PRODUCTION LINES

11.1 PRODUCTION LINES

Consider the representation of a generic manufacturing facility depicted in a rather abstract form in Figure 11.1. The manufacturing facility is a production line composed of manufacturing stages consisting of workstations with intervening buffers to hold product flowing along the line. Each workstation contains one or more machines, one or more operators (possibly robots), and a work-in-process (WIP) buffer. Upon process completion at a workstation, departing jobs join the WIP buffer at the next workstation, provided that space is available; otherwise, such jobs are typically held at the current workstation until space becomes available in the next buffer.

Many practical models may be formulated as variations on the generic production line of Figure 11.1, or with additional wrinkles. For example, a model may call for one or more repetitions of a certain process or a set of processes, or some of the workstations may process jobs in batches. In other cases, the transfer of jobs from one workstation to another is of central importance, so that transportation via vehicles or conveyors is modeled in some detail (see Chapter 13 for a detailed treatment of this subject). Eventually, jobs depart from the system, and such departures can occur, in principle, from any workstation. In general, production lines employ a push regime, where little attention is given to the finished-product inventory. The manufacturing line simply produces (pushes) as much product as possible under the assumption that all finished products are to be used. Otherwise, when the accumulation of finished products

224 Modeling Production Lines

Operator

Incoming material storage

Work station 1

Work station 2

Work station K

Figure 11.1

A generic production line.

becomes excessive, the manufacturing line may stop producing, at which point the push regime is switched to a pull regime (the process only produces in response to specific demands; see the examples in Chapter 12). Push and pull types of manufacturing systems are studied in detail in Altiok (1997) and Buzacott and Shanthikumar (1993).

More generally, storage limitations in workstations give rise to a bottleneck pheno- menon, involving both blocking and starvation. The sources of this phenomenon are space limitations and cost considerations, which impose explicit or implicit target levels for storage between stages in a production line. Space limitations (finite buffers) in a downstream workstation can, therefore, cause stoppages at upstream workstations–

a phenomenon known as blocking. Blocking policies differ on the exact timing of stoppage. One policy calls for an immediate halt of processing of new jobs in the upstream workstation as soon as the downstream buffer becomes full. The upstream workstation is then forced to be idle until the downstream buffer has space for another job, at which point the upstream workstation resumes processing. This type of blocking is often called communications blocking, since it is common in communications systems (see Chapter 14). Another policy calls for processing the next job, but holding it (on completion) in the upstream machine until the downstream buffer can accommo- date a new job. This type of blocking policy is called production blocking, since it often occurs in manufacturing context. Various types of blocking mechanisms are discussed in detail in Perros (1994).

It is important to realize that blocking tends to propagate backwards to successive workstations located upstream in the production line. Similarly, some workstations may experience idleness due to lack of job flow from upstream workstations. This pheno- menon is called starvation for obvious reasons. It is further important to realize that starvation tends to propagate forward to successive workstations located downstream in the production line. Blocking and starvation are, in fact, the flip sides of a common phenomenon and tend to occur together—a bottleneck workstation can be identified, which separates a production line into two segments, such that upstream workstations experi- ence frequent blocking and downstream workstations experience frequent starvation.

Another type of (forced) idleness in production lines is caused by failures. Usually,

a failed workstation is taken in for repair as soon as possible and resumes operation once repair is completed; the alternating periods of operation and failure are referred to as uptimes and downtimes, respectively. Clearly, a failed workstation can become a bottleneck workstation, causing blocking in upstream workstations and starvation in

Modeling Production Lines 225 downstream workstations. In the presence of failures, one needs to devise procedures

for handling interrupted jobs (those being processed when a failure strikes). For instance, an interrupted job may need to be reprocessed from scratch or may merely need to resume processing. In some cases, however, the interrupted job is simply discarded.