A falling dart impact tester measures the resistance of plastic film to failure when a weighted dart strikes it under specified conditions. The usual result is the dart mass associated with a 50% probability of film failure, determined from a series of impacts on separate specimens. A higher result indicates greater resistance to that particular impact under the stated test conditions.

The measurement describes how a film responds to a sudden, concentrated load. It is useful for comparing films that must tolerate impacts during handling or packaging, but it does not identify every property that contributes to package durability. Dart shape, drop height, specimen preparation and the definition of failure all influence the result.

What happens when the dart strikes the film?

A film specimen is secured across an opening in a clamp. A dart with a rounded striking surface is released from a specified height and hits the exposed area. The film stretches around the dart, and the impact either causes failure or leaves the specimen intact according to the applicable test procedure.

A single impact gives a pass-or-fail observation at one dart mass. It does not establish the film’s impact resistance on its own. Separate specimens are tested at different masses so that the result reflects the transition between impacts that the film survives and impacts that cause failure.

During impact, the film absorbs energy through deformation and other processes around the point of contact. Its response reflects the combined behavior of the material and the film structure: a tough resin, for example, can still produce a weak result if the finished film contains defects or has been processed into an unfavorable structure.

Reading the result: mass, energy and failure probability

The common reported value is a mass, often expressed in grams, rather than a direct reading of the force at impact. It represents the dart mass at which approximately half of the specimens are expected to fail under the specified conditions. The value is a statistical estimate from the test series, not a guarantee that every specimen will survive just below that mass or fail just above it.

That distinction matters when results are close. If two films produce similar failure masses, the difference should be considered alongside the test method, specimen variability and any available measure of precision. A small numerical difference alone may not establish a meaningful difference in performance.

The dart also has gravitational potential energy before release. For a given mass and drop height, that energy is calculated as mass × gravitational acceleration × drop height. It provides a way to describe the severity of the drop, but it should not automatically be treated as the energy absorbed by the film. After impact, some energy can remain in the moving dart or be lost through other parts of the system.

A report expressed in mass is therefore incomplete without the associated drop height and test method. The same dart mass released from a different height reaches the film with a different impact severity. Conversely, two tests with the same calculated drop energy can still produce different failure behavior if their dart geometry or specimen support differs.

What counts as failure?

The failure criterion is defined by the procedure. A visible rupture, puncture or split through the film is different from a dent or permanent stretching that leaves the specimen intact. This distinction affects the estimated failure mass because the result is calculated from the recorded failures and non-failures.

Damage near the clamp also deserves attention. A tear caused by a wrinkle, a damaged edge or poor seating may not represent the response of a properly mounted specimen. The applicable method determines whether such an observation is valid and how it should be handled; it should not simply be reclassified to make the result look consistent.

Why test conditions change the number

The falling dart result belongs to a defined test setup. It is tempting to compare two reported masses directly, especially when both are described as dart impact strength. That comparison is useful only when the conditions are sufficiently alike.

Dart head and drop height. The shape and size of the striking surface influence how the load is distributed. A smaller contact area concentrates deformation differently from a broader one. Drop height changes the energy and impact velocity at a given mass. Test methods may specify different combinations of these features, so results obtained under different configurations should not be treated as interchangeable.

Clamping and exposed area. The film needs consistent support around the impact zone. If it slips during impact, the dart can draw additional material into the opening and alter the observed response. Excessive distortion, creases or inconsistent tension during mounting can also change how the film stretches. The clamp arrangement and specimen positioning are therefore part of the measurement, not incidental details.

Temperature and conditioning. Plastic films change their response to rapid loading as temperature changes. A film tested after being stored in a cold area may behave differently from the same film after conditioning at the specified laboratory temperature. Specimens also need time to reach the intended condition throughout the material, particularly when their recent storage environment was different.

Specimen condition. Pinholes, scratches, folds and local thickness variations can become starting points for failure. Sampling from a roll also matters: specimens taken from one small area describe that area, while samples distributed across the width and length can reveal variation in the finished material. The sampling plan should match the question being investigated.

How the test series estimates impact resistance

One common approach uses a sequence in which the dart mass changes after each impact. If a specimen fails, the next mass is reduced; if it survives, the mass is increased. The recorded sequence is then evaluated according to the chosen procedure to estimate the mass associated with 50% failure. Other prescribed approaches use groups of specimens tested at selected masses.

Each impact requires a fresh specimen. A film that has already been struck may contain stretched or damaged regions even when no rupture is visible, so a second drop onto the same area would not provide an independent observation. The mass increments, number of specimens and calculation rules are set by the method; changing them informally affects the meaning and comparability of the estimate.

The distribution of failures can be informative as well as the final value. If failure occurs over a narrow range of masses, the specimens responded relatively consistently in that series. A wider transition may point to variation in film structure, thickness or test conditions. It does not by itself identify the cause, so the individual observations are useful when an unexpected average appears.

What film properties influence the outcome?

Thickness has a direct bearing on impact resistance because a thicker specimen generally has more material available to deform and absorb energy. A higher dart failure mass for a thicker film does not automatically show that its material formulation is tougher. When the purpose is to compare materials or processing conditions, thickness and its variation need to accompany the result.

Dividing failure mass by thickness can be useful as an additional comparison, but the relationship between thickness and impact resistance is not necessarily linear. A normalized figure should not replace the measured result or be assumed to predict the performance of a much thinner or thicker film.

Film structure and processing affect the way a crack starts and spreads. Resin selection, blending, layer arrangement, orientation and cooling conditions can all change impact behavior. In a multilayer film, a layer that contributes barrier or sealing properties may also influence how the whole structure deforms. The test gives a result for the complete specimen; it does not isolate the contribution of each layer.

Directionality can matter too. Films are often drawn or stretched differently along the machine and transverse directions. A dart impact loads the film in more than one direction around the contact point, but the resulting tear pattern can still reflect directional differences. A long split in one direction and a more localized puncture are both possible failure appearances; the reported failure mass alone does not describe that difference.

Surface defects and contamination introduce a separate issue. Two specimens from the same nominal film can respond differently if one contains a weak spot at the impact location. Retaining information about where specimens were sampled and what the failures looked like makes it easier to distinguish material variation from a mounting or handling problem.

Dart impact versus other strength measurements

Dart impact resistance is a response to a rapid impact on a clamped sheet. Tensile testing measures behavior while a specimen is pulled, generally under a different loading speed and geometry. Tear resistance concerns the propagation of a tear under a specified test arrangement. A film can perform well in one of these tests and less well in another because the loading conditions and failure mechanisms differ.

A dart result also differs from puncture resistance measured with a probe in a controlled penetration test. The contact shape, rate of loading and support arrangement can produce different stress patterns. A high falling dart value is useful evidence of resistance to the specified impact; it does not establish resistance to a sharp corner pressing slowly against a package.

Nor does it directly measure drop performance of a filled package. A finished pack includes seals, folds, contents and air space, all of which affect how an impact is distributed. The film may pass a sheet impact test while a seal fails during a package drop, or a package may withstand a drop because the contents and geometry reduce the load at the film surface.

Making a useful comparison between films

For a meaningful side-by-side comparison, the results need the same test procedure and relevant setup: dart configuration, drop height, specimen support, conditioning and failure criterion. Film thickness and construction should be recorded with the result. If any of these differ, the comparison may combine a change in material performance with a change in test severity or specimen geometry.

Consider two packaging films with equal reported dart failure masses. If one is substantially thinner, the equal result describes their performance as tested; it does not mean their material structures respond identically. If the films were tested at different temperatures or with different dart heads, even the apparent equality requires caution.

For packaging applications, the result is most informative when connected to the expected contact. A broad impact against a supported package, a sharp projection inside a pack and a drop onto a hard surface impose different loads. The falling dart test supplies a controlled reference for film impact resistance, while application-specific testing establishes how the complete package responds to its intended conditions.