Leveling Volume and Product Mix with Heijunka

Why it matters

Heijunka means production leveling. Instead of turning every order change into a production wave of the same size, the aim is to produce a steadier volume and product mix within a defined time horizon. Volume leveling addresses the pattern of total output across periods; mix leveling addresses how that total is spread across products. Both have to be examined at the same time. If the total quantity stays the same every day but one product is made entirely in the morning and another in the evening, large waves can arise in materials and workload. Merely lining up colors in turn does not remove a shortage of total capacity either.

Leveling does not give authority to change demand. The order quantity, the product definition, the acceptance condition and the delivery window are preserved. The production sequence can be changed within the planning horizon; but for a product to be made later, it must be shown that the customer can wait or that the opening stock is sufficient. A plan that meets the average daily need may not meet every delivery within the day. For this reason, cumulative production and cumulative need are compared per product at the same points in time. A correct total is a necessary closure; for on-time delivery, the time order must also be correct. The stock and space burden of early production is added to this comparison.

The mix ratio is not divided equally by the number of products. Having three products does not mean that each product is one third of the total. Demand shares are derived from order quantities and converted into a repeating sequence of whole numbers. A small sequence makes the plan visible; however, every changeover may require setup, first-piece inspection and material change. Whether frequent repetition is feasible depends on the duration of these tasks. Producing the same daily quantity in large batches or in small repeats does not have to create the same setup load. While building the ratio calculation in this lesson, we will keep setup adequacy open as a separate item to be confirmed.

A leveling decision does not replace reality with the average. If demand has changed noticeably, keeping the old ratio creates a surplus in one product and a shortfall in another. On the other hand, changing the sequence entirely at every small deviation can also disrupt the setup and replenishment routine. The appropriate approach is to define a specific review window, visible stock limits and explicit replanning conditions. It must be known which version of demand the plan was prepared for. That way workers are not asked to hurry through a vague list; which product is needed in which time window becomes a clear production decision.

Formula