Tote might be half full..

One of the key challenges in warehouse operations today is the ever-growing number of SKUs. A higher SKU count inevitably leads to more orderlines (touches) per order, increasing the throughput requirements—particularly in automation setups like tote-based ASRS systems.

The immediate thought to tackle this complexity often leans toward using totes with multiple partitions to house several SKUs in one bin. The hypothesis is simple: with multiple SKUs in a tote, the likelihood of fulfilling multiple orderlines with a single presentation increases, reducing overall tote presentations. But is this hypothesis truly that straightforward? Is the solution as simple as it seems?

Let’s examine this idea objectively—balancing its promised benefits against the complications it inherently brings.

The Promised Benefits

A tote with multiple SKUs can efficiently accommodate a larger variety of items, especially when storage volume per SKU is low. This approach optimizes space utilization and improves picking flexibility by reducing the total number of presentations needed. With fewer tote presentations, the system achieves higher operational efficiency.

Moreover, spreading SKUs across the system enables better organization, lowering the lead times for items to reach picking stations. This setup can be a boon when the need for compact storage and faster retrieval aligns.

The Subtle Complications

Yet, as we lean closer, complications emerge under a magnifying glass.

The probability of picking multiple SKUs from a tote hinges heavily on the affinity of SKUs stored together. Maintaining this affinity, however, is no small feat. Consider a tote housing both fast- and slow-moving SKUs: the fast-moving items will deplete swiftly, leaving the tote underutilized until its slower-moving companions are also picked.

A potential workaround is introducing an "add-to-stock" mechanism during putaway or compaction activities. This ensures better utilization of totes but risks disturbing SKU affinity—assuming it was even achievable to begin with. Additionally, throughput may take a hit, as partially filled totes are cycled out for refilling rather than simply putting away newly filled ones.

A high SKU spread across multiple totes does offer better picking efficiency and reduces reliance on a single tote. However, this strategy has its limitations. It can lead to simultaneous depletion of totes containing the same SKU, lowering overall storage utilization—especially in batch-managed or FIFO/FEFO systems, where the advantages of such spread are marginal at best.

In a nutshell

In an ideal world, single-SKU totes would be the epitome of efficiency, yet the realities of warehouse operations often necessitate a multi-SKU approach. The benefits of mixed-SKU totes are undeniable, but the challenges they bring cannot be ignored. Even with careful management, the tote might end up being half full.