Optimizing automated sorting in warehouses: The minimum order spread sequencing problem

In warehouses, order consolidation processes are inevitable whenever picking orders are assembled under a zoning and/or batching policy. In our specific warehouse setting, bins containing partial orders picked under a zoning and batching policy are intermediately stored in an automated storage/retrieval system (ASRS) and, afterwards, released on a conveyor system supplying the consolidation area. Here, a fully-automated sorter merges partial orders dedicated to the same customer in packing stations where, finally, the customers' cardboard boxes are packed. For sequencing the release of bins from the ASRS we derive an elementary optimization problem, which aims to minimize the spread of orders in the release sequence so that picking orders are quickly assembled at their packing stations. The resulting problem is formalized, computational complexity is proven, and efficient solution procedures are provided. Additionally, managerial aspects are addressed with the help of a comprehensive simulation study.