August 1, 2019

By Darren Dolcemascolo

Continuous flow is a key principle of Lean and the Toyota Production System.  It means that product or information is moved through operations, from step-to-step, with no work in progress (WIP) one piece at a time at the pace of the customer demand downstream.  It is a True North principle that we work toward.  We want to get closer and closer to this ideal.  Whether we are performing a service or building a product, the ideal situation is to provide the next downstream customer with what they need, when they need it, and in the right quantity.

What are some of the benefits of achieiving continuous flow?  An operation that has achieved continuous flow has:

• Eliminated WIP (Work-In-Progress) inventory with the exception of a single unit at each step in the operation.
• Minimized Lead Time Through the process (or Turnaround Time).  The total time through the process should be the number of operations X the cycle time at each operation.  There is very little waiting.
• Increased the ability to detect a defect. By moving one unit of work at a time from one step to the next, defects are discovered more quickly and fewer overall defects are created.  (Consider that if product moves in large batches with lots of waiting, many defects can be created, and they may not be detected for a long time.)

Continuous Flow is not easy to achieve.  In order to move toward continuous flow, we need to work on the following:

• Achieving consistent quality and repeatability.  We must minimize variation and develop a high capability process.
• Improving equipment or systems reliability.  If we are utilizing equipment or other systems in the process, these systems must be reliable. 
• Developing processes that can be scaled to takt time, the rate of customer demand.  For example, if we need to process one unit every minute, we need to develop a process that can be scaled to 1 unit/minute at each operation.

Before we achieve these conditions, we must have some buffer of inventory between operations (e.g. ,a FIFO line/WIP queue or kanban system).  As we get closer to continuous flow, we reduce the buffer incrementally until continuous flow is possible.

• Repeatable administrative processes such as claims processing, mail processing, and order processing.
• Some healthcare processes.
• Assembly processes
• Other manufacturing processes for which equipment can be right-sized to achieve continuous flow.

Continuous flow is one of the key concepts within lean manufacturing; in most cases, a piece of a value stream can be transformed into a continuous flow operation. While continuous flow is not always achievable for an entire door-to-door value stream, lean organizations must continually improve their processes in an attempt to get closer and closer to true continuous flow. This will reduce inventory levels, reduce lead time, and improve customer service levels.

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