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Heijunka

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

Heijunka 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.