Heijunka / Level Production
Leveled production refers to the practice of averaging model mix and production volume over a specified period. This approach is essential for creating a consistent workflow that minimizes fluctuations in production requirements. At the final assembly line, it is crucial to produce a continuous sequence of various models while limiting production fluctuations. By leveling both the volume and mix of production at this final stage, we also stabilize the output requirements for all upstream processes. This allows for smaller incremental production runs, resulting in more frequent changeovers, reduced batch sizes, and lower levels of work-in-process (WIP) inventory. For instance, consider a downstream demand of 100 units per day. This can be fulfilled either by producing a large batch of 1,000 units every 10 days or by implementing a Just-in-Time (JIT) system that delivers 100 units each day. The former scenario, with a lot size of 1,000, typically leads to an average inventory of 500 units, whereas the daily lot of 100 units results in an average inventory of just 50 units. The benefits of smaller lot sizes are significant: Reduced Inventory Costs: The capital tied up in materials and inventory is significantly lower, freeing up resources for other investments. Optimized Warehouse Space: The need for extensive storage facilities is diminished, as less inventory is on hand. Enhanced Responsiveness: The ability to quickly address manufacturing issues improves, reducing downtime and inefficiencies. Lower Defect Rates: The risk of producing large quantities of defective parts decreases, as smaller batches can be monitored and adjusted more easily. Balanced Workload: Leveling production requirements ensures that employees and equipment are not subjected to excessive strain, fostering a more sustainable work environment. To mitigate the negative impacts of production variability, companies like Toyota emphasize the importance of smoothing production levels. This concept, often referred to as load smoothing, aims to flatten the peaks and fill the valleys of production demand, creating a steady flow on the assembly line. Ideally, this leveling process should achieve zero fluctuation at the final assembly line, ensuring a consistent output. In the case of automotive manufacturing, the number of product varieties can soar into the thousands when accounting for different combinations of car size, style, body type, engine specifications, and transmission options. By embracing Heijunka, organizations can enhance efficiency, reduce waste, and improve overall operational performance, ultimately leading to a more responsive and customer-focused production system. Leveling the mix. The sorter rearranges customer orders into a predefined sequence by item type. The items are selected, for example, to minimize total changeover time or to break up large batches of demand and spread them across the day. Leveling the quantity. The sorters also defines for each item the maximum quantity of that item that should be proced on one pass through the sequence. Two already well-known reasons for leveling production in an assembly process are to be able to serve a variety of customers in a short lead time, and to limit the bullwhip effect, aka the “Forrester effect.” The latter states that any unevenness in assembly is increasingly aimplified as the demand is transmitted to upstream processes. Since upstream processes must hold enough inventory to meet demand spikes, the amount of inventory – that is, lead time – in a value stream will be lower when the downstream assembly process operates in a level fashion.