In short: Load leveling, Production leveling, production smoothing
A technique 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. Unfortunately this is rarely the case because actual customer demand fluctuates.
If a line is designed to cope with peak productions, it is clear that there will be wast at moments this peak is not needed. 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. In the 1940s it took two to three hours, in the 1950s it dropped from one hour to 15 minutes, now it takes few minutes.
The Heijunka box allows easy and visual control of a smoothed production schedule.
A typical heijunka box has horizontal rows for each product. It has vertical columns for identical time intervals of production. In the illustration on the right, the time interval is thirty minutes. Production control kanban are placed in the pigeon-holes provided by the box in proportion to the number of items to be built of a given product type during a time interval.
In this illustration, each time period builds an A and two Bs along with a mix of Cs, Ds and Es. What is clear from the box, from the simple repeating patterns of kanbans in each row, is that the production is smooth of each of these products.
This ensures that production capacity is kept under a constant pressure thereby eliminating many issues.