Why Wrap Film Needs the Falling Dart Impact Test

Wrap films, particularly those used in packaging, need to withstand significant stress and impact during shipping, handling, and storage. Impact strength is a key property that ensures the film can absorb and distribute energy without tearing or breaking when subjected to external forces. The Falling Dart Impact Test is used to simulate real-world impact conditions to determine a film’s durability.

 

Dart Impact Test

Why Wrap Film Needs an Anti-Impact Feature

Wrap films are exposed to sharp edges, heavy objects, or rough handling in real-life scenarios. Without a sufficient anti-impact feature, the film could tear, compromising product protection. A film with poor impact strength could lead to higher material waste, reduced product shelf life, and potential damage to goods, resulting in customer dissatisfaction and additional costs.

Guiding Standards for Falling Dart Impact Test

ASTM D1709–Standard Test Methods for Impact Resistance of Plastic Film by the Free-Falling Dart Method

ASTM D1709 is the primary standard used for testing the impact resistance of plastic films using the Falling Dart Impact Test. It specifies the procedure for determining the impact strength of plastic films and sheets by measuring the height at which a dart is dropped to cause failure in the specimen. The test helps in classifying the resistance of films to impacts under specific conditions.

 

Know More About ASTM D1709

ISO 7765-1 – Plastics film and sheeting — Determination of impact resistance by the free-falling dart method Part 1: Staircase methods

ISO 7765-1 provides a procedure similar to ASTM D1709 for evaluating the impact resistance of plastic films and sheets. It specifies the method for determining the energy required to cause failure in films less than 1 mm in thickness when impacted by a free-falling dart. The test is designed to identify the height at which the dart will cause 50% of the specimens to fail under standardized conditions, providing a measure of the material’s impact resistance.

Know More About ISO 7765-1

Test Overview

The Falling Dart Impact Test is a widely used method for determining the impact resistance of materials, particularly plastic films, including wrap films, polyethylene, polypropylene, and other flexible packaging materials. This test evaluates how well a material can withstand impact when subjected to a free-falling dart, which simulates real-world stress and handling conditions. Below is a detailed description of how the Falling Dart Impact Test is performed, based on ASTM D1709, ISO 7765-1, and best practices.

The most used testing technique is the staircase method. A uniform missile weight increment is employed during test and the missile weight is decreased or increased by the uniform increment after test of each specimen, depending upon the result (fail or not fail) observed for the specimen.

According to the different darts(sizes, materials, weights, etc) and dropping heights, Method A and Method B are both indicated in the two ASTM and ISO standards.

Method A employs a dart with a 38 mm +1mmdiameter hemispherical head dropped from a height of 0.66 m±0.01 m. This method may be used for materialswhose impact resistance requires masses of about 0.05 kg toabout 2 kg to fracture them.

Method A dart and shaft
Method A dart and shaft
Method A increment weights
Method A increment weights

Method B employs a dart with a 50mm±1mm diameter hemispherical head dropped from a height of1.50m±0.01m. lts range of applicability is from about 0.3 kg to about 2 kg.

Method B dart and shaft
Method B dart and shaft
Method B increment weights
Method B increment weights

1. Test Setup and Equipment

Test specimens are taken from several rolls of film, and where possible, from several production runs of film. Strong conclusions about a specific property of a film cannot be based on a single roll of film.

Testing Machine: The test is performed using a FDT-01 Dart Impact Tester, which consists of a rigid frame that holds the specimen (film or sheet) and a dart assembly that is dropped from a predetermined height onto the specimen. 

Dart and Drop Height: Select method A or method B for use, as required by the relevant material specification or as agreed between the interested parties. Method A and B have its own dart and drop heights. FDT-01 Dart Impact Tester has menu to indicate which method is used.

 

2.Sample Preparation

Size: The impact area for the falling dart impact test is φ120mm, therefore, usually a square sample 150mm*150mm, or a long strip with width 150mm is used. 

Loading: The sample is held by a two-piece annular specimen clamps having an inside diameter of 125mm. Upper or movable clamp is pneumatically operated for user-friendliness.The clamp contacting surfaces are covered with rubber baskets to avoid slippage.

3. Missile Weight and Increment Weight ΔW Selection(or Δm in ISO)

For a starting point, select a missile weight near the expected impact failure weight. Add the necessary number of incremental weights onto the dart shaft and put the locking collar into place so that the weights are held securely in place. 

4. Start a Test

Activate the dart electromagnetic release mechanism and put the dart into position. Release the dart. If the dart bounces off the specimen surface, catch the dart after it bounces to prevent both multiple impacts with the specimen surface and damage to the hemispherical contact surface of the dart resulting from impact with metal parts of the apparatus.

5. Evaluation

If the first specimen fails, decrease the missile mass by ΔW,if the first specimen does not fail, increase the missile mass by ΔW, continue testing successive specimens, decreasing or increasing the missile mass by ΔW between drops depending upon whether the preceding specimen did or did not fail.

6.Continuation

After 20 specimens have been tested, count the total number, N, of failures, (X’s). If N= 10 at this point, testing is complete.If not, complete testing as follows:

If N< 10, continue testing additional specimens until N=10, then stop testing.

If N> 10, continue testing additional specimens until he total number of non-failures(O’s) reaches 10, then stop testing.

7.Calculation

Unlike the manual calculation formula described in standards, FDT-01 Dart Impact Tester gives the results of impact energy (in Joule) and impact mass (in gram) directly without any delay. 

FAQs about Dart Impact Test

What is the Falling Dart Impact Test, and why is it important for packaging materials?

The Falling Dart Impact Test is a widely recognized procedure for evaluating the impact resistance of plastic films and flexible materials. It simulates real-world scenarios where packaging materials might be subjected to sudden impacts or drops. This test helps manufacturers determine the tensile strength, impact toughness, and durability of materials used in packaging, ensuring they can withstand harsh handling conditions during transport and storage. By assessing materials like wrap films, the test ensures that packaging will perform well under stress, reducing the risk of damage during shipping or use.

How is the Falling Dart Impact Test performed?

The Falling Dart Impact Test follows a specific procedure:

  • Specimen Preparation: The test specimen, typically a thin plastic film, is cut to standard dimensions (usually 150 mm x 150 mm) and conditioned in a controlled environment.
  • Test Setup: The specimen is placed horizontally between rigid supports in FDT-01 Dart Impact Tester, and a weighted dart is dropped from a pre-determined height.
  • Dart Impact: The dart strikes the specimen, and the energy imparted to the film is calculated based on the dart’s weight and the drop height.
  • Failure Measurement: The test continues until 50% of the specimens fail. The energy (measured in joules) and mass required for the specimen to fail is recorded as the impact strength.

This method provides reliable results on how a material will perform under dynamic conditions such as drops, impacts, or shocks during use.

How do the results of the Falling Dart Impact Test relate to real-world applications?

The Falling Dart Impact Test provides valuable insight into how materials will perform under practical conditions such as drops during transportation, handling, or end-use applications. For instance, in packaging, materials tested for impact strength help determine the likelihood of film rupture or damage during shipping or handling, ensuring the integrity of the product. The results are used for material selection in industries like food packaging, pharmaceuticals, and electronics, where impact resistance is critical for protecting contents from physical damage.

Can the results obtained from Method A and Method B be comparable? 

The specified data obtained by the two test methods cannot be compared directly nor with those obtained from tests employing different conditions of missile velocity, impinging surface diameter, effective specimen diameter, and thickness. However, the values obtained by these test variables are highly dependent on the method of flm fabrication.

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