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Batch Picking and Sortation

CHOOSING AN ORDER PICKING PROCEDURE

Traditionally a picking list for a customer order is given to a picker who travels the pick path, adding cases to a pallet or roll cage as he/she goes. Large orders may require interrupting the process to drop off full pallets as they are completed. Alternatively a large order may be segmented and assigned to several pickers, thereby reducing the elapsed picking time.

Order picking has several advantages, such as:

  1. Simple paper pick lists may be used.
  2. One person can be made responsible for an entire order.
  3. Products may be easily grouped into families along the pick path to ensure that policies, such as placing the heaviest products on the bottom of an outbound pallet, may be easily observed
  4. Activity based location assignment can significantly reduce the pick path for an order.
  5. Since pickers are each picking one order and operating independently, it is easy to assign work, keep them busy and measure results.

However, as product offerings increase (SKU proliferation), the warehouse begins to fill with more and more material. Consequently, the pick path gets longer, and as orders or order quantities become smaller, the path must be traveled more frequently. Congestion and delays can occur near fast moving products in the larger volume facilities.

The challenge in larger facilities is to develop a system that maintains most of the advantages of smaller, order picking environments, while addressing the problem of increasing levels of “not” picking.

Batch Picking

One alternative is to batch the picking requirement for several orders and then sort the cartons that are picked to individual orders as the picker goes along. This is known as multi-order picking or cluster picking. In its simplest form, a picker may take two pallets at a time along the pick path and simultaneously pick two orders. This approach can be extended by pulling a “train” of several pallets or roll cages. In this way most of the benefits of simple order picking are maintained. The technique is limited by the length of the train to about 4 or 5 orders. As the train size grows, aisle widths and congestion become bigger factors.

Wave Picking

The next variation is radical. An interesting and even more powerful concept is to accumulate the requirements for a larger number of orders (50-100); pick them as a batch and convey the cartons to a high speed sortation machine and then divert them to chutes/spurs dedicated to each order. At the end of the spur they are loaded onto a pallet or roll cage by a second operator. This is known as “wave picking”. The general principle is to handle each case efficiently two times, rather than inefficiently once. Clearly, the rate for each of the two handing steps, batch picking and container loading, must be considerably more than twice as fast as the single step in traditional serial order picking to make the concept worthwhile.

The table in Figure 2 shows that labor savings sufficient to justify the cost of mechanization only occurs when the rate for picking individual orders is expected to be slow and the total case picking requirement is high. This occurs when the same product is required in small quantities for many orders at the same time. This is a common occurrence in centralized distribution of high volume consumer products to a chain of retail stores. Groceries are a typical example. Shoes, Hardware, and PC Software are other examples.

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While batch picking is more efficient, it only starts to pay off at higher volumes

Because very popular products may be ordered several cases at a time for each store/order, they generally can be picked at higher rates in the traditional order picking fashion. The midrange and slower moving products are more time consuming to pick and represent the best opportunities for wave picking and sortation. Once a major equipment investment can be justified for those products, it sometimes makes sense to use the same method for the fast movers as well, if there is sufficient capacity.

CHOOSING MATERIAL HANDLING EQUIPMENT

The picking of cases of product in wave picking environments may be accomplished in a number of ways. Several common ones are listed below. A few are illustrated and described below.

Pick-to-pallet on pallet jack

This is the simplest and most common way to accumulate cartons for a batch of customer orders. The investment in equipment is low, however, it is difficult to pick from any location except at floor level, so the pick path may be quite long. Aisles must be wide enough to accommodate pickers passing one another and most probably, lift trucks for storage and replenishment moves. Cases must be rehandled to induct them onto the sorter.

Pick 2-4 batches to pallet or roll cage train

This method is very similar to picking to a pallet jack, but permits several batches to be combined to reduce the number of times that a picker must travel the pick path. To make the picking most efficient, a special pick list aggregating the requirements for the batches and then indicating the allocation by batch must be prepared.

Continued on page 3


Published in March 2013

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