Dart impact strength is measured using ASTM D1709, often called the free-falling dart method. A round-headed dart of a known weight is dropped from a fixed height, typically 0.66 meters, onto a clamped sample of film. Using the staircase method, the dart's weight is incrementally increased or decreased based on whether the film fails or survives the previous drop, and after enough trials a statistical weight is calculated at which 50 percent of samples fail. That value, expressed in grams, is the film's dart impact strength, often abbreviated as dart drop or simply dart.
This test measures the film's ability to absorb a sudden, localized impact rather than a slow, steady pull, which makes it especially relevant for predicting real-world performance when a package is dropped, thrown, or handled roughly, such as in shipping and distribution. Dart impact strength is heavily influenced by resin choice, with LLDPE generally outperforming LDPE and especially HDPE at comparable gauges, by molecular orientation from the blown film process, and by gauge itself, since a thicker film generally absorbs more impact energy.
Dart impact strength is one of the most frequently requested mechanical property specifications for packaging film, particularly for heavy-duty bags, shipping sacks, and produce packaging, because it correlates well with how a package will actually survive handling and transit rather than just how it performs in a controlled lab pull test.
Glossary
Dart Impact Strength
A standardized test measuring how much energy a film can absorb before a free-falling weighted dart punches through it, reported as a single weight value.
Example
A heavy-duty trash bag spec might require a minimum dart impact strength of 150 grams at a given gauge, giving the converter a clear, testable benchmark for how much sudden impact the bag needs to survive without puncturing.
Nuance
Dart impact and tensile strength are sometimes assumed to track each other directly, but a film can have high tensile strength while still failing a dart impact test, because tensile strength measures resistance to a slow, steady pull while dart impact measures resistance to sudden, localized impact energy. The two tests reveal different failure modes and both matter depending on the end use.