Poor unit load stabilization practices contribute to hundreds of millions of dollars in unsaleable goods annually as well as unfortunate workplace injuries and deaths. Investigators have identified load shifting, ripped or loose packaging, crushing, water damage, and other types of contamination as the major causes of these types of problems.
In the context of the materials handling environment, unit loads often experience a wide range of horizontal and vertical forces throughout the supply chain — both static load bearing forces and forces caused by movement, impacts, and vibrations, known as dynamic forces.
As an illustration of these forces, consider a loaded tractor-trailer traveling along a highway. Environmental factors, such as potholes, poor suspension system, rough road surfaces, flexing of the trailer floor, and-or improperly balanced tires can cause unit loads to experience vertical impacts and vibrations. Depending on the design of the unit load, these random vibrations can lead to large stresses, causing displacements of product and packaging within the load.
Displacements are typically greatest at the top of a unit load and in between the top deck boards of the pallet and the bottom layer of product. During the unit load materials handling process, packages in the top layer of a load will experience accelerated dynamic stresses that accumulate from layers below — similar to a spring. The bottom layer will experience more static stress caused by the total weight of the load from the layers stacked above. Together, these stresses can create instability and damage packaging and product within unit loads during shipping and handling.
Due to the forces at play around and within unit loads, load stability depends on a variety of factors that include: the design of the pallet, the design of the packaging (primary and secondary), stacking patterns, and materials handling equipment.
Stabilizing Loads
Another key component to unit load stability is the applicability and use of various load stabilization materials and practices. Common techniques typically include stretch and shrink wrap, banding or strapping, break away adhesives, and a relatively new product called stretch hoods.
A stretch hood is a tubular plastic film that is heat sealed at the top, stretched horizontally, pulled down over a unit load, and released under the pallet. They offer excellent vertical and horizontal film tension that provides long-term stability. The smooth surfaces of the stretch hood also allow for high product visibility, and it provides a waterproof barrier to the top and sides of a unit load.
The goal of external load stabilizers is to effectively secure unitized packaged products together and maintain the stacked integrity of the load on the pallet throughout the supply chain storage and distribution system.
Over the years, researchers at Virginia Tech’s Center for Unit Load Design have identified three important considerations when choosing an effective unit load stabilizer:
• Shape of the unit load
An effective stabilizer for a generic unit load may not apply to a unit load that is strangely shaped with awkward protrusions. Even small differences in unit loads, such as whether or not the load covers the entire footprint of the pallet, may change the load stabilizer needed for the application.
• Density and fragility of the product
Determining the density and fragility of a product will help establish how much containment force a load stabilizer should exert on the product without damaging it.
• Material handling environment.
Having complete knowledge of exactly how far the unit load will travel, the method of transport, and the equipment used to handle the unit load is critical for selecting the correct load stabilizer.
Although load stabilization methods vary in terms of effectiveness and cost, employing the best applicable technique can have a significant impact on reducing the cost of secondary and tertiary packaging, reducing product damage and reclamation and ensuring workplace safety.
Of particular interest, manufacturers of the newly developed stretch hoods claim the product can improve vertical load stability and help reduce shifting and protrusions that come from within the unit load by helping to connect the load to the pallet. Until now, however, little research has been conducted to support these claims.
Researchers at Virginia Tech recently completed a study evaluating stretch hoods and their effect on load shift during simulated shipping and handling. The goal of the research was to measure and compare the load shift characteristics within unit loads stabilized with 400ga stretch hooding, horizontal stretch wrap (both 80ga and 63ga were evaluated) and polyester strapping. Load shift was induced by vertical vibration and horizontal impact testing according to standard ASTM protocol.
Virginia Tech Study
For this study, test loads were constructed using 45 corrugated boxes that were column-stacked on a pallet: nine boxes per layer, five layers high. To simplify the research, each corrugated box was packed with cut-to-fit dimension lumber.
Figure 2 shows typical test units used in this study. The unit on the left (A) was stabilized with a 400ga ethylene/acetate co-polymer stretch hood (wrapping down to the bottom of the pallet). The center unit (B) represents both the 80ga and 63ga horizontal stretch wrap stabilized units; stretch wrapped test loads were wrapped using a common pattern of three layers around the top and bottom with approximately 50% overlap through the middle. The unit on the right (C) depicts the test units stabilized with ½-inch wide polyester strapping (torque = 60 inch/pounds); a total of 4 straps per load were used, two on each side.
Vibration tests were conducted using a random tractor trailer simulation according to ASTM D 5415 – Evaluating Load Containment Performance of Stretch Wrap Films by Vibration Testing. Impact tests were conducted according to ASTM D 5414 – Evaluation of Horizontal Impact Performance. Five replicates were used for each load stabilization method and each test method.
Results were determined by the following criteria:
• Container displacement: The container displacement is defined as the displacement of one test container layer relative to another. This characterizes how much lean occurs in the unit load.
• Pallet Container Displacement: The pallet container displacement is defined as the amount of displacement between the bottom layer of test containers and the top deck boards of the pallet. This is an important measure because it helps to determine how much the unit load slipped off the pallet during testing, creating overhang. Lean and overhang are indicators for loss of integrity and safety in a load.
The results measured and compared the relative performance of the different load stabilizers in terms of cumulative average and cumulative maximum displacement. Summary tables of testing results are shown in Tables 1 and 2. Effectiveness of 100% indicates the best performing stabilizer. The relative performances were calculated using the best performing stabilizer as a benchmark.
To appreciate magnitude of displacements, displacements measured after vibration testing were relatively small and insignificant for each stabilizing technique. The cumulative average maximum displacements both between container layers and between the containers and the pallets were all less than 0.85-inches.
For impact testing, cumulative average maximum displacements were of greater significance and ranged from 2 inches for test loads stabilized with stretch hoods to approximately 4.5 inches for loads stabilized with 63ga stretch wrap.
The stabilization techniques studied in this research vary in cost. The results presented here apply only to the specific products tested.
Conclusions
• Horizontal impact testing resulted in significantly more load shifting than vertical vibration testing.
• Strapping was the most effective stabilizer during vibration testing, followed by 63ga stretch wrap, 80ga stretch wrap, and stretch hooding.
• Stretch hooding was the most effective in resisting displacement during impact testing, followed by strapping, 80ga stretch wrap and 63ga stretch wrap.
• Though not statistically significant, test units stabilized with 80ga stretch wrap performed slightly better than those wrapped with 63ga stretch wrap.
(Jim Bisha is a Virginia Tech graduate research assistant; Marshall White is a consultant and former director of Virginia Tech Center for Unit Load Design; Ralph Rupert is director of the Va. Tech Center for Unit Load Design; Peter Hamner is a research associate at the Virginia Tech Center for Unit Load Design. For more information, contact Peter Hamner at the Virginia Tech Center for Unit Load Design at phamner@vt.edu.)
