Industrial production is always subject to fluctuations – in both directions. The Japanese Heijunka method represents an ideal tool for smoothing out the flow of production.
There are numerous lean production concepts that have become commonplace in western Europe, such as the 7 Muda and the continuous improvement process (CIP). The Heijunka concept, however, is much less well known, which is regrettable, because it is one of the building blocks of the Toyota Production System (TPS). What’s more, a lot of other lean ideas are based on it, such as the pull principle.
Heijunka is about keeping production at a consistent level, so that external fluctuations have no impact. It also prevents the bullwhip effect, which is when minor demand fluctuations in a process result in spiralling demand fluctuations in upstream processes. In the worst-case scenario, this effect can culminate in material bottlenecks that interfere with the Kanban pull systems so popular in industry.
Mura = imbalance
To properly understand Heijunka, it is important to know exactly how the bullwhip effect is inhibited. After Muri (excess strain) and Muda, Mura is the third component in the Japanese notion of waste (3M model). Translated from the Japanese, it means “unevenness”, “irregularity” or “lack of uniformity”. In other words, it describes the absence of balance in the production workflow, which leaves production completely exposed to common market fluctuations. This can in turn lead to both excessive strain and debilitating waits due to material bottlenecks. Mura can also therefore be seen as a combination of Muda and Muri.
Heijunka: Counteracting imbalance
The principle of Heijunka provides the perfect answer to Mura – it is about levelling and smoothing production. Heijunka ensures that the material flow is harmonised in terms of volumes and mix. However, ensuring uniformity is not the only aim. Ideally, the same quantity of products should be produced at all times.
Heijunka is particularly ideal for production processes with a multi-stage make-up.
So how does Heijunka differ from conventional production levelling and smoothing? Standard levelling involves dividing the products that are to be manufactured into day rates. By contrast, the aim of Heijunka is to lower batch sizes towards the ideal of a batch size of 1. To achieve this goal in a gradual process, setup times need to be lowered.
However, that doesn’t mean that each variant of a product also ought to be manufactured at least once a day. That is not possible. Heijunka helps to reduce batch sizes and therefore increases the number of setup processes that are required. To prevent unnecessary waiting times, setup times have to be minimised (e.g. by applying the SMED methodology). Heijunka is particularly ideal for production processes with a multi-stage make-up.
Picking and Heijunka boxes
Heijunka also requires a high degree of efficiency in material supply and picking. Numerous parts groups have to be available simultaneously – without possibilities for substitution or conversion. As a result, there are a lot of tools at the work bench that need to be stowed in a space-saving manner and in line with the rules of ergonomics. Naturally, the same applies to the arrangement of machinery.
In Heijunka, everything has to be organised so that products run through the production process without having to be held in temporary storage along the way. A Heijunka box is the ideal planning and visualisation tool for the entire process. It is a schedule in which products are arranged vertically and times are marked horizontally. Each individual pigeon hole or cell in the box is a time slot and cards are used to allocate these slots to the relevant products.