Coefficient of Friction (COF) Testing of Films: Why Slip Decides Packaging Line Speed

When film jams on a forming collar, bags refuse to open, or stacked sacks slide off a pallet, the property in question is friction. Coefficient of friction (COF) testing quantifies film slip per ASTM D 1894 (sled method) and the classic inclined-plane method — and converters specify COF windows as tightly as tensile strength.

A C.O.F. Tester (Inclined Plane Tester) gives packaging labs this measurement in a simple, robust format.

Static vs Kinetic COF

Static COF is the force ratio needed to start sliding; kinetic COF is the ratio to keep it sliding. Machines care about kinetic COF (film moving over rollers and collars); bag-opening and stacking behaviour depend more on static COF. In the inclined-plane method, the angle at which a standard sled begins to slide gives static COF directly as the tangent of that angle — elegant, fast and free of load cells.

Film-to-film and film-to-metal COF are different numbers; measure the pairing your process actually uses.

What Moves COF in Production

• Slip additives — erucamide and oleamide bloom to the surface over 24–48 hours; COF measured too soon after extrusion reads high.
• Corona treatment — raises surface energy and COF; over-treatment makes film grabby.
• Recycled content — variable additive carryover makes COF drift; regular testing is essential for recyclate-rich films.
• Temperature and humidity — amide slip behaviour is climate-sensitive, so condition specimens before testing.

Specifying and Testing COF

Typical packaging films target kinetic COF around 0.2–0.4 film-to-film — low enough to run, high enough to stack. Agree the test method, sliding pair, and conditioning time with your customer, then verify each lot. COF joins dart impact and seal testing in the daily routine of a film testing lab.

Frequently Asked Questions