|Methods and techniques|
Heijunka or production levelling converts uneven customer pull into even and predictable manufacturing process. It reduces mura (unevenness) in production process. Unevenness leads to Forrester effect (see below) - any unevenness creates yet increased unevenness on the next stages and finally leads to imbalance of the process. Thus the fluctuation in performance is a cause of waste. The organization has to keep additional capacity, inventory, staff and is unable to optimize process.
Implementing heijunka requires stability, predictability and flexibility. It is one of the most difficult methods of lean.
How heijunka works? - an example
Let's assume the enterprise manufactures five types of products: A, B, C, D, E. Whole Lean manufacturing system is implemented, including Kanban methodology. The production levelling will work that way:
- The customer buys 7 products D.
- Delivery causes release of 7 kanban cards.
- Without heijunka, kanban cards would be transferred to the last workplace in production process. But with heijunka they are transferred to heijunka box (see below).
- The heijunka box determines the sequence of production and the volume for each type of product. Let's assume, that sequence is: B(5)->E(3)->A(12)->D(5)->C(3). Volume for each product was presented in parentheses.
- The manager can put only 5 kanban cards into heijunka box (because of D(5)) and following 2 cards will be added in next batch. This limits the Forrester effect on earlier workplaces and suppliers, removes mura and reduces required capacity.
The heijunka box is a tool using to sort orders (kanban cards). On the production line that manufactures many types of products an optimal sequence of production is determined. It stems from typical size of orders, SMED optimization, TPM program, Just in time, etc. For each type of product optimal volume is also determined.
The heijunka box has many shelves - one for each product. Each shelf contains information how many products can be manufactured in one batch/day. Each day, managers transfer kanban cards from heijunka box to workplaces. Then they fill the box with kanban cards for another day.
The heijunka can be, and these days often is, implemented using computer networks and ERP systems.
Types of heijunka implementation
- Volume levelling. Manufacturer levels the production in long-term on average demand and keeps buffer inventory for variability in demand.
- Type levelling. Each day every product is manufactured in small number and heijunka box is used to level the production. The additional flexibility is required for changeover.
- Demand levelling. Sales influence demand to make it more predictable. Every promotional activity should be discussed with production department before it happens.
Benefits of heijunka
The main benefits are:
- Meeting customer demand in total over a given period of level production
- Finished good inventory to make up for short periods of higher demand
- Predictable work schedules
- Stability transmitted to supplier reduces inventory over the entire supply chain
- Overtime savings
- Happy employees
Heijunka vs. Just in time
- Meeting customer demand upon request
- Reduced finished goods inventory
The downside of planning using JIT is:
- Unpredictable work schedules
- High variability for supplier cascade varying customer demand back upstreams
- Overtime and idle time occasionally
- Bullwhip effect
The heijunka requires full implementation of lean manufacturing, often including suppliers. It is very sensitive to problems with:
- delays of supplies
- equipment out of order
- imbalance in sales
Any of these can destroy sophisticated plan of smooth production. The lean is about value, not ideas. Therefore if heijunka is not yet possible, and can induce more problems than profits, it should be postponed.
- lack of tools for large-scale levelling
- increased finished goods inventory can be seen as anti-lean
- cannot be implemented immediately
- predictions are imperfect
- depends on direct customer contact
- required standardization of work
- reduction of workers flexibility
- requires discipline and planning
The Forrester effect is know also as bullwhip effect and was described in the beer game. It arises in the chain that consists of several companies or workplaces that cannot communicate directly, but only using orders. Each unevenness of the customer order in the first company in the chain leads to yet greater unevenness in the following companies. This can lead to overproduction or reduced level of production, high idle time or overtime for employees, etc. The reason for that is that managers try to chase the demand. They treat is as a stable trend, while it isn't stable at all.
- Coleman, B Jay; Vaghefi, M Reza. (1994) Heijunka (?): A key to the Toyota production system, Production and Inventory Management Journal, 35.4
- Reyner A.,, Fleming K., (2004) Presentation about Heijunka product and production levelling (PDF)
- Furmans K. (2005), Models of heijunka-levelled kanban-systems (PDF)
- Heijunka Product & Production Levelling presentation
Author: Slawomir Wawak