Heijunka: production leveling

In one line

Heijunka (leveling) means producing the same quantity and the same mix of products every day, even when customer orders arrive unevenly.

What it is

Manuel Tornell starts from a problem you can see in any Lean plant: one of the principles is to match production to what customers ask for (just in time) and to start up when there is demand (pull), but orders fluctuate a lot, sometimes from one week to the next and in ways that are hard to predict. Following them exactly as they come would mean some weeks with machines and people running flat out, with overtime and more risk of safety problems, quality problems and breakdowns, and other weeks with underused resources. On top of that, to respond to the fluctuations you would have to increase the stock of parts from suppliers.

That is why the goal of Heijunka, according to the article, is to balance the production schedule instead of always manufacturing exactly against the orders of the moment. And it does not aim only at eliminating the waste that lengthens delivery times (muda): it also targets the overburdening of people and machines (muri) and the unevenness of schedules caused by internal problems such as breakdowns, lack of materials or quality problems (mura). The three words appear in the Toyota Production System and in the eight mudas. The article draws on Jeffrey Liker, Las claves del éxito de Toyota (Gestión 2000, 2006).

How it is applied

Applying Heijunka, says Tornell, means taking the total volume of orders for a given period and leveling it so that the same quantity and the same mix are produced every day. In steps:

1. Take the orders for the period (week, month) by product.
2. Divide by the production days to get the daily quantity of each one.
3. Interleave the mix: instead of making all of product A and then all of product B, alternate them in small, repetitive batches.
4. Put the pattern in plain view, for example on a Heijunka box.
5. Keep the pattern and review it when the underlying demand changes, not with every order.

The thesis from the Universidad Estatal de Milagro describes the Heijunka box (citing Guerrero, 2017) as a tool that lets you schedule the operation through work slots: each slot is a time interval, which can range for example from 15 minutes to an hour, in which certain products are made. This reduces batch sizes and improves flexibility and inventory reduction. The thesis also cites Kogel (2016), who credits Heijunka with predictability (demand is leveled), flexibility (through shorter changeover times) and stability (it averages the volume and type of production over the long term). It also relates it to takt time and to pitch, which it links to the number of parts per package.

Try it

What leveling demands

Tornell points out two practical conditions:

• Short model changeovers. With Heijunka you change product many times a day, so changeover time weighs heavily on availability. Tools such as SMED separate the changeover tasks that require the line to be stopped (internal) from those that can be done while the line is running (external), and convert as many as possible to external.
• A small finished-goods inventory. To protect against sudden fluctuations, the article recommends a small buffer of finished goods: even if it looks like a waste, it can eliminate far more waste, because it lets you keep a stable flow and leveled production. The article adds that roughly 80% of the line's production should be done applying Heijunka.

Real example

The article's own example: a line makes three types of parts, A, B and C, and the monthly orders are 2,000 of A, 1,600 of B and 400 of C, with 20 production days planned. Each day the three parts are made in the same quantity: 100 of A, 80 of B and 20 of C. This mixes the production, separates the part types and levels their volume in a predictable sequence. It is the case preloaded in the calculator above, which also proposes a repeating cycle (5 of A, 4 of B and 1 of C) with the mix interleaved. That sequence order is this site's proposal; Tornell describes the principle, not that sequence.

People and machines also benefit from Heijunka: some parts need more labor than others, and knowing the sequence together with standardized operations lets you plan and balance the workload. You can also agree on stable, leveled orders with suppliers.

A connection to maintenance, specific to this site and not from the sources: an even load makes the windows for preventive maintenance predictable and avoids the overload weeks in which the breakdowns that the article mentions tend to appear.

Template to use

Product	Orders for the period	Production days	Daily quantity	Parts per cycle
(product A)			orders ÷ days	daily quantity ÷ greatest common divisor
(product B)
(product C)

Heijunka box: a grid with the products in the rows and the time intervals (for example, every 30 minutes) in the columns. In each slot goes the card or production order that is released in that interval.

So you do not have to build it by hand, there is a leveling spreadsheet in Excel with the formulas already done: it calculates the daily quantity and the repeating cycle, and includes the Heijunka box to fill in. The values it comes with are examples and must be deleted.

Benefits

• It reduces load peaks and overtime, and with them the risk of quality, safety and machine failures.
• It lowers inventory and the hidden problems associated with it, because you make what the customer orders in the period.
• It lets you plan people, machines and supplier purchases at a steady pace.

Limitations to keep in mind

• Without short model changeovers, changing product several times a day destroys availability: work on SMED first.
• It needs some stability in the underlying demand and a buffer of finished goods; with very erratic demand or made-to-order products, it applies only partially.
• The Milagro thesis describes an application project at an Ecuadorian plastic recycling company, focused on meeting deliveries; it serves as an example of how the Heijunka box is adapted to order scheduling, but the results of that application are not reproduced here.

In summary

Heijunka is not about making more or less: it is about making evenly. You take the order volume for the period, divide it by the days and spread the same mix across each working day, with short changeovers and a small buffer of finished goods. The calculator gives you the daily quantity and an interleaved cycle to get started.