There are many reasons why the performance of a once robust batch order picking and sorting system degrades. As equipment and components age, failures can become more prevalent. Hardware problems are easy to spot and usually quickly fixed. However, as it relates to system performance, the problem may not be so obvious.

Early warning signs are unusually frequent alert signals that are being triggered by full line backups. In addition to being annoying, system throughput takes an immediate hit and worse yet, depending on when these interruptions occur, customer shipments can be delayed.

So, if equipment failure is not the problem, what’s causing the backups and flow interruptions?

The Cause & Effect of Operational Changes on System Performance

Many Distribution Centers have adopted batch order picking methods within their order fulfillment process to better control and manage daily requirements. Over time customer order profiles change (such as smaller but more frequent orders) and operational procedures are adjusted to suit. But the material handling system (equipment and controls) remains, for the most part, unchanged. As a result the system may become out of sync.

For example; batch order picking and sorting operations, especially those employing fixed segregation between pick batches, may see productivity on their sorter decrease near the end of the current batch and go to near zero until the next batch is released to the sorter. Regardless of system design specifics, it is critical to maintain proper timing between picking, sorting, palletizing, and shipping operations. If picking and shipping are not synchronized, it will eventually cause the following operational issues:

• Due to changes in order profiles often additional picking labor is added. This creates a situation where pickers (in order to stay productive) pick ahead of the palletizers. That is to say that while some pickers are still picking orders for the current batch others are working on a future batch.
• Cases belonging to those future batches, cannot be assigned an active divert lane. So they start accumulating on the sorter recirculation loop until it becomes full, which then interrupts inbound case flow to the sorter causing those conveyor lines to backup.
• Cases belonging to orders for the current batch (and perhaps stragglers from the previous batch) end up accumulating behind cases belonging to a future batch. Thus, cases for the current and/or previous batch are inhibited from reaching the sorter in a timely manner.
• The system is out of sync and palletizing/shipping lanes are not accessible for cases belonging to the current batch.
• Although pickers are being effectively utilized, palletizers and shipping personnel are standing idle waiting for the system to sort through cases belonging to multiple batches, thus reducing throughput and productivity while delaying the completion of customer orders.

The congestion caused by the above operation left unchecked will eventually fill up the available accumulation lines shutting down picking and shipping operations. Adding more accumulation conveyor will delay the problem, but not solve it. It may be better (certainly less expensive) to test different batch picking strategies like limiting the pickers to no more than two batches at a time. You may also want to review critical path conveyor speeds to determine if they can and should be increased. And, make sure that the operation is in sync by having the right amount of labor at the right place at the right time.

System Solutions for Improving Performance

Sortation strategies, that eliminates the need for segregated batches is a practical solution to increasing performance. There are a few relatively simple ways to implement this capability:

• You can establish more than one palletization station for each sortation divert lane. This allows the next batch to be started while waiting for the last case from the previous batch to be sorted and palletized. Typically, the PLC initially provided with the sorter has this capability built-in, and just needs to be activated. If that is not the case, updated software should be considered.

• If you don’t have the room for more than one palletization station per divert lane, consider only including half the divert lanes into one batch and the second half into the second batch. Although this reduces the size of the pick batch, which ultimately can increase the man-hours required to pick, the overall time to complete batches is reduced because you have eliminated the time of low efficiency on your sortation system.

Most system providers suggest that maintaining sorter capacity during batch transitions can improve overall throughput by 10-25% depending on the severity of your batch overlap problem. This is important to know because that level of improvement will go a long way toward justifying adding batch overlap software to your current WMS or perhaps adding a Warehouse Control System (WCS) which will provide many other performance enhancing benefits.

Although it is best to implement these strategies while designing a new system, they can be implemented within existing system layouts. Your business requirements and rules will determine the best way to implement overlapping batches.

Final Thoughts

While the above deals with one of the most common problems associated with batch picking and sorting, in the typical distribution system the rate at which cases are introduced into the system from receiving, warehousing, picking, special processing, and staging areas, varies greatly throughout the production cycle. In addition, overtime the mix of product and volumes are bound to change.

Having the proper amount of accumulation will help smooth out spikes in flow, thereby optimizing system performance. By analyzing each inbound production area it is possible to estimate how much accumulation is required under various flow scenarios to keep all areas running at their target design rate.

For some operations computer simulations and emulations can be effective tools for companies to use to better understand system performance, test operational alternatives, identify potential bottlenecks, and understand the likely result of changes to an existing system – see (DC Automation Raises Operational Complexity Issues For Many Companies).

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