Lean tools

Heijunka / production levelling

In French: Heijunka (lissage de la production)

Heijunka spreads volume and variety evenly over the period, instead of producing orders in their order of arrival with the peaks and troughs that entails.

What it is

Producing each order as soon as it arrives looks responsive. In practice, it passes all the irregularity of the order book on to the workshop: an overloaded week followed by a slack one, overtime then under-employment, and a capacity sized on the peak.

Levelling works on two dimensions. Volume: producing a comparable quantity every day rather than everything at the end of the month. Mix: alternating references instead of handling them in big blocks — rather than AAAA BBBB CC, producing AABC AABC. The repeated sequence makes the load predictable for upstream as for logistics.

This is the right place to introduce the three enemies of the Toyota system: muda (waste), mura (unevenness) and muri (overburden). Heijunka tackles the second, and many of the wastes one tries to treat directly are only the consequence of the unevenness that produces them.

What it requires

Alternating references means changing over often, hence changing over fast. That is why heijunka and SMED go together: without changeover reduction, levelling the mix is economically untenable, and the workshop goes back to big batches.

It also means accepting a modest stock of finished goods to absorb the gap between levelled production and actual demand. It is a deliberate choice — a little stock in a chosen place, to avoid a lot of unevenness everywhere else.

The common mistake

Levelling the volume and forgetting the mix. The same quantity is indeed produced every day, but always in big blocks of the same reference — and upstream keeps suffering jolts, station by station. Levelling volume alone is the easier and less useful half of the two.

On the shop floor — Cosmetics

A workshop concentrated its production at the end of the month to hit an invoicing target. The last three weeks ran on overtime, the first was under-loaded, and the rework rate doubled during the tight period. Levelling cost a little stock and removed both problems.

The other tools illustrated by this trade: plastics and chemicals.

Frequently asked questions

What does heijunka mean?

The Japanese word refers to the act of making flat, of evening out. Applied to production, it consists of spreading the load and the variety of references evenly over the period, instead of reproducing in the workshop the jolts of the order book as they arrive.

What is the difference between heijunka and takt time?

Takt is a ratio, calculated from available time and demand: it indicates the pace to hold. Levelling is a scheduling practice: it decides how volume and references are spread so that this pace is sustainable day after day. The first sets the target, the second makes reaching it possible.

Why does heijunka require fast changeovers?

Because alternating references frequently mechanically multiplies the number of changeovers. If each one ties up the equipment for a long time, productive time collapses and the workshop goes back to large batches to survive. Reducing changeover duration is therefore the economic condition of levelling, not an improvement to run in parallel.