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ABC's of Package Tests: Coefficient of Friction

Coefficient of Friction (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.

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.

Packaging materials are typically cut, fed, and passed through production at extreme speeds. Films and cardboard are usually coated with inks and lacquers, which impact their surface characteristics. Materials that have an uneven or rough surface texture will generate a considerable amount of friction and are likely to affect the production line efficiency. Testing the surface’s frictional qualities would not only ensure that the raw materials used are appropriate for machinery but would also prevent the possibilities of damage or jams caused at the time of the packaging process.

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.