Unit Load Design Approach Offers Insights Into Effect of Deck Stiffness on Package Compression

A stiff pallet deck can help reduce stress on packages; may provide opportunities for overall packaging cost reductions

Dr. Mark White, Professor Emeritus, Department of Sustainable Biomaterials, Virginia Tech, Blacksburg, Virginia and President of White and Company LLC a Division of Ongweoweh Corporation, Ithaca, New York, spoke this April at ISTA TRANSPACK in Orlando, Florida. Using the BestLoad(™) supply chain simulator as an instructional tool, he presented the science behind the Systems Design Process for unit load design. A selective summary of the paper appears below. To read the complete paper, click here.

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By way of introduction, unit load systems have three primary components; the pallet, layers of packaging, and the equipment used to move, store, and ship unit loads. Today these components are typically designed by vendors with little interaction between designers. This Component Design Process has led to supply chains operating with significant avoidable cost. To improve the safety, sustainability, and efficiency of these supply chains, designers must make a fundamental shift to a Systems  Design Process. To accomplish this we need to research how these components mechanically interact and to educate and train a new population of logistics and packaging professionals, knowledgeable in the design of all components. These new professionals must be provided tools that model how the components interact. In this paper the process of systems based design of supply chains has been demonstrated using a new supply chain simulator Best Load™.

Dr. White observes that with a wood pallet, it is widely recognized that the bearing area is a function of deck design and specifically spacing between deck boards of the top and bottom decks of the pallet.  What is not well understood is how the stiffness of the pallet decks affects the bearing area and consequently the compression stress on the packaging and its contents.

White further concludes:

The following trends are evident from both measured and predicted stresses.  Compression stress on the packaging,  increases as the packaging stiffness increases.  However, the compression stress is inversely related to the pallet deck stiffness.  The connection fixity significantly influences pallet deck stiffness.  According to these test results, the manipulation of the pallet deck design can significantly alter the effective bearing area and consequently alter the maximum compression stress to which the packaged product is exposed when unitized and moved through supply chains.  The concept is to use the pallet to reduce the stress on the packaging and then reduce the cost of packaging.

Two case studies were covered in the presentation.

5 Gallon Pails      One example involves unit loads of 90 mil, 5 gallon, HDPE, plastic pails of coatings. The maximum compression stress on the pails occurs when these unit loads are stacked stored in a warehouse, three unit loads high. Each pail contains 45.8 pounds of coating material. The total weight of the product and packaging on each pallet was 1649 pounds. The Best Load™ analysis further shows that the bottom pail in the six interior pail columns are under the most compression.

A column of the current 90 mil, 5 gallon plastic pails (filled), was tested in compression. The average load at failure was 2190 pounds. An alternative, lower cost, 75 mil, 5 gallon pail was similarly tested and the average load at failure was 1604 pounds. This is 27 percent less resistant to compression. Therefore, as a first approximation of a pallet deck to safely support product in the 75 mil pail, the compression stress must be reduced from the current 22.06 psi to less than 15 psi . To accomplish this, the pallet deck must be stiffened. Since the pallet boards act as beams, it was decided to reduce the span between stringers by adding a fourth stringer and by recessing the outer stringers 1 inch. To reduce pallet cost the stringer width was reduced from 1.5 inches to 1.25 inches. The structural analysis using Best Load™ of this new pallet, predicts a maximum compression on the pails of 13.70 psi. Prototypes of the proposed unit load with the new pallet design and 75 mil pails were tested in the field to verify performance. Within three months the new unit load designs were implemented.

The cost per pallet increased by $0.98 and the cost per pail decreased by $0.31. The net savings per unit load was $10.11. This was an annual packaging spend reduction of $209,199.00

bottled water

Bottled Water     The second case study involves the stacking of bottled water. The stacking patterns on pallets can significantly affect the compression stresses on packaged product. The figure above is a Best Load™ analysis of two stacking patterns of shrink wrapped, 24 packs of 16.9 oz. bottles of water on a block class wood pallet. The stresses are greatest on the bottom layer of 24 packs. Notice the top two layers are interlocked for stability and the unit load is stretch wrapped. A change in stacking pattern alters the maximum compression stress by 29 percent. This is a potential reduction of maximum compression stress from 10.2 to 7.2 psi. Whether manually or automatically stacking product on pallets such changes in patterns have little or no effect on the cost of assembling unit loads. Therefore this potential reduction in compression stress is free and will significantly reduce leakers and related damage.

In the final analysis, it pays to take a broader view when selecting packaging and pallets. By taking a systematic approach to unit load design, you are unleashing the potential to reap new savings.

For further information, please visit www.whiteandcompany.net.