Impact Strength Testing of Plastic Finished Products: Standards, Methods and Applications

Impact strength testing is one of the most critical quality control procedures for plastic finished products, evaluating a material's ability to resist sudden shocks and dynamic loads during handling, transport, and end use. Accredited third-party laboratories perform these inspections in accordance with international standards such as ISO 179, ISO 180, ASTM D256, and their GB and JIS equivalents to verify product safety, durability, and regulatory compliance. This article provides a comprehensive technical overview of plastic impact strength inspection, covering sample types, test items, testing methods, industry applications, and frequently asked questions.

Technical Overview

Impact strength describes the capacity of a plastic material or finished product to absorb energy during rapid deformation or fracture under a sudden blow. Because plastics are viscoelastic, their mechanical response depends strongly on strain rate, temperature, part geometry, and stress concentration. A component that performs well in slow tensile testing may still fail catastrophically under a drop, collision, or shock load in service. Impact strength inspection is therefore essential for validating material formulation, processing quality, weld line integrity, and long-term durability.

Testing is performed using pendulum-type impact testers (Charpy, Izod, and tensile impact configurations) as well as falling-weight and drop-tower systems for films, sheets, pipes, and complete products. Results are typically expressed in kJ/m² for ISO-based pendulum tests, J/m or ft·lb/in for ASTM Izod tests, and in joules or failure mass for dart and drop-weight tests. Accredited third-party laboratories conduct these evaluations under ISO/IEC 17025 quality management systems, supporting incoming material control, process validation, design verification, product certification, and compliance with customer and regulatory specifications.

Testing Samples

Standard Molding Specimens

Material data sheets are usually verified on standardized bars injection or compression molded directly from granules or compound pellets. Typical geometries include 80 mm × 10 mm × 4 mm bars for ISO 179 and ISO 180 testing, and 63.5 mm × 12.7 mm bars with 3.2 mm or 6.4 mm thickness for ASTM D256 testing.

Specimens Machined from Finished Products

To evaluate the actual processing history of a production part, specimens are cut and machined from finished products such as housings, panels, and moldings in accordance with ISO 2818. These samples capture the influence of gate location, weld lines, orientation, cooling rate, and annealing on real-world impact performance.

Plastic Films and Flexible Sheets

Packaging films, agricultural films, geomembranes, and flexible laminates are tested as clamped discs or squares. Free-falling dart and instrumented dart-drop methods are applied to determine the mass or energy at which 50% of specimens fail.

Pipes, Fittings, and Profiles

Full pipe sections, injection-molded fittings, and extruded window or conduit profiles are tested in their as-manufactured condition using tup or falling-weight impact rigs designed for cylindrical and profiled geometries.

Complete Products and Assemblies

Functional impact verification is performed on entire products such as crates, containers, helmets, appliance housings, and packaged goods, evaluating the combined performance of material, design, and assembly.

Testing Items

Core impact strength inspection items for plastic finished products include the following:

• Charpy impact strength, notched and unnotched (ISO 179-1, ASTM D6110, GB/T 1043.1, JIS K 7110 series)
• Instrumented Charpy impact testing (ISO 179-2)
• Izod impact strength, notched and unnotched (ISO 180, ASTM D256, GB/T 1843)
• Tensile impact strength (ASTM D1822)
• Free-falling dart impact resistance of plastic film (ASTM D1709, ISO 7765-1, GB/T 9639.1)
• Instrumented dart-drop impact of film (ASTM D4272, ISO 7765-2)
• Gardner and drop-weight impact resistance of rigid flat specimens (ASTM D5420, ASTM D5628)
• Instrumented puncture impact of rigid plastics (ISO 6603-2, ASTM D3763)
• Resistance of thermoplastic pipes to external blows (ISO 3127, ASTM D2444, GB/T 14152)
• Low-temperature impact performance and brittleness temperature (ISO 974, ASTM D746)

Testing Methods

Charpy Impact Testing

In the Charpy configuration, the specimen is supported horizontally on two anvils set 62 mm apart and struck at the center by a swinging pendulum, with the notch positioned on the side facing away from the striker. Standard pendulums deliver nominal energies of 1 J, 2.75 J, 5.5 J, 11 J, or 22 J, selected so that the absorbed energy is between 10% and 80% of the pendulum capacity. Impact strength is calculated as the absorbed energy divided by the cross-sectional area under the notch and reported in kJ/m². ISO standards define notch types A (0.25 mm radius), B (1.00 mm radius), and C (0.10 mm radius), and classify failure modes as complete break, hinged break, partial break, or non-break.

Izod Impact Testing

In the Izod configuration, the specimen is clamped as a vertical cantilever and struck by the pendulum at the notched face above the clamp. ASTM D256 reports results in J/m or ft·lb/in, calculated from the energy absorbed divided by the specimen width, while ISO 180 and GB/T 1843 report in kJ/m². Both notched and unnotched (or reversed-notched) testing can be performed to separate notch sensitivity from inherent material toughness.

Falling Dart and Drop-Weight Testing

For films, ASTM D1709 and ISO 7765-1 employ the staircase (bracketing) method, in which the dart mass is increased or decreased after each pass or failure to determine the F50 mass at which 50% of specimens fail. For rigid flat parts, ASTM D5420 and ASTM D5628 determine the drop height or mass producing 50% failure. Instrumented puncture testing per ISO 6603-2 and ASTM D3763 drives a 20 mm hemispherical striker into a clamped specimen at a controlled velocity of approximately 4.4 m/s, recording the full force-displacement curve and reporting peak force, deflection and energy at peak force, and total absorbed energy, along with ductile or brittle failure classification.

Tensile Impact Testing

ASTM D1822 measures the energy required to fracture a dumbbell-shaped specimen attached between a pendulum crosshead and a fixed grip, effectively combining high-speed tension with pendulum impact. The result is expressed as energy per unit cross-sectional area and is particularly useful for materials too flexible or tough to fracture cleanly in Izod or Charpy testing.

Pipe Impact Testing

Thermoplastic pipes are evaluated for resistance to external blows using a falling tup. ISO 3127 and GB/T 14152 apply the staircase or round-the-clock method at specified temperatures, while ASTM D2444 determines the impact energy at which 50% of samples fail, verifying resistance to transportation and installation damage.

Specimen Preparation and Conditioning

Notches are machined with a dedicated milling cutter or shaping tool, and notch geometry is verified before testing. Specimens are conditioned to equilibrium under standard laboratory atmospheres of 23 °C ± 2 °C and 50% ± 5% relative humidity in accordance with ISO 291 or ASTM D618, typically for 16 to 88 hours depending on thickness. For low-temperature evaluation, specimens are conditioned in a refrigerated chamber at temperatures such as 0 °C, −20 °C, −30 °C, or −40 °C and tested within seconds of removal to prevent temperature drift.

Applications

Impact strength inspection serves a wide range of industries in which plastic components must withstand shock loading:

• Automotive: bumpers, interior trim, dashboards, battery housings, and under-hood components, frequently validated at −30 °C or −40 °C for cold-climate safety compliance
• Packaging: shipping sacks, heavy-duty films, bottles, injection-molded crates, and pallets evaluated for drop and transport durability
• Building and construction: PVC, PE, and PP pipes and fittings, window profiles, corrugated sheets, and roofing panels assessed for installation and service impact resistance
• Electrical and electronics: equipment housings, enclosures, connectors, and switch components where impact performance supports both mechanical safety and electrical enclosure integrity
• Consumer goods and appliances: housings, handles, lids, and containers subjected to daily drops and knocks
• Medical devices: device housings, infusion components, and packaging verified after sterilization or accelerated aging
• Sports and personal protective equipment: helmets, guards, and protectors where impact energy absorption is a primary safety requirement

FAQ

What is the difference between Izod and Charpy impact testing?

The Charpy test strikes a horizontally supported beam at midspan, while the Izod test strikes a vertically clamped cantilever specimen above the clamp. The stress states differ, so the two methods produce different numerical values and results cannot be converted between configurations.

Why are notched specimens used?

The notch introduces a controlled stress concentration that promotes brittle fracture and provides a measure of notch sensitivity. Notched results reveal how cracks, scratches, and sharp features in a real part may reduce toughness, while unnotched results reflect the inherent ductility of the material.

How many specimens are required?

Pendulum tests typically require five to ten specimens per condition to obtain a reliable mean value. Staircase dart-drop methods generally require twenty or more drops to establish a statistically valid F50 failure energy or mass.

At what temperatures is testing performed?

Standard testing is conducted at 23 °C and 50% relative humidity after adequate conditioning. Low-temperature testing at 0 °C down to −40 °C is commonly specified for outdoor, automotive, and cold-chain applications to characterize the ductile-to-brittle transition.

What causes low impact strength results?

Common causes include incorrect or contaminated resin, excessive regrind content, moisture in hygroscopic materials, improper melt temperature or injection pressure, poor weld line fusion, excessive orientation, degradation from long residence times, and poor notch machining quality.

Can finished products be tested without cutting specimens?

Yes. Drop-weight, dart, and functional drop tests are performed directly on complete parts or assemblies, providing a realistic assessment of how design features, ribs, and assembly joints behave under impact.

How long does impact strength inspection take?

After specimen preparation and conditioning, mechanical testing is usually completed within a few hours. A typical laboratory report for a standard sample set, including conditioning, testing, and data review, is issued within three to seven working days.