Flexural strength testing is a fundamental mechanical evaluation used to determine the bending performance, rigidity, and load-bearing capability of injection molded plastic components. Conducted in accordance with internationally recognized standards such as ISO 178, ASTM D790, and GB/T 9341, this test enables manufacturers to verify material quality, validate molding processes, and confirm that finished parts can withstand real-world bending loads. This article provides a comprehensive technical guide covering test principles, specimen requirements, procedures, results, and typical applications.
Technical Overview
Flexural strength, also referred to as bending strength, is the maximum stress that a material sustains at the point of fracture, or at a defined conventional deflection limit, when subjected to a bending load. For injection molded parts, this property is one of the most widely measured mechanical indicators because bending stresses are among the most common service conditions encountered by plastic components in real applications.
Injection molded parts differ from isotropic bulk materials in several important ways. The molding process induces molecular orientation, residual stresses, weld lines, and—for fiber-reinforced grades—preferential fiber alignment along the flow direction. These factors make the flexural behavior of an injection molded part highly dependent on gate location, melt temperature, packing pressure, cooling rate, and flow path geometry. Flexural strength testing therefore serves both as a material qualification tool and as a means of validating that the molding process has produced parts with the expected mechanical performance.
Principle of the Test
The test is typically performed as a three-point bending configuration, in which a rectangular specimen is supported on two parallel cylindrical supports and loaded at mid-span by a loading nose moving at a constant rate. The applied force and resulting deflection are recorded continuously. From the force–deflection curve, key parameters such as maximum stress, modulus, and strain at break are calculated using standard beam-bending equations.
Governing Standards
- ISO 178 – Plastics – Determination of flexural properties (the primary international method)
- ASTM D790 – Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials
- GB/T 9341 – Chinese national standard, technically aligned with ISO 178
- ISO 294-1 – Preparation of test specimens by injection molding
- ISO 2818 – Preparation of test specimens by machining
- ISO 291 / ASTM D618 – Standard atmospheres for conditioning and testing
Testing Samples
Standard Injection Molded Bars
The preferred specimen for ISO 178 is a rectangular bar of 80 mm × 10 mm × 4 mm, produced directly in a dedicated test-surface mold in accordance with ISO 294-1. Using molded bars rather than machined plaques preserves the as-molded skin–core structure and molecular orientation, giving results that reflect true processing conditions. Specimens must be free of flash, voids, sink marks, and visible defects, and the face that contacts the loading nose should not be machined unless unavoidable.
Machined Specimens from Finished Parts
When mechanical performance of an actual production part must be verified, specimens can be cut and milled from flat regions of the component following ISO 2818. Machining must be performed carefully to avoid introducing heat, cracks, or rough edges that act as stress concentrators. The specimen thickness governs the required support span, since a span-to-thickness ratio of 16:1 (±1) is maintained in accordance with ISO 178. Results from machined specimens should be interpreted with caution, as machining removes the molded skin layer and may alter the measured values.
Typical Materials Tested
- Commodity thermoplastics: PP, PE, PS, ABS, PMMA
- Engineering thermoplastics: PA6, PA66, POM, PBT, PET, PC, PC/ABS blends, PPO alloys
- High-temperature and specialty plastics: PPS, PEI, PSU, PEEK
- Glass- or mineral-filled grades, such as PA6-GF30 or PBT-GF30, where fiber orientation strongly influences results
Testing Items
A standard flexural test program on injection molded parts typically reports the following parameters, all expressed with SI units and rounded according to the governing standard:
- Flexural strength (MPa) – the maximum flexural stress sustained by the specimen, either at break or at the conventional deflection limit
- Flexural stress at break (MPa) – the stress at the moment of fracture, when break occurs before the deflection limit
- Flexural stress at 3.5 % strain (MPa) – the conventional reporting point for ISO 178 when no fracture occurs
- Flexural modulus (MPa) – the ratio of stress to strain in the initial linear elastic region of the force–deflection curve
- Flexural strain at break (%) – the nominal strain on the outer surface of the specimen at fracture
- Full force–deflection curve data for engineering analysis and comparison
For materials that yield in bending without fracturing, ISO 178 requires the test to be stopped when the outer-fiber strain reaches 3.5 %, and the corresponding stress is reported instead of a breaking strength. This is common for ductile polyolefins such as PP and PE.
Testing Methods
Specimen Conditioning
Unless otherwise specified, specimens are conditioned in a standard laboratory atmosphere of 23 °C ± 2 °C and 50 % ± 10 % relative humidity in accordance with ISO 291 or ASTM D618. Minimum conditioning periods are typically 40 hours for ISO testing and 40 to 88 hours for ASTM testing, depending on the material. For hygroscopic polymers such as PA6 and PA66, the moisture state of the specimen must be documented because it significantly affects stiffness and strength.
Three-Point Bending Procedure (Primary Method)
The specimen is placed on two supports set at a span of 16 times the specimen thickness (64 mm for a standard 4 mm bar). A loading nose with the specified cylindrical radius applies force at mid-span at a constant crosshead speed, typically approximately 2 mm/min for standard ISO specimens. For ASTM D790 Procedure A, the crosshead rate is calculated from the span, specimen depth, and a nominal strain rate of 0.01 mm/mm per minute. The test proceeds until the specimen breaks or until the strain limit is reached.
For a three-point configuration, flexural stress is calculated from classical beam theory using the relationship σ = 3FL / (2bh²), where F is the applied force, L is the support span, and b and h are the specimen width and thickness, respectively. The flexural modulus is derived from the slope of the initial linear portion of the stress–strain curve.
Four-Point Bending
Four-point loading, in which the force is applied through two loading noses symmetrically positioned within the span, creates a region of pure bending with no shear between the loading points. This configuration reduces the localized compression effects seen under the central loading nose and is preferred for certain materials and research applications. The loading geometry and span-to-thickness ratio are adjusted according to the requirements of the selected standard.
Number of Specimens and Data Treatment
A minimum of five valid specimens per condition is generally required. Where material anisotropy is expected—particularly with glass-filled compounds—specimens should be tested in both the flow direction and the transverse direction, and the results reported separately. Statistical parameters such as mean, standard deviation, and coefficient of variation should accompany each reported value.
Influencing Factors to Control
- Specimen dimensions and their measurement accuracy, especially thickness, which enters the stress equation as a squared term
- Molding conditions: melt temperature, injection speed, holding pressure, and mold temperature
- Moisture content for hygroscopic resins
- Test temperature and conditioning environment
- Surface defects, gate residue, or weld lines located within the gauge region
Applications
Flexural strength testing of injection molded parts supports a wide range of engineering, quality, and regulatory activities across industries:
- Automotive: qualification of interior trim, brackets, clips, battery housings, connectors, and under-hood structural components required to sustain bending loads over a wide temperature range
- Electronics and electrical: verification of housings, enclosures, structural frames, and insulating components against assembly loads and consumer handling
- Consumer appliances: validation of panels, handles, bases, and load-bearing supports for long-term reliability
- Medical devices: mechanical characterization of device housings, instrument components, and sterilization-resistant plastic parts
- Material selection and comparison: ranking of candidate resins and reinforced grades during product development
- Injection molding process validation: confirming that tooling changes, gate relocation, or parameter adjustments do not degrade mechanical performance
- Incoming inspection and supplier quality control: batch verification against datasheet values in accredited third-party laboratories
- Failure analysis: comparing failed field parts against control specimens to identify processing or material root causes
FAQ
What is the difference between ISO 178 and ASTM D790?
Both standards use three-point bending with a 16:1 span-to-depth ratio, but they differ in specimen geometry, conditioning requirements, and reporting conventions. ISO 178 uses a preferred 80 mm × 10 mm × 4 mm bar and stops the test at 3.5 % strain for non-breaking materials, while ASTM D790 defines its own specimen, speed, and termination criteria and permits a 32:1 span-to-depth ratio for certain materials under Procedure B. Results from the two standards are similar in trend but should not be compared directly without technical justification.
Why test flexural strength instead of, or in addition to, tensile strength?
Flexural testing is often simpler and more economical because specimen gripping is not required, and it directly reflects the bending loads that many injection molded parts experience in service. For thin, flat molded components such as clips, panels, and housings, bending is frequently the dominant failure mode, making flexural data more representative of actual performance than tensile data alone. Ideally, both properties are characterized for structural applications.
Can specimens be taken directly from finished molded parts?
Yes. Flat sections of production parts can be machined into rectangular bending specimens following ISO 2818, provided the resulting dimensions satisfy the span-to-thickness requirements and the surfaces are free of machining damage. This approach is common for part qualification and failure analysis, although the removal of the molded skin layer must be considered when interpreting the results.
Why do glass-filled parts show different flexural strength in different directions?
During injection molding, glass fibers align predominantly along the flow direction. Specimens loaded with fibers parallel to the beam axis exhibit substantially higher flexural strength and modulus than specimens loaded transverse to the flow. Reporting both flow-direction and cross-direction values is standard practice for reinforced compounds.
How many specimens are needed for a valid test?
Both ISO 178 and ASTM D790 require a minimum of five valid specimens per material condition, with additional specimens recommended for highly variable or anisotropic materials. All invalid results, such as tests affected by specimen defects or improper seating, must be documented and repeated.
What could cause unexpectedly low flexural strength in production parts?
Common root causes include insufficient packing pressure leading to voids or sink marks, low melt or mold temperature producing poor weld-line strength, degradation from excessive residence time or drying failures, excessive use of regrind, and moisture absorption in hygroscopic resins. A comparison of failed parts against properly molded reference bars, conducted in an accredited third-party laboratory, is an effective way to isolate the cause.