A scheduling method in multi-item production adjusting to urgent orders based on job modifiability

The target of this research is to address scheduling problems in order to ship multi-item orders in a rice milling process on the same day the order is placed. Milled rice can be stored as stock for some days and can be shipped from stock to satisfy orders. The setup time for switching milled items are needed and work hours in a day are given in the process. Scheduling is generated from the producers\´ order forecast, orders and stock before milling. When new orders during milling (urgent orders) come and exceed the work-in-process stock, the schedule must be modified in order to satisfy the orders. The purpose of this problem is to minimize the number of over time hours worked, to reduce switching time and to maximize throughput. In this problem, because urgent orders account for 90% of the whole, stocks often run out and the need to mill new items often occurs in multi-items. Therefore, this research proposes a modifiable scheduling method to minimize the number of switches and over time hours worked when milling starts in a day and when urgent orders are received. In scheduling, items to be milled and the amount to mill are first decided using an inventory control theory. The milling sequence is decided using "modifiability", which is defined by the reducibility of milling time and the necessity of modification of items to be milled for modifiable scheduling against urgent orders. In a case where the estimated end time runs over work hours, milling amounts which has smaller stock-out probability are reduced in order of increasing. The proposed method has been applied to a real problem. It has been confirmed that this method can generate the solutions 37% better than producer\´s solutions