Manual order consolidation with put walls: the batched order bin sequencing problem

Batching and zoning are popular picking strategies to speed up picker-to-parts order fulfillment systems. On the negative side, these strategies require a subsequent order consolidation to separate the customer orders. In a widespread setup implemented by many online retailers put walls are applied to consolidate orders manually. A batch of orders picked under a batching and/or zoning strategy and, thus, distributed over multiple bins arrives via a conveyor system in the consolidation area. Here, a logistics worker retrieves the items from the successively arriving bins and—guided by a put-to-light mechanism—places them into the put wall. The wall is a simple reach-through rack separated into multiple shelves each temporarily dedicated to a specific customer order. On the other side of the wall reside packing workers, who empty shelves and pack completed orders into cardboard boxes. We aim to optimize the bin sequence in which a batch arrives at the consolidation area, such that the probability of starving packers waiting idle for completed orders is reduced. For this purpose, we formulate the batched order bin sequencing problem and derive suited optimization procedures. In our computational study, we investigate under which circumstances optimized bin sequences are especially valuable to reduce the packers' idle times.