Coefficient
of Friction – “COF”
Working in virtually
every facet of our lives, friction is the unseen force that allows, or hinders
the movement of two surfaces that are in contact with one another. A packaging film’s “coefficient of friction”
(COF) provides a relative indication of frictional characteristics. Controlling COF gives processors the ability
to optimize performance and avoid problems in forming, transporting, and
storing of packages.
- In
HFFS (horizontal form fill and seal) systems, too much friction of the sealant
side of the film can lead to film dragging or jamming as it passes over metal
plates.
- In
VFFS (vertical form fill and seal) systems, too much friction of the sealant
side of the film can cause poor film feeding over metal forming collars,
inconsistent package sizes, and squealing.
- In
either system, too much friction can result in lateral slipping that leads to
poor seals (leakers).
- Too
little friction on the outside can cause packages to slip or fall-off-of inclined
conveyor belts.
- Too
much friction on the outside can slow packages’ progress down delivery chutes.
- Too
little friction on the outside can result in packages sliding off of stacks or
pallets.
COF is determined by
dividing the force needed to move one surface across another by the force
perpendicular to the surfaces. COF values
range from 0.00-1.00, with lower COF values indicating lesser resistance to
sliding, or higher “slip.”
- 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.
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.
The first step to creating and
optimizing the perfect packaging film for your product and process is talking
with an expert. To learn more about
optimizing coefficient of friction for your product and process, to be sure
you’re using the best packaging film for your application, contact Flair today.