Economic Lot Scheduling

In a job shop,eac h job has its own identity and its own set of processing requirements. In a flexible assembly system,there are a number of different types of jobs and jobs of the same type have identical processing requirements; in such a system,set up times and setup costs are often not important and a schedule may alternate many times between jobs of different types. In a flexible assembly system an alternating schedule is often more efficient than a schedule with long runs of identical jobs. In the models considered in this chapter,a set of identical jobs may be large and setup times and setup costs between jobs of two different types may be significant. A setup typically depends on the characteristics of the job about to be started and the one just completed. If a job’s processing on a machine requires a major setup then it may be advantageous to let this job be followed by a number of jobs of the same type. In this chapter we refer to jobs as items and we call the uninterrupted processing of a series of identical items a run. If a facility or machine is geared to produce identical items in long runs,then the production tends to be Make-To-Stock,whic h inevitably involves inventory holding costs. This form of production is,at times, also referred to as continuous manufacturing (in contrast to the forms of discrete manufacturing considered in the previous chapters). The time horizon in continuous manufacturing is often in the order of months or even years. The objective is to minimize the total cost,whic h includes inventory holding cost as well as setup cost. The optimal schedule is typically a trade-off between inventory holding costs and setup costs and is often repetitive or cyclic.