvs developed a simulation, which showed that no amount of inventory was as effective as short production cycles in achieving high customer service levels. So, to make short cycles possible, workers minimized changeover losses by dedicating production lines by size. Because a size changeover took most of a production day, dedicating lines by size avoided size changeovers entirely.
Luvs diapers were produced at six sites across North America. The strategy had been for each site to be self-sufficient in product mix in order to minimize distribution costs. Instead, leaders decided to dedicate product families to specific plants, accepting the interplant shipments as an additional way to simplify changeovers.
The next part of the strategy was to produce every size and language variation every week, in a repeating cycle. From a finished product inventory standpoint, there was no longer cycle stock for the high-volume products, as production occurred every day to match demand. The low volume count variations required cycle stock, but only to cover one week.
The line operators now had stable sequences and got really good at the remaining changeovers and startups from changeovers. Customer service remained high, and out-of-stocks were avoided. Production efficiency improved from about 70% to the high-80% range, and inventory turns tripled.
Proven advantages
Diaper producers are no more able to go all the way to one-piece flow than potato chip manufacturers, but Luvs demonstrated the dramatic benefits achieved by simply aiming in that direction. Strategically reducing batch sizes, implementing cellular flow and cutting changeover times can lead to dramatic improvements in customer service, production efficiency and inventory turns. The goal isn't necessarily a lot size of one, but to continuously optimize flow and eliminate waste, securing a more agile and profitable future.
Master the principles of production flow, inventory management and lean operations with ASCM’s Certified in Planning and Inventory Management program. You’ll gain the know-how to successfully work across all functions of the supply chain, effectively manage risks and disruptions, and leverage technology to streamline processes.
