Heijunka in English means production leveling. This lean tool created by Toyota means 2 different, but related things in production. One is volume leveling, and the other one is product type or mix leveling.
Simpler it can be explained by having the correct number of components for the demand of products and type of product.
Heijunka keeps an average volume of components and produces the right product mix, as customer demand.
The example below illustrates the Heijunka methodology used in car manufacturing.

The first table represents a mass producer (without Heijunka):

The main disadvantage of this method is, if the customer, for example, wants to buy a black car on Monday, he must wait until Friday, to get the desired product. Traditional mass production can’t fulfill on-time customer’s needs. Another important aspect is, what happens if instead of blue cars, the orders for the orange cars, suddenly will increase. In this case, the producer needs space, to deposit the already manufactured blue cars, and to use more capacity, to satisfy the clients.

To avoid such a waste of resources, the new schedule might look like, as in the table below:

Lean producers who accepted the idea of leveling by volume and mix, needed a very good schedule system to control the production.
The Heijunka box or board invented by Toyota helps visualize, the optimized production schedule. It’s a system that visualizes the orders of each product and, according to the average demand, it levels the production in sequence, for achieving an optimal flow.
The vertical columns are dedicated to each workday, and the horizontal rows represent the product type.
The box or board depends on preference, contains Kanban cards, so in this way, the employees know exactly which type of product will be produced, how many of each product, at what time can be the changeover expected.

See the examples below how it can be used the Heijunka:

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