FASE III: REVISIÓN Y EVALUACIÓN DEL CONTROL INTERNO

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4.3.1

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The main reason to schedule the press lines first is the limited capacity of the press silos, which are the buffer capacity between the grind and mix line and the press lines. By scheduling the press lines first, we determine when a job can be processed at the grind and mix line, such that it fits in the press silo. A second reason is that we want at least the jobs of the same product, with the same due date, next to each other at the same press lines. This is easier to accomplish when we schedule the press lines first.

The most challenging part in the connection between the grind and mix line and the press lines is when to release a job for the grind and mix line such that it fits in the press silo, without blocking the grind and mix line because it must wait. We choose to prevent blocking when creating a schedule because in general, the operators do not start production at the grind and mix line if they know that the product cannot move to the destination press silo. If the line is blocked, operators cannot react on last minute changes, e.g. producing another product first. Therefore, they delay the start of production until the press silo is available. By scheduling the press lines first, we are able to determine when there is space in the press silo to store a job. Hence, when the job can be produced best at the grind and mix line. Therefore, the job gets a release date and a due date for the grind and mix line. Given the capacity of the press silo and the finish times at the press, we determine when a job may finish at the grind and mix line. Given the minimum process time at the grind and mix line for the job, we determine its release date. We determine the due date for the grind and mix line based on the start date of the press. Important to note is the assumption that only one press silo can be used for a product. In reality, both dedicated press silos can be used to store jobs of a certain product. We choose to use this assumption because it is complex to implement a rule that uses both press silos, within the limited time for this research. When we would use two silos, we must take care of which silo is currently used by the press and of rules that take care of the question: do I use the second silo, or do I postpone production at the grind and mix line? In the end, it does not change the schedule at the press lines. Only the interval in which jobs can be scheduled at the grind and mix line becomes smaller by our approach for part of our jobs, which can result in a somewhat worse solution. However, the grind and mix line has excess capacity. So, the impact is limited.

If we would start scheduling at the grind and mix line, scheduling the press lines becomes rather complex. First of all, we would also schedule job-by-job, but now we schedule all stages at once for each job, instead of dividing the problem in two sub problems. At stage four we need to select a press silo dedicated to a line where the job is allowed to be scheduled. But if no press silo is available, we block the grind and mix line until an allowed press silo becomes available. If we schedule a job at the press silo such that it is full afterwards, we have to update the release dates of the remaining jobs of the product. These additional procedures take more running time. Getting the jobs of the same product and with the same due date after each other on the same press line also can become more difficult. Whether this is still possible depends on the sequence at the grind and mix line.

The last disadvantage of starting to schedule at the grind and mix line is an important reason for us to start at the press lines when we create a schedule. We want to produce jobs of the same product, with the same due date, after each other at the same press line. An important reason for this is the fact that the press lines are the bottleneck for ForFarmers in Deventer (Section 2.5). Recall, e.g. from toy problem 2, that when jobs of the same product are scheduled after each other, we save start-up time

41 and time it takes to empty the cooling machine. Thereby, we have more time available to pelletize jobs. By creating first a schedule for the press lines, we can relatively simple schedule jobs of the same product at the same press line, without needing all kind of difficult and complex control rules.

4.3.2

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Figure 4.7 shows a logic flowchart that illustrates the steps to create a schedule. The blue rectangles are steps that consist of sub steps. We discuss these sub steps later this section. First, we sort the jobs on their due date. Each job can have a different due date, so we reduce the probability of finishing a job late by producing the earliest due dates first. Recall the EDD-rule in Section 3.2.2. After scheduling the jobs at the press lines, we sort the jobs again on due date for the grind and mix line. The pellet type jobs get a due date for the grind and mix line equal to their start time at the press. For meal products, we use their production due date. Given the release and due date for the grind and mix line, we schedule each job at each machine of the grind and mix line. We schedule the jobs job-by-job at each line. For example at the grind and mix line this is necessary. To determine when a job can start unloading at a certain stage, we need to know the time at which the previous job finishes unloading at the next stage. Therefore, we need to schedule the previous job at all stages of the line first.

Create Schedule

Product type = pellet

Determine press line for

Job Schedule Job at Press (Stage 5) Yes Yes

All pellet jobs scheduled?

Select (next) job in list

No

No

Sort jobs on due date (Grind & Mix Line)

Select (next) job in list

Are stages 1, 2 and 3 scheduled?

Select (next) stage

Plan job at first machine of stage (There is only one machine) Adjust start date of press silo

All jobs scheduled?

Stop Yes Yes No No Schedule job at press silo (Stage 4)

Sort jobs on due date