1. Principles and Scope of the Standard
ASTM D3420 is a test method designed to determine the impact-puncture resistance of plastic films using a pendulum apparatus. The principle is based on measuring the energy absorbed by a film specimen when it is punctured by a hemispherical head mounted on a free-swinging pendulum. The kinetic energy lost by the pendulum during puncture corresponds directly to the impact resistance of the film.
This method offers a fundamental advantage over conventional uniaxial tensile tests: it stresses the film at strain rates closer to those encountered in real-world conditions (drops, impacts during transport, handling). The results therefore reflect the dynamic biaxial tensile behaviour of the material, providing a more realistic assessment of its toughness than static tests.
The standard applies to the full range of plastic films used in packaging: polyethylene (PE-LD, PE-LLD, PE-HD), polypropylene (PP, OPP, BOPP), polyester (PET), polyamide (PA) and multilayer films (coextrusion, laminates).
1.1 The Two Test Procedures
The standard defines two procedures identical in principle but differing in specimen geometry and impact head size:
| Parameter | Procedure A | Procedure B (Spencer) |
|---|---|---|
| Specimen holder aperture | 60 mm diameter | 89 mm diameter |
| Impact head diameter | 25.4 mm (1 in.) | 19 mm (0.75 in.) |
| Typical application | Thin films (< 50 µm) | Thick or robust films (> 50 µm) |
| Energy range | Lower (fragile or very thin films) | Higher (resistant films) |
Procedure B, known as the “Spencer” test, is historically the most widely used in the packaging industry and serves as the default reference when a specification simply mentions “ASTM D3420 pendulum impact test”.
1.2 Distinction from Related Test Methods
Several standards characterise the impact resistance of plastic films. The difference lies in the loading mechanism, impact speed and type of result:
| Standard | Method | Use Case |
|---|---|---|
| ASTM D3420 | Pendulum impact: puncture energy at intermediate speed | Packaging films, production quality control |
| ASTM D1709 | Free-falling dart impact (dart drop): failure mass | Blown films, PE grade qualification |
| ASTM D4272 | Falling weight impact on clamped specimen | Rigid films, thick sheets |
| ISO 7765-1 | Free-falling dart impact (staircase or fixed energy method) | ISO equivalent of D1709, European markets |
Results from these standards are not directly comparable: D3420 provides a puncture energy (in joules), D1709 a failure mass (in grams), and D4272 an impact energy (in joules) but with a different geometry. The choice of method depends on the film type, its thickness and the specification requirements.
2. Role in Packaging Film Evaluation
2.1 Characterising Dynamic Toughness
The primary objective of ASTM D3420 is to quantify a film’s ability to resist puncture under rapid impact. This property is critical for applications where the film is subjected to sudden mechanical stresses: package drops, contact with sharp edges, transport vibrations.
The biaxial loading exerted by the hemispherical head reproduces a realistic failure mode. Unlike the uniaxial tensile test, which measures resistance in a single direction, the pendulum impact stresses the film in all directions around the contact point, revealing weaknesses related to molecular orientation, thickness non-uniformities or structural defects.
2.2 Production Quality Control
ASTM D3420 is widely used as a reception and in-process monitoring test. Film manufacturers specify minimum impact energy thresholds depending on the intended application. A stretch film for palletisation typically requires a higher impact energy than a shrink film for bundling.
Statistical tracking of pendulum impact results across production batches enables detection of process drifts (extrusion temperature variation, resin batch change, blow-up ratio misadjustment).
2.3 Interpretation Limitations
The pendulum impact test has limitations that must be considered:
- The measured energy depends on the film thickness. To compare different materials, results must be normalised by thickness (in J/µm or mJ/µm).
- The result is sensitive to film orientation (machine direction vs. transverse direction): oriented films often exhibit marked anisotropy.
- Test temperature significantly influences impact resistance, particularly for polymers near their glass transition temperature.
- The test does not reproduce slow puncture conditions (e.g., a film resting on an edge during storage).
Important: Pendulum impact energy depends on thickness, orientation and temperature. Any comparison between different materials must specify all three parameters. An isolated result without context has no technical value.
3. Test Procedure
3.1 Specimen Preparation
Specimens are conditioned for at least 40 hours at 23 ± 2 °C and 50 ± 10% RH (or in accordance with ASTM D618). Test pieces are cut into squares of sufficient dimensions to be clamped in the specimen holder (typically 100 mm × 100 mm for Procedure A and 125 mm × 125 mm for Procedure B).
A minimum of 10 specimens per direction (machine direction and transverse direction) is recommended to obtain statistically representative results.
3.2 Test Execution
The procedure involves four steps:
- The specimen is clamped between the two plates of the specimen holder, centred on the circular aperture (60 mm or 89 mm depending on the procedure).
- The pendulum is released from its starting position (defined angle). Its hemispherical head passes through the film, puncturing it.
- The apparatus records the pendulum’s return angle after puncture. The difference in kinetic energy between the start and arrival positions corresponds to the energy absorbed by the film.
- The impact energy is calculated and reported in joules (or millijoules for very thin films).
The apparatus must be calibrated in accordance with the standard’s requirements. Verification of the free-swing friction energy (test without specimen) is mandatory before each test series to correct for mechanical losses.
3.3 Data Collection and Analysis
The key parameters reported in the test report are:
| Parameter | Description |
|---|---|
| Impact energy | Energy absorbed during puncture (in J or mJ) |
| Normalised energy | Energy normalised to film thickness (J/µm or mJ/µm) |
| Procedure used | A (60 mm) or B (89 mm, Spencer) |
| Direction | Machine direction (MD) and/or transverse direction (TD) |
| Thickness | Average film thickness at the impact point (µm) |
| Standard deviation | Measure of result dispersion, reproducibility indicator |
The free-swing friction energy must be systematically subtracted from the raw measurements. Results in machine direction (MD) and transverse direction (TD) must be reported separately, as anisotropy is essential information for formulation and extrusion line setup.
4. Regulatory Framework and Industrial Applications
4.1 Standards References
ASTM D3420 is part of a broader standards ecosystem. The main cross-references:
- ASTM D1709: free-falling dart impact resistance test (dart drop), the most common method for blown films.
- ASTM D4272: falling weight impact test for rigid sheets and films.
- ISO 7765-1: ISO dart impact method, international equivalent of D1709.
- ASTM D618: conditioning of plastic specimens before testing.
- ASTM D882: tensile properties of thin plastic films, complementary for mechanical characterisation.
The current version in force is D3420-21, developed by ASTM International Subcommittee D20.19.
4.2 Industry Applications
Food Packaging
Food packaging films (pouches, flow wrap, lidding, stretch film) must resist impacts and punctures throughout the supply chain, from packing to point of sale. ASTM D3420 verifies that the film maintains its mechanical integrity against transport and handling stresses.
Industrial and Logistics Films
Stretch films for palletisation, shrink films and protective covers require high impact resistance to maintain unitised loads and protect products during transport. Tracking pendulum impact energy ensures consistency of mechanical properties from batch to batch.
Technical and Multilayer Films
Multilayer barrier films (PE/PA, PE/EVOH/PE) used in modified atmosphere packaging (MAP) combine barrier properties with mechanical resistance. ASTM D3420 validates the contribution of each layer to the overall resistance of the laminate and detects delamination issues under impact.
Agriculture and Geomembranes
Agricultural films (mulching, greenhouses, silage) and waterproofing geomembranes are subjected to punctures from stones, roots or tools. D3420 qualifies their resistance to mechanical damage under conditions close to real-world use.
5. Best Practices for Reliable Testing
5.1 Choosing a Laboratory
Result reliability depends directly on laboratory competence. Selection criteria:
- ISO/IEC 17025 accreditation, guaranteeing technical competence and metrological traceability.
- Calibrated equipment: impact pendulum with interchangeable heads (Procedure A and B), angle measurement system or digital energy sensor, micrometer for thickness measurement.
- Systematic free-swing energy verification: friction loss correction is essential for result accuracy.
- Rigorous conditioning: compliance with the 40-hour conditioning period and temperature/humidity tolerances is essential for reproducibility.
5.2 Practical Recommendations
To maximise the value of D3420 testing:
- Systematically specify the procedure used (A or B), direction (MD/TD) and thickness in every test report.
- Test a minimum of 10 specimens per direction for sufficient statistical representativeness.
- Normalise results by thickness when comparing films of different grades or suppliers.
- Complement the pendulum test with a dart impact test (ASTM D1709) and a tensile test (ASTM D882) for a complete mechanical characterisation of the film.
- For multilayer films, check for delamination at the impact point, which indicates insufficient interlayer adhesion.
Food packaging, logistics, technical multilayer films and agriculture are the main user sectors. Any sector where plastic films play a mechanical protection role benefits from this standard for material selection, production control and supplier qualification.