Plastic Burning Test Methods: Flammability Standards, Procedures and Classifications

Plastic burning test methods are standardized laboratory procedures used to evaluate the ignitability, burning rate, and self-extinguishing behavior of polymeric materials under controlled flame exposure. Based on internationally recognized standards such as UL 94, IEC 60695-11-10, ISO 1210, and GB/T 2408, these tests generate the flammability classifications required for safety compliance in the electronics, appliance, and automotive industries. This article explains the main combustion test methods, specimen requirements, classification criteria, and typical applications.

Technical Overview

Plastic burning tests are designed to measure the relative flammability of plastic materials by exposing precisely dimensioned specimens to a standardized ignition source under strictly controlled laboratory conditions. During each test, key parameters such as afterflame time, afterglow duration, flame propagation speed, and the ignition of a cotton indicator by flaming drips are recorded and compared against defined pass criteria. The results allow manufacturers, certification bodies, and accredited third-party laboratories to assign a flammability classification that predicts how a material will behave in the event of an electrical fault, overheated component, or external fire exposure.

The most widely applied combustion test frameworks include the UL 94 series from Underwriters Laboratories, the IEC 60695-11 series for fire hazard testing, ISO 9772, ISO 9773, ISO 1210, and ISO 4589, the ASTM D635, D3801, D5048, and D2863 methods, the Chinese national standards GB/T 2408 and GB/T 2406, and automotive specifications such as ISO 3795 and FMVSS 302. Although the apparatus and acceptance criteria differ between these standards, they all evaluate the same fundamental question: how quickly does the material ignite, how far does the flame travel, and does the material extinguish itself once the ignition source is removed.

Testing Samples

Bar Specimens for 50 W Horizontal and Vertical Tests

For the UL 94 HB and V methods, IEC 60695-11-10, ISO 1210, ASTM D635, ASTM D3801, and GB/T 2408, standard bar specimens measure 125 mm in length and 13 mm in width. The thickness must correspond to the intended end-use application, and materials are normally evaluated at both the minimum and maximum commercial thickness. Five bars are typically tested for the vertical burning method and three bars for the horizontal burning method, with a second set often required after oven aging to confirm long-term stability of the flame-retardant system.

Plaque and Bar Specimens for 500 W Tests

The 5VA and 5VB methods under UL 94 and IEC 60695-11-20 require both 125 mm × 13 mm bars and flat plaques, generally 150 mm × 150 mm, at the nominal thickness of the finished part. The plaque test determines whether the flame burns through the material, which is the distinguishing criterion between the 5VA and 5VB classifications.

Foam and Thin-Film Specimens

Cellular plastics for the HBF, HF-1, and HF-2 classifications are tested as 150 mm × 50 mm specimens in accordance with UL 94 and ISO 9772. Thin, flexible films that cannot support themselves as flat bars are tested as 200 mm × 50 mm strips wrapped around a mandrel per ISO 9773 and the UL 94 VTM protocol.

Finished Parts and Oxygen Index Specimens

Glow-wire and needle-flame tests are performed either on complete end products or on representative cut sections of the enclosure or component. For the limiting oxygen index method under ISO 4589-2, ASTM D2863, and GB/T 2406, specimens typically measure 80 mm to 150 mm in length and 10 mm in width, with a thickness of 4 mm or the actual product thickness.

Conditioning Requirements

Prior to testing, specimens must be conditioned at 23 °C ± 2 °C and 50 % ± 5 % relative humidity for a minimum of 48 hours. Specimens for the second conditioning set are commonly oven-aged at 70 °C for 168 hours, cooled, and then reconditioned before testing, in order to simulate thermal aging of the flame retardant.

Testing Items

Depending on the product standard and the intended application, plastic materials are evaluated against one or more of the following flammability items:

• Horizontal burning classification: UL 94 HB, ISO 9772 HBF, ASTM D635, and GB/T 2408 FH-1/FH-2/FH-3.
• Vertical burning classification at 50 W: UL 94 V-0, V-1, V-2; IEC 60695-11-10 V-0, V-1, V-2; ASTM D3801; and GB/T 2408 FV-0/FV-1/FV-2.
• Vertical burning classification at 500 W: UL 94 5VA and 5VB, and IEC 60695-11-20.
• Foam material classifications: UL 94 HBF, HF-1, and HF-2.
• Thin material classifications: UL 94 VTM-0, VTM-1, and VTM-2 per ISO 9773.
• Limiting oxygen index (LOI) per ISO 4589-2, ASTM D2863, and GB/T 2406.
• Glow-wire flammability index (GWFI) and glow-wire ignition temperature (GWIT) per IEC 60695-2-12 and IEC 60695-2-13.
• Needle-flame test per IEC 60695-11-5 and GB/T 5169.5.
• Automotive interior horizontal burning rate per ISO 3795, FMVSS 302, and GB 8410.

Testing Methods

Horizontal Burning Test (UL 94 HB, ASTM D635, ISO 9772)

The bar specimen is clamped horizontally, and a 25 mm blue premixed flame is applied at an angle of 45 degrees to one end for 30 seconds or until the flame front reaches the 25 mm reference mark. The operator records the time required for the flame to travel between the 25 mm and 100 mm marks in order to calculate the linear burning rate. A material receives the HB classification when the burning rate does not exceed 75 mm/min for thicknesses below 3.0 mm, does not exceed 40 mm/min for thicknesses between 3.0 mm and 13 mm, or when burning ceases before the flame reaches the 100 mm mark. HB represents the lowest flammability rating in the UL 94 system and is generally acceptable only for less critical, decorative, or mechanically protected parts.

Vertical Burning Test at 50 W (UL 94 V, IEC 60695-11-10, GB/T 2408)

The specimen is clamped vertically with its lower end 10 mm above the burner. A 20 mm high, 50 W methane flame is applied to the center of the lower edge for 10 seconds, removed, and reapplied for a second 10-second interval after the specimen stops burning or glowing. Afterflame times after each application (t1 and t2), total afterflame plus afterglow time, and whether flaming drips ignite surgical cotton placed 300 mm below the specimen are all recorded. The classification criteria are as follows:

• V-0: afterflame after either application does not exceed 10 seconds for any specimen, and the sum of afterflame times for all ten applications does not exceed 50 seconds; no flaming drips ignite the cotton.
• V-1: individual afterflame times do not exceed 30 seconds and the cumulative afterflame time does not exceed 250 seconds; no flaming drips ignite the cotton.
• V-2: identical timing criteria to V-1, but flaming drips that ignite the cotton indicator are permitted.

For all three classes, no specimen may burn up to the clamp, and afterflame plus afterglow must not persist beyond 30 seconds for V-0 or 60 seconds for V-1 and V-2 following the second flame application.

500 W Vertical Burning Test (UL 94 5VA and 5VB, IEC 60695-11-20)

The 500 W method applies a higher-energy flame five times for 5 seconds each to the bar specimen, and additionally to plaque specimens. To obtain the 5VB classification, afterflame and afterglow must not exceed 60 seconds after the fifth application and flaming drips must not ignite the cotton, although the plaque may develop a burn-through hole. The 5VA classification, the most severe UL 94 rating, additionally requires that the plaque not be burned through. Materials rated 5VA are commonly specified for enclosures exposed to potential arcing or high-energy fault conditions.

Needle-Flame Test (IEC 60695-11-5, GB/T 5169.5)

The needle-flame test simulates small fault conditions such as a failing component or an overloaded conductor. A 12 mm flame produced by a 0.5 mm bore burner is applied to the specimen for a specified duration, commonly 5 to 30 seconds depending on the product standard. The specimen passes if flaming ceases within the specified time, surrounding parts are not ignited, and tissue paper placed beneath the specimen does not burn.

Glow-Wire Tests (IEC 60695-2 Series)

In the glow-wire flammability test, an electrically heated glow wire at temperatures between 550 °C and 960 °C is pressed against the finished part for 30 seconds. The part passes the GWFI criterion at a given temperature if any flame or glowing extinguishes within 30 seconds after removal of the glow wire and the underlying tissue paper does not ignite. The GWIT method determines the temperature 25 K above which the material does not ignite during exposure. Household appliance standards in the IEC 60335 series routinely require glow-wire verification at 750 °C or 850 °C for live parts and enclosures.

Limiting Oxygen Index (ISO 4589-2, ASTM D2863, GB/T 2406)

The oxygen index method mounts a small vertical specimen in a glass chimney through which a controlled mixture of nitrogen and oxygen flows. The test determines the minimum oxygen concentration, expressed as a volume percentage, that will just support candle-like combustion for 3 minutes or over a 50 mm length. Higher LOI values indicate better intrinsic flame resistance; engineering materials with an LOI above approximately 27 % are generally regarded as self-extinguishing, which makes the method particularly useful for comparing candidate materials during design and material development.

Automotive Horizontal Burning Test (ISO 3795, FMVSS 302, GB 8410)

Interior materials for road vehicles are tested in the horizontal orientation with the specimen held above a horizontal burner. The measured burning rate between gauge marks must not exceed 102 mm/min, and materials that extinguish before the first mark is reached automatically comply. This test is a regulatory requirement for seat covers, headliners, door panels, and other occupant-compartment components.

Applications

Plastic combustion test results directly determine whether a material may be used in safety-critical applications and are cited in nearly every product safety standard for electrotechnical equipment. Typical application scenarios include the following:

• Enclosures and internal supports for IT equipment, power supplies, chargers, and consumer electronics, where V-0 at the declared minimum thickness is routinely mandated.
• Household appliance housings, switch assemblies, and terminal blocks verified through glow-wire and needle-flame requirements of the IEC 60335 series.
• Connectors, sockets, relays, circuit-breaker housings, and other unattended electrical components requiring 5VA or GWFI 960 °C performance.
• Automotive interior trim, textile composites, and battery module components evaluated against FMVSS 302, ISO 3795, and GB 8410.
• Wire and cable insulation, sheathing, and conduit compounds qualified through LOI and vertical flame methods.
• Material research and development, incoming quality control, and supplier qualification programs where LOI and UL 94 data support material selection and flame-retardant formulation optimization.

Because flammability classifications are thickness-dependent, certification documents must always state the tested thickness, and design changes that reduce wall thickness require re-evaluation of the flame rating.

FAQ

What is the difference between UL 94 V-0, V-1, and V-2?

The three classes are distinguished by afterflame time and dripping behavior. V-0 requires individual afterflame times of no more than 10 seconds and a cumulative time of no more than 50 seconds with no flaming drips. V-1 allows individual afterflame times up to 30 seconds and a cumulative limit of 250 seconds with no flaming drips. V-2 matches the V-1 timing requirements but tolerates flaming drips that ignite the cotton indicator.

Are UL 94 and IEC 60695-11-10 results interchangeable?

The 50 W horizontal and vertical burning methods of UL 94, IEC 60695-11-10, ISO 1210, and GB/T 2408 are technically aligned in apparatus and criteria, and results are widely accepted as equivalent in practice. However, full UL 94 certification also includes aging treatments, follow-up service requirements, and program-specific documentation, so manufacturers should confirm with the certification body or accredited third-party laboratory which scheme applies to their target market.

How many specimens are required for a vertical burning test?

A standard 50 W vertical burning evaluation uses five bar specimens in the as-conditioned state. If a second set after oven aging at 70 °C for 168 hours is required, ten bars in total are prepared. Additional specimen sets may be needed when several thicknesses are being qualified.

Which material thickness should be submitted for testing?

Flammability performance degrades as thickness decreases, so tests must be performed at the minimum thickness that will actually be molded in production. When a part contains multiple wall thicknesses, each relevant thickness should be evaluated, and the reported rating is only valid at or above the tested thickness.

What is the difference between V-0 and 5VA?

V-0 is obtained with a 50 W flame applied twice for 10 seconds to bar specimens, whereas 5VA and 5VB use a 500 W flame applied five times to both bars and plaques. A 5VA material additionally resists burn-through of the plaque, making it the most demanding classification in the UL 94 system and the preferred choice for enclosures near potential arc sources.

How should manufacturers select the appropriate burning test method?

Selection follows the applicable end-product safety standard for the target market, such as IEC 62368-1 for audio-visual and IT equipment, IEC 60335-1 for household appliances, or the automotive interior regulations. When no specific requirement exists, UL 94 vertical burning at the intended thickness combined with oxygen index data provides a robust baseline for comparing material flammability.