Designing WIP Systems BY RICHARD ETRA
ment is not producing due to scheduling, short order mix with a high ratio of setup to run time or, scheduled or un- scheduled downtime. 3. Customer orders may be split in the corrugator schedule, and space is required to consolidate complete orders prior to delivering them for conversion. Each plant’s physical configuration and throughput requirements differ, but the basic requirements of a WIP system are adequate storage capacity and the ability to store and retrieve material without choking the corrugator or starving the converting equipment. For our purposes, we will assume that the system is conveyorized. The corrugator discharges to a transfer aisle with one or more cars depending upon throughput, which then feed these WIP conveyors. Ideally, the corruga- tor discharge should be situated in the center of this aisle, but this may not be possible due to plant configuration. In a plant whose layout can accommodate many rel- atively short conveyors, the WIP conveyors discharge to a second transfer aisle which feeds the converting ma- chines. On the other hand, if the space allocated for WIP is rel- atively narrow and long, conveyors may be laid out in two, or even more, storage banks. The corrugator discharges to the first transfer aisle which feeds the first bank of con- veyors. That bank, in turn, discharges to a second transfer aisle which feeds the second bank. The second bank then
Each plant requires space to store work in process (WIP). This can be sheets waiting to be converted, or multi-pass
product that has been processed at one or more operations, and is awaiting further processing. This article will primarily focus on corru- gator plants with a preponderance of single pass product, but the same concepts apply to sheet plants, and product awaiting further processing.
Richard Etra
A WIP system is essentially a buffer that stores material from one operation until it can be processed. Determin- ing how much WIP storage is required depends upon how much material needs to be stored, and for how long. It is required in a corrugator plant primarily for one or more of the following reasons: 1. The rate at which board can be produced on the cor- rugator exceeds the plant’s capacity to convert it. Among other things, this may be because of disparate production rates or different shift schedules. For example, a high- speed corrugator may produce enough board in two shifts to supply converting equipment for three shifts. Of course, this cannot continue indefinitely. 2. The corrugator is operating, but converting equip-
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April 4, 2022
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