Heijunka is the levelling of production in both volume and product mix over time, so operations run in a steady rhythm rather than in bursts. Instead of producing Monday’s orders on Monday, an average of demand is produced each day.
Uneven load (mura) creates alternating overload and idle time, and forces the organisation to size itself for the peaks. Levelling makes the capacity requirement predictable and enables takt, flow and small batches.
Heijunka presupposes short changeovers (SMED); otherwise frequent switching between variants is too expensive.