Since the 80’s, the Japanese production techniques and philosophies spread among the Western manufacturing companies. This was possible because the Toyota Motor Company experience was indeed a success. The so-called “Toyota Production System” (TPS) seemed to be the “one best way” to manage a manufacturing production site.
On the other side, it is also well known that not every implementation of Lean Production was a success, especially in Western companies: some enterprises – together with the consultancy firms that should have supported them – forgot that there are some main hypotheses and issues to comply with, in order to achieve Toyota-like results. On top of this, certain requisites are not related to a mere managerial approach, but depend on exogenous conditions, e.g. market behavior or supplier location; thus, not every company can successfully implement a TPS system.
One critical requirement for a TPS approach to be effective is that the production plan should be leveled both in quantity and in mix. This is indicated by the Japanese term heijunka (平準化), which stands for “leveling” or “smoothing”. Here, we will focus our attention on why leveled production is a key factor for JIT implementation, and specifically we will describe and analyze some approaches to deal with the leveling problem.
At first, the original Toyota Production System is briefly recalled, with specific regard to the Just In Time (JIT) approach to manage inventories in production. JIT is a stock replenishment policy that aims to reduce final product stocks and work-in-process (WIP); it coordinates requirements and replenishments in order to minimize stock-buffer needs, and it has reversed the old make-to-stock production approach, leading most companies to adopt "pull” instead of “push” policies to manage material and finished product flows. However, in case of unleveled demand, stock levels in JIT may grow uncontrolled.
Secondly, kanban-based production is described: kanban, a Japanese word meaning “visual record”, is a card that contains information on a product in a given stage of the manufacturing process, and details on its path of completion. It is acknowledged as one of the most famous technique for material management in the JIT approach. Here we will present some common algorithms for managing kanban queues, along with their criticalities in terms of production smoothing requirements and reduced demand stochasticity. Some of the JIT-derivative approaches will be recalled as well: CONWIP, Quick Response Manufacturing, Theory of Constraints and the Just-In-Sequence approach.
Then, a review on the mixed-model JIT scheduling problem (MMJIT), along with the related solving approaches, is presented. Despite the huge literature on MMJIT mathematical programming approaches, here it will be described why the real-world production systems still prefer the simpler kanban approach and the old (1983) Goal Chasing Method algorithm. In the end, an overview on simulators advantages to test alternative heuristics to manage JIT production is presented.
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