We had a great response from the last Ask Steve post. Several of you asked about Total Applied Stretch and how it factors into load containment. So, I thought that would be an excellent follow up question to answer.
Total Applied Stretch is exactly what the name implies. It is the total amount the film that has been stretched from its starting point on the roll to its ending point on the load. It is the sum of the pre-stretch (stretch in the wrapper’s carriage) and the applied tension (stretch between the wrapper and the load). It is critical to load containment because the further you stretch the film, the stiffer (resistant to further stretch) it becomes.
In my example from the last Ask Steve, two pallets identically wrapped with the same Force-to-Load will not have the same containment if the Total Applied Stretch is different. If the film resists “secondary stretch” or further stretch of the film when a force is applied during transportation it will have superior load containment performance (because of higher Total Applied Stretch).
Let’s use our imagination for just a minute to illustrate this concept. You have 9 cubes or blocks on a table and organize them in three rows of three forming a square. Then you place a rubber band around the outside perimeter of the blocks. The rubber band pulls the blocks together because as it retracts it applies force around the perimeter. This is the unitizing force we measure as Force-to-Load. But if you hold the two outer blocks on either side with one hand and push the center block with the other, you will see the center row of blocks move. This happens because the rubber band continues to stretch, even though it initially seemed tight.
Now for the second part of the illustration. Take those nine blocks, but this time, wrap a string around them. Pull the string tight as you tie it. The string also pulls the blocks together as it applies a unitizing force around the perimeter, and it may well be the same Force-to-Load as the rubber band when no external force is applied. But unlike the rubber band, if you push on the block in the center, that row cannot move because the string does not stretch.
Secondary stretch is a load containment enemy, and Total Applied Stretch can predict how likely secondary stretch will happen.
By nature (and name) all stretch films stretch, it’s kind of their thing! Diving a little deeper, all films have a modulus of elasticity, which is the relationship between the force required to stretch over the distance (or percentage) the film is stretched. Films become stiffer as they are stretched, to a point where they reach a maximum, and that is just before they break. For stretch film to be effective for load containment it must resist stretching under the forces applied to the load during transportation. If the film stretches when those forces are applied, the load will move, and load failure can/will occur.
Knowledge can be a powerful thing! Now you understand why it is important to focus not only on Force-to-Load, but on Total Applied Stretch when evaluating a load containment standard. But, how do you know what percent of Total Applied Stretch is right for your application? Generally, most films will reach a peak performance around 200% to 250%. But, keep in mind, that while they stiffen, they will puncture easily on the corner of a pallet or sharp edge of a tier sheet or box corner. Once the film is pierced, the entire web will break. This is the limiting factor for both Total Applied Stretch and your load containment.
We use science to achieve extremely high levels of Total Applied Stretch (without crushing the load) with our patented Rapid Bander, Rapid Roper, and Rapid Roper Plus. And in doing so, we have completely changed the level of load containment that can be achieved with averages of 275% to 390% Total Allied Stretch.
We will be happy to evaluate your unique load characteristics and to help you develop the right standard for load containment, it’s kind of our thing!
As always, thanks for asking!