Stretch wrappers come in a multitude of styles and sizes. This article discusses turntable stretch wrappers, but many of the same concepts and essential parts can be found in other machine styles. Specifically, we will look at film carriages with pre-stretched capabilities vs. without.
First, what is a stretch wrap machine with pre-stretch capabilities? Pallet wrappers with these capabilities pull and elongate film before it is applied. Stretching can be accomplished in different ways but is commonly done with film rollers that stretch as the material passes through the carriage.
Film Carriage Without Pre-stretch
When a machine does not come with pre-stretch capabilities, the initial cost is less, but depending on usage and film consumption, the savings from pre-stretching can be substantial over time. There are tension capabilities without pre-stretching rollers, but they are less robust. In the image below, a, b, and c in Figure 3-4 hold the film correctly on the shaft. The plate and roll mount holder fit within the core to hold it in place, and the fastening screw tightens it.
Figure 3-5 below shows tension adjustment on a machine without pre-stretch rollers. Other wrappers can have additional rollers with a tightening spring on top for tension. Either option places tension on secondary rollers to stretch the film as pallets are wrapped.
Tightening in this manner can substantially increase stretch rates compared to wrapping without, but it still can present problems. Because the secondary tension roller pulls on the material during the application, sharp edges from the corners of the pallet or possibly from the products themselves can limit the tension and overall stretch that can be obtained without the material tearing.
Optimal wrapping with no sharp edges can reach over 150% stretch. An advantage of no pre-stretch rollers is the ability to run a machine film that has already been stretched. These machine prestretch rolls are cheaper and require less energy to apply. The major drawback to wrappers without prestretch rollers is not obtaining the total stretch rate. Many machine films will stretch up to 250%; the film is wasted by reaching only a 150% stretch rate.
Film Carriage With Pre-stretch
Stretch wrap machines with pre-stretch rollers pull film before leaving the film carriage. In addition to the elongation before application, machines have a tension adjustment on the main panel to control the amount of stretch while wrapping the pallet. The tension adjustment to the pallet can be changed based on the edges and corners while still reaching a full stretch because of the prestretch rollers.
The film can stretch before reaching the pallet because of the pre-stretch rollers seen in the image below, figure 3-3. The black rollers grip and pull the film. The gripping rollers are why prestretch film cannot be run with this carriage. The black rollers will rip through a thinner film that cannot fully stretch.
Like a film carriage without prestretch rollers, the roll is mounted onto a shaft. From there, it goes into the prestretch rollers to the dancing roller. The final dancing roller on the right-hand side of Figure 3-3 helps keep film taught when applied to the pallet.
The film can expand by over 300% with pre-stretch rollers and optimal tension settings. This increase can make a 5,000 ft roll more than 17,000 feet long. In addition to reduced waste, pallets will be unitized and protected from toppling.
Conclusion
There are many styles and varieties of pallet wrappers. These illustrations and explanations are one variation. The additional cost of pre-stretch rollers can pay for itself over time with film savings. Loading either carriage should be effortless after a time or two.
Choosing the cheaper option with no pre-stretch rollers can provide benefits if using a pre-stretched machine film. This wrap has already been pulled up to 90% of its stretch capabilities. A carriage without pre-stretch rollers can reach 150% stretch rates for the standard wrap, but it may not maximize some machine film’s full potential.
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