Manufacturing products is not for the faint hearted. The coordination of resources required to convert raw material to the final product can be exhausting on a normal day. To add to this, customer orders may fluctuate from day to day — leaving the plant guessing on what and how much to produce.
Wouldn’t it be nice if customer orders were constant and everything run smoothly in a factory? What if you did not have to worry about machines breaking down? Or erratically fluctuating customer orders? Or raw materials never running out in your factory?
That ideal situation is what entices companies to want to implement lean manufacturing. This management paradigm has been widely implemented — with varying degrees of success. Many of the Japanese terms used in lean have entered into common everyday usage.
Almost everyone now knows what Muda, Hoshin-Kanri and Jidoka mean. One term that even the most ardent disciples of lean don’t use often is Heijunka. This is partly because it is an advanced tool that is only used in advanced lean stages. It requires a mature lean environment because if implemented badly it may result in staff demotivation.
Aspects of production such as changeover and total productive maintenance must be at an advanced stage of implementation for heijunka to work.
This article looks at heijunka as a production leveling tool. We will discuss how to successful use heijunka in low volume high mix production situations.
Heijunka Definition
Heijunka is a term used in lean manufacturing to describe a method of stabilizing production. This is achieved by taking into account all the products demanded by the customer and ensuring each is manufactured within a specified period — usually a week.
Typical factories prefer running on product type/family continuously before changing over to the next. This mental paradigm is influenced by extremely long changeover times. When changeover times are long, producing different products in one shift becomes impossible.
The same applies in an environment where preventive maintenance of equipment is non-existent. In such situations, people want to make as much of a product as possible before the next breakdown occurs.
Heijunka will therefore only work where changeover times have been reduced significantly and where machines do not breakdown. For changeover, Single Minute Exchange of DIe (SMED) must be implemented. Machine breakdowns are effectively eliminated through an Autonomous Maintenance program — which part of Total Productive Maintenance (TPM).
Heijunka in Lean Manufacturing
One of the best known principles of lean manufacturing is Just In Time — the concept of producing goods only when needed. The idea with JIT is that resources should be used to produce only what is in demand at the moment.
This has a number of benefits:
- Reduced inventory carrying costs
- Less space required
- Better visibility of customer demand
Despite these advantages, JIT can be difficult to implement. The tight inventory levels means that there is no room for complacency when it comes to stocking. Also, any supply side disruptions can cripple operations as Toyota experienced after a recent earthquake.
The problems arising from disruption of JIT systems has seen companies roll back on the system. Supply chain experts are asking whether it is time to rethink JIT. Instead of trashing just in time altogether, the best thing would be to redesign the system to mitigate risks.
Increasing the buffer before assembly, having multiple suppliers and relocating production closer to market are possible solutions.
Why heijunka is necessary
Heijunka is a tool useful that helps in producing the whole range of goods demanded by the customer. This is achieved without having to respond to erratic demand spikes. In a counterintuitive approach to lean, heijunka uses demand forecasting as well as inventory buffers.
Heijunka is still a more superior approach than batch processing. This is because it aims at producing all the products all the time. Batching, on the other hand, produces massive inventories of one product before moving to the next. This eats into production capacity that could have been used in satisfying present demand.
Producing Every Product Every Cycle (EPEC) is meant to even-out demand over a specific time period. This is done by dividing the available production time into small periods where specific amounts are produced.
The amount to be produced is determined by using the total demand over a certain period of time. For example, production figures could be determined using the current weekly or monthly demand. This is a more accurate form of demand forecasting because it takes into account current market realities and not long term past performance. Should there be a fluctuation, production adjustments can be made to accommodate them.
How heijunka stabilizes flow of value
Heijunka helps in preventing the chaos that is caused by the bullwhip effect. To be effective in doing this, change-over times must be reduced. This makes it possible to switch between production runs so as to meet demand in terms of variety and quantity.
Once the demand for a the specified period of time is determined, the next step is spreading out production of all products. As stated earlier, the aim is to produce every product during each production cycle or interval.
Typically, when faced with a variety of products, most people would opt to produce each type in full before moving to the next. This kind of batch thinking is caused by the fear of changeovers — which are typical long and costly in a non-lean environment.
Products are grouped by their characteristics as follows:
- High runners
- Medium runners
- Low runners
The high runners are the products with the highest demand — these should be included in every production slot in the cycle. Medium runners will be included in every other production slot, while the low runners can be put in the last slots of the cycle.
Does heijunka conflict with lean?
It is easy to fall into the trap of thinking that lean is a set of tools that are to be used religiously. This dogmatic approach makes people lose sight of the fact that it is the thinking behind lean that is important — not just the tools. The lean tools are as a result of applying the principles.
The fact that heijunka results in having a buffer of inventory may look anti-lean on the face of it. But if we remember that lean is about flowing value to the customer with the least amount of waste, that compromise is understandable. It is better to have some inventory if it helps in stabilizing the entire production system.
The takt time — the rate at which the market is demanding a product — is an important metric in lean. So long as every process is working as close as possible to takt time, then customer demand will be met.
Conclusion
Lean manufacturing is about flowing value to the customer in the most efficient way. Heijunka is an advanced lean tool for doing exactly that. To be at a level where an organization is implementing heijunka means that they have addressed other important facets such as changeover and cycle time improvement. It also means that the people doing the work have shifted their mindsets from a batch to a lean thinking paradigm.
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