Heijunka (Production Leveling)

Heijunka operates on two dimensions. Volume leveling ensures that the total production quantity is distributed evenly across the available time. Instead of producing 1,000 units in the first three days of the week and nothing in the last two, volume leveling produces 200 units every day. This prevents the feast-and-famine cycle where some days the factory is overwhelmed and other days it is idle. Mix leveling goes further by distributing different product types evenly within each day or shift. If weekly demand is 500 units of A, 300 of B, and 200 of C, mix leveling produces A, B, and C in a repeating pattern every day — perhaps 100A, 60B, and 40C per day, sequenced in a repeating A-A-B-A-C-A-B pattern. Mix leveling requires fast changeovers (because you switch between products frequently) but dramatically reduces the demand variability that upstream processes and suppliers experience. The combination of volume and mix leveling creates the smoothest possible production flow.

Implementing heijunka manually is complex, which is why many factories resist it. But digital scheduling tools make leveled scheduling practical and visual. The traditional tool is the heijunka box — a physical grid with rows for products and columns for time slots, loaded with kanban cards. Modern equivalents use digital production calendars like LinePlanner, where each time slot (shift or day) shows the planned product mix and quantity. To implement heijunka, first calculate the daily demand for each product based on monthly or weekly orders. Then determine the pitch — the smallest practical batch size for each product, considering changeover time and minimum run requirements. Distribute the daily demand for each product across the available shifts, creating a repeating pattern. The pattern should minimize consecutive changeovers on the same product family to reduce total changeover time. Review and adjust the heijunka pattern weekly as demand changes. The result is a stable, predictable production rhythm that operators can follow without daily schedule chaos.

The benefits of heijunka cascade through the entire manufacturing system. Upstream processes receive a steady, predictable demand signal instead of erratic spikes, enabling them to operate more efficiently and with less safety stock. Lead times shrink because products no longer wait in large batches — a daily mix of all products means every product is produced every day. Quality improves because smaller, more frequent batches mean defects are detected sooner (if today's batch of Product B has a defect, you find it today — not at the end of a 500-unit batch next week). However, heijunka has prerequisites: changeover times must be short enough to make frequent product switches economical (target under 10 minutes using SMED techniques), operators must be cross-trained to handle multiple products, and the production scheduling system must support mixed-model sequencing. These prerequisites are themselves valuable improvements, making heijunka a forcing function for broader lean progress.