Impact testing of injection molded parts is a fundamental mechanical evaluation used to quantify the toughness, notch sensitivity, and failure behavior of plastic components under rapid loading conditions. This article provides a comprehensive technical overview of impact test methods for injection molded plastics, covering key standards such as ISO 179, ISO 180, ASTM D256, and GB/T 1043, typical specimen configurations, measurable test items, laboratory procedures, and industry applications in automotive, electronics, appliances, and consumer products.
Technical Overview
Impact testing evaluates the ability of an injection molded plastic part to absorb energy during a sudden, high-speed collision or shock load. Because thermoplastic materials exhibit strain-rate sensitivity, their behavior under impact can differ dramatically from their behavior in quasi-static tests such as tensile or flexural testing. A component that performs well in a slow bend test may still fail in a brittle manner when dropped, struck, or subjected to a sudden load.
Impact performance of injection molded parts is governed by multiple factors, including polymer grade, molecular weight, additive package (impact modifiers, fillers, glass fiber content), mold design, gate location, weld line formation, molecular orientation, residual stress, and wall thickness. Processing parameters such as melt temperature, mold temperature, injection speed, and holding pressure also significantly influence the final impact resistance. For this reason, impact testing is typically performed on specimens cut from production tools, standard test plaques, or actual molded components rather than compression-molded reference material alone.
Impact tests for injection molded parts are standardized by international and national standards organizations including ISO, ASTM, EN, GB, JIS, and UL. Common methods include pendulum impact (Izod and Charpy), falling dart and instrumented puncture impact, and high-speed tensile impact. Testing is routinely carried out by accredited third-party laboratories, in-house quality control departments, and research facilities.
Testing Samples
Notched Pendulum Impact Bars
Notched specimens are the most widely used configuration for measuring notch sensitivity. The notch acts as a stress concentrator that promotes brittle fracture, allowing differentiation between ductile and brittle material behavior.
- ISO specimens: typically 80 mm × 10 mm × 4 mm bars, injection molded per ISO 294 or ISO 10724, with a Type A notch of 0.25 mm radius machined per ISO 2818.
- ASTM specimens: typically 63.5 mm × 12.7 mm bars at 3.2 mm or 6.4 mm thickness, notched to a depth of 2.54 mm with a 0.25 mm root radius per ASTM D256.
- Notch types include Type A (0.25 mm radius), Type B (1.0 mm radius), and Type C (0.10 mm radius) for ISO testing.
Unnotched Pendulum Impact Bars
Unnotched specimens measure the total energy absorption capability of the material without artificial stress concentration. These results are useful for materials such as glass-filled or highly brittle compounds where notched values may be very low and difficult to differentiate. The same ISO 80 mm × 10 mm × 4 mm bar geometry or ASTM bar geometry is used without machining a notch.
Flat Plaques and Panels
Injection molded flat plaques, typically 60 mm × 60 mm up to 100 mm × 100 mm with thicknesses of 1 mm to 4 mm, are used for falling dart and instrumented puncture impact testing. Plaques may be gated to produce weld lines or fiber orientation patterns representative of actual production parts.
Finished Components and Assemblies
Complete injection molded parts, such as housings, clips, trim panels, covers, and structural brackets, can be tested directly in drop tests or with customized fixtures. Testing actual parts captures the combined effects of design features such as ribs, bosses, snap fits, corner radii, and knit lines.
Special Conditioning States
- As-molded specimens tested within a defined time window after molding.
- Specimens conditioned to equilibrium at 23 °C and 50% relative humidity per ISO 291 or ASTM D618.
- Moisture-sensitive materials (such as polyamide) tested both dry-as-molded and after moisture conditioning.
- Specimens aged, heat-treated, or exposed to chemicals prior to impact evaluation.
Testing Items
Depending on the method and specimen configuration, the following parameters are typically reported:
- Notched Izod impact strength, expressed in J/m (ASTM D256) or kJ/m² (ISO 180).
- Unnotched Charpy impact strength, expressed in kJ/m² (ISO 179-1, GB/T 1043.1).
- Notched Charpy impact strength (ISO 179-1, ISO 179-2 for instrumented testing).
- Total puncture impact energy in joules and peak impact force in newtons from instrumented drop tower testing (ASTM D3763, ISO 6603-2).
- Energy to peak force, deflection at peak force, and deflection at failure from force-displacement curves.
- Tensile impact strength per ASTM D1822 or ISO 8256 for thin or highly ductile materials.
- Ductile-to-brittle transition behavior across a temperature range, such as from 23 °C down to -40 °C.
- Failure mode classification: complete break, hinged break, partial break, or non-break.
- Statistical results including mean value, standard deviation, and coefficient of variation.
Testing Methods
Pendulum Izod Impact (ASTM D256, ISO 180, GB/T 1843)
In the Izod configuration, the notched specimen is clamped vertically as a cantilever beam, and the pendulum hammer strikes the notched side of the specimen above the clamp. The absorbed energy is calculated from the difference between the initial and final pendulum heights, corrected for friction and windage. The hammer energy must be selected so that the specimen absorbs between 10% and 80% (ISO) or 20% and 80% (ASTM typical guidance) of the available energy to ensure valid results. Results are reported as energy per unit width (J/m) in ASTM format or energy divided by cross-sectional area (kJ/m²) in ISO format.
Pendulum Charpy Impact (ISO 179-1, ASTM D6110, GB/T 1043.1)
In the Charpy configuration, the specimen is supported horizontally on two anvils and struck at mid-span by the pendulum, typically in edgewise (1eA, 1eU) or flatwise orientation. Charpy testing is the preferred European and Chinese method and is particularly suitable for materials that show excessive flexure or slip in the Izod clamp. Instrumented Charpy testing per ISO 179-2 provides a force-deflection record that separates initiation and propagation energy.
Instrumented Puncture Impact (ASTM D3763, ISO 6603-2)
A flat plaque or cut disk is clamped around its perimeter and struck at its center by a hemispherical-tipped dart propelled by a falling weight or servo-driven actuator at speeds commonly between 1 m/s and 4.4 m/s. A load cell behind the striker tip records the full force-time history, producing force-displacement curves from which peak force, total energy, and failure mode are extracted. This method is the industry standard for evaluating the ductile versus brittle puncture behavior of panels and is widely applied to automotive interior and exterior trim materials.
Falling Dart Impact (ASTM D5628, ISO 7765)
In the staircase or bracketing method, a dart with a specified geometry is dropped from increasing or decreasing heights onto the clamped specimen until a statistical failure energy or failure height (F50) is determined. This method is simple and robust for quality control comparisons.
Tensile Impact (ASTM D1822, ISO 8256)
The specimen is attached between a pendulum crosshead and a fixed grip and is broken in tension at high speed. This approach suits very thin sections or highly ductile materials that do not break cleanly in flexural impact.
Test Conditions and Procedure
- Specimens are conditioned per ISO 291 or ASTM D618, typically at 23 °C ± 2 °C and 50% ± 5% relative humidity for a minimum of 40 hours unless otherwise specified.
- Specimen dimensions, notch geometry, and notch machining quality are verified before clamping, since notch root radius deviations strongly affect results.
- A minimum of five and preferably ten specimens per condition are tested to establish statistical validity.
- For low-temperature evaluation, specimens are conditioned in a temperature-controlled chamber and tested within a short transfer time to prevent warming.
- Energy corrections for friction, air resistance, and bounce are applied per the applicable standard.
- Test reports include material identification, molding conditions, specimen geometry and notch type, test temperature, hammer energy, individual and mean results, failure mode, and the applicable standard.
Applications
Impact testing of injection molded parts supports material selection, design validation, production quality control, and failure analysis across many industries:
- Automotive: bumpers, fascias, interior trim panels, instrument panel components, door modules, air ducts, battery housings, and under-hood brackets tested at ambient and subzero temperatures to meet OEM specifications.
- Consumer electronics and electrical products: housings, enclosures, connectors, and structural frames where drop and impact durability are essential.
- Household appliances: control panels, handles, knobs, and internal components requiring resistance to accidental knocks and drops.
- Packaging: containers, closures, and thin-wall parts evaluated for transport and drop resistance.
- Medical devices: impact evaluation of device housings and components after sterilization cycles or chemical exposure.
- Toys, sporting goods, and tools: validation of safety margins under rough handling.
- Pipes and fittings: injection molded fittings and accessory parts tested for toughness in water, gas, and industrial systems.
- Material development: comparison of impact-modified grades, recycled content formulations, and glass-fiber reinforced compounds.
FAQ
What is the difference between Izod and Charpy impact tests?
Izod tests use a vertically clamped cantilever specimen struck near the clamp, and results are usually reported in J/m under ASTM D256. Charpy tests use a horizontally supported beam struck at mid-span, with results reported in kJ/m² under ISO 179 or GB/T 1043.1. The methods produce different numbers on the same material and are not directly interchangeable, so the test method must always be stated with the result.
Why is a notch machined into the specimen?
The notch creates a controlled stress concentration that forces brittle fracture and amplifies differences between materials. Notched results reveal notch sensitivity, which is critical because real parts contain radii, weld lines, gate marks, and scratches that act as stress concentrators. Unnotched tests measure overall energy absorption, while notched tests measure resistance to crack propagation from a flaw.
Why do some parts pass laboratory tests but fail in the field?
Field failures often result from combined factors not captured by a single laboratory test, including impact at low temperature, aging and UV degradation, chemical exposure, residual stress, sharp internal corners, poor weld line strength, or impacts at locations away from the tested gate region. Instrumented testing of actual parts under realistic temperatures and support conditions provides a more representative assessment.
How can the impact strength of an injection molded part be improved?
Common strategies include selecting impact-modified or higher molecular weight grades, increasing wall thickness at load-bearing areas, enlarging corner and rib root radii, relocating gates and weld lines away from high-stress zones, raising mold temperature to reduce residual stress, optimizing holding pressure to avoid sink and voids, and drying hygroscopic resins properly before molding.
Do impact results depend on test temperature?
Yes, strongly. Many thermoplastics, especially polypropylene and polyamide, transition from ductile to brittle behavior as temperature decreases. Impact qualification for outdoor, automotive, and cold-chain applications therefore typically includes testing at specified low temperatures such as -20 °C, -30 °C, or -40 °C in addition to room temperature.
What units are reported, and can they be converted?
ASTM Izod results are reported in J/m, while ISO and GB Charpy and Izod results are reported in kJ/m². Because the units are defined differently (energy per unit width versus energy per cross-sectional area), a direct numerical conversion is not valid; data should be compared only within the same standard and specimen geometry.