Ask Steve
September 20, 2024
Question from Allison: Will equipment condition affect our load containment?
Hi, Allison,
Great question! The simple answer is yes, absolutely. But there is more to the story. The extent to which machine condition affects load containment will depend on the type of film used, the type of load to be contained, the transportation method moving the product to its destination, and finally, the part of the equipment that may be underperforming. We will look at all of those.
Let’s start at the beginning. Higher levels of load containment typically are achieved only with a performance film, which needs to be stretched more than commodity films. The film is stretched in the machine before it is applied to the load, then stretched again as it is being applied. The first stretch is called pre-stretch, which occurs in the machine’s pre-stretch carriage. The film is pressed around a pair of textured driven rollers. (The texture helps grip the film, preventing it from slipping on the roller.) The infeed roller and the exit roller rotate at different speeds. There are three ways this can happen. The input roller may be smaller in diameter than the exit roller. While they turn the same number of revolutions, because the exit roller circumference is much larger, the film stretches as it is pulled through. If the film does not slip, enough force is generated to stretch the film between the rollers. Another type of machine may gear the rollers to rotate at different rates or revolutions per minute by using a belt or chain to connect them. This produces the same result by stretching the film between the rollers. More sophisticated machines may use servo drives, where each roller has an independent motor, the speed differential between the rollers is electronically controlled rather than having them mechanically connected. And finally, some machines may employ a combination of these methods. Regardless of how it is accomplished, here is an example to help visualize pre-stretch; 1 foot of film goes into the carriage while 3 feet of stretched film comes out, and this is expressed as a percent (%) of stretch or its pre-stretch. Most machines come standard with 170% or 200% pre-stretch. If you are using high performance film you may need adjust pre-stretch to as much as 250%, or if you use our Rapid Bander / Rapid Film, which gives the best load containment, it will need to be set to 275% to 300%. It is always necessary to adjust the speed difference of the rollers to provide the required pre-stretch, based on the film you are using, to achieve proper load containment.
The objective is to ensure your load arrives in good condition. Now we will discuss how the type of load factors in. Some loads have contents that are easily crushed; therefore, you may need to use much higher pre-stretch and less applied tension (stretch between the carriage and the load as it is being wrapped). High pre-stretch will increase the film’s stiffness or resistance to further stretch without having to apply so much force that it crushes product. In this case, the machine’s pre-stretch performance is even more critical.
There is also a relationship between machine condition and transportation method. I have previously described the three primary forces that act on a load during transportation. We optimize load containment based on how the product is being transported. Normal over-the-road transportation applies moderate and repetitive forces to the load. Secondary stretch (how much stretch is left in the film after the load is wrapped) will yield additional stretch when a force is applied over an extended period. This happens when traveling up or downhill, sweeping turns, or starting and stopping. To counter the effects of secondary stretch, you need to use higher levels of pre-stretch. High impact forces are more sensitive to tension (or the film’s gripping force that is directly applied to the load) than they are to secondary stretch.
Now for the machine condition. We will consider how worn pre-stretch rollers, gearing / belts, or improperly adjusted idler rollers (which press the film into the driven pre-stretch rollers) will reduce the percentage of pre-stretch. Without measuring it, if your machine is set to 250% pre-stretch, you may not have any reason to believe that is not happening, but in reality, it may be more like 200% or less. This decrease in pre-stretch reduces the total stretch of the film after it is applied to the load, increases the film usage, reduces the unitizing force, and more importantly, decreases your overall load containment. So, the bottom line is that you pay more and get less.
When it comes to assigning maintenance priorities, the stretch wrap machine is often overlooked. Upstream equipment (filling machines, palletizers, etc.) get the focus because typically when one of those machines is underperforming, it is quite obvious. The stretch wrapper’s performance degradation may be more subtle than other equipment, and because of that, scheduled maintenance may often be postponed (and then maybe forgotten altogether). It is important to remember that this machine is your key defense against costly load failures.
We know that it is hard to find qualified maintenance technicians and most companies are significantly understaffed in that department. To fill the gap, we have a nationwide footprint of highly qualified Field Service Engineers that can evaluate the condition of your equipment, recommend service that requires immediate attention, along with things that should be done to optimize performance. We do not operate the service program as a profit center, so no one is pushing parts or labor hours that are not necessary, just to meet their sales quota. They focus on you and your equipment and to ensure you receive the value you expect and deserve.
Thanks for asking!
