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Cliff Holste

Supply Chain Digest
Material Handling Editor

Logistics News - Sorting It Out

Cliff Holste is Supply Chain Digest's Material Handling Editor. With more than 30 years experience in designing and implementing material handling and order picking systems in distribution, Holste has worked with dozens of large and smaller companies to improve distribution performance.

September 6, 2017

Sorting It Out: The Cause & Effect of Operational Changes on Order Fulfillment System Performance

Solutions for Improving 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 operations may become unbalanced or out of sync. For example; batch order picking and sorting operations 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:

Holste Says...

Regardless of system design specifics, it is critical to maintain proper timing between picking, sorting, palletizing, and shipping operations.

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  • 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.

The following picking and sorting strategies will improve system performance.

  • You can establish more than one palletizing/sort location at the end of each sort lane. This allows the next batch to be started while waiting for the last case from the previous batch to be sorted and palletized. The person stacking the pallets would make the final sort. Typically, the system controller (PLC initially provided with the sorter) has this additional sorting 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 palletizing station per divert lane, consider dividing 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.

Most system providers suggest that minimizing low volume periods 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.

Final Thoughts

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.

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