Why does stretch film have a tacky side and does it really matter if it is facing the load or facing outside the load?
Hi Brian
Thanks for sending in a great question! This will help a lot of people understand something that very few do today.
Let’s set the groundwork for the answer by covering a few basic concepts.
- The job of the stretch film is to unitize and contain the load when external forces are applied during transportation.
- We know that when stretch film is stretched, and the force stretching the film is released, the stretch film, because of its memory, will contract by some percentage.
- When the film contracts, it applies a force against the load (Force-To-Load) which is one of the measurements used in a load containment standard. (As a point of reference, when you pick up an object the force you apply to the object as you grip it is the same as the force-to-load the stretch film applies to the load.)
- This containment force draws all the elements of the load (boxes, bags, pails, etc.) together, resisting independent movement.
- Independent movement of components within the load will allow an uneven distribution of force during transportation and it is the single greatest contributor to load failure.
I know this may seem like a lot to absorb, but if you understand these concepts, you will be able to visualize what role the tacky surface plays in load containment and why its direction is important.
And now for the answers to your questions.
I will address the direction of the tack first: it should always be facing the inside of the load, and I am sure you will agree there is a good reason for this. It grips the side of the load when applied, which marginally contributes to load containment. But more importantly, If the tack is on the outside of the load, there is a high probability of damage when a trailer is loaded or unloaded because the stretch film on the pallets will stick together while one is stationary, and the other is being moved. Wraps of stretch film may be torn from the load but the worst-case scenario is that the stationary pallet may be pulled over or toppled by the pallet being moved. So, how do you know which side is which? Most all film manufacturers have an arrow marked inside the core to indicate how the roll should be loaded on your machine. Depending on the configuration of the carriage and direction of wrap the arrows may be up or down when the roll is loaded. If there is any question, you can unwind a small amount of film from the roll and rub outside-to-outside surfaces together, then do the same with inside-to-inside surfaces. One should have little to no friction and the other will tack together. The side with the tack should be applied against the load.
The answer to the “why does it have a tacky surface” question is a little more involved. Drawing on the basic concepts we reviewed earlier, when film is stretched it will contract or “recover” a percentage when the stretch force, in our case machine tension, is released at the end of the wrap cycle. Each layer applied to the load will tack or stick to the previous layer when it is under tension, and with each applied layer, the containment force it generates against the load when it contracts is multiplied. For many years, the answer to load containment challenges was simply to apply more layers or wraps to the load. But (and you knew there would be a but…), there is a point of diminishing returns where simply adding more layers generates so much containment force that it begins crushing the contents. So, while adding wraps maximized load containment, that doesn’t necessarily mean you have solved a load failure problem. It also means that you are probably putting on far more film than is necessary.
To truly optimize load containment and minimize film usage, you must reach the level of stretch necessary to remove most of the stretch in the film, which limits continued film stretch when forces are applied during transportation. There is a fine line though, because when you reach the stretch needed to maximize film performance, any puncture will cause a break in the web and then productivity suffers, and with web breaks (and the adjustments to minimize them) come load failures.
We have approached this age-old challenge by using science and incorporating some principles of physics. Our Rapid Bander technology incorporates reinforcement filaments that allow us to stretch the film several times more than conventional film, taking most of the stretch out. Yes, we still have a tack side, for all the reasons we discussed, but we do not rely on it as the primary ingredient of our load containment recipe. The result is better load containment (by reducing secondary stretch), even if force-to-load measurement are the same, with some added benefits; you will apply about half the film, and half the wraps.
Thanks for asking!