Mixed/Level-Loaded Production provides a system for advanced scheduling of production activities. This tool allows you to reduce inventory, decrease lead-times, and produce the variety of products your customers want, as they want them. Many Lean tools should already be in place to properly use and maintain a Heijunka scheduling system.
Level Production is a technique for reducing the muda waste and vital to the development of production efficiency in the Toyota Production System and Lean Manufacturing. The general idea is to produce intermediate goods at a constant rate, to allow further processing to be carried out at a constant and predictable rate. Ideally production can easily be leveled where demand is constant but in the real world where actual customer demand appears to fluctuate two approaches have been adopted in lean: Demand leveling and production leveling through flexible production.
On a production line, as in any process, fluctuations in performance increase waste. This is because equipment, workers, inventory and all other elements required for production must always be prepared for peak production. This is a cost of flexibility. If a later process varies its withdrawal of parts in terms of timing and quality, the range of these fluctuations will increase as they move up the line towards the earlier processes. This is known as demand amplification.
To prevent fluctuations in production, even in outside affiliates, it is important to try to keep fluctuation in the final assembly line to zero. Toyota's final assembly line never assembles the same automobile model in a batch. Production is leveled by making first one model, then another model, then yet another. In production leveling, batches are made as small as possible in contrast to traditional mass production, where bigger is considered better. When the final assembly process assembles cars in small batches, then the earlier processes, such as the press operation, have to follow the same approach. Long changeover times have meant that economically it was sound to punch out as many parts as possible. In the Toyota Production System this does not apply. Die changes (changeovers) are made quickly (SMED) and improved even more with practice.