Order release controls the flow of work unto the shop floor. Jobs are not released immediately but flow into a pre-shop pool of order from which they are released in time to meet due dates while keeping the work-in-process within limits or norms.
Empirical research has reported that the use of order release control can lead to reductions of more than 25% in work in process, 15% in lead times and 20% in the percentage of tardy jobs. Similar, Thürer et al. (2012) observed in their simulated job shop environment that at the appropriate workload norm level:
throughput times - i.e. the time from job release to completion - and work-in-process, is cut in half;
percentage tardy can be reduced by more than 25%; and
lead time, i.e. the time from job arrival to completion, is reduced by 10%.
The Workload Control order release mechanism outlined here incorporates both a periodic and a continuous element. Both elements are important: the periodic release mechanism allows the workload to be balanced - creating an even flow of work - while the continuous release mechanism allows premature work center idleness or starvation to be avoided - creating a swift flow of work. In line with the principle of input/output control the release decision is dependent on the capacity gap created by the output.
The Periodic Element:
At periodic time intervals - e.g. once a day or once a week - jobs are considered for release comparing the job's workload and the workload of all uncompleted operations for a particular work center (either directly queuing in front of this work center or still upstream) against workload limits or norms.
Continuous Element (Starvation Avoidance):
In addition to the above periodic release procedure: if the load of any work center falls to zero, a job with that work center as the first in its routing is released from the pre-shop pool.
Key Definitions
Periodic Release
At periodic time intervals - e.g. once a day or once a week - all the jobs in the pool are considered for release.
First, jobs in the pool are sorted according to e.g. earliest due date or planned release date. Then, jobs are selected for release, beginning with the first job in the sequence. This selection decision can be summarized as follows:
The workload of each operation in the job’s routing contributes to the load of the corresponding work centers. Note, that the load contribution is converted according to the corrected aggregate load approach – i.e. the original load of an operation is divided by the position of the corresponding work center in the routing of the job.
If norms are not violated – that is the workload of all work center in the routing of the job (e.g. the work centers which will be visited) plus the contributed workload of the job is smaller or equal than the workload norm for each particular work center – then the job is released onto the shop floor and its load is assigned to the load of the work centers in its routing until the operation is completed. When an operation is completed, the assigned load is removed from the load of the corresponding work centers; i.e. in line with the principle of input/output control the release decision is dependent on the capacity gap created by the output. If one or more norms are violated, then the job is retained in the pre-shop pool and its load contribution is reset to zero.
Steps 2 and 3 are repeated until all jobs in the pool have been considered for release at least once according to the determined sequence.
In this example, there are six work centers with equal capacity. The workload norm for each work center is set to 10 hours. The current workload of each work center consists of the direct load queuing in front of the work center and the upstream load of all released orders which still have to arrive at the work center. The job considered for release has three operations: the first operation is at work center 3 and has a processing time of 2 time units; the second is at work center 1 and has a processing time of 2 time units; and finally, the third operation is at work center 5 and has a processing time of 3 time units. The load each operation of the job would contribute to the load of the corresponding work center follows the corrected aggregate load approach, i.e. the first operation contributes 3 divided by 1 time unit, the second operation contributes 2 divided by 2 time units, and the third operation contributes 3 divided by 3 time units. In our example, the job is not released as it violates the norm at the first operation (at work center 3); therefore, it is retained in the pool until the next release period.
Corrected Aggregate Load Approach
For most periodic release mechanism the procedure is similar (Land & Gaalman, 1998). Periodic release methods differ from each other in the way a job contributes to the current load of work centers over time; in other words, the treatment of the load to be processed at a certain work center directly queuing at the work center and still upstream (i.e. indirect). A job becomes part of the direct load of the first work center in its routing as soon as it is released, and the full contribution lasts until completion at this work center. For the second work center, the job again contributes to the workload but it will only be part of the direct load of the second work center for, roughly speaking, 50% of the time that it contributes. Thus to ensure a similar direct load level at the first and second work center the workload norm for the second work center should be different. An alternative is the corrected aggregate load approach which divides the contributed workload by the position of the work center in the routing of the job. In other words, 100% of the load is contributed at the first work center, 50% at the second work center, 33.33% at the third work center, and so on. It has been shown in the Workload Control literature how this conversion leads to the most stable direct load buffer, i.e. the load queuing directly in front of the work center (see Oosterman et al., 2000).
Summarizing, the corrected aggregate load approach compensates for the fact that the workload at the second work center in the routing of the job will only be part of the direct load of this work center for, roughly speaking, 50% of the time, that the workload at the third work center in the routing of the job will only be part of the direct load of this work center for, roughly speaking, 33.33% of the time, and so on. This facilitates the setting of an appropriate workload norm which can be oriented at the desired direct load level in front of an work center.
Continuous Release
In contrast to periodic release methods, which take the release decision in periodic time intervals continuous release methods may take the release decision at any moment in time. Continuous order release methods apply a workload trigger. A critical load is determined which, if violated, triggers the release procedure thereby pulling orders from the pool until the critical load is no longer violated. This may allow the next job to be selected even if its load contribution will exceed the critical load (i.e. there is no maximum workload constraint).
Periodic and Continuous Release
Periodic release rules allow the workload to be balanced. Yet, if the workload norm is tight it may happen that a work center remains without work due to uncertainty in future workload estimation. Thus periodic release may actually introduce additional idleness which is known as premature work center idleness (Kanet, 1988; Land & Gaalman, 1998). As a consequence the Workload Control order release mechanism outlined here incorporate both a periodic and a continuous element. The periodic release mechanism allows the workload to be balanced - by selectively releasing work to the shop floor, it is able to smooth the workload seen by each work center, which allows non-repetitive manufacturing to implement the heijunka principle of lean (Thürer et al., 2012) - while the continuous release mechanism allows premature work center idleness or starvation to be avoided. This enables managers of small to medium sized make-to-order companies to achieve the same leveling of workload to capacity as through the use of lean tools in repetitive manufacturing. However, it does this while allowing their customers to obtain highly customized products.