Heijunka, a Lean Manufacturing tool to level production variations
Heijunka is a Japanese term meaning levelling, making customers production smooth, reducing differences by smoothing and planning the activity. It is a Lean Manufacturing tool and is used to implement the JIT method, just in time. Thus it participates in the TPS, Toyota Production System.
The levelling phase, the basic principle of Heijunka
The principle of smoothing is based on the planning in time of production according to an average quantity to be produced. This quantity is calculated in advance to establish some routine, a simple and repetitive process by producing the same quantity of product each day. Thus a kind of average rhythm, stability of production is launched. In order to reduce possible variations in the management of customer orders, production volume, finished product storage and costs.
However, it is not possible for all manufacturers to produce the same quantity of products every day because they have several ranges, several types of products. This implies a different production line, a different assembly time. This is why this smoothing technique exists for each type of product. As a result, delivery times, costs will be reduced and quality increased. Quality, cost and lead time factors are therefore improved, which represents the fundamentals of Lean Manufacturing.
Understand an example of Heijunka deployed in companies
Take a factory that manufactures 3 types of products in different volumes, quantities. Here is the representation of the 3 products, one for a different color. Now imagine the average monthly number of orders for each product line.
- The company manufactures 1 product per hour, with 7 hours of work in a day.
- It is open 5 days a week, 20 days for one month.
- This represents 140 products per month.
Without Heijunka, the company will mass produce and chain the most consumed products ending with the least consumed products. This avoids the need to change series and moulds between each product to minimize time wasted. This is what it looks like:
Now, let us imagine, with this operation, that the company receives an order from a customer at the beginning of the month, wishing the 3 products. We'll have to wait until the end of the month to deliver the client. This represents logistics and management costs of at least 20 working days. This duration is not acceptable either for the company or for the customer.
The company may initially produce the following C products in small quantities. But the delivery time will always be as long from the time the sales order is placed in the middle of the month and the C products have been consumed. So there can be an improvement of a few days of delivery but it is still not optimal.
The Heijunka method therefore offers the best improvement in terms of flows. To begin with, the production must be distributed in a repetitive, rhythmic manner. Let's take the week as a mesh. The mesh represents the base, the base, the plane that we will repeat.
- Per month, the company must produce 140 products.
- These 140 products are produced over 1 month, that represents 35 products manufactured per week
- De plus il existe 3 gammes pour notre entreprise. Ainsi, on divise par 4 le besoin mensuel du produit en volume pour trouver la quantité à produire par gamme par semaine. Par exemple pour le produit A, le besoin mensuel est de 82. Etalé sur 4 semaines cela représente 82 divisé par 4. Soit 20.5 produits.
- Create 20.5 products is impossible. So we round up. By looking to always arrive at the same quantity per week, 35 for our example.
- Finally as we rounded, we will miss products for the range rounded down and one on more for the range rounded up. Thus a levelling will be done on this last week.
According to the Heijunka, the company with its 3 product categories will have the following manufacturing schedule. The Kanban method can facilitate the production volume in order to respect the smoothing, the created rhythm.
Thus with this method, delivery times are reduced to a minimum, 3 days can be enough for the sales order to be delivered. The performance of the Heijunka can be verified by Takt Time.
Essential tools to implement the Heijunka method
Discover SESA SYSTEMS creations for the improvement of production logistics.