Work in progress blocks the money; it hides problems and makes the process untidy. Therefore many people interpret lean is about making the work in progress zero.
In lean manufacturing WIP is identified as one of the main wastes a process contain. It also treats WIP as the mirror of waste the system contains. Every imperfection in the system creates a requirement for WIP. But lean manufacturing never talks about making work in progress zero at any cost. Lean concepts are built around analyzing the bigger picture. Optimizing the processes so that it will provide best value to the customer is one of the key concepts to remember about lean manufacturing. Removing wastes from the system, creating value to the customer are equally important when it comes to lean.
In an ideal manufacturing process, you should not have any WIP apart from the parts are being processed in work areas. But in practice manufacturing facilities are hard to run like this. It will have so many constrains which will prevent this. Machine capacity requirements, technical requirements of machinery and limitations in human resources are few of them. On the other hand it is difficult to equip a manufacturing facility to handle the peak requirements. These will cerate idling time in off-peak periods. This is a cost and non value adding from the customer’s point of view.
So it is obvious to say we need some mechanism to level out the production loads. This will prevent overloading machinery and people in the peak seasons. On the other hand people and machinery will be used optimally throughout. Production leveling is an important aspect of lean manufacturing and it is called “Heijunka” in lean context. Production leveling has two aspects into it. First the volume of production must be leveled out. Secondly the product mix must be leveled out.
Heijunka is very useful if the fluctuations of demand are predictable. Understanding the fluctuations and making the prediction requires good data collection and analyzing mechanisms. The rhythm of the internal processes will be based on TAKT time of the process. On the other hand SMED techniques will help production leveling. If the setup times are higher it will increase the resistance towards change.
It may seem like Heijunka is going against lean concepts in the first glance. It can create some work in progress in the system. It also depends on the predictions rather than the actual demand. But if we look at core concepts of lean it makes sense to level out production load in the bigger picture. It will reduce the cost of operation and will reduce stress on people and machinery. More importantly production leveling will create value to the customer.
In simple terms Heijunka is making internal processes stable with the fluctuating external demands.