Heijunka
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Heijunka (平準化) is Japanese term that refers to a system of production smoothing designed to achieve a more even and consistent flow of work. Heijunka as a concept is closely related to lean production and just in time manufacturing.
Heijunka means two different, but related, things. One is the leveling of production by volume. The other is leveling production by product type or mix.
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[edit] Leveling by volume
If for a family of products that use the same production process there is a demand that varies between 800 and 1200 units then it might seem a good idea to produce the amount ordered. Toyota's view is that production systems that vary in the required output suffer from mura and muri with capacity being 'forced' in some periods. So their approach is to manufacture at the long-term average demand and carry an inventory proportional to the variability of demand, stability of the production process and the frequency of shipments. So for our case of 800-1200 units, if the production process were 100% reliable and the shipments once a week, then the production would be 1000 with minimum standard inventory of 200 at the start of the week and 1200 at the point of shipment. The advantage of carrying this inventory is that it can smooth production throughout the plant and therefore reduce process inventories and simplify operations which reduces costs.
[edit] Leveling by product
Most value streams produce a mix of products and therefore face a choice of production mix and sequence. It is here that the discussions on economic order quantities take place and have been dominated by changeover times and the inventory this requires. Toyota's approach resulted in a different discussion where it reduced the time and cost of changeovers so that smaller and smaller batches were not prohibitive and lost production time and quality costs were not significant. This meant that the demand for components could be leveled for the upstream sub-processes and therefore lead time and total inventories reduced along the entire value stream. In order to simplify leveling of products with different demand levels a related visual scheduling board known as a heijunka box is often used in achieving these heijunka style efficiencies. Other Production leveling techniques based on this thinking have also been developed. Once leveling by product is achieved then there is one more leveling phase, that of "Just in Sequence" where leveling occurs at the lowest level of product production.
The use of heijunka as well as broader lean production techniques helped Toyota massively reduce vehicle production times as well as inventory levels during the 1980s.
