How do you test for load containment and is it the same as an ISTA test?
Hi Art,
Thanks for submitting your question. It is something that should be asked before sending product to any lab for containment testing. The test protocol we use in our lab is very specialized and different from some other labs, and from the test protocol used by ISTA.
To understand the difference, let’s take a closer look at our objectives. The ISTA test is a pass/fail test where a certain threshold is met, similar to Underwriter’s Lab. As an example, when you see the UL label on a product, it means that product has met certain requirements, but it DOES NOT indicate how well it performs. The ISTA test criteria was developed based on conventional methods of load containment. So, passing an ISTA test does not guarantee loads will not fail during transportation. Other test labs may use equipment like vibration tables and are typically more focused on testing primary and/or secondary packaging. As a result, some of the test methods they use do not apply when measuring load containment.
In contrast to most labs, we test loads to the point of failure, so the limits of load containment can be quantified. To optimize load containment, it is critical to know how much force will be necessary to cause failure. While our methods may be similar, our objective is quite different.
From our studies, we know that there are three separate and distinct types of force that a load may experience during transportation. For our purpose, it is critical to apply each one separately so we can quantify its effect on the load, since some loads may be more susceptible to only one of the applied forces. What we do to optimize for load containment may vary based on which of the force types most affect the load. Therefore, it is only logical that we incorporate three testing methods: each simulating a real-world condition. After analyzing the data and video that we collect from each of these tests, we optimize the load containment, focusing on the specific force that effects the load the most.
Now, let’s take a step back for just a moment and review what those forces are and how we apply them to test load containment. Imagine a load about the size of your hand and it weighs one pound. Every time the truck turns, travels uphill / downhill, accelerates or brakes, the forces that act on the pallet would be similar to your hand pressing against the side of the imagined load. If you are turning right, you would press the right side, and the opposite if you are turning left. If you are going uphill or accelerating, you would be pushing against the front of the load. If you were decelerating or going downhill, you would be pushing against the back. Most normal transportation forces generate about .5g max, which means that you would be applying about ½ pound of force (.5 x load weight). If your load weighs 2,000 pounds, that would be 1,000 pounds of side force that the stretch film must contain. That force is applied and removed over a relatively long period of time (not instantaneous). Although if you are riding in that truck, it may seem instant to you. From a previous Ask Steve, I talked about inertia and momentum (a body remains in motion until a force equal and opposite acts upon it). When the driver turns the wheel, steps on the gas, or brakes, the force applied to the load is limited by the traction the tires provide and dampened by the movement of the suspension. Even the most abrupt changes the driver makes take some fractions of a second to transfer to the load. In contrast, trucks backing hard into dock bumpers, rail car coupling, and trailers running up over curbs are examples of high impact forces that are applied instantaneously, and they affect the load quite differently. An impact force can generate 7g or so, and do so in under 30 milliseconds, less than a blink of your eye. Again, if our load weighs 2,000 pounds, we have just applied 14,000 of force over a fraction of a second to the side of the load that the stretch wrap must contain.
Our Transportation Simulator will generate the “normal over-the-road forces” to the load, up to .84g by tilting the pallet, allowing gravity to create the side force. We begin with a nominal tilt angle holding for a long period or dwell, then returning to 0 degrees. We perform the test again, increasing the tilt by a couple of degrees and continuing with this procedure until the load fails. Vibration over a range of frequency can be applied during the test to simulate road vibration, which reduces friction between the layers of product. If there is stretch left in the film after the load is wrapped, this test will quantify its effect on containment. We scan the side of the load to create a digital image or profile, so we can identify where movement occurs with each successive test and measure any offset. We also video the test, so each frame can be meticulously analyzed to identify the nature of movement within the load.
For high impact forces, such as a truck backing into a dock and hitting the bumpers hard, railcar coupling, and trailer curb jumping, we place the test pallet on a sled which rolls on a long frame. The frame is adjusted to a precise angle and then the sled is accelerated by gravity over a precise distance to a point of impact. With this test equipment, we can generate well over 40g of force within a few milliseconds, although we would not expect any load to survive that level of force. We start with an impact of about 1.75g and with each successive test, we incrementally increase the impact force to a point of load failure. Profile scans and video of the load allow us to analyze movement within the load. In addition, we have data loggers that measure the force and velocity the load experiences at the point of impact.
The final test is a horizontal acceleration test. In this test we place the pallet on a horizontal sled and accelerate it over a specific distance using precise hydraulic controls. This test simulates panic braking where the driver locks up the wheels. The force that panic braking generates is usually under 2.5g, which is the limit of traction the tires have to a good road surface, so we begin our test there. As with the other tests, we capture the profile, video, g force applied and velocity profile.
Once the load is fully optimized and we have collected all the data, we publish a comprehensive report of our findings, conclusion, and recommendations. This report also includes all the data we collected.
So, while the test methods we use may be very similar, how we apply them is very different, and is a critical tool in optimizing load containment, reducing loss from unnecessary damage while using only the amount of stretch film you need.
Thanks for asking!