Literally translated, the Japanese term heijunka (heijun-ka” means “leveling,” though for the purposes of Lean manufacturing, a more accurate translation would be “production smoothing.” Heijunka seeks to accurately meet demand while reducing waste in the manufacturing process. It helps smooth out the workload to eliminate unevenness in productivity (mura) and the overburden of machines, managers, and workers (muri). Heijunka was critical to the success of the Toyota Production System and is now a key component to Lean.
Although though closely related, production leveling refers to either leveling by volume or leveling by product type. Manufacturing in accordance to long-term average demand and keeping a ‘cushion’ in inventory that is proportional to the variability in demand is leveling by volume. It adds stability to production processes as well as shipping speed. On the other hand, type leveling refers to making every product every day but developing a schedule to reduce changeover times; this approach usually utilizes a heijunka box.
Businesses employing heijunka frequently rely on a production system in which they can switch from one product to another at will and with minimal downtime. Though a system like this may lack the overall production capacity of more dedicated, single-purpose production systems, it makes up for this deficit by offering the freedom to rapidly create a small batch of a product when and if customer demand emerges. This supports the just-in-time method of production.
Lean experts recommend implementing heijunka in the later stages of Lean, after identifying and improving value streams. Organizations will find more success if the Lean philosophy is already a part of the culture and is embedded into the process and material cycles.
Not only does this reduce the material wastes and inevitable production surpluses inherent to large batch production, it also limits day-to-day production variation. In comparison to mass production, the leveling of production in heijunka can make processes more predictable, more stable, and more flexible.