What Is Coefficient of Friction Testing: Standards, Values & Film Specs

How the Sled Test Works: Equipment and Procedure

The physical setup for COF testing is straightforward and consistent across both major standards. Film is laid flat on a horizontal test plane. A weighted sled — a flat block covered with a second piece of the same film or the target mating surface — is placed on top. The load frame then pulls the sled across the specimen at a controlled, constant speed using a thin line routed through a pulley system. A load cell attached to the line records the pulling force continuously throughout the stroke.

The force trace produced by the instrument has a characteristic shape: a sharp peak near the start of motion, followed by a lower, roughly flat region as sliding continues. The peak force is used to calculate static COF; the average force across the sliding portion is used to calculate kinetic COF. Both values use the same formula — COF = measured force / sled weight — which means the sled weight must be fixed and documented for any comparison between runs to be valid.

The test is fast, typically completed in under two minutes per specimen, and the equipment required is standard laboratory hardware: a calibrated load frame (instruments from manufacturers such as Shimadzu or Instron are commonly used in packaging labs) paired with a precision sled of defined weight and a matched load cell. The same fundamental sled method underlies both ASTM D1894 and ISO 8295, which is why the two standards produce comparable — though not directly interchangeable — results.

Understanding the distinction between these two values is not academic. Static COF governs plastic-on-plastic interactions — how stable stacked bags remain when palletized or cartoned. Kinetic COF governs plastic-on-metal interactions — how smoothly film advances over machine surfaces. A film that passes on one measure can still fail on the other, and a test certificate that reports only a single COF value without specifying which type was measured is incomplete for qualification purposes.

ASTM D1894 vs. ISO 8295: Key Differences B2B Buyers Need to Know

Both ASTM D1894 and ISO 8295 describe the same underlying sled method and measure the same two outputs — static and kinetic COF — but treating them as equivalent when comparing supplier test reports is a common and costly mistake. The differences in test parameters are specific enough that a film tested under one standard can return a meaningfully different numerical result than the same film tested under the other.

ASTM D1894: Parameters and Configurations

ASTM D1894 is the North American standard and the most widely cited reference in global flexible packaging specifications. It specifies a sled weight of 200 g and a test speed of 150 mm/min. The standard defines five different apparatus configurations — different arrangements of the specimen and sled surface to simulate film-on-film, film-on-metal, and other real-world contact conditions. Which configuration is used matters: the same film can produce different COF values depending on whether it is tested film-side-up against itself or against a metal platen. ASTM D1894 applies to plastic films greater than 0.254 mm thick and to sheeting thinner than 0.254 mm.

ISO 8295: Where the Numbers Diverge

ISO 8295 is the international equivalent and is more prevalent in European specifications and in film purchase orders from European grocery or pharmaceutical converters. The methodology is identical in structure, but two specific parameters differ. Test speed is 100 ± 10 mm/min rather than 150 mm/min. ISO 8295 also does not use a fixed 200 g sled for all specimens: for films thinner than 0.075 mm, the 200 g sled is used, but for thicker films the sled weight increases to 500 g. That 300 g difference in normal force changes the denominator of the COF calculation directly, producing a lower calculated COF value for the same frictional force. Two suppliers quoting COF from the same film but using different ISO sled configurations — one at 200 g, one at 500 g — will return numbers that are not directly comparable.

The practical implication for procurement teams: before comparing COF values from two different test reports, confirm that both reports name the same standard, the same sled weight, and the same configuration. A report that states only "tested to ISO 8295" without specifying which sled weight was used is ambiguous and should be queried before being accepted as a qualification document. Requesting a retest under a fully specified configuration — rather than assuming the default was used — is standard practice among procurement teams who manage multi-supplier film programs.

What COF Values Actually Mean for Packaging Film

COF is expressed on a scale from 0 (no friction — a theoretical surface offering no resistance) to 1 (very high friction). In practice, packaging films for automated processing lines fall in a much narrower band, and the consequences of landing outside that band arrive at the machine, not in the lab.

Test data across packaging film applications places the useful processing range for most flexible films at 0.1 to 0.4. Within that range, polyethylene films show typical static COF values of 0.1 to 0.4 depending on surface treatment — a wide spread that reflects how much additives such as slip agents, anti-block compounds, and corona treatment shift surface energy. Film producers typically report lower COF values after corona treatment or the addition of slip additives, and higher values with anti-block agents; retesting after any formulation change is standard practice in converter qualification programs.

For VFFS applications specifically, the acceptable window is narrower than many engineers expect. The outside surface — the layer that contacts pull belts and forming collars — needs enough friction to give the belts grip. The inside surface — the sealing layer — needs to slide easily over itself to prevent the tube from resisting advancement. Industry-accepted targets reflect this asymmetry: inside COF of 0.200–0.300, outside COF of 0.300–0.400, with outside exceeding inside by approximately 0.100. When inside COF crosses above 0.400, VFFS line operators report pull belt slippage, reduced line speed, tracking problems, and seal failures. When a film is too slippery — inside COF below 0.200 — filled bags can slide and collapse when stacked during transit.

The counterintuitive takeaway is that a film that slides too easily is just as likely to cause a line qualification failure as one that is too sticky. Both failure modes have different root causes and different corrective paths, but both show up as downtime and rejected production runs. A film that arrives with a full inside-and-outside COF profile — tested against the correct surface pair — gives line engineers the information needed to pre-qualify the roll before it reaches the machine.

Materials and Applications: Where COF Testing Is Specified

The sled test method was developed for plastic films and sheeting and remains most heavily used in that segment. Polyethylene films — including LLDPE, LDPE, and multilayer laminates — account for the largest share of COF tests run in commercial packaging labs. ASTM D1894 explicitly covers plastic films greater than 0.254 mm thick and sheeting below that threshold, which encompasses the full range of flexible packaging substrates from thin barrier films to heavier form fill seal stock. Coated papers and polymer sheet laminates are also commonly tested under ASTM D1894 when the coating formulation affects surface slip properties.

Film producers supplying European grocery converters typically specify ISO 8295 as the governing standard in their quality plans. North American form fill seal machinery builders and their packaging customers generally reference ASTM D1894 in qualification documentation. When a converter operates lines qualified under both regional specifications — as is common for global contract manufacturers — film suppliers are expected to provide test results under both standards or confirm that their COF values were generated under a fully specified protocol that allows cross-standard interpretation.

Beyond packaging, the sled method applies to ceramic tiles and flooring products (where ISO 8295 variants are referenced in safety specifications), and the COF concept is central to brake and friction component design, though those applications use application-specific standards rather than ASTM D1894 or ISO 8295.

Surface treatment changes are one of the most common reasons film rolls from the same supplier fail incoming COF inspection. Converters running continuous production programs typically retest any film lot where the formulation, extrusion parameters, or surface treatment process has changed — even if the change appears minor — because small shifts in slip additive concentration or corona treatment intensity can move COF by 0.05 to 0.10, enough to push inside surface values across the 0.300 threshold where machine problems begin.

How to Specify COF in a Film Purchase Order

A COF test result is only as useful as the specification it is measured against. The most common gap in film qualification programs is not a failure to test, but a failure to specify — a purchase order that references COF without naming the standard, the configuration, the surface, or the acceptance range leaves both the supplier and the receiving lab without a shared reference point.

A complete COF clause in a film purchase order should contain seven elements: the standard and year edition; the apparatus configuration number (for ASTM D1894, one of five); the surface pair being tested (film-on-film or film-on-metal); the surface being tested (inside sealing layer or outside print/contact layer); the sled weight; the test speed; and the acceptance range with separate limits for static and kinetic COF. Using the VFFS inside surface as an example, a well-formed clause might read: "Tested to ASTM D1894, Configuration A (film-on-film), inside (sealing) surface, sled 200 g, speed 150 mm/min; static COF 0.200–0.300; kinetic COF: report actual value, flag if greater than static COF by more than 0.050."

Packaging converters who include a full COF clause in their film specification typically report fewer line qualification failures on incoming rolls, because the supplier's quality team is testing against the same parameters the line qualification used. When a supplier's test report arrives without a named configuration, the appropriate response is to request a retest under the specified configuration before accepting the certificate — not to assume that the default configuration matches the one used in the buyer's lab.

For procurement teams managing multiple film suppliers — some supplying under ASTM D1894, others under ISO 8295 — the safest approach is to require each supplier to report both the standard used and the configuration parameters on every test certificate. That single requirement eliminates the most common source of ambiguity in multi-supplier film qualification programs and makes cross-supplier COF comparisons valid rather than misleading.