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The research into multi-product production/inventory control systems has mainly assumed one of the two strategies: Make-to-Order (MTO) or Make-to-Stock (MTS). In practice, however, many companies cater to an increasing variety of products with varying logistical demands (e.g. short due dates, specific products) and production characteristics (e.g. capacity usage, setup) to different market segments and so they are moving to more MTO-production. As a consequence they operate under a hybrid MTO-MTS strategy. Important issues arising out of such situations are, for example, which products should be manufactured to stock and which ones on order and, how to allocate capacity among various MTO-MTS products. This paper presents the state-of-the-art literature review of the combined MTO-MTS production situations. A variety of production management issues in the context of food processing companies, where combined MTO-MTS production is quite common, are discussed in details. The authors propose a comprehensive hierarchical planning framework that covers the important production management decisions to serve as a starting point for evaluation and further research on the planning system for MTO-MTS situations.
The last major performance breakthroughs in job shop control stem from the 1980s and 1990s. We generate a new search direction for designing job shop control policies, providing a key to delivery improvements. Based on a common characteristic shared by the most effective job shop control policies, we posit that control should have a specific focus during high load periods. A probability analysis reveals that substantial periods of high load are common, and even occur under assumptions of stationarity and moderate utilization. Subsequent simulations show nearly all tardy deliveries can be attributed to high load periods; and that the success of the best control policies can be explained by their ability to switch focus specifically during these periods, from reducing the dispersion of lateness to speeding up the average throughput time. Building on this, we demonstrate that for example small capacity adjustments targeted at handling high load periods can improve the percentage tardy and other delivery-related performance measures to a much greater extent than the best existing policies. Sensitivity analysis confirms the robustness of this approach and identifies a performance frontier reflecting the trade-off between capacity resources used and delivery performance realized. We conclude that a paradigm shift in job shop research is required: instead of developing single policies for application under all conditions, new policies are needed that respond differently to temporary high load periods. The new paradigm can be used as a design principle for realizing improvements across a range of planning and control decisions relevant to job shops.
In the case of production environments with job shop characteristics, much research has been done on partial control such as priority dispatching. The development of comprehensive control concepts lags behind. However, the principles of workload control (WLC) have been elaborated to more comprehensive production control concepts. WLC concepts buffer the shop floor against external dynamics by creating a pool of unreleased jobs. The use of workload norms should turn the queueing of orders on the shop floor into a stationary process which can be characterised by an equilibrium. This paper compares and discusses the concepts of WLC. Assumptions of stationarity implied in the workload norms are exposed. A subdivision of workload definitions is chosen as a starting-point to trace assumptions of stationarity. The assumptions highlighted relate to the shop floor situation and make demands upon the job release function. An obvious conflict between timing and balancing within the job release function leads to an examination of stationarity requirements on the job pool contents.The analysis of stationarity requirements within existing production control concepts provides guidelines for developing production control concepts for job shops working under dynamic circumstances.
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