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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

August 22, 2012

Understanding Key to Obtaining High Sorting System Productivity

Optimizing the Number of Diverts with the Number of Pallet Build Positions Reduces Incremental Labor and Increases Overall System Productivity

In a typical DC batch order fulfillment operation, selecting the type of automated sorting equipment is primarily based on the volume of cases that need to be sorted. Providers of automated material handling systems offer a large range of case sorting equipment designed for a specific volume range and sorting capability. Selecting the type of sorter then becomes a matter of capacity and handling characteristics relative to the items being sorted.

Once the type of automated sorting equipment has been determined, the next step in the planning process is determining the number of diverts (sort positions) and after-sort pallet build positions. Getting this mix correct is critical to system performance. If there are too few diverts then the number of individual customer orders that can be processed through the system will be constrained. If there are too many diverts then the cost of the sorting system may exceed the budget and/or the available physical space.

Holste Says:

No matter how efficient batch-order picking and sorting system are what happens in after-sort pallet build impacts directly on overall system productivity.
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Understanding Cost & Operational Tradeoffs

The two key business requirements that must be satisfied are customer orders and pallet builds. The sorting system must be capable of processing a given number of orders per day and pallet builds per day to satisfy sales and/or production demand. This can be achieved by having more diverts with fewer after-sort pallet build positions per divert, or fewer diverts each with more pallet build positions. The tradeoffs are incremental labor cost verses equipment cost.

After-sort line pallet build productivity declines dramatically as the number of manual pallet positions increases, because: (a) it takes more time to determine which pallet the carton goes onto (even when supported by auto ID technologies), and (b) the more pallet positions the more cycle time walking back and forth while building each pallet load and the greater the potential for errors and injuries. However, each additional divert position increases the length and cost of the sorter.

All that said the number of diverts is also based on such factors as:

Number of orders per batch
Number of batches per day
Number of pallets per order
Number of cases per order

System planners need to find the right balance between initial system investment and incremental labor cost.

In some projects, this comparative analysis may be impacted by physical and/or budget constraints. Facility and operational requirements may place limits on the number of diverts or the length of the sorter. Available funding may also place limits on the size and configuration of the sorter.

Another factor for some DCs occurs when orders are batched and picked by carrier. This adds an additional level of granularity to the analysis.

Conclusions & Suggestions

First - the cost of the physical sorter is a onetime capital investment; whereas labor is incremental and on-going. This favors more diverts with fewer after-sort pallet positions per divert. Typically, the number of manual pallet build positions per divert will be in a range of 1 to 6 with 3 being a good compromise.

Second - it is logical that for every DC operation there is a point on a curve where additional diverts do not yield higher pallet build productivity. That would be the theoretical optimum number of diverts.

It is also important to monitor and adjust pallet build labor so that after-sort lanes do not frequently backup to the point where incoming cases must be re-circulated. This will reduce system throughput capacity and productivity.

From a safety prospective – it is inherently dangerous to have forklift traffic in and around where people are physically building pallet loads. There should be some type of barrier where completed pallets can flow from the palletizing side to the pickup side.

If the completed pallet loads are stable enough, they can be moved via forklift truck or conveyor system to a centrally located automatic stretch-wrapper. If manual wrapping at each pallet build position is necessary, then there must be sufficient space between pallets.

Final Thoughts

No matter how efficient batch-order picking and sorting system are what happens in after-sort pallet build impacts directly on overall system productivity. Coming up with the best ratio of diverts to pallet build positions involves many variables and in the final analysis, there may not be a “correct” mathematical answer. Therefore, it’s a good idea to allow for some flexibility.

As the adoption of robotic mixed SKU palletizing becomes more prevalent, manual palletizing in the DC will become less of a factor. Supply Chain Digest will provide more up-to-date information on this important development in our upcoming Automatic Case Picking 2012 report now in production and scheduled for release this fall.

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