Testing standard
ASTM D1894
Standard Test Method for Static and Kinetic Coefficients of Friction of Plastic Film and Sheeting
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ASTM D1894 measures the static and kinetic coefficients of friction of plastic film and sheeting. A weighted sled faced with one material is drawn across a plane faced with the other, and the force is recorded. The peak needed to start it moving gives the static coefficient; the steadier force to keep it moving gives the kinetic one.
At a glance
- Test type
- Shear
- Published by
- ASTM
- Material
- Plastics, polymers & films
- Runs on
- Series 7200 and Series 9000
What the test does
A sled of specified mass, faced with one test material, is placed on a horizontal plane faced with the other. The sled is drawn along the plane at a constant speed through a low-friction pulley or a horizontal load train, and the force needed to move it is recorded continuously. The peak needed to start it moving gives the static coefficient; the steadier force needed to keep it moving gives the kinetic coefficient.
What it measures, and why it matters
Two numbers come out: the static coefficient of friction, which governs whether a stack of film will start to slide, and the kinetic coefficient, which governs how it behaves once moving. For packaging film these decide whether a bag opens on a form-fill-seal line, whether a stack of sheets destacks one at a time, and whether a palletised load stays put. Slip additives migrate to the surface over days, so the figures also track how far a film has bloomed since extrusion.
Both coefficients are dimensionless ratios, which makes them look more transferable than they are. They describe one pair of surfaces at one normal load and one speed, and changing any of the three changes the answer — often substantially, because polymer friction is not well described by the classical model in which the coefficient is independent of load and area. That is the practical reason a specification has to name the sled mass and the speed as well as the method.
Two surfaces, not one material
Friction is not a property of a material. It is a property of a PAIR of surfaces at a given load and speed, which is why both faces are specified and both are recorded.
- Sled face
- One test material, wrinkle-free
- Plane face
- The other, or a specified reference surface
- Which side faces which
- RecordedA coextruded film is rarely symmetrical, so film-to-film results depend on which surfaces meet.
- Handling
- By the edges onlyA fingerprint changes the answer, and not subtly.
- Time since manufacture
- Worth recordingDakSlip additive migrates to the surface over days. Comparing a film tested fresh with one tested a month later confuses blooming with formulation.
Test speed and geometry
- Sled
- Specified massIt sets the normal force, which is the divisor in both coefficients.
- Speed
- Constant and low
- Pull direction
- HORIZONTALA pull line that is not level adds or subtracts a component of the sled's weight, and the error is silent.
- Data capture
- Fast enough for the static peakDakThe break-free peak is brief. Slow capture rounds it off and under-reports the static coefficient.
Calculations
μs = F_peak / N
- F_peak
- force at which the sled breaks free, N
- N
- normal force — the sled's weight
μk = F_avg / N
- F_avg
- average force over the steady sliding portion, N
Averaged over the running portion, excluding the break-free peak and any end effects — the same discipline every peel method uses.
How the test runs
- 01Condition both specimens in the standard atmosphere.
- 02Handle the film by the edges and note which surfaces will meet.
- 03Face the sled and the plane, smoothing out wrinkles without stretching.
- 04Check the plane is level and the pull line horizontal.
- 05Set the low constant speed and a data rate fast enough to catch the peak.
- 06Start the pull and record force continuously.
- 07Take the break-free peak as the static value.
- 08Average the steady sliding portion for the kinetic value.
- 09Record which surfaces were in contact and the time since manufacture.
Watch the test
What the report has to contain
- Reference to ASTM D1894
- Both materials, and WHICH SURFACES were in contact
- Sled mass and contact area
- Test speed
- Conditioning atmosphere and time since manufacture
- Static coefficient of friction
- Kinetic coefficient of friction
- Number of specimens, mean and standard deviation
What the machine must be capable of
The forces are very small — often well under 10 N — so a low-capacity load cell with clean resolution at the bottom of its range matters far more than frame capacity. The plane must be level and the pull horizontal, or a component of the sled's weight is added to the friction force. A constant, low speed is needed, and the data capture must be fast enough to resolve the static peak, which is brief.
The whole measurement lives at the bottom of the load range, so the load cell is chosen for its resolution rather than its capacity. The plane must be genuinely level, since even a small tilt adds a component of the sled's weight to the measured force and biases every result in the same direction. Where a pulley is used it has to run freely, because its own friction is added to the film's and cannot be separated afterwards.
What goes wrong in practice
Contaminated surfaces are the commonest fault: a fingerprint changes the answer, and the effect is not subtle. A pull line that is not horizontal adds or subtracts weight. Slow data capture rounds off the static peak and under-reports it. And comparing a film tested a day after extrusion with one tested a month later confuses additive migration with a formulation difference.
Beyond contamination, two errors are common. A sled faced with film that has been stretched during application reads differently from one applied relaxed, because the surface texture changes under tension. And running successive tests over the same track on the plane polishes it — the second and third results drift downward for a reason that has nothing to do with the material, which is why a fresh track is used for each specimen.
Friction is not transferable
| ASTM D1894 | ISO 8295 | |
|---|---|---|
| Scope | Plastic film and sheeting | Plastic film and sheeting |
| Sled mass and speed | Specified | Specified, and not identical |
| Reports | Static and kinetic | Static and kinetic |
| Interchangeable | No | No |
Because a coefficient depends on the pair of surfaces, the normal load and the speed, no figure transfers between methods without a correlation study. A specification quoting a coefficient of friction has to name the method, the surfaces and the sled.
Questions we are asked about this test
What is ASTM D1894?
It is the ASTM test method for static and kinetic coefficients of friction of plastic film and sheeting. A sled of specified mass faced with one material is drawn at constant speed across a plane faced with the other, and the force to start it and to keep it moving give the two coefficients.
What is the difference between the static and kinetic coefficients?
The static coefficient comes from the peak force needed to break the sled free from rest; the kinetic one from the steadier force needed to keep it sliding. Static governs whether a stack of film will start to slip; kinetic governs how it behaves once moving. They are normally different, and the static value is usually the higher.
Why does it matter which surfaces are in contact?
Because friction is a property of the pair, not of the material. A coextruded film is rarely symmetrical — one side may carry slip additive and the other not — so film-to-film results depend entirely on which surfaces meet. That has to be recorded or the result cannot be reproduced.
Why record the time since manufacture?
Because slip additives migrate to the film surface over days after extrusion. A film tested the day it is made and the same film tested a month later can give quite different coefficients, and without the dates that difference looks like a formulation change rather than normal blooming.
Why is the pull line horizontal?
Because any tilt adds or subtracts a component of the sled's weight from the measured force, and the normal force is the divisor in both coefficients. The error is silent — the trace looks entirely normal — which makes checking the geometry more important than checking the numbers.
Why do my friction results drift across a run?
Usually the track. Sliding the sled repeatedly over the same path polishes the plane and redistributes any slip additive, so successive results fall for reasons that have nothing to do with the material. Using a fresh track for each specimen, and fresh film on the sled, removes it.
Can I use one coefficient for film against metal and film against film?
No. Friction is a property of the pair of surfaces, so film against a steel plane and film against itself are different measurements with different values. Both are legitimate and both are used — film-to-film for stacking and destacking, film-to-metal for behaviour on a forming machine — but they are not interchangeable and the pairing is reported.
Running ASTM D1894 on the Series 7200 and Series 9000
Dak verifies against whichever standard the method names, and where a class applies our frames sit a class tighter than it asks.
| The method asks for | Dak supplies | |
|---|---|---|
| Capacity | Very low — often well under 10 N. Clean resolution at the bottom of the load cell decides whether the test means anything. | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | ASTM E4 | Verified to ASTM E4, and to ISO 7500-1 Class 0.5 |
| Gripping | Sled of specified mass, a level plane, and a horizontal pull line through a low-friction pulley. | Wedge, vice-action, pneumatic and hydraulic grips, built to the specimen |
| Environment | Ambient: standard laboratory atmosphere. Time since manufacture is recorded, as slip additive migrates. | 3009 series chambers, −150 °C to +400 °C — temperature only |
This page describes the method as practised. The governing text is the current edition from the issuing body. Tell us what you are testing and we will answer with the machine, the fixture and a quotation.