with Luke Venechuk, Sr Packaging Engineer
What happens when films are run at high wrapping speeds? Why do films breaks increase so much for these applications? Will the same film perform differently at differently at 12, 24, and 40 RPM?
There are some people in our industry that believe film breaks increase at high speeds because the film heats up and lose its integrity. To find out whether this theory is correct Highlight ran films at different speeds and measured its properties to compare.
The graph below shows a single film being ran at speeds equivalent to 12, 24 and 40 RPM:
In the graph above we can see that the force to stretch the film (on the left-hand Y-axis) increases slightly as the application speed increases, this is about a 2% increase if we look at the force to stretch at 200% (stretch level is the bottom X-axis, and 200% is within the middle red circle).
We can also see that the stretch at break or ultimate stretch does not change significantly (the right hand red circle).
The increase in force is due to the fact that we’re doing the same work, stretching the film to the same level, in a shorter amount of time, which means we have to pull a little harder to end up with the same result.
There are some small differences that take place for the film at different speeds, but it’s certainly not anything that would cause the film to break.
So what is causing film breaks at high speeds?
Highlight found that the cause of most film breaks at high application speeds is due to poor corner compensation. Corner Compensation is a system within most stretch wrappers that increases the film payout rate for the corners of the load and slows the film speed on the sides of the load while wrapping, usually by changing the speed of the prestretch rollers throughout the wrap cycle
Here’s a top view of a rotating load:
Corner Compensation is needed as the film demand is constantly changing as the four-sided load rotates. Here’s an example of a changing demand rate through one revolution of wrapping:
As we wrap faster and faster, the equipment has less time to change its film speed, so the rate of acceleration for the film delivery gets exponentially higher.
At 12 RPM the film goes from 131 to 200 ft/min in .66 seconds. At 50 RPM the film goes from 550 to 835 ft/min in .16 seconds.
This rapid acceleration creates more stress and is the reason why we see more film failures at high wrapping speeds. This is compounded by the fact that many corner compensation systems perform worse as they age, becoming “out of sync” with the wrapping needs of the rotating load.
What’s the solution to all of this?
The correct approach with the film or the equipment can reduce these film breaks.
For the film: high ultimate stretch levels allow films to absorb sudden spikes in stretch levels without breaking. It may be possible that very stiff films will resist stretching when exposed to spikes in stress.
For the equipment: The best solution is to fix the corner compensation system. Highlight responds to corner demand and changing film feed rates without the constant acceleration and deceleration of the prestretch roller, where the most stress on the film occurs. By moving our corner compensation from the high stress area of the prestretch section to the low stress area after that we can almost completely eliminate film breaks due to corner compensation at high speeds.
For more information visit www.highlightindustries.com or call us at 800-531-2465