Words like “containment force” and “pre-stretch” get tossed around often in the field of packaging and logistics, but what does “pre-stretch” really mean and how can increasing your pre-stretch knowledge save you money?
When a stretch wrapper is used to apply stretch film to a pallet, the film is elongated using a pre-stretch carriage on a wrapper. The film gets pulled between the rollers and is stretched prior to being applied to the load. This is called pre-stretch.
Why is this important?
Film that is stretched properly will hold the load more securely, be more cost effective, and lead to a more productive plant. This is because properly maintained pre-stretch tends to minimize film breaks when wrapping skids. As you can see, there are a variety of reasons why you would want to replace your pre-stretch rollers.
What are some signs that your rollers need to be replaced (in order of amount of occurrences):
The rollers contain debris, cuts, and rough edges.
The rollers are bald. Similar to a car tire, after much use the tread or grooves tend to smooth out.
The roller shafts are worn where they spin in their top and bottom bearings.
What are the common causes for having to replace your rollers?
When the film gets stuck around the rollers, many machine operators tend to cut the film on the pre-stretch rollers using a pallet knife. What the machine operators don’t realize is, as they are cutting film from the rollers, often times they end up cutting the roller as well. If your pre-stretch rollers have cuts or gaps on the surface, it will puncture holes into the film as the film is pulled tightly across the rollers. Once film encounters a snag, the tear tends to run like a stocking until it ultimately breaks.
If rollers are worn down to a smooth surface, they should be replaced. The rubber on the rollers should be soft and should have ridges. Like tires or shoe treads, some rollers have a specific pattern that is proprietary to the manufacturer. The purpose of the ridges is to grip the film to stretch it. If the pre-stretch rollers are worn or smoothed out, the rollers in the pre-stretch carriage cannot grip the film properly. Instead of being stretched, the film slides across the rollers causing friction and heat. When film slides across pre-stretch rollers, the risk of film breaks is dramatically increased. Film that slides (rather than stretches) across the rollers causes inconsistent pre-stretch, therefore the ultimate amount of stretch applied to the load is less than the ideal 70%. When the amount of stretch applied to a load is not consistent, additional stretch film may be used, or damages occur during shipping. Either way, it’s not good for your bottom line.
Why is this a problem?
When pre-stretch rollers are cut, damaged or worn, operators will notice the film breaks more frequently. When this happens the most common response from machine operators is to reduce the force-to-load by dialing it back on the control panel or in the touchscreen. However, reducing the force-to-load doesn’t actually solve the problem since the origin of the issue is the rollers. Reducing the force-to-load eventually presents a bigger problem, because often the loads don’t have enough holding force to say together.