Working in
virtually nearly every facet of our lives, friction is the invisible
force that allows, or hinders the movement of two surfaces that are in contact
with one another. Coefficient of
Friction is the ratio of the force that maintains contact between an object and
a surface and the frictional force that resists the motion of the object.
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.
F 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.
F 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.
F In either system, too much friction can
result in lateral slipping that leads to poor seals (leakers).
F Too little friction on the outside can
cause packages to slip or fall-off-of inclined conveyor belts.
F Too much friction on the outside can slow
packages’ progress down delivery chutes.
F Too little friction on the outside can
result in packages sliding off of stacks or pallets.
You can see that
various applications require different fictional properties. Therefore,
Coefficient of Friction (COF) measurement is very important and used for
process control to ensure the consistent production of a kind of packaging
(films etc).
Test Principles
As mentioned
above, COF is a simple ratio equal to the force required to slide on surface
over another, divided by the force perpendicular to the contacting surfaces.
COF =
|
Force to cause sliding of film surfaces
|
sled weight
|
It’s a unitless coefficient.
We can see that coefficient of friction should be between 0 and 1. Higher
values indicate more resistance to sliding. There are two kinds of coefficient
of friction – static and kinetic COF. Static COF is a higher value and is related
to the force to get movement started.
International Standards
ISO 8295 -- Plastics
-- Film and sheeting -- Determination of the coefficients of friction
ASTM D1894 --
Standard Test Method for Static and Kinetic Coefficients of Friction of Plastic
Film and Sheeting
Coefficient of Friction Tester
Labthink Coefficient of Friction Tester MXD-02 provides a digital display of
static and kinetic Coefficient of Friction (COF). It is designed according to
ISO 8295 & ASTM D1894. It has a high accuracy test result (0.5% of reading
value).
Weight of sled is standardized
by ASTM D1894, which is 200g.
