Often people try to isolate a mechanical factor for VFFS inefficiency, for instance, sealing. And that's often the case. But before jumping to conclusions it's always important to take a system view of the entire mechanical stack-up, environmental factors, and film.
Everyone's aware of a significant change - like between film suppliers - but sometimes very subtle factors can manifest in frustrating ways. One example is the variety of ways in which coefficient of friction (CoF) affects overall system performance and how overlooking this simple film spec and have effects on the performance of your operation.
Before we break down specific solutions to help solve for lesser-than-ideal CoF film specs, let's break down what coefficient of friction is why it matters for VFFS packaging operations.
What is Coefficient of Friction?
The coefficient of friction is defined by Encyclopaedia Britannica as the “ratio of the frictional force resisting the motion of two surfaces in contact to the normal force pressing the two surfaces together”.
There are two different kinds of friction at play when two surfaces are in contact- static friction and kinetic friction. Static friction is the friction between two objects that keeps the objects at rest- like sitting a heavy object on a sloped surface - high static friction will keep it in place.
To move the object the static friction between these objects must be overcome and kinetic friction takes over. Kinetic friction is the friction occurring between two objects that are moving relative to each other.
How do Coefficient of Friction and Packaging Film Relate?
The coefficient of friction of your film plays a huge part in the success (or potential issues) of your packing operations, especially for vertical form fill seal machines. It’s important that VFFS packaging film have a very low coefficient of friction so the film creates as little resistance as possible when traveling through all of the rollers of the film carriage and over the forming collar.
When the film comes over the forming collar, it makes over a 90-degree direction change, which has the potential to generate higher kinetic friction. This friction can put stress on the film, causing wrinkling, or in the worse cases, tearing of the film material.
Sustainable Packaging Materials and Coefficient of Friction
Many producers today are navigating sustainable packaging demands from their retailers and customers but many of these renewable and recyclable materials can come with unique challenges like higher (even incrementally) coefficient of friction that can cause issues with output speed.
This is one of the main factors to consider when evaluating paper packaging for your products. The coefficient of friction of plastic film is much lower than paper films, so production tactics and machinery innovations are often coupled with this change to help keep OEE high.
VFFS technologies to help run films with a higher Coefficient of Friction
If higher friction caused by your packaging material is unavoidable, there are material handling strategies that can help to prevent wrinkling and tearing of your film caused by the added stress. Relying on just the draw-down belts below the forming collar to pull the film adds stress to the film as it travels over the collar. It’s possible to spread out the energy needed to move the film, such as using a power unwind on the back of the machine, which “pushes” the film from the back as the draw-down belts “pull”.
Not only does this help to reduce the stress on the film, but it can also have the added bonus of being able to use larger rolls of film, resulting in less downtime and labor costs.
Films with higher CoF also put higher stress on your wear parts, particularly the forming set. Over time, the edge of the forming collar can be worn down by this friction but simply reinforcing the edge can help to prolong the life of this spare part.
Your First defense- Partners in Packaging
It’s never a bad idea to loop in your suppliers when evaluating any production changes, whether they relate to your film, product doses, bag styles changes, etc. Often, machinery vendors are willing to test film samples for you and make operation recommendations based on their findings.
ROVEMA even has a database of nearly 200 sustainable film options from multiple films suppliers and their testing/spec recommendation results so that customers can make a sustainable film transition efficiently and confidently.
Machinery vendors and film suppliers often have close relationships and ones that truly consider themselves as your business partners will work collaboratively to assist you in taking a new packaging material or other innovation to market.