Heijunka

Heijunka is one of the elements of JIT (Just In Time) Manufacturing of Toyota Production System.

Definition

The process of smoothing the type and quantity of production over a fixed period of time

Benefits

• Enables production to efficiently meet customer demands (more flexibility)
• Avoids batching
• Reduces finished goods inventories
• Reduces capital costs
• Stable work force
• Reduces production lead time

Heijunka Example

• Level Production by Quantity of Items

Routinely received orders for 500 units/wk:

• 200 units on Monday
• 100 units on Tuesday
• 50 units on Wednesday
• 100 units on Thursday
• 50 units on Friday

Level production at 100 units/day

Keep small stock of finished goods

• Leveling Production by Type of Goods
• Product A – 5 units/wk
• Product B – 3 units/wk
• Product C – 2 units/wk
• Product D – 2 units/wk

Mass Production: AAAAABBBCCDD

Lean Level Production: AABCDAABCDAB

Conversion Steps

1. Determine the total monthly demand for each type of product
   • Example:
     • Product A = 1000 units
     • Product B = 200 units

2. Establish the weekly product requirement
   • Example:
     • Product A = 1000/4 = 250 units
     • Product B = 200/4 = 50 units

3. Determine the number of days during the production month
   • Example:
     • 4 weeks x 5 days = 20 days

4. Calculate the daily requirement for each product type
   • Example:
     • Product A = 1000/20 = 50 units
     • Product B = 200/20 = 10 units
     • Total daily production = 60 units

5. Compute the build ratio and the production frequency for each product type
   • Example:
     • Product A = 50/10 = 5
     • Product B = 10/10 = 1
     • Production frequency = 60/10 = 6

6. Determine the build frequency
   • Example:
     • Product A – 5 units will be produced every 6 units
     • Product B – 1 unit will be produced every 6 units

7. Set a build schedule
   • Example:
     • BAAAAABAAAAABAAAAA

Heijunka Box

A tool used to level the mix and volume of production by distributing kanban within a facility at fixed intervals

• Each horizontal row is for one part number
• Each vertical column represents identical time intervals for paced withdrawal of kanban
• The slots represent the material and information flow timing
• The kanban in each slot represents one pitch of production for one part number

Pitch = the takt time multiplied by the pack-out quantity

• A: 20 min
• B: 10 min
• C: 40 min

Product D: Product E = 2:1 – they share a production process with a pitch of 20 min.

• Heijunka consistently levels demand by short time increments (instead of releasing a shift, a day or a week’s worth of demand to the floor)
• It levels demand by mix ensuring that products d and e are produced in a steady ratio with small batch sizes.