Heijunka is the levelling of production in both volume and mix, so a process runs at a steady rate rather than reacting to the immediate shape of demand.
It exists because demand variation amplifies as it travels upstream. A modest swing at the customer end becomes a larger one at assembly and larger again at the supplier, since each stage adds buffer against the variation it observes. The result is alternating overload and idleness at every level. Producing to a steady rate, with inventory absorbing the difference, stops the amplification where it starts.
Raznolikost izdelkov je enako pomembna kot njihova količina. En mesec izdelka A, ki mu sledi mesec izdelka B, je povsem izravnan po količini in izjemno neizravnan po raznolikosti — vsak dobavitelj, ki dobavlja za A, miruje, medtem ko se proizvaja B. Izravnava raznolikosti pomeni majhne količine vsakega izdelka v ponavljajočem se zaporedju, kar je cenovno dostopno le, če so preklopi poceni. Zato se Heijunka in SMED skoraj vedno uvajata skupaj.
The trade is real: levelling requires holding finished inventory, which sits against the instinct to minimise it. A defined, visible amount of stock in exchange for stability across the whole chain is usually a net gain, but it is a genuine cost rather than a free improvement.
