Mixing the modes to improve case storage density

To reduce transportation costs for products arriving from overseas, cartons are normally hand-stacked in the shipping containers. This works quite well for reducing shipping costs, but if we’re not careful, we may give away the savings in storage and handling, once we open the container door.

Storing cartons of product would seem, on the surface, to be a relatively simple matter.
Put them on a pallet, stack them on decked rack, put several on a mini-load tray, place them on shelving in the pick face or even put them in carton flow rack. But, which one, and how much?

Most problems have a simple part, and a complicated part. Clearly, when a container arrives with large numbers of cartons for only a few SKUs, then it seems logical to palletize them for storage. However, each SKU is likely to create several full pallets, and one partial pallet. This partial pallet will underutilize a full pallet storage position. The more SKUs, and the more receipts, the more storage capacity we lose.

Providing some short pallet positions in the rack is a viable approach. However, these positions are less “cube efficient” and it’s hard to get the mix of “shorts” and “talls” just right.

A second alternative is to hand-stack the residual cartons on shelving or decked rack.
Often, this is done with the cartons 2-high per level, or even 2-deep. Generally, cartons are stored randomly in the nearest empty location. Standard shelving, with 4-5 levels can be served with input and output conveyors, providing the possibility of relatively high throughput capacity.

If this storage mode is in decked pallet racking, accessed with orderpicking trucks operating in narrow aisles, the storage density is very high. But, the efficiency of put-away and retrieval is far less than walking. When the number of cartons in storage for a single SKU is small, they may be placed directly in the pick face. Picking may progress from one location to the next as they are exhausted, or the top and bottom levels, which are less convenient for picking, may be used as reserve positions.

Storing cartons in carton flow rack is generally considered to be too expensive, unless the receipts are frequent enough and small enough that the entire inventory can be held in the flow lane. A major grocer, who can control the flow from their suppliers takes advantage of this opportunity for one-step handling.

Every once in a while, the characteristics of the inventory and activity open the door for higher levels of automation. With today’s much faster mini-load cranes (approaching 150 dual cycles per hour), cartons may be stored singly on a shelf, or several on a standard tray.

Mini-load storage turned out to be a good solution for an apparel manufacturer, conveying the cartons to the induction stations of a high-speed sortation system. Space is reserved at the front of the storage system for partially filled cartons returning from the sorter so that they may be accessed quickly the next time that SKU is needed.

In choosing which one, or what combination of these storage modes to use, we must consider:

• What is the range of carton sizes and the expected inventory in each?
• How many cartons of each SKU do I have in inventory?
• Do I need to manage inventory on a FIFO basis?
• Do I need to separate inventory by lot code or receipt date?
• How many cartons of a SKU are in a typical receipt?
• How will cartons be drawn from, and perhaps returned to inventory?

Because we can’t predict the profile of the inventory very far into the future, it’s best to bias the mix of storage modes slightly toward the most flexible alternative.

ABOUT THE AUTHOR

James M. Apple, Jr. is a Director in The Progress Group. Prior to co-founding The Progress Group in 1991, he was a Partner with Coopers & Lybrand's SysteCon division. During 1992-1995 he served as a Senior Systems Advisor with Vanderlande Industries, a major conveyor and systems provider in Europe.

Jim is an internationally recognized thought leader in the area of facility design and integrated distribution systems. His contributions to the improvement of distribution practices have been recognized by his receipt of the prestigious Reed-Apple Award, which is given for lifetime contributions to the advancement of the material handling profession. Jim has also received the Institute of Industrial Engineers' Facilities Planning and Design Award. He has written numerous articles and handbook chapters on warehousing and logistics operations and is a popular speaker on logistics seminar and conference programs.

Prior to SysteCon, Jim worked as an Industrial Engineer with IBM, was Supervisor of Facilities Planning for the Oldsmobile Division of General Motors and was Executive Vice President for an automotive aftermarket parts supplier. He holds B.S. and M.S. degrees in Industrial and Systems Engineering from the Georgia Institute of Technology.