Plastic Film Falling Ball Impact Test: Principles, Standards, and Test Methods

The plastic film falling ball impact test, also known as the free-falling dart impact test, is a widely adopted method for evaluating the impact resistance and dynamic toughness of plastic films and thin sheeting. Based on international standards such as ASTM D1709 and ISO 7765-1, the test determines the falling mass at which 50% of film specimens fail, delivering critical data for packaging design, material selection, and quality control. This article explains the testing principle, applicable sample types, test procedures, calculation methods, and typical industrial applications.

Technical Overview

The falling ball impact test measures the ability of a plastic film to withstand the concentrated dynamic stress produced when a hemispherical-headed projectile, usually called a dart or ball, is dropped from a fixed height onto a horizontally clamped specimen. Because packaging films are frequently subjected to drops, knocks, and puncture-type loading during filling, transport, and handling, this test is one of the most important indicators of practical film toughness.

The core parameter obtained from the test is the impact failure mass, commonly designated F50, which is the dart mass at which 50% of the tested specimens fail. This value can be converted into impact energy using the relationship E = m × g × h, where m is the dart mass, g is gravitational acceleration, and h is the drop height. For example, a 100 g dart falling from 0.66 m delivers approximately 0.65 J of kinetic energy.

The test is primarily governed by the following standards, and results should always be reported together with the method used:

• ASTM D1709 — Standard Test Methods for Impact Resistance of Plastic Film by the Free-Falling Dart Method (Method A and Method B)
• ISO 7765-1 — Plastics film and sheeting, determination of impact resistance by the free-falling dart method, Part 1: Staircase methods
• ISO 7765-2 — Part 2: Instrumented puncture test on a laboratory scale
• EN ISO 7765-1 — European adoption of the ISO staircase method
• GB/T 9639.1 — Chinese national standard for impact resistance of plastic film and sheeting by the free-falling dart method, staircase procedure

Testing Samples

Single-Layer Polyolefin Films

Common test materials include low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), and high-density polyethylene (HDPE) films, as well as cast polypropylene (CPP) films. These films are widely used in bags, liners, and flexible packaging where drop and impact resistance is a key performance requirement.

Composite and Laminated Films

Multilayer structures such as PET/PE, PA/PE, BOPP/CPP laminates, aluminized films, and retort pouch materials can be tested to verify that lamination and printing processes have not degraded impact performance. For laminates, the test helps confirm interlayer adhesion and overall structural toughness.

Specialty and Functional Films

Agricultural mulch and greenhouse films, heavy-duty shipping sack films, stretch hood films, barrier films, and sterile medical packaging films are also evaluated. For very thin or highly oriented films, the test distinguishes brittle from ductile failure behavior, which is essential for material development.

Specimen Requirements

Specimens are typically cut into square pieces large enough to fully cover the clamping ring, with a common size of approximately 150 mm × 150 mm or larger. Specimens must be flat, free from creases, wrinkles, visible defects, and surface contamination. A sufficient number of specimens is required because the staircase method is statistical in nature.

Testing Items

Depending on the purpose of the evaluation, the following results can be reported:

• Impact failure mass (F50): the dart mass at which 50% of specimens fail, expressed in grams
• Impact failure energy: the kinetic energy corresponding to the F50 mass at the specified drop height, expressed in joules
• Pass/fail verification: confirmation of whether a film withstands a specified dart mass dropped from a specified height, commonly used for incoming inspection and release testing
• Failure mode assessment: classification of the fracture as ductile tearing or brittle cracking, which supports material diagnosis
• Comparative ranking: relative impact resistance of different resin grades, additives, thicknesses, or processing conditions

Testing Methods

Apparatus

The instrument consists of a vertical guide tube, an electromagnetic or mechanical release mechanism, a dart with a hemispherical head and detachable incremental weights, a specimen holder with a clamping ring (typically with an inner diameter of about 127 mm), and a padded catch base. The clamping ring, often fitted with an O-ring, must grip the specimen firmly without slippage during impact.

Specimen Conditioning

Specimens are conditioned in a standard laboratory atmosphere of 23 ± 2°C and 50 ± 5% relative humidity in accordance with ISO 291 or ASTM D618, typically for at least 40 hours before testing. Testing is then performed under the same atmospheric conditions, because temperature and moisture significantly influence the ductility of polymeric films.

Method A (Light-Weight Dart)

Method A uses a dart with a 38.1 mm diameter hemispherical head dropped from a height of 0.66 m. It is suitable for most thin, flexible packaging films with low to moderate impact strength, covering an impact energy range appropriate for general commercial materials.

Method B (Heavy-Weight Dart)

Method B uses a larger dart with a 50.8 mm diameter head dropped from a greater height of 1.52 m under ASTM D1709, or from 1.0 m under ISO 7765-1. Method B is intended for heavier, tougher films such as heavy-duty sacks and thick agricultural films whose impact resistance exceeds the range of Method A.

Staircase (Bruceton) Test Sequence

The classical procedure follows a staircase approach. Testing begins at an estimated mean failure mass. If the specimen fails, the dart mass is decreased by a fixed increment for the next specimen; if the specimen passes, the mass is increased by the same increment. This adaptive sequence concentrates the test points around the true failure threshold, improving statistical efficiency. Testing continues until a sufficient number of failures and survivals is recorded, commonly involving at least 20 specimens in total with a minimum number of results in each outcome group as specified by the standard.

Calculation of Results

The impact failure mass F50 is calculated from the staircase data using the statistical formula defined in the standard, which combines the lowest dart mass used, the weight increment, and the weighted distribution of failures or survivals across the test levels. The F50 value, together with the drop height, is then used to derive the impact failure energy. All results must be reported with the test method, dart diameter, drop height, specimen thickness, conditioning atmosphere, and number of specimens tested.

Failure Criterion

A specimen is considered to have failed if the dart passes completely through the specimen, or if any visible hole, crack, or tear caused by the impact is observed. The specimen is inspected immediately after each drop, and the outcome is recorded before proceeding to the next mass level.

Applications

The falling ball impact test supports decision-making across the entire plastic film value chain, from resin development to finished package qualification:

• Food and beverage flexible packaging: verifying that pouches, bags, and laminates survive filling lines and distribution hazards
• Heavy-duty industrial packaging: qualifying films for shipping sacks, FIBC liners, and construction films
• Agriculture: evaluating greenhouse films and mulch films for resistance to hail, impact, and mechanical damage
• Medical and pharmaceutical packaging: confirming the integrity of sterile barrier films against puncture and drop loads
• Material research and development: comparing impact modifiers, slip and antiblock additives, resin blends, and processing parameters
• Incoming quality control: verifying conformity of purchased film against contractual impact resistance specifications
• Certification support: generating data packages for submission to accredited third-party laboratories or regulatory verification

FAQ

What is the difference between the falling ball test and the free-falling dart test?

The two terms describe essentially the same type of test. In the film industry, the standardized method is formally called the free-falling dart method, in which a dart with a hemispherical head is dropped vertically onto a clamped film. The term falling ball impact test is a widely used informal name for the same procedure, as well as for related ball-drop quality tests.

When should Method A or Method B be selected?

Method A, with a smaller dart and lower drop height, is suitable for thin, general-purpose packaging films. Method B, with a larger dart and higher drop height, should be used when the impact resistance of the film exceeds the practical mass range of Method A, as with heavy-duty sack films. If results must be comparable between laboratories, the same method must be used by all parties.

How many specimens are required for one test?

Because the result is statistical, a typical staircase determination involves at least 20 specimens, and testing continues until the required minimum numbers of failures and survivals are obtained. For simple pass/fail verification at a fixed mass and height, ten specimens are commonly tested.

What factors can influence the test results?

Key influences include film thickness and gauge uniformity, material temperature and conditioning history, dart head condition, drop height accuracy, clamping pressure, specimen slippage, additive migration, and prior mechanical orientation of the film. Strict control of these variables is essential for reproducible results.

Can results from ASTM D1709 and ISO 7765-1 be directly compared?

Not directly. Although both standards use the staircase principle, Method B differs in drop height between the two standards, so F50 values are not interchangeable without conversion. When comparing data, always confirm that the method, dart diameter, and drop height are identical.

How is the impact failure energy calculated?

The impact failure energy is calculated from the F50 mass using the equation E = m × g × h. For instance, an F50 of 150 g obtained at a drop height of 0.66 m corresponds to an impact energy of approximately 0.97 J, which quantifies the toughness of the film at the moment 50% of specimens fail.