- COF < 0.25 is considered LOW COF, HIGH-SLIP
- COF > 0.45 indicates HIGH COF, LOW/NON-SLIP
COF is reported in two values: static (µs) - the force needed to begin movement, and kinetic (µk) – the force required to maintain movement. Generally, Kinetic COF is used for roll films. Static COF is of greater concern for stacked or palletized items.
Here's the coefficient of friction formula:
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μ = f÷N
Where μ is the coefficient of friction, f is the force of friction, and N is the normal force.
Coefficient of friction is broken down into two categories: Static and Kinetic. Here's the difference:
- Static COF is a measure of the force required to begin movement. In the packaging industry, static coefficient of friction is generally used to measure the friction between bags when they are stacked or cartoned (plastic on plastic).
- Kinetic COF is a measure of the force required to maintain movement. In the packaging industry, kinetic coefficient of friction is generally used for rolls of plastic films processed on form fill seal packaging machines (plastic on metal).
Customized COF’s are achieved by adding a “slip agent” to a film resin during production. The traditional approach to reducing COF involves adding a compound that is incompatible with the film resin, and will migrate to the surface of the film over time. Non-migratory slip agents offer benefits in the area of thermal stability and consistency, but can affect film clarity.
COF can be affected by a number of factors including antiblock additives, corona treatment, antistats, inks, varnishes, adhesives. Because laboratory testing cannot simulate every element of any packaging process, COF should not be considered an indication of system-specific performance. It is, instead, a means of reproducing frictional properties shown to be successful under a given set of conditions.
How do you measure coefficient of friction?
Coefficient of friction is usually measured using a friction testing device. Basically, the plastic packaging film is dragged across a stainless steel surface and the force required to move the film is measured. The person doing the testing then does some mathematical calculations to arrive at the coefficient of friction. The resulting number will range from 0 to 1.
Why is COF important in packaging?
So what does COF have to do with automated packaging lines? A lot. More specifically for form fill seal machines, packaging film COF refers to the level of 'slip' that the packaging material has when interacting with different parts of the machine.
For vertical form fill seal machines in particular, packaging film interacts with a machine component called pull belts. The pull belts press against the outside of the film, sandwiching it between the belts and the forming tube, helping to draw it through the packaging system. To optimize performance, there must be enough friction on the outside of the film to overcome the friction on the inside. In other words, the inside of the film must be 'slippery' enough to be able to slide over the forming tube assembly (FTA) when the pull belts draw the film through the machine. The image below illustrates this concept:
What is the ideal range for VFFS film coefficient of friction?
The packaging material processed through a vertical form fill seal (VFFS) machine must be within an ideal range for COF in order to be compatible with the machinery. Below are industry-accepted ranges for ideal coefficient of friction values:
Inside COF: 0.200 – 0.300
Outside COF: 0.300 – 0.400
‘Perfect film’ will have a COF difference of 0.100 from inside to outside, with the outside being the higher number. However, film suppliers indicate that ‘perfect film’ is nearly impossible to achieve in practice. The most important thing is to make sure your film is always more slippery on the inside than the outside.
What happens if inside COF is higher than outside?
In short, this is bad. When inside coefficient of friction is higher than outside, a number of VFFS machine malfunctions can occur, including:
- Pull belt slippage
- Excessive pull belt wear
- Reduction in packaging speed
- Poor film web tension
- Inconsistent package dimensions
- Film tracking problems
- Seal problems (leakers)
- Film registration problems
- 'Squealing' noises when the machine is in motion
To put it simply, if the packaging film's inside COF is higher than outside COF, your VFFS machine may not be able to run consistently and efficiently, if at all
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