Determination of Impact Resistance Mass of Plastic Films by the Free-Falling Dart Method

The free-falling dart impact test is the internationally recognized method for determining the impact resistance mass of plastic films and thin sheeting, quantified as the dart mass that causes failure of 50% of tested specimens. Governed by ASTM D1709, ISO 7765-1, and GB/T 9639.1, this test is essential for quality control, material qualification, and product development in the flexible packaging, film extrusion, and converting industries. This article explains the test principle, sample requirements, methods, data evaluation, and typical applications of dart drop impact mass measurement.

Technical Overview

Dart drop impact testing, formally described as the determination of impact resistance of plastic film by the free-falling dart method, measures the ability of a flexible plastic film to absorb a localized, high-speed impact. In this test, a dart with a hemispherical head and adjustable mass is released from a fixed height onto a film specimen held horizontally between two annular clamping rings. The specimen either fails, defined as any puncture, tear, split, or crack that penetrates through the film, or survives the impact.

Because plastic films exhibit inherent statistical scatter in failure behavior, the result is not derived from a single impact. Instead, the staircase (up-and-down or Bruceton) procedure is applied: the dart mass is increased after each non-failure and decreased after each failure. From the resulting sequence, the dart mass at which 50% of the specimens would fail, designated F50 and expressed in grams, is calculated statistically. This value is the dart impact mass, or impact failure mass, and represents the standard index of film toughness against impact.

The principal governing standards are ASTM D1709 (Standard Test Methods for Impact Resistance of Plastic Film by the Free-Falling Dart Method, Methods A and B), ISO 7765-1 (staircase method), and GB/T 9639.1, which is technically aligned with the ISO procedure. Instrumented alternatives, such as ISO 7765-2 and ASTM D4272, measure the full force–displacement response during puncture and report total absorbed energy, and are often used as complementary techniques in accredited third-party laboratories.

• Key output: dart impact failure mass F50, in grams, at 50% failure probability
• Equivalent energy: E = m × g × h, where m is the F50 mass, g is gravitational acceleration, and h is the drop height
• Applicable materials: flexible plastic films and thin sheeting, typically up to approximately 1 mm in thickness

Testing Samples

Polyethylene Films

Low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), metallocene LLDPE, and high-density polyethylene (HDPE) blown or cast films are the most frequently tested materials. Typical products include shopping bags, liner films, lamination sealant webs, and heavy-duty sacks, where dart impact resistance is often a contractual specification.

Polypropylene Films

Cast polypropylene (CPP) and in-line polypropylene (IPP) films used as sealant or thermoforming layers in food packaging are routinely evaluated, since impact toughness directly affects down-gauging potential and pack integrity on high-speed filling lines.

Flexible Packaging Laminates

Multilayer structures such as PET/PE, PA/PE, and foil-containing laminates can be tested to verify that lamination, printing, and converting operations have not degraded the impact performance of the finished structure.

Agricultural and Technical Films

Greenhouse films, mulch films, silage films, stretch films, and construction membranes are tested because field performance depends heavily on resistance to impact from handling, stones, hail, and mechanical loading.

Specimen Preparation and Conditioning

Specimens are cut from the film roll as strips or square sections large enough to completely cover the annular clamp opening, and are free from wrinkles, gauge bands, visible defects, and printed or damaged areas unless these are deliberately under evaluation. Unless otherwise specified, specimens are conditioned in a standard laboratory atmosphere of 23 °C ± 2 °C and 50% ± 5% relative humidity in accordance with ISO 291 or ASTM D618 for the minimum period stated in the governing method before testing.

Testing Items

The scope of a typical dart impact test program includes the following reported items:

• Dart impact failure mass F50 by Method A (38.1 mm dart head, 0.66 m drop height), in grams
• Dart impact failure mass F50 by Method B (50.8 mm dart head, 1.52 m drop height), in grams
• Equivalent mean failure energy, calculated as E = F50 × g × h and expressed in joules, where requested
• Compliance assessment against customer or product specifications, reported as pass or fail with the numeric result
• Supplementary data: average film thickness, specimen orientation (machine direction versus transverse direction sampling), dart mass increment used, and conditioning details

Testing Methods

Method A – 38.1 mm Dart, 0.66 m Drop Height

Method A employs a dart with a 38.1 mm diameter hemispherical head dropped from a height of 0.66 m, producing an impact velocity of approximately 3.6 m/s. It is suitable for most general-purpose packaging films, and dart masses are adjusted in fixed increments, commonly on the order of a few grams to tens of grams, selected according to the expected resistance of the material.

Method B – 50.8 mm Dart, 1.52 m Drop Height

Method B uses a larger 50.8 mm diameter dart head released from 1.52 m, giving an impact velocity of approximately 5.5 m/s. It is generally applied to heavier-duty or tougher films whose impact failure mass under Method A conditions would be impractically high, and it typically uses larger mass increments, for example 15 g.

Staircase (Up-and-Down) Test Procedure

The test sequence follows the staircase principle defined in ASTM D1709, ISO 7765-1, and GB/T 9639.1:

• Clamp the specimen securely between the annular rings, ensuring a flat, taut surface free of wrinkles and consistent clamping pressure for every specimen
• Mount a dart of estimated starting mass and release it smoothly onto the center of the specimen without initial velocity or lateral swing
• Inspect the specimen: if the film is punctured or ruptured, record a failure and reduce the dart mass by one increment for the next impact; if the film survives, record a non-failure and increase the mass by one increment
• Continue the sequence until a statistically valid number of impacts, commonly at least 20, has been accumulated, including both failures and non-failures, using fresh specimen areas or new specimens for each impact

Statistical Calculation of F50

The 50% failure mass is computed from the staircase data set using the standard arithmetic given in the test methods, generally expressed as F50 = F0 + ΔF × [(A/B) − 0.5], where F0 is the dart mass at the lowest level at which the counted events occur, ΔF is the mass increment, A is the sum of the products of level number and event count at each level, and B is the total number of counted events. The events counted are normally the failures or the non-failures, whichever group is smaller, and the sign of the half-increment correction term is applied as specified in the governing standard. The final result is reported in grams together with the method used, dart increment, drop height, and mean specimen thickness.

Failure Criteria and Influencing Factors

Failure is defined as any break-through of the film caused by the dart, including punctures, splits, tears, and cracks; partial marks or dents that do not penetrate the full thickness are non-failures. Results are sensitive to thickness variation, specimen wrinkles, clamping pressure, room temperature and humidity, film orientation, and operator judgment of failure, so strict adherence to the standardized procedure and consistent technique are essential for reproducible interlaboratory agreement.

Applications

Dart impact mass data are used throughout the plastics and packaging value chain for material and product evaluation:

• Incoming quality control of film rolls and sealant webs purchased by packaging converters and brand owners
• Process and resin qualification by film extruders, including comparison of LDPE, LLDPE, and metallocene grades and optimization of blend ratios
• Down-gauging studies that verify whether thinner structures retain sufficient impact toughness for the intended application
• Evaluation of recycled content, additives, and processing changes on film toughness during research and development
• Verification of compliance with customer specifications, purchasing agreements, and industry data sheets, often through accredited third-party laboratories for independent certification
• Failure analysis of packaging that has ruptured during filling, transport, or storage, in combination with complementary tests such as tensile strength, Elmendorf tear, and puncture resistance

FAQ

What does the dart impact mass F50 mean?

F50 is the dart mass, in grams, at which 50% of specimens are statistically expected to fail under the specified drop conditions. A higher F50 indicates greater impact resistance and, in practical terms, a tougher film that is less likely to puncture or split during handling, filling, or transport.

Which standards govern the free-falling dart test?

The primary standards are ASTM D1709 for Methods A and B, ISO 7765-1 for the staircase method, and GB/T 9639.1 as the corresponding national method. Instrumented puncture methods ISO 7765-2 and ASTM D4272 may be used when a full energy-absorption profile is required.

What is the difference between Method A and Method B?

Method A uses a 38.1 mm dart head and a 0.66 m drop height, while Method B uses a 50.8 mm dart head and a 1.52 m drop height. Method B delivers roughly twice the impact velocity and is intended for tough, heavy-gauge films that would require excessively large dart masses under Method A conditions. Results from the two methods are not directly comparable.

How many specimens are required?

The staircase method is a statistical procedure that typically requires a minimum of about 20 impact events, with additional impacts improving the confidence of the calculated F50. Each impact must be performed on an undamaged area of film so that prior impacts do not influence the result.

How is the dart mass converted into impact energy?

The mean failure energy is calculated from E = m × g × h, where m is the F50 mass converted to kilograms, g is 9.81 m/s², and h is the drop height in meters. The result is reported in joules when energy rather than mass is required by the specification.

Can rigid or thick plastics be tested by this method?

No. The free-falling dart method is designed for flexible films and thin sheeting. Rigid plastics are evaluated with pendulum impact tests such as Izod or Charpy impact in accordance with ASTM D256, ISO 179, and ISO 180, and thicker sheeting is assessed with dedicated instrumented impact methods.

How should a testing laboratory be selected?

Testing should be performed by accredited third-party laboratories or qualified in-house facilities that operate calibrated drop towers, maintain standard conditioning atmospheres, and follow the documented procedures of ASTM D1709, ISO 7765-1, or GB/T 9639.1, including full reporting of method conditions alongside the F50 result.