The authors are Marshall S White, Ph.D., Professor EmeritusDepartment of Sustainable Biomaterials, Virginia Tech and Laszlo Horvath, Associate Professor and Director of the Center for Packaging and Unit Load Design
There are two important principles of unit load design:
- The cost of the empty packaging on top of the pallet is significantly more than the cost of the pallet.
- The most important measure of unit load design sustainability is the mass (weight)/use of the empty packaging and pallet.
From these two principles, we know the most effective way to reduce packaging costs in the unit load is to focus on the cost of the packaging on top of the pallet. Typically, this cost reduction will coincide with a reduction in packaging mass, a smaller ratio of packaging weight to product weight, and a more sustainable unit load design.
Research at the Virginia Tech Center for Packaging and Unit Load Design has confirmed that increasing the stiffness of a pallet deck reduces the compression stresses on rigid packaging such as corrugated containers (boxes) and plastic pails. If these packages with product inside, are to be palletized, the most important design criterion is compression strength. As shown, as the pallet deck sags or deforms due to the weight of the packaged product the compression stresses become higher and more concentrated. A stiffer pallet deck will deform less and distribute the compression forces more uniformly, and therefore the maximum stresses are reduced. This clearly implies that pallet stiffness can be used to reduce packaging costs, product damage, and leakers of liquids when bottles or pails fail.
This interaction between packaging and pallets was recently studied by Chandler Quesenberry¹ and Mary Paz Alvarez Valverde², both, recent graduate students in the Packaging Systems and Design program at Virginia Tech.
The effect of pallet deck stiffness on the compression strength of corrugated containers
According to Chandler Quesenberry, increasing the wood pallet deck stiffness by simply increasing the thickness of the deck boards by ¼ of an inch resulted in an effective increase in average box strength by 16.3%. He indicates that this is approximately the difference in strength of a 40 ECT and 32 ECT box. Based on then 2019 prices of a unit load containing 48, 16” X 10” X 10” boxes, switching from a 40 ECT to a 32 ECT box saves about $4.31/unit load. At the time the increase in pallet cost was $3.24 for a net savings of $1.07/unit load. While this is not much it adds up depending on number of unit loads shipped. The mid-size manufacturer usually ships 200-400 thousand pallets a year, thus $1.07 savings per unit load could results in a $214-$428,000 annual savings. When one compares the weight reduction of the packaging to the weight increase of the pallet, the weight of the unit load does not change significantly. Thus, the change does not significantly change the sustainability of the unit load.
However, if this unit load contained five layers instead of four and 60 boxes instead of 48, the metrics change significantly. The net cost savings would be $2.00 per unit load and the reduction in packaging and pallet mass per unit load would be 1.7 pounds. Thus, the new, stiffer, pallet design results in a more sustainable unit load design. Saewhan Kim who is currently a Ph.D. student at Virginia Tech packaging program, conducted an extensive study to quantify the environmental impact of the proposed holistic unit load design idea by Quesenberry. He found that the environmental impact of the unit load system can decrease as much as 23% if the interaction between pallet stiffness and corrugated boxes are used to optimize the unit load.
The effect of pallet deck stiffness on the compression strength of plastic 5 gallon pails
According to Mary Alvarez Valverde, a similar increase in pallet deck stiffness results in an effective increase of average 5 gallon plastic pail compression strength of 14%.
This corresponds to the difference in compression strength of a 90 mil wall thickness pail and a 75 mil wall thickness plastic pail. Based on current prices the price difference between the 90 mil and 75 mil pails is about $0.54/pail. In the three-layer, 36 pail unit load this a $19.44 savings in pail spend per unit load. However, the stiffer pallet is more expensive, so the net savings would be about $11 to $12 per unit load. Additionally, the 36, 75 mil pails weigh about 13 pounds less than the 36, 90 mil pails. The stiffer pallet will weigh about 8 pounds more so the net reduction in packaging and pallet mass per unit load is about 5 pounds. The unit load design is now more sustainable not just because of the overall weight reduction but also because 8 pounds of fossil fuel based plastic packaging is exchanged to 8 pounds of wood fiber-based packaging.
How to stiffen the decks of wood pallets
Based on these two examples one can simply increase the thickness of the deck boards. However, this method is relatively expensive. If one remembers that the deck boards act as beams and remember that beams hate spans or said another way, the shorter the span between stringers the stiffer and stronger the deck, the least expensive way to stiffen a deck would be to add a wing. Another way would be to add another stringer. Both of these design options reduce the span and stiffen the deck of the pallet. Also remember that dry deck boards are stiffer than green deck boards.
How stiff is stiff enough?
The Best Load™ and the Pallet Design SystemTM (PDS) software can be helpful. However, laboratory and field testing are always recommended, much as was done in the two examples. Standard tests could tests based on ASTM D1185 to verify the load capacity of the pallet and ISTA 3E to ensure that the fully built unit load is safe to transport through the supply chain. Virginia Tech’s Center for Packaging and Unit Load Design is very experienced in conduction such tests. These tests should be followed by field trials to verify design performance.
These examples demonstrate a fundamental principle that within most unit loads, stiffer pallets are better. In addition to reducing compression stresses on packaging, such pallets, better stabilize unit loads in transit. They better protect the product from damage due to impacts or shocks during handling. Stiffer pallets are typically more expensive, but this cost can be more than offset by the associated reduction in packaging cost. Stiffer pallets result in more sustainable unit load designs. Unit loads with stiffer pallets are safer to use.
- Quesenberry C., L Horvath, J Bouldin, M.S. White. The effect of pallet top deck stiffness on the compression strength of asymmetrically supported corrugated boxes. Packaging Technology and Science. 2020; 1-12. https://doi.org/10.1002/pts.2533
- Valverde M. P. A., L Horvath. Effect of wooden pallets characteristics on the compression strength of palletized plastic pails. Packaging Technology and Science. 2022; 35(10):719-727. Doi:10.1002/pts2672
- Kim, S. Investigation of the environmental effect of unit load design optimization using physical interaction between pallets and corrugated boxes. [Master Thesis], Virginia Tech, Blacksburg, VA.