Ask Steve
October 21, 2024
Question from Adrian: What impacts load containment, I have heard several opinions, but I am not sure which one to believe?
Hi Adrian,
Thanks for sending in your question, and yes, there are a lot of opinions on the subject. So, for now, let’s forget the opinions and look to science for the answer! We study the science of load failures every day in our lab to understand how various forces that act on a load during transportation (on the road, but also in your warehouse operation) affect it. And it’s all about physics!
Applied forces come in three basic flavors, impact, panic stops, and normal transportation forces. Countering impact forces (very high force over a very short duration) requires as much unitizing force as you can apply. This Force to Load is like the force you apply to a coffee cup when you grasp it. Normal transportation forces cover traveling down the road turning, going uphill and downhill, starting and stopping. While Force to Load is still important, eliminating secondary stretch (how much the film will continue to stretch when a force is applied) is what will reduce load failures. Lastly, panic stopping is somewhere in between, so both Force to Load and minimizing secondary stretch are important. These examples highlight how matching stretch wrap technology to the type of load and method of transportation is key to minimize failures.
Let’s take this opportunity to dive in a little deeper. Movement is the root cause of load failures! What I mean by movement is any individual movement of any component in the load (components defined as primary packaging, secondary packaging, tertiary packaging - stretch wrap, straps, corner boards, etc), or between the load and pallet. If there is no movement within the load it is considered a solid object, and as such, when a force is applied, it is evenly distributed throughout the entire solid body. When a load is in motion and is not accelerating or decelerating then all components have the same inertia or kinetic energy. However, if there is movement within the load as a force is applied (changing the state of inertia or kinetic energy such as with acceleration or deceleration), then the force is no longer evenly distributed throughout the load. Various parts of the load will have different levels of inertia, and that concentrates significantly more force in those areas. The result is a shifting of the load layers. The more the layers shift, the more the center of gravity also shifts, applying additional downward force on the leading edge at the bottom of the load. Eventually, the downward force exceeds the structural limit of the primary package and failure occurs.
Now that we know that movement within the load is our enemy, what next? Since we cannot always control the forces applied during transportation, we have to determine, within reason, how much force we need to withstand (since a pallet load of the product is typically not a solid object). A Load Containment Standard is a compromise that includes considerations like: How do you normally ship product (Full Truckload, Less Than Truckload, Intermodal, Over the Ocean, etc.)? Are your pallets double stacked? How much do your pallets weigh? How much is a pallet of product worth? Is there an environmental impact if a load fails? What load shift requirements do your customers have? Those are the driving factors that are used to establish a load containment standard, or how much force your load can withstand before failure.
Once you have clearly defined your standard, each element of your packaging must be evaluated based on its contribution to meeting your standard. Sometimes changes that look good on paper have significant unintended consequences. For example, we helped a customer whose marketing department changed the look of a carton. It looked great, but immediately they began having load failures. The failures were caused by the reduced friction of the new printing on the carton compared to the original packaging, and as a result, the layers shifted on each other much easier. We were able to fix that using our patented stretch wrap technology, but without that, they would have had to make a very tough decision.
Another example, that we have seen many times, is when a company downgauges its packaging to achieve sustainability goals. Again, unintended consequences. When I look at a plastic bottle for instance, I don't just see a bottle, I see a structural member. Think of each layer on the pallet as if it were a floor in a building. Consider that the bottle on the bottom layer must withstand the force of perhaps ten filled bottles above it (twenty if pallets are double stacked). Downgauging reduces the structural integrity of the bottle. It may withstand the weight from above in a static condition, but when dynamic transportation forces are applied, and if there is layer shift, the bottle may no longer be able to withstand that additional pressure.
Shrink bundling is also a key contributor to load containment. Does it pull the individual components together tightly? Is the Coefficient of Friction high enough to resist shifting? Is the pattern of the shrink bundled components stable (i.e. width equal to, or greater than, 2/3 of the length)? Are tier sheets used to help with weight distribution? Are the pallets you use in reasonably good condition, in other words, will the stretch film be able to grip the corners properly?
This is just a sampling of what needs to be considered when talking about the impact to load containment. Using technology, we can help you achieve your load containment standard, even if some of the elements are less than ideal, and it will be backed up with science vs someone’s opinion.
Thanks for asking.
