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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

February 27, 2013

Boost DC Shipping Capacity By Eliminating Sorting System Congestion And Smoothing Out Flow

Enhancements for Increasing DC System Throughput

Aside from their many labor saving benefits, automated sorting systems often represent the most critical shipping capacity constraint for even the most technically advanced operations. Distribution Digest has discussed this issue with several DC operations managers and uncovered a number of opportunities to squeeze a little more shipping capacity from them. The following is a short list of suggested operational and control changes that can potentially improve system performance:

Holste Says:

If you haven't done so already, now is a good time to have your existing WMS evaluated to make sure you are utilizing all of the available built-in and appropriate functionality.
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  • Test different wave picking strategies like limiting the pickers to no more than two active pick waves at a time. This will increase system throughput by reducing congestion on the sorter caused by an excessive amount of re-circulated cases.

  • Review critical path conveyor speeds, including the sorter itself, to determine if they can and should be increased. This may be as simple as changing drive sprockets – first check with your system provider.

  • Bypass the sorting system for single-line orders (common among Internet orders) and those that may be completed with a sub-set of popular products.

  • At the central merge, invest in up-to-date merge control logic that can reduce the gap distance between cases to just a few inches thereby increasing throughput capacity. This will require some changes to the PLC that controls the merge – first check with your system provider.

  • Adjust labor in upstream operations to maintain a steady workflow through notorious bottlenecks areas such as: Picking, Packaging, Weighing & Manifesting, etc. Adding or reducing labor, as required to prevent accumulation lines from becoming full and shutting down upstream production areas, is an on-going system management issue.


Another shipping capacity enhancing opportunity can be found in the typical order picking operation:

  • Pick-&-Pass is a popular discrete order picking method where cartons and/or tote boxes containing picked products are conveyed from one picking zone to the next in series. The productivity challenge is to maintain a level workload in each zone. If there is no queue of work coming into the zone, then the picker is idle. If there is a surge of work, congestion can occur. Either way, throughput slows down. Worse yet, when left unchecked, either condition can potentially starve downstream operations.


In this type of picking operation it is common to have a high occurrence of single line orders or orders that can be completed in a single pick zone (typical of Internet orders). These orders can be held back in the WMS and used to fill lulls in the flow of multi-zone orders. This will increase productivity by reducing congestion caused by a surge in single line orders, while interleaving work into otherwise idle zones.


  • In another Pick-&-Pass example - as pickers move along the face of the flow rack or down an aisle of bin shelving, their productivity can be improved considerably by reversing the sequence of locations for the next order to be picked so that the return trip is used productively.

Most WMS packages can easily provide the above capabilities, especially when picking is directed by RF or Voice Directed technologies.

Going Multi-Modal Enhances Shipping Capacity

Long before the current business downturn began customer orders were getting smaller while order frequency was increasing. To better manage this dynamic, more adaptable multiple data collection technologies are emerging that enable pickers to pick orders and collect data while using a single PDA (Personal Digital Assistant) device. This capability is referred to as “multi-modal”.

For example, let’s say that in a voice directed picking application, the operator must record a lot number for tracking purposes (a common requirement in many consumer goods operations). Voice will direct the pickers to the location and tell them what to pick. Then, they scan the lot number because that’s more accurate and faster than speaking a lengthy number. With multi-modal, these technologies, along with RFID, are on one device and system, thereby eliminating the need to manually switch from the voice application to the scanning application. The time saved will net an increase in shipping capacity.

For more insight into this developing technology see Dan Gilmore’s recent interview with Mark Wheeler of Motorola.

Final Thoughts

If you haven’t done so already, now is a good time to have your existing WMS evaluated to make sure you are utilizing all of the available built-in and appropriate functionality. You will find that there are many functions imbedded in the standard modules that can be activated to improve shipping capacity.



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