What is HEIJUNKA (平準化)
Heijunka in short: Load leveling, Production leveling, production smoothing.
Load leveling is a technique to produce intermediate goods at a constant rate. This allows 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.
Preventing ‘Demand Amplification’
If a line is designed to cope with peak productions, it is clear that there will be waste 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. 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.
Using very small batches
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. This is 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.
Changeover time versus small Batch Size
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.
Using a Heijunka Box
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.
How does a heijunka box work?
In the illustration, each (time) slot 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 the illustration above, each time slot builds a product A and two B’s along with a mix of C’s, D’s and E’s. What is clear from the box, from the simple repeating patterns of kanbans in each row, is that the production is smooth for each of these products.
This ensures that production capacity is kept under a constant pressure thereby eliminating many issues.