Plastic flexural strength and Vicat softening temperature testing are two of the most widely used methods for evaluating the mechanical rigidity and heat resistance of polymeric materials. Performed in accredited third-party laboratories in accordance with ISO, ASTM, and GB standards, these tests deliver essential data for material qualification, product design, and quality control. This guide explains the test principles, sample requirements, testing procedures, typical results, and applications of flexural strength and Vicat heat resistance testing for plastics.
Technical Overview
Flexural strength testing measures the ability of a plastic material to resist deformation under bending load. The test is performed in a three-point bending configuration, in which a rectangular specimen rests on two parallel supports and a loading nose applies force at midspan until the specimen yields or breaks. The test yields flexural strength, flexural stress at conventional deflection, and flexural modulus, all of which are fundamental parameters for rigid and semi-rigid thermoplastics, thermosets, and fiber-reinforced composites.
Vicat softening temperature (VST) testing evaluates the short-term heat resistance of thermoplastics. A flat-ended needle with a cross-sectional area of 1 mm² is pressed against the surface of a specimen under a defined load of 10 N or 50 N while the specimen is heated at a constant rate in a liquid bath. The temperature at which the needle penetrates 1 mm into the material is recorded as the Vicat softening temperature. A higher VST indicates better resistance to softening and deformation at elevated temperature.
Together, these two tests provide a combined picture of a plastic material's structural stiffness and thermal endurance, making them indispensable for material comparison, incoming material inspection, and compliance verification in industries such as automotive, electronics, construction, and household appliances.
Testing Samples
Standard Flexural Test Specimens
For ISO 178 and GB/T 9341 testing, the preferred specimen is a flat bar of 80 mm × 10 mm × 4 mm, machined or molded directly to shape. For ASTM D790, a common specimen size is 127 mm × 12.7 mm × 3.2 mm. Specimens must be flat, free of voids, sink marks, and flash, and the width-to-thickness ratio should generally be greater than 2.5.
Vicat Test Specimens
Vicat specimens should be flat pieces with a smooth lower surface of at least 10 mm × 10 mm and a thickness typically between 3 mm and 6.5 mm. Thicker samples should be machined evenly from one side to bring them within range. Specimens may be taken from compression-molded or injection-molded plaques, or cut directly from finished parts.
Finished Products and Components
Samples can also be prepared from pipes, sheets, profiles, housings, and other molded components. In such cases, the sampling location, direction relative to melt flow, and any surface treatments must be documented, since processing conditions and orientation can significantly affect both flexural and thermal results.
Conditioning Requirements
Unless otherwise specified, specimens should be conditioned at 23 ± 2 °C and 50 ± 10% relative humidity in accordance with ISO 291 or ASTM D618 prior to testing, typically for a minimum of 40 hours, to ensure reproducible and comparable results.
Testing Items
The core measurable parameters covered by these two test methods include:
- Flexural strength (MPa) — the maximum bending stress sustained by the specimen
- Flexural stress at 3.5% strain (MPa) — reported when the specimen does not break before conventional deflection, per ISO 178
- Flexural modulus (MPa) — the ratio of stress to strain in the elastic region, indicating material stiffness
- Flexural stress at break or at specified deflection (MPa)
- Vicat softening temperature, VST/A/50, VST/B/50, VST/A/120, VST/B/120 (°C) — depending on the load (10 N or 50 N) and heating rate (50 °C/h or 120 °C/h) selected
Testing Methods
Flexural Strength Test Method (ISO 178 / ASTM D790 / GB/T 9341)
The specimen is placed on two supports and loaded at midspan at a constant crosshead speed. Under ISO 178, the support span is set at 16 times the specimen thickness (64 mm for a 4 mm specimen), and the nominal test speed is 2 mm/min, corresponding to a strain rate of 1% per minute. Under ASTM D790 Procedure A, a span-to-depth ratio of 16:1 and a crosshead rate calculated to produce an outer-fiber strain rate of 0.01 mm/mm per minute are typically used. The flexural strength is calculated from the maximum force using the relationship of applied load, support span, specimen width, and thickness. When a material exhibits large deflection without fracture, flexural stress at 3.5% strain is reported instead of a breaking strength.
Vicat Softening Temperature Test Method (ISO 306 / ASTM D1525 / GB/T 1633)
The specimen is positioned in a temperature-controlled bath, usually filled with silicone oil or another transparent heat-transfer liquid, with the flat-ended needle resting on its upper surface. A constant load of 10 N (Method A) or 50 N (Method B) is applied, and the bath temperature is raised at 50 ± 5 °C/h or 120 ± 10 °C/h. The test ends when the needle penetrates 1 mm into the specimen; the temperature at that moment is the Vicat softening temperature. Standard condition combinations are designated as A50, B50, A120, and B120, and the specific method used must always be stated with the result because different conditions yield different temperatures.
Test Conditions and Report Content
A complete test report should include the material identification, specimen dimensions and preparation method, conditioning atmosphere, test standard and method designation, span, loading rate, heating rate and load for Vicat testing, individual and mean results, and any deviations from the standard. All testing should be performed with calibrated universal testing machines and Vicat softening point apparatus traceable to national or international measurement standards.
Applications
Flexural strength and Vicat softening temperature data are used throughout the plastics value chain, from resin development to end-product certification. Typical application scenarios include:
- Automotive engineering: interior trim, dashboards, bumper structures, under-hood components, and battery housings that must retain stiffness at elevated temperature
- Electrical and electronics: connectors, switch housings, charger enclosures, and circuit breaker parts requiring verified thermal endurance
- Pipes, fittings, and valves: quality control of PVC, PP, and PE systems against product standards
- Building and construction: profiles, panels, flooring, and transparent glazing materials exposed to solar heat gain
- Household appliances: kettle bodies, oven handles, microwave cookware, and hairdryer components in contact with heat sources
- Material research and development: comparison of resin grades, filler loading, glass fiber content, and heat stabilizer formulations
- Regulatory compliance and supplier qualification: verification of incoming material data sheets and lot-to-lot consistency
FAQ
What is the difference between Vicat softening temperature and heat deflection temperature (HDT)?
Vicat softening temperature (ISO 306 / ASTM D1525) measures the temperature at which a needle penetrates 1 mm under a small static load, reflecting the softening point of the polymer surface. Heat deflection temperature (ISO 75 / ASTM D648) measures the temperature at which a specimen deflects a defined distance under a bending stress of 0.45 MPa or 1.8 MPa, reflecting stiffness retention under load. Vicat values are generally higher than HDT values for the same material, and the two tests are often requested together for a complete thermal profile.
Why do Vicat results differ between A50, B50, A120, and B120 conditions?
The applied load and heating rate directly influence the measured softening point. A higher load (50 N versus 10 N) causes penetration at a lower temperature, while a faster heating rate (120 °C/h versus 50 °C/h) generally produces a higher reading due to thermal lag. For this reason, the test condition must always accompany the reported value, and comparisons should only be made between results obtained under identical conditions.
How do ISO 178 and ASTM D790 flexural results compare?
Both standards use three-point bending, but differences in specimen dimensions, support span, and crosshead speed can lead to slightly different strength values. Flexural modulus results are usually closely comparable, while strength values may differ by a small margin. When reporting data for international projects, the test standard used should always be clearly identified.
What sample quantity is required for testing?
For flexural testing, at least five specimens are normally tested for isotropic materials; for oriented or fiber-reinforced materials, ten specimens may be required. For Vicat testing, at least two to three specimens are typically tested, and the mean value is reported. Additional samples should be supplied to allow for retesting in case of anomalous results.
Can specimens be taken directly from finished products?
Yes. Specimens can usually be machined from flat regions of pipes, sheets, panels, and molded parts, provided the resulting dimensions meet the standard requirements and the surface is smooth. The sampling position and orientation should be recorded, since glass fiber orientation and weld lines in molded parts can significantly affect the measured values.
How long does testing take?
Flexural testing of a standard specimen set is typically completed within one working day after specimen preparation and conditioning. Vicat testing duration depends on the heating rate and the expected softening point, generally ranging from thirty minutes to two hours per specimen. Overall turnaround, including conditioning and reporting, is commonly three to five working days at accredited third-party laboratories.