Continuous flow is defined as producing and moving one item at a time, or a small and consistent batch of items, through a series of processing steps as continuously as possible, with each step making just what is requested by the next step. Other names for continuous flow are one-piece flow, single-piece flow, and make one, move one. Flow supports lean by focusing on producing what’s needed when it’s needed.
The benefits of continuous flow are that it results in fewer opportunities for operator error, improved quality output, and higher overall efficiency. In order to achieve continuous flow, all required materials, tools, equipment, and information need to be available.
There are basically two types of production systems: push or pull systems. In a traditional push system, work is “pushed” downstream to the next process step as it is completed, whether or not the next process center has requested it or is ready to receive new work. When using a push system, work moves through the various process steps based on a production forecast and is not dependent on actual customer orders. Using a push system results in a buildup of excess work-in-process, or WIP, and finished goods. In addition to the high cost of maintaining inventory, WIP has to wait for open capacity in the next process step causing the overall cycle time for the product to increase.
A pull system, in simple terms, is one where no one from an upstream process produces a product, service, or item until the downstream process asks for it. It is different from the traditional push system because it does not rely on a production schedule to initiate work but on actual customer demand. The product or service is “pulled” as the customer needs or wants it. Pull, therefore, controls the amount of inventory and triggers replenishment based on consumption or demand.
There are two types of pull systems frequently used: sequential and replenishment. A sequential pull system pulls products from the previous step as it is produced thus enabling continuous flow and limiting or controlling the inventory between steps. In a sequential system, the scheduling department must set the right mix and quantity of products to be produced. This can be done by placing production kanban cards in a rack, called a heijunka box, at the beginning of each shift. These instructions are then sent to the upstream process. Each following process then produces the sequence of items delivered to it by the preceding upstream process.
A sequential system creates pressure to maintain short and predictable lead times. In order for this system to work effectively, the pattern or sequence of customer orders must be well understood. If orders are hard to predict, production lead times must either be very short (less than order lead time) or an adequate store of finished goods must be held.
A replenishment pull system pulls products from a storage location called a supermarket. A supermarket is a storage location that holds an amount of each product it produces. Each process simply produces to replenish what is withdrawn from its supermarket. Typically, as the material is withdrawn from the supermarket by the downstream customer process, a kanban will be sent upstream to the supplying process to withdraw the product. This will authorize the upstream process to replace what is withdrawn.
Each process is responsible for the replenishment of its supermarket, so daily management of the system is relatively simple and kaizen opportunities are easy to see. The disadvantage of a supermarket system is that a process must carry an inventory of all part numbers it produces, which may not be feasible if the number of parts is large.
Through the implementation of continuous flow and pull systems, organizations can address multiple issues to reduce cost and waste:
- wasted time
- wasted resources
- wasted space
- inefficient communication
- poor and/or inconsistent work quality