Plastic After-Flame Time Testing: Standards, Methods, and Flammability Classification

Plastic after-flame time testing measures how long a plastic specimen continues to burn after a standardized ignition source is removed, and it forms the core basis of flammability classifications such as UL 94 V-0, V-1, and V-2 under IEC 60695-11-10, ISO 1210, and ASTM D3801. This guide explains the test principle, specimen requirements, procedures, and acceptance criteria used by manufacturers and accredited third-party laboratories worldwide.

Technical Overview

After-flame time is defined as the period during which a plastic specimen continues to flame after the ignition source has been removed under specified laboratory conditions. It is recorded after each application of the test flame (commonly designated t1 and t2) and, together with after-glow time, dripping behavior, and ignition of an indicator medium, it determines the flammability classification of a polymeric material.

The principal test methods for after-flame time evaluation are the 50 W (20 mm) horizontal and vertical flame tests described in IEC 60695-11-10, ISO 1210, UL 94, ASTM D635, and ASTM D3801, together with the technically aligned Chinese national standards GB/T 5169.16 and GB/T 2408. Related methods include the 500 W flame test of IEC 60695-11-20 for 5VA/5VB classifications and the needle-flame test of IEC 60695-11-5 and GB/T 5169.5 for finished components.

These small-scale tests serve three purposes: quality control of production materials, comparison and screening of candidate materials during design, and demonstration of compliance with product safety requirements. Because results depend strongly on specimen thickness, geometry, and conditioning, a classification applies only to the specific form tested and is not, by itself, a measure of the fire hazard of a finished product in a real fire scenario.

Testing Samples

Vertical Burning Specimens (V-0 / V-1 / V-2)

Standard specimens are rectangular bars of 125 mm × 13 mm, cut or molded at the thickness intended for end use (commonly 1.6 mm and 3.2 mm). A minimum of five specimens is tested as one set; where ignition of the cotton indicator occurs or results are marginal, additional sets may be required. Specimens are conditioned for at least 48 hours at 23 ± 2 °C and 50 ± 5 % relative humidity, and a further set is typically oven-aged for 168 hours at 70 ± 2 °C to confirm that the classification is maintained after thermal aging.

Horizontal Burning Specimens (HB)

Bars of 125 mm × 13 mm × thickness are marked with reference lines at 25 mm and 100 mm from the ignited end. Three specimens are normally tested, and the burning rate between the reference marks is used for the HB classification.

Plaque Specimens (5VA / 5VB)

The 500 W flame test requires flat plaques of 125 mm × 125 mm at the relevant thickness, in addition to bar specimens, to assess whether the flame burns through the material.

Finished Components (Needle-Flame Test)

Complete parts, sub-assemblies, or sections cut from end products are tested in their installed orientation, since the needle-flame test evaluates the behavior of a component as actually used rather than a molded bar.

Special Material Forms

Thin flexible films are tested on a mandrel in accordance with ISO 9773 or the VTM procedures, while cellular and foamed plastics are evaluated under ISO 9772 for HF-1 and HF-2 classifications, using appropriately sized specimens mounted to simulate end-use orientation.

Testing Items

The following parameters are recorded and evaluated during after-flame time testing:

• After-flame time after each application of the ignition source (t1 and t2), in seconds
• Total after-flame time for the complete specimen set (sum of all individual after-flame times)
• After-glow time, meaning continued glowing combustion after flaming ceases
• Whether flaming or incandescent drips ignite the cotton indicator placed below the specimen
• Whether the flame front reaches the specimen clamp or the upper reference mark
• Burning rate in mm/min for horizontal tests, measured between the 25 mm and 100 mm marks
• Whether the specimen burns through to the rear surface in plaque tests
• The resulting classification: HB, V-0, V-1, V-2, 5VA, 5VB, VTM, or HF series

Testing Methods

Vertical Burning Test (V Classification)

The specimen is clamped vertically in a draft-free enclosure, with a layer of surgical cotton placed 300 mm below its lower edge. A burner producing a 20 ± 1 mm blue flame fueled by methane of at least 98 % purity is applied to the bottom edge of the specimen for 10 ± 0.5 seconds and then withdrawn. The after-flame time t1 is timed from removal of the burner until flaming stops. A second 10-second application is made immediately after flaming ceases, and the after-flame time t2 is recorded, along with after-glow duration and any dripping that ignites the cotton.

• V-0: each after-flame time ≤ 10 s; total after-flame time for the set ≤ 50 s; no ignition of cotton by flaming drips; no after-flame or after-glow reaching the clamp
• V-1: each after-flame time ≤ 30 s; total after-flame time ≤ 250 s; no ignition of cotton by flaming drips
• V-2: each after-flame time ≤ 30 s; total after-flame time ≤ 250 s; flaming drips permitted only if they ignite the cotton briefly without sustained flaming

Horizontal Burning Test (HB Classification)

The specimen is mounted horizontally with the free end exposed to a 25 mm test flame applied at an angle for 30 seconds or until the 25 mm mark is reached. The time for the flame front to travel from the 25 mm mark to the 100 mm mark is measured. Materials that extinguish before the 100 mm mark, or that burn at no more than 40 mm/min (thin sections) or 75 mm/min (sections 3 mm and thicker), qualify for the HB classification.

Needle-Flame Test

A small 12 mm butane flame is applied to a component for a severity time selected from the standard series, commonly 5 to 30 seconds. After removal of the flame, the after-flame time is measured; a typical acceptance criterion requires after-flaming to cease within 30 seconds without igniting surrounding parts or spreading to adjacent materials.

Flame Verification and Test Environment

The test flame is verified before use with a copper-block calorimeter, requiring a temperature rise from 100 °C to 700 °C in 44 ± 2 seconds for the 50 W flame, in accordance with IEC 60695-11-4. Tests are conducted inside a ventilated enclosure with the draft suppressed during measurement, and all timing is performed with calibrated instruments traceable to national standards.

Applications

After-flame time testing is a fundamental compliance and quality-control activity across all industries that use plastic materials in electrotechnical and consumer products. Typical application areas include:

• Information technology and audio-video equipment enclosures and internal components, supporting requirements of IEC 62368-1 and GB 4943
• Household appliances and unattended appliances, where material flammability is combined with glow-wire requirements under IEC 60335-1
• Connectors, switches, terminal blocks, and other current-carrying parts that may be exposed to abnormal heat or fault arcing
• Wire and cable insulation and sheathing materials, printed circuit board laminates, and electronic packaging
• Medical electrical equipment, lighting products, batteries, and power tools
• Automotive interior components, where equivalent horizontal and vertical burn requirements are frequently specified
• Research and development of flame-retardant compounds, incoming material inspection, and periodic production quality monitoring

Manufacturers routinely submit materials to accredited third-party laboratories to obtain recognized flammability classifications, which are then referenced in product safety files, datasheets, and customer specifications.

FAQ

Q: What is the difference between after-flame time and after-glow time?

A: After-flame time is the period of visible flaming combustion after the ignition source is removed, while after-glow time is the period of glowing combustion without flame that may follow flaming. Both are recorded separately because glowing particles can also ignite adjacent materials.

Q: Why does the V-0 rating require a total after-flame time of 50 seconds or less?

A: The limit of 50 seconds applies to the combined after-flame times of all ten flame applications across the five-specimen set. It ensures that self-extinguishing behavior is consistent across the material batch rather than relying on a single favorable specimen.

Q: How does specimen thickness affect the result?

A: Thinner specimens generally burn more readily, so a classification is only valid for the thickness tested. Products must use material at a thickness equal to or greater than the tested thickness, or the material must be retested at the thinner dimension.

Q: What happens if only one specimen in a set fails the criteria?

A: Under the standard decision rules, if the failure is limited to a single specimen meeting relaxed conditions, another set of five specimens is tested and all specimens in the new set must pass for the classification to be granted.

Q: Is a passing after-flame time test sufficient to prove fire safety?

>A: No. These are small-scale comparative tests on specific specimens. Product safety standards combine them with glow-wire tests, needle-flame tests on components, and fault-condition evaluations to assess the overall fire risk of the finished product.