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About the Author

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.

Logistics News

By Cliff Holste

January 20, 2016

Logistics News: Optimizing DC Picking & Sorting System Performance

Managing the Bell Curve Effect

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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Previous Columns by Cliff Holste

Sorting It Out: Shippers Looking To Increase System Capacity Are Surprised To Find It May Already Exist!

Sorting It Out: For Shippers - Benefits Of Real-Time Control In The DC Are Huge!

Sorting It Out: Shippers Looking to Improve Operations Choose Customer Centric Approach

Sorting It Out: Productivity is a Crucial Factor in Measuring Production Performance

Sorting It Out: Packaging Construction Impacts on Logistics Operations


In the typical batch order picking and sorting system, volume on the sorter decrease near the end of the current batch and goes to near zero until the next batch can be released to the sorter. In this type of sorting system, the least productive period occurs at the beginning and at the end of each batch order picking cycle. This is referred to as the bell curve effect. Maintaining sorter capacity during batch transitions can improve overall throughput by 20-25%. Unfortunately, the typical batch order picking/sorting operation does not incorporate a picking strategy to minimize these losses.

Regardless of system design specifics, it is critical to maintain proper timing between picking, sorting, palletizing, and shipping operations. If these operations are not synchronized, the following issues are likely to occur:


  • When order fulfillment volume increases additional picking labor is added. This creates a situation where pickers (in order to stay productive) pick the next batch of orders before the current batch is completed. As a result, 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 to a divert lane until the current batch is completed. So they accumulating on the recirculation conveyor loop until it backs-up, 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 batch are inhibited from reaching the sorter in a timely manner.


At this point, 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, palletizing 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 condition 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.

An effective remedy is 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. Forecasting software is available (perhaps already included with the initial WMS) that can optimize batch size and order release timing.

Also, consider making the following operational changes:


a) Establish multiple palletizing stations for each sortation divert lane. This allows the next batch to be started while waiting for the last cases from the previous batch to be sorted and palletized. Typically, the controls (WMS/PLC) initially provided with the sorting system has this capability and just needs to be programmed. If that is not the case, updated software should be considered.

b) 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 other half into the next 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 reduced the time of low volume on the sortation system. The effect is a much flatter bell curve.

There are other order fulfillment strategies available from industry service providers, such as continuous-flow or wave-less picking, that eliminates the need for segregated batches. These types of solutions require the implementation of specialized software programs and a high level WMS.

Final Thoughts

A company’s specific business requirements and rules determine the best way to implement overlapping batches. 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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