Horizontal and Vertical Burning Tests for Plastic Materials: UL 94, IEC 60695-11-10 and GB/T 2408 Fl

Horizontal and vertical burning tests are the internationally recognized small-scale flammability methods used to classify how polymeric materials ignite, burn, and drip when exposed to a low-energy flame. Based on standards such as UL 94, IEC 60695-11-10, ISO 1210, ASTM D635/D3801, and GB/T 2408, these tests assign the widely cited V-0, V-1, V-2, and HB ratings that underpin material selection and product safety compliance worldwide. This article explains the test principles, specimen requirements, classification criteria, and typical applications of horizontal and vertical burning determination.

Technical Overview

Horizontal and vertical burning determination is a set of laboratory-scale flammability tests in which standardized bar or plaque specimens are exposed to a calibrated premixed gas flame of nominal power 50 W (approximately 20 mm blue flame height) produced by a specified burner. Depending on the orientation of the specimen, the method classifies materials as either vertical burning categories (V-0, V-1, V-2) or horizontal burning categories (HB, often subdivided as HB 40 and HB 75 in IEC 60695-11-10). Related methods extend the same principle to 500 W flame tests (5VA, 5VB per UL 94 and IEC 60695-11-20) and to thin flexible materials (VTM-0, VTM-1, VTM-2).

The principal reference standards are UL 94, IEC 60695-11-10, ISO 1210, GB/T 2408, and GB/T 5169.16 for the 50 W horizontal and vertical methods, together with ASTM D635 (horizontal), ASTM D3801 (vertical), ASTM D4804 (thin flexible vertical), ASTM D4986 (foam horizontal), and IEC 60695-11-3/IEC 60695-11-20 for the 500 W method. These standards are largely harmonized, so a test performed by an accredited third-party laboratory to one standard is generally accepted across regulatory schemes.

The results are used to demonstrate compliance with material flammability requirements in product safety standards such as IEC 62368-1 (audio/video and IT equipment), IEC 60335-1 (household appliances), and IEC 60601-1 (medical electrical equipment), as well as for material datasheets, qualification programs, and quality control. It is important to note that these tests are comparative, small-scale screening methods: a rating indicates relative burning behavior under controlled laboratory conditions and is only valid for the specific thickness tested, not a direct measure of fire hazard in a full-scale fire.

Testing Samples

Specimens must be representative of the material as supplied and are normally prepared by injection molding or cutting from sheet or finished parts. Conditioned bars are tested in the thickness of intended use, because the classification depends strongly on specimen thickness.

Rigid Plastic Bars

Standard bar specimens measure 125 mm x 13 mm in length and width, with the thickness being the actual material thickness (typically up to 13 mm). For a vertical classification under UL 94, two sets of five bars are required: one conditioned at 23 +/- 2 degrees C and 50 +/- 10% relative humidity for at least 48 hours, and one additionally aged at 70 degrees C for 168 hours and cooled in a desiccator. For a horizontal classification, three bars per material and thickness are customary.

Thin and Flexible Materials

Films and thin flexible materials that distort, shrink, or cannot be clamped as rigid bars are tested in the rolled configuration described for VTM classifications (UL 94, ASTM D4804, ISO 9773). Specimens are typically 200 mm x 50 mm and are wrapped on a mandrel to form a self-supporting tube before vertical flame exposure.

Cellular and Foam Materials

Flexible and rigid foams are evaluated with dedicated horizontal (and where required vertical) foam methods such as ISO 9772 and ASTM D4986, which yield the HBF classification, or the HF-1 and HF-2 categories for vertical exposure. Foam specimens are cut with precise dimensional tolerances because cell structure strongly influences burning behavior.

Sections Cut from Finished Products

Where complete parts or enclosures must be assessed, specimens may be cut from the actual device, provided the resulting test piece retains the geometry, thickness, and surface finish of the end-use part. This approach verifies that processing, pigments, and part design have not degraded the base resin rating.

Testing Items

A horizontal and vertical burning determination typically includes the following evaluations:

• Vertical burning classification at 50 W: V-0, V-1, or V-2, per UL 94, IEC 60695-11-10, ISO 1210, ASTM D3801, and GB/T 2408.
• Horizontal burning classification at 50 W: HB (HB 40 / HB 75), per UL 94, IEC 60695-11-10, ISO 1210, ASTM D635, and GB/T 2408.
• Thin-material vertical classification: VTM-0, VTM-1, VTM-2 per UL 94, ISO 9773, and ASTM D4804.
• 500 W flame classification: 5VA and 5VB per UL 94, IEC 60695-11-20, and ASTM D5048.
• Individual afterflame times after the first and second flame applications (t1 and t2) and afterglow time (t3).
• Total afterflame time summed over all specimens and applications.
• Occurrence of flaming or non-flaming drips and whether they ignite the cotton indicator pad placed below the specimen.
• Whether burning or glowing reaches the clamp or the upper gauge mark.
• Horizontal burning rate in mm/min over the 75 mm measured span, where applicable.

Testing Methods

Vertical Burning Test (50 W, V Classifications)

The bar specimen is clamped at its upper end so that its lower longitudinal edge is positioned 10 mm above the top of an unignited, vertically mounted burner. A cotton indicator pad is placed 300 mm below the specimen. The burner is adjusted to deliver a 20 mm high blue flame, and the flame is applied to the center of the lower edge of the specimen for 10 seconds, then removed while the afterflame time t1 is recorded. After flaming ceases, the flame is applied a second time for 10 seconds, and the afterflame time t2, the afterglow time t3, and any dripping that ignites the cotton are recorded. The classification criteria are:

• V-0: each afterflame time is 10 seconds or less; the sum of afterflame times for the five specimens does not exceed 50 seconds; no flaming drips ignite the cotton; no afterflame or afterglame reaches the clamp; afterflame plus afterglow after the second application does not exceed 30 seconds.
• V-1: each afterflame time is 30 seconds or less; the sum of afterflame times does not exceed 250 seconds; no flaming drips ignite the cotton; afterflame plus afterglow after the second application does not exceed 60 seconds.
• V-2: identical timing criteria to V-1, but flaming drips are permitted provided the material still meets the afterflame and afterglow limits, including when drips ignite the cotton.

If any single specimen in a set fails the criteria, a second set of the same number of specimens is tested, and the combined results determine the final classification according to the decision rules of the applied standard.

Horizontal Burning Test (50 W, HB Classifications)

The bar specimen is clamped horizontally, and gauge marks are drawn 25 mm and 100 mm from the free end. The burner is inclined at 45 degrees beneath the free end, and the 20 mm blue flame is applied for 30 seconds or until the flame front reaches the 25 mm mark, whichever occurs first. The time for the flame front to travel the 75 mm span between the 25 mm and 100 mm marks is measured, and the linear burning rate is calculated as 60 multiplied by 75 and divided by the measured time in seconds, expressed in mm/min. The classification criteria are:

• HB 40: burning ceases before the 100 mm mark, or the burning rate does not exceed 40 mm/min for thicknesses of 3.0 mm to 13 mm and 75 mm/min for thicknesses below 3.0 mm.
• HB 75: the burning rate does not exceed 75 mm/min, applicable where the slower HB 40 limit cannot be met.
• Under UL 94 these are reported collectively as the HB rating, and any specimen that does not meet the applicable limits requires supplementary testing of additional specimens.

Supplementary 500 W and Thin-Material Methods

For the 5VA and 5VB classifications, both bar and plaque specimens are exposed to a 500 W flame approximately 125 mm high, applied five times for 5 seconds each with defined intervals. Both classifications require that afterflame plus afterglow after the fifth application does not exceed 60 seconds and that flaming drips do not ignite the cotton; the more stringent 5VA additionally requires that the plaque does not exhibit burn-through. Thin flexible materials follow the VTM procedure with rolled specimens and criteria analogous to the V classifications.

Reporting

The test report identifies the applied standard, material designation and color, specimen thickness, conditioning history, the number of specimens, all measured afterflame and afterglow times, observations of dripping and cotton ignition, and the resulting classification. Because ratings are thickness-specific, the report must clearly state the thickness at which the classification was obtained.

Applications

Horizontal and vertical burning data are referenced throughout industry and regulation. Typical applications include:

• Enclosures and internal parts of information technology, audio, and video equipment under IEC 62368-1, where V-rated or 5V-rated materials are required near potential ignition sources.
• Unattended and attended household appliances under IEC 60335-1, which prescribe minimum flammability ratings for non-metallic parts based on their location and function.
• Medical electrical equipment under IEC 60601-1, connector bodies, switches, sockets, terminal blocks, and other current-carrying or adjacent insulating components.
• Printed circuit board laminates, adhesive tapes, insulating films, sleeves, and cable accessories used inside electronic assemblies.
• Automotive interior non-metallic components, complementing horizontal burn-through requirements such as FMVSS 302 and OEM material specifications.
• Resin producer datasheets, material qualification databases, and incoming inspection programs, where the V-0 and HB ratings serve as standardized comparison points between competing materials.
• Research and development screening of flame-retardant formulations, where small-scale ratings guide iterative compound optimization before full certification testing.

FAQ

What is the difference between the vertical and horizontal burning tests?

The vertical test exposes a vertically clamped specimen to a flame from below, which is the more severe orientation because hot combustion gases rise and preheat the specimen; it yields the V-0, V-1, and V-2 ratings. The horizontal test exposes a horizontally supported specimen at 45 degrees and measures either self-extinguishment before the 100 mm mark or a linear burning rate; it yields the HB classifications and is generally suited to materials in less demanding locations.

What do V-0, V-1, and V-2 actually mean?

V-0 indicates the fastest self-extinguishing behavior: afterflame within 10 seconds per application and 50 seconds total for the specimen set, with no flaming drips. V-1 allows 30 seconds per application and 250 seconds total, also without flaming drips. V-2 meets the same timing as V-1 but permits flaming drips that may ignite the indicator cotton.

How many specimens are required?

For horizontal testing, three bar specimens per material and thickness are standard. For vertical testing, five bars per set are required, and UL 94 additionally requires a second set aged at elevated temperature, making ten specimens in total for a full V classification at one thickness.

Why is the cotton indicator pad used?

The cotton pad positioned below the specimen detects flaming drips or particles. This is a safety-critical observation because burning droplets can ignite secondary materials located beneath the part in real equipment, which is why V-0 and V-1 prohibit drips that ignite cotton.

Which standard should be specified: UL 94, IEC 60695-11-11-10, or GB/T 2408?

The 50 W methods of UL 94, IEC 60695-11-10, ISO 1210, and GB/T 2408 are technically harmonized, so the choice usually follows the end market and the product safety standard invoked. Accredited third-party laboratories can typically issue reports to multiple equivalent standards from a single test series.

Is an HB rating acceptable, or is V-0 always required?

It depends on the location and function of the part. Product safety standards such as IEC 62368-1 and IEC 60335-1 prescribe different minimum ratings depending on proximity to ignition sources, operating temperatures, and whether the part is attended. HB materials are accepted for many low-risk positions, while parts near arcing contacts or in unattended appliances usually require V-rated or higher classifications.

Does the rating apply to all thicknesses of the same material?

No. Flammability classification is thickness-dependent, and a V-0 rating at 3 mm does not guarantee the same behavior at 1 mm. Each relevant thickness must be tested and reported separately, and the minimum qualified thickness should be respected during product design.

How long does testing take?

Excluding sample preparation, conditioning of 48 hours at standard atmosphere, and, for UL 94 vertical tests, the additional 168-hour thermal aging, are the dominant time requirements. Most accredited third-party laboratories complete the full conditioning and test sequence within one to two weeks, subject to workload and the number of thicknesses submitted.