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Heijunka is defined as a technique for reducing unevenness in a production cycle, which in turn reduces waste. Below, we'll discuss how to implement heijunka, its benefits and more.
Heijunka (pronounced hi-JUNE-kuh) is defined as a technique for reducing unevenness in a production cycle, which in turn reduces waste. It's a Japanese term that means "leveling" and is a key lean manufacturing method first used by the Toyota Production System (TPS) to develop production efficiency. Toyota realized that batching isn't sustainable and that production systems can't continuously respond to sporadic orders without suffering from uneven productivity levels, inconsistent quality, and overburdening of machines and employees, all of which results in waste.
Heijunka is often referred to as production leveling in the United States, but the two terms have the same goal: reducing unevenness in production by avoiding batching, which results in smaller inventories, lower capital costs and less overburdening of employees. Heijunka lets you produce products at a steady pace, allowing you to react to fluctuations based on your average demand. It does this in two ways: leveling by volume and leveling by type/product.
However, customers frequently change their orders or volume/demand increases and decreases. What if a customer decides he actually needs the orange shirts to be gray? What if the volume of gray shirts suddenly drops, and people start ordering more black shirts? This would leave the typical mass producer scrambling to figure out how to make more gray shirts while the orange shirts just sit in inventory. To avoid this type of waste, a heijunka production schedule might look like this: OORGBOORGBOR, adding emphasis to ensure changeover times are efficient and popular items are in a buffer inventory.
Batching is popular among manufacturers because it's viewed as a cost-effective approach; there are minimal costs associated with changeover and prepared stock to meet demand. When it was first created, Ford Motor Co. was a mass producer. When the Model A was invented, Henry Ford was famous for telling the public they could have any color they wanted, as long as it was black. What was later discovered, and what Toyota found out, is that this so-called "cost-effective approach" had some pitfalls that could end up doing more harm than good. When you batch production, you more than likely will run into a few problems.
Heijunka aims to mitigate these problems by enabling companies to produce products as closely as possible to what demand requires. This is achieved by making smaller batches and creating more than one type of product at the same time. Using this approach allows organizations to roll with changes in demand while keeping inventory to a minimum.
Heijunka is frequently related to or referred to in conjunction with a lean manufacturing technique known as just-in-time (JIT) production. These two concepts both try to regulate inventory. However, JIT doesn't produce the leveling seen with heijunka but instead stipulates that products be produced when they are needed in the quantity required. Heijunka aims to level the amount and types of products manufactured in a given production cycle by producing a consistent quantity of multiple products throughout a given cycle.
JIT invokes what is known as the "bullwhip effect," a phenomenon in which forecast inefficiencies lead to increasing swings in inventory when trying to respond to sporadic customer demand as you move further up the supply chain.
Heijunka and JIT have a few subtle differences, according to Honeywell's internal lean consultants, Amy Reyner and Kweku Fleming. In their Lean/Six Sigma course for MIT, they compare the two side-by-side.
|Heijunka vs. JIT|
|Meets customer demand upon request||Meets customer demand in total over a period of level production|
|Smaller finished goods inventory||Uses finished goods inventory to make up for short periods of higher demand|
|High variability for supplier production, varying customer demand upstream||Stability for supplier, reduces inventory over the entire supply chain|
|Occasional overtime||Overtime savings|
|Induces the bullwhip effect||Calm upstream|
Three factors affect the implementation of heijunka:
Implementing heijunka starts with knowing your takt time and taking the following core concepts into consideration.
Planning out changeover times this way ensures only the individuals responsible for the changeover are on the floor, so you're not paying people just to wait around.
A heijunka box is a visualization tool used for scheduling production that is leveled by type or volume. Originally a physical schedule placed on a wall, the heijunka box can either be a physical tool or a virtual tool using a software program. It's divided into a grid pattern with rectangular slots or boxes. Columns or vertical rows represent specific time periods, usually days of the week or individual shifts. Horizontal rows represent the type of product needing to be produced.
The heijunka box contains cards or "kanban" (usually color-coded) in various box slots that denote which type and in what quantity products will be produced per a specific time period. Using a heijunka box helps manufacturers implement their production sequence and evenly distribute the production of various products.
As you can see from the heijunka box illustration, each horizontal row is reserved for one type of product. Each vertical column represents identical time intervals for when a card is to be withdrawn or a product is to be produced. For example, if a shift starts at 8 a.m. and the card withdrawal interval is every 30 minutes, this means that every 30 minutes, the material handler takes the appropriate number of cards and gives them to the production process.
Each box or slot in the heijunka box signifies the timing of information flow and material. The cards in each box represent one pitch of production for one product type. Pitch is the amount of time needed to make one batch of product. Pitch can be calculated by multiplying takt time by the pack-out quantity. For example, if the takt time is 1 minute, and the pack-out quantity is 30, then 1 minute x 30 units = a pitch of 30 minutes.
In our illustration, the pitch for the orange shirts is 30 minutes, and there is one card in the box for each time interval. The pitch for the gray shirts is 15 minutes, so there are two cards in each box. The red shirts have a pitch of 60 minutes, so there is one card in every other box. Now, let's say the blue and black shirts share a production process with a pitch of 30 minutes. The demand dictates a ratio of two black shirts to every one blue shirt. This gives us one card for black shirts in the first two intervals of the shift or day and one card for blue shirts in the third shift or day, and so on.
The goal of manufacturers should be to meet customer demand as smoothly and with the most predictability as possible, even though customer buying behavior is often unpredictable. Matching your production rate to customer demand as closely as possible helps create a level production process with minimal waste. Heijunka aims to do just that, providing multiple benefits along the way and targeting waste found in inventory processes.