Heijunka, flexible and smoothed production

Heijunka means sequencing, or smoothing of production.
The objective of Heijunka is to absorb sudden fluctuations in market demand by producing several different models in small batches on the same line. It is the principle of “one piece flow”.
The practice of Heijunka also allows mudas to be eliminated by making it easier to standardise work. Good sequencing practice reduces the need for line side labour.
Why does the practice of Heijunka, a basic building block of lean manufacturing, provide a high productivity increase?
For better amortisation of fixed investments in lines, the use of resources (factories, shopfloor, machinery) has to be maximised by improving their use so that more is produced with existing resources. This is achieved by switching from a line dedicated to a single product - which is therefore sensitive to sales variations - to a flexible line capable of manufacturing several types of product. Variations in demand for each product are absorbed by the flexibility of the production tool: sudden fluctuations and variability are reduced.
A concrete example: a single-product mass production line on which demand drops  by 30% suffers a sudden production variation of 30%. With variability at this level, it is impossible to standardise and improve a workstation by Kaizen.
On the other hand, if a lean manufacturing line used to assemble 4 models suffers a 30% drop on one of these 4 models, the line is only affected by 0.25 (one quarter) x 30% = 7.5%.
Disturbance is considerably reduced and may even be compensated by the possible increased demand for the other 3 models. There is less variability on a line like this and standardised work for added value is possible. The Kaizen attitude improves the standard continuously.
In short, Heijunka allows line loads to be smoothed by mixing the order of product manufacture. This assists stability and standardisation of work. The second objective of Heijunka is to assemble different models on the same line while eliminating Mudas by standardised work.
The application of Heijunka allows production in the same order as customer demand. The Heijunka practice distributes and balances production over all available means, rather than allowing dedicated resources to suffer from sudden fluctuation in demand.
Contrary to the perceived idea, it is easier to optimise workstation balance when the lines are multi-mode. Why is this so? Because by working more on one product, less work is required on the next: basic tasks are multiplied, broken down and therefore more easily divided into basic units.
By multiplying the tasks, and distributing them better, and by standardising them by carefully-studied smoothing, better use can be made of the working time available for creating value. Heijunka is therefore able to reduce the various mudas while assigning greater value to the Lean production line.
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Lean Manufacturing factory

1. Heijunka on line: e.g.: 2 people-carriers, 1 two-door, 1 saloon car…, 2 peoplecarriers, 1 two-door, 1 saloon car…
2. Lightened logistics, small trains, setting up flows.
3. Small containers, less stock.
4. Line side compression, concentration on added value, reduction of mudas.
5. Heijunka flexible multi-product line: better use of production resources.
6. Operators creating added value.