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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

October 10, 2012

Understanding the Root Cause of System Generated Shipping Delays

Adjust Picking Strategy to Reduce Shipping Delays

I have been involved in a number of DC system assessments over the years and found that it is a common belief among DC operations managers that shipping delays are often the result of an insufficient amount of accumulation conveyor between picking, sorting and shipping. It’s easy to understand how they arrive at this opinion after observing the system in operation and noticing how frequently alert signals are being triggered by full-line backups. In addition to being annoying, system productivity and throughput capacity are taking a major hit, and even worse many shipments are being delayed and we know how quickly that can turn ugly.

Because there is a great deal of volume fluctuation in the typical distribution system operation it’s very difficult to estimate just how much accumulation conveyor is needed to keep all operations running smoothly. Accumulation conveyor is relatively expensive and, therefore, it is prudent to minimize the amount used in a system. Computer simulations are good tools for system designers to test the preliminary system design under varying volume conditions in order to determine the best location and the optimum amount of buffering. Still, most system designers rely on experience to guide them.

Holste Says:

Because there is a great deal of volume fluctuation in the typical distribution system operation it's very difficult to estimate just how much accumulation conveyor is needed to keep all operations running smoothly.
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However, in an actual system operation, the key question is - what’s causing the backups and how can they be minimized?

There may be several operational factors that are contributing to this problem. As an example, let’s look at a typical batch-order picking, sorting, and palletizing system operation. In this type of segregated picking operation it is critical to maintain proper timing between the picking and palletizing operations. If not, you can expect the following problems to occur:


  • When order pickers pick ahead of the palletizing operation (start the next batch before the current batch is completed), cases belonging to large orders from the current batch will frequently end up accumulating on the sorter recirculation loop behind cases belonging to the next batch of orders. Consequently, some of the after-sort palletizing/shipping lanes will not be available for the next batch.

  • Inbound cases belonging to the next batch will begin accumulating on the sorter recirculation loop until it becomes full. At this point a condition known as “system gridlock” occurs leading to a total system shutdown.

  • Palletizing operation are idle waiting for the system to sort through cases belonging to the current batch thus reducing system throughput and resulting in shipping delays.

The congestion caused by the above pick ahead strategy left unchecked will quickly fill up the available accumulation capacity. In this case, adding more accumulation conveyor will delay the problem, but not solve it. It may be better, and 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, adjust labor deployment making sure that the right amount of labor is at the right place at the right time.

A system sorting strategy that eliminates segregated batches and allows for overlapping is the best solution. There are various strategies to implement this capability. You can establish more than one palletizing station for each after-sort lane. This allows the next order pallet to be started while waiting for the last few cases for the pallet from the current batch to be completed.

If you don’t have the room for more than one palletizing station per after-sort lane, consider dividing half the after-sort lanes into one batch and the second half into the second batch. Although this reduces the size of the 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 (time between batches) on your sorting system. Maintaining sorter capacity during batch transitions can improve overall throughput by 10-25% depending on the severity of the batch overlap problem.

Final Thoughts

Although it is best to implement batch overlap strategies while designing new order fulfillment systems, these strategies can be implemented within existing system layouts. Your business requirements and rules will determine the best way to manage overlapping batches.

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