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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

September 11, 2013



Managing Dynamic Change Within A Fixed System

The Cause & Effect of Operational Changes on System Performance


Holste Says:

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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

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Logistics News : Will DC Automation Drive Some Shippers to 3PL Service Providers?

Logistics News : Can a Custom Built DC Cost Less Than A General Purpose Building?

Logistics News : E-Commerce Verses Conventional Order Fulfillment

Logistics News : When Is The "Best" Time To Upgrade DC Operations?

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Over the past several decades, many Distribution Centers have adopted batch order picking and sorting methods to increase productivity and better manage operating cost. Recently, the wide adoption of multi-channel marketing strategies have lead to customer order profile changes such as smaller, more frequent orders, and demand for next day or even same day shipping. But, for the most part, material handling systems (equipment and controls) have remained unchanged from a functional perspective. Existing system operations may be out of sync and not capable of supporting current requirements.

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. Most picking and shipping systems do not have sufficient capacity to make up for these frequent production losses.

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, the following issues are likely to occur:


 
  • Due to changes in order profiles often 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. 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 accumulating on the recirculation 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 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, 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 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.

A more cost effective solution 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.

 

System Solutions for Improving Performance

Batch overlap is one of the most common problems associated with picking and sorting systems. Continuous-flow or waveless picking and sorting strategies, that can eliminate the need for segregated batches by expanding the capabilities of the WMS, may be a solution to increasing performance for some companies. Other possibilities are:


 
  • Establish multiple palletizing stations 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 palletizing 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 20-25% 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 associated project costs.


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 – see (Managing Complex DC Systems).

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