Plastic Burning Duration Testing: Afterflame Time Evaluation of Polymeric Materials

The plastic burning duration test measures how long a plastic specimen continues to burn after removal of a standardized ignition source, providing a quantitative basis for flammability classification. This article explains the test principles, sample requirements, test items, and methods defined by standards such as UL 94, IEC 60695-11-10, ISO 1210, and GB/T 2408, and outlines how the resulting afterflame time data supports material selection and compliance for electrical, electronic, and automotive products.

Technical Overview

The plastic burning duration test, commonly referred to as the afterflame time test, is a small-scale laboratory flammability test used to evaluate the combustion behavior of polymeric materials. A conditioned bar or plaque specimen is exposed to a standardized gas flame for a specified period, after which the ignition source is removed and the duration of continued flaming is precisely recorded. The key measured parameters include afterflame time, afterglow time, flame spread distance or linear burning rate, and whether flaming drips ignite an indicator cotton pad placed beneath the specimen.

The test is primarily governed by the UL 94 flame testing framework, which is harmonized at the international level with IEC 60695-11-10 (50 W horizontal and vertical test flames), IEC 60695-11-3 (500 W test flame), ISO 1210, ISO 9772, ISO 9773, and ISO 10351. In China, the corresponding national standards include GB/T 2408 (horizontal burning), GB/T 5169.16 (equivalent to IEC 60695-11-10), and GB/T 5169.17 (equivalent to IEC 60695-11-3). ASTM counterparts include ASTM D635 for horizontal burning and ASTM D3801 for vertical burning. Although the test is a comparative, small-flame method rather than a full-scale fire simulation, the resulting classifications such as V-0, V-1, V-2, HB, and 5VA are widely referenced in product safety standards and material specifications worldwide.

Testing Samples

Specimen geometry, preparation, and conditioning directly affect the reproducibility of burning duration results. Materials are typically tested at the minimum thickness used in the end product as well as at standard reference thicknesses, because flammability ratings are thickness-dependent.

Molded Bar Specimens

For the 50 W horizontal and vertical tests, standard specimens are bars of 125 mm × 13 mm, cut or molded at the intended thickness, with the preferred reference thickness being 3.0 mm. Injection-molded, compression-molded, or cut-from-sheet specimens must be free of flash, burrs, and surface defects that could influence flame behavior.

Foam Plastic Specimens

Cellular materials such as polyurethane, polystyrene, and polyolefin foams are tested as 150 mm × 50 mm blocks at natural thickness in accordance with ISO 9772 or UL 94 HBF procedures, because thin bar specimens do not represent the burning behavior of foamed structures.

Thin and Flexible Materials

Films, flexible sheets, and thin fabrics that distort or shrink away from the flame in the standard vertical bar test are evaluated with a mandrel-supported configuration in accordance with ISO 9773 or UL 94 VTM procedures, yielding classifications VTM-0, VTM-1, and VTM-2.

Finished Product Sections

Cut sections from enclosures, connectors, insulation parts, and decorative components may be tested to verify that the manufactured part, including processing effects, pigments, and fillers, achieves the intended burning duration classification.

All specimens are conditioned at 23 °C ± 2 °C and 50% ± 5% relative humidity for a minimum of 48 hours prior to testing. For certain classification requirements, an additional set of specimens is conditioned at elevated temperature and humidity to confirm performance under less favorable conditions. A complete test generally requires two sets of five specimens per thickness.

Testing Items

Depending on the selected method and standard, the burning duration experiment reports the following items:

• Afterflame time after the first flame application (t1), recorded in seconds
• Afterflame time after the second flame application (t2), recorded in seconds
• Total afterflame time per specimen (t1 + t2) and the sum for each set of five specimens
• Afterglow time, meaning continued glowing combustion without flame after flaming ceases
• Linear burning rate in millimeters per minute for horizontal (HB) testing
• Time for the flame front to travel between the 25 mm and 100 mm reference marks
• Whether flaming particles or drips ignite the cotton indicator placed below the specimen
• Damage length, damaged zone extent, and whether flaming reaches the specimen clamp or 100 mm gauge mark
• Final flammability classification, such as V-0, V-1, V-2, HB, HB40, HBF, 5VA, 5VB, VTM-0, VTM-1, or VTM-2

Testing Methods

Horizontal Burning Test (HB)

The specimen is clamped horizontally with the long axis parallel to the burner. A 20 mm high 50 W premixed blue flame is applied to the free end at a 45° angle for 30 seconds, then removed. The laboratory records whether the specimen burns to the 25 mm reference mark, the time to travel from the 25 mm to the 100 mm mark, and the resulting burning rate. Materials that do not burn past the 100 mm mark, or that burn slower than the specified rate limits, are classified HB or HB40. This method, specified in UL 94, ASTM D635, ISO 1210, and GB/T 2408, is typically used for less critical components.

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

The specimen is suspended vertically above a horizontal layer of dry surgical cotton. A 20 mm 50 W flame is applied to the lower edge of the specimen for 10 seconds, removed, and reapplied for another 10 seconds after flaming stops or after the specimen self-extinguishes. Afterflame times t1 and t2, afterglow, and cotton ignition by drips are recorded. Classification criteria include:

• V-0: afterflame time of each specimen ≤ 10 s, total afterflame time per set ≤ 50 s, no cotton ignition, afterglow ≤ 30 s, and no flaming to the clamp
• V-1: afterflame time of each specimen ≤ 30 s, total afterflame time per set ≤ 250 s, no cotton ignition, afterglow ≤ 60 s
• V-2: identical timing limits to V-1, but flaming drips that ignite the cotton are permitted

This procedure follows UL 94, IEC 60695-11-10, ISO 1210, ASTM D3801, and GB/T 5169.16, and is the most frequently specified burning duration method for electronic enclosures and live parts.

500 W Vertical Test (5VA / 5VB)

For the most demanding applications, bar and plaque specimens are exposed to a higher-energy 125 mm premixed flame in accordance with UL 94, IEC 60695-11-3, ASTM D5048, and GB/T 5169.17. The flame is applied five times for 5 seconds each. A 5VA rating requires that flaming and glowing cease within 60 seconds after the fifth application, no drips ignite cotton, and plaques are not burned through; 5VB permits a hole in the plaque but retains the extinguishing and drip requirements.

Thin-Material and Needle-Flame Tests

Flexible films and thin sheets are wrapped on a mandrel and tested per ISO 9773 for VTM classification. For very small components or fault-simulation scenarios, the needle-flame test of IEC 60695-11-5 applies a 12 mm flame for 10 to 30 seconds and records burning duration, glow duration, and whether an underlying tissue paper layer is ignited.

Applications

Burning duration test data are applied across many industries and product categories:

• Electrical and electronic equipment enclosures, switches, relays, and connectors, where product safety standards referencing UL 94 or IEC 60695 ratings are mandatory
• Wire and cable insulation, spacers, and terminal blocks requiring defined afterflame limits
• Household appliances and power tools, whose safety standards reference vertical burning classifications for live-part carriers and enclosures
• Automotive interior components, where burning duration complements horizontal burn-rate requirements such as ISO 3795 and OEM material specifications
• Building products, furniture, and transportation interiors that specify flame-retarded polymers
• Research and development of halogenated, halogen-free, and intumescent flame-retardant compounds
• Incoming quality control and supplier verification of resin lots, masterbatches, and compounded grades by accredited third-party laboratories
• Regulatory compliance documentation, material datasheets, and certification dossiers for global market access

FAQ

What is the difference between afterflame time and afterglow time?

Afterflame time is the period during which a specimen continues to burn with a visible flame after the ignition source is removed. Afterglow time is the period of glow combustion without flame that may follow flaming, and both parameters are recorded because glow can reignite adjacent materials.

Why is the flame applied twice in the vertical test?

The two 10-second applications simulate repeated exposure to an ignition source, such as an electrical arc or an overheated component. The second application evaluates whether the material, already heated and partially pyrolyzed, still self-extinguishes within the time limits of the target classification.

Does a higher thickness improve the burning duration result?

In general, thicker specimens dissipate heat more slowly relative to their mass and often achieve better ratings; conversely, thin sections burn more readily. For this reason, a classification is only valid at or above the thickness actually tested, and end products must be evaluated at their minimum wall thickness.

How many specimens are required?

A standard vertical or horizontal classification normally requires one or two sets of five bar specimens per thickness. If any single result falls outside the criterion of the intended class, additional sets or a lower classification may be required per the decision rules of UL 94 and IEC 60695-11-10.

Can burning duration results predict full-scale fire hazard?

No. The test uses a small, well-defined ignition source and measures relative extinguishing behavior under laboratory conditions. It is suitable for material comparison, quality control, and compliance screening, but full-scale fire risk assessment requires supplementary tests such as cone calorimetry, oxygen index, glow-wire, and large-scale fire performance evaluations.

Which classification should a design engineer specify?

The required class depends on the product safety standard applicable to the end device. V-2 or V-1 is commonly accepted for general enclosures, V-0 for parts near unsecured live connections or supporting current-carrying components, and 5VA or 5VB for high-demand fire enclosure applications. The specification must always state the minimum tested thickness along with the class.