Plastic flammability rating testing evaluates how plastic materials ignite, burn, and extinguish under controlled flame exposure, providing the flammability classifications required for product safety compliance. Based on internationally recognized standards such as UL 94, IEC 60695-11-10, ISO 1210, and GB/T 2408, these tests assign ratings such as HB, V-0, V-1, V-2, 5VA, and 5VB that determine whether a material is suitable for electrical enclosures, electronic components, and other safety-critical applications. Accredited third-party laboratories perform these tests to support regulatory compliance, material certification, and new product development.
Technical Overview
Plastics are organic polymer materials that are inherently combustible, meaning their flammability behavior must be characterized before they can be used in electrical devices, appliances, vehicles, and building products. Plastic flammability rating testing subjects standardized specimens to a defined ignition source under strictly controlled laboratory conditions and evaluates key parameters such as ignition resistance, afterflame time, afterglow time, burning rate, flaming drips, and burn-through behavior. The results are expressed as a flame class rating that engineers and regulators use to select materials appropriate for each application.
The most widely applied framework is the UL 94 standard from UL (Underwriters Laboratories), which is technically aligned with IEC 60695-11-10 and IEC 60695-11-20, ISO 1210 and ISO 9773, ASTM D635, D3801, D4804, D4986 and D5048, and the Chinese national standards GB/T 2408 and GB/T 5169.16. Complementary methods include the glow-wire tests of IEC 60695-2-10/-2-11/-2-12/-2-13 (GB/T 5169.10 to 5169.13), the needle-flame test of IEC 60695-11-5 (GB/T 5169.5), and the limiting oxygen index (LOI) methods of ISO 4589-2, ASTM D2863, and GB/T 2406.2. Together, these tests form the technical basis for material declaration in safety standards such as IEC 62368-1, IEC 60335-1, and IEC 60950-1 (legacy).
Testing Samples
Standard Bar Specimens
For horizontal burning (HB) and vertical burning (V-0, V-1, V-2) tests, specimens are molded or cut bars of 125 mm in length by 13 mm in width, at the actual product wall thickness or at standard reference thicknesses such as 1.5 mm and 3.0 mm. Because flame ratings are thickness-dependent, each thickness must be tested and reported separately. Typically, two sets of five bar specimens are prepared so that both the primary condition and any verification condition can be evaluated.
Plaque Specimens for 500 W Tests
The 5VA and 5VB classifications require, in addition to bar specimens, flat plaques of at least 150 mm by 150 mm at the relevant thickness. The plaque test evaluates whether the 500 W flame causes a burn-through hole, which is the distinguishing criterion between 5VA (no hole) and 5VB (hole permitted).
Thin Materials and Foams
Films and thin sheeting below the scope of the bar tests are classified under VTM-0, VTM-1, or VTM-2 using rolled specimens per ISO 9773 and ASTM D4804. Flexible and rigid cellular foams are evaluated under HF-1, HF-2, or HBF classifications using specimens per ISO 9772 and ASTM D4986.
Conditioning Requirements
Unless otherwise specified, specimens are conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity for a minimum of 48 hours prior to testing. For certain end-product and glow-wire evaluations, additional conditioning such as 168-hour humidity exposure or thermal aging at 70 °C or 125 °C may be required by the applicable product standard.
Testing Items
- Horizontal burning test (HB rating) — burning rate and extinguishing distance along a horizontal specimen.
- Vertical burning test with a 20 mm (50 W nominal) flame — V-0, V-1, V-2 classifications.
- Vertical burning test with a 500 W flame — 5VA and 5VB classifications, including plaque burn-through assessment.
- Thin-material vertical burning — VTM-0, VTM-1, VTM-2 classifications.
- Cellular foam flammability — HF-1, HF-2, HBF classifications.
- Needle-flame test (IEC 60695-11-5) — simulation of small fault-source ignition.
- Glow-wire tests (IEC 60695-2-11/-2-12/-2-13) — glow-wire flammability index (GWFI) and glow-wire ignition temperature (GWIT).
- Limiting oxygen index (ISO 4589-2, ASTM D2863, GB/T 2406.2) — minimum oxygen concentration to sustain candle-like burning.
- Smoke density (ISO 5659-2, ASTM E662) — specific optical density of smoke generated during combustion.
- Automotive horizontal burn rate (ISO 3795, FMVSS 302, GB 8410) — burn rate of interior trim materials.
Testing Methods
Horizontal Burning (HB)
Per UL 94, ASTM D635, ISO 9772 (foams), and GB/T 2408, a 25 mm blue flame from a specified burner is applied at a 45° angle to one end of a horizontally clamped bar for 30 seconds. The burning rate between the 25 mm and 100 mm reference marks is measured. For specimens 3 mm to 13 mm thick, the rate must not exceed 40 mm/min; for specimens less than 3 mm thick, 75 mm/min. A specimen that extinguishes before the 100 mm mark also passes. HB is the lowest UL 94 classification and is acceptable only for limited applications.
Vertical Burning 20 mm Flame (V-0, V-1, V-2)
Per UL 94, IEC 60695-11-10, ISO 1210, ASTM D3801, and GB/T 5169.16, a 20 mm high blue flame is applied for 10 seconds to the lower end of a vertically clamped bar, removed, and then reapplied for a second 10-second period after extinguishing. Timing of afterflame and afterglow is recorded, and a surgical cotton pad below the specimen detects flaming drips. The classification criteria are as follows:
- V-0: afterflame time not exceeding 10 s per application, total afterflame not exceeding 50 s for all ten applications, no flaming drips, and afterglow not exceeding 30 s.
- V-1: afterflame not exceeding 30 s per application, total not exceeding 250 s, no flaming drips that ignite the cotton indicator, and afterglow not exceeding 60 s.
- V-2: identical time criteria to V-1, but flaming drips that ignite the cotton are permitted.
Vertical Burning 500 W Flame (5VA, 5VB)
Per UL 94, IEC 60695-11-20, and ASTM D5048, a 125 mm high, 500 W premixed flame is applied five times for 5 seconds each to both bar and plaque specimens. Both classifications require no flaming drips and afterflame plus afterglow within 60 s after the fifth application. The 5VA classification additionally requires that the flame does not burn through the plaque, making 5VA the most severe UL 94 rating available for polymeric materials.
Glow-Wire Testing
Per IEC 60695-2-10 and related parts, an electrically heated glow-wire at a fixed temperature (typically 550 °C, 650 °C, 750 °C, 850 °C, or 960 °C) is pressed against the specimen for 30 seconds. The glow-wire flammability index (GWFI, IEC 60695-2-12) identifies the highest temperature at which the material either does not ignite or self-extinguishes without igniting surrounding materials. The glow-wire ignition temperature (GWIT, IEC 60695-2-13) identifies the temperature 25 K or 30 K above the highest temperature at which no ignition occurs in three consecutive tests. Glow-wire data are required by IEC 60335-1 and IEC 60695-2-11 for unattended appliances and fire-enclosure components.
Limiting Oxygen Index (LOI)
Per ISO 4589-2, ASTM D2863, and GB/T 2406.2, a small vertical specimen is ignited in a top-down manner inside a glass chimney while the oxygen/nitrogen ratio is varied. The LOI is the minimum oxygen volume fraction that just supports burning for 3 minutes or over 50 mm of specimen length. Engineering convention generally regards LOI values above approximately 27 % as indicating a self-extinguishing material, and LOI data are widely used for research, quality control, and material comparison rather than direct product certification.
Needle-Flame Testing
Per IEC 60695-11-5 and GB/T 5169.5, a 12 mm needle flame from a specified burner is applied to the specimen for durations such as 5, 10, 20, 30, 60, or 120 seconds to simulate the thermal stress from a small fault condition such as a failing component or overloaded connection.
Applications
Flammability ratings are prerequisites for material acceptance across many regulated industries:
- Consumer electronics and IT equipment: fire enclosures, housings, and internal parts of products covered by IEC 62368-1 typically require V-1 or better at the minimum wall thickness.
- Household appliances: IEC 60335-1 mandates glow-wire temperatures up to 750 °C or 850 °C for live parts and 960 °C for unattended appliances, alongside UL 94 ratings for certain components.
- Connectors, switches, and PCB laminates: thin-walled molded parts and copper-clad laminates are commonly specified at V-0 at 1.5 mm or less.
- Wire and cable accessories: insulation materials, spacers, and mounting hardware rely on V-2 or V-0 ratings and needle-flame performance.
- Automotive interiors: trim materials must satisfy ISO 3795, FMVSS 302, and GB 8410 with a burn rate not exceeding 100 mm/min.
- Medical electrical equipment: IEC 60601-1 requires minimum flammability ratings of V-1 or better for enclosure and barrier materials.
- Railway and building applications: combined flammability, smoke density, and toxicity assessments under standards such as EN 45545-2 and regional building codes.
FAQ
What is the difference between V-0, V-1, and V-2?
All three are vertical burning classifications from a 20 mm flame test. V-0 requires the shortest afterflame times (≤10 s each, ≤50 s total) and no flaming drips; V-1 allows longer afterflame times (≤30 s each, ≤250 s total) with no flaming drips; V-2 matches the V-1 time criteria but permits flaming drips that may ignite the cotton indicator. V-0 represents the best performance of the three.
Why does the same material show different ratings at different thicknesses?
Flame retardance depends strongly on geometry. Thicker sections dissipate heat more effectively and char more readily, so a compound may achieve V-0 at 3.0 mm but only V-1 or HB at 1.5 mm. Ratings must therefore always be quoted together with the tested thickness.
Are UL 94, IEC 60695-11-10, and GB/T 2408 equivalent?
The vertical burning methods of UL 94, IEC 60695-11-10, ISO 1210, ASTM D3801, and GB/T 5169.16 are technically harmonized, so results obtained by any of these methods under equivalent conditions are generally accepted interchangeably by regulators and accreditation bodies.
How many specimens are needed?
A standard vertical burning series uses five bar specimens; a second set of five is prepared in case of marginal or non-uniform results or when two thicknesses are being characterized. Glow-wire and LOI methods each define their own specimen counts, typically a minimum of three per temperature or oxygen concentration.
What is the difference between UL 94 testing and glow-wire testing?
UL 94 evaluates the response of a material specimen to an open gas flame, whereas glow-wire tests evaluate the response of a material or finished part to a heated resistive element, simulating overheated electrical contacts. Product standards often require both, because they model different real-world ignition scenarios.
How long are test reports valid?
For material data reports issued by accredited third-party laboratories, validity is typically accepted for the period stated in the report and as agreed with the certifying body or customer, provided the material formulation has not changed. Long-term regulatory acceptance usually requires ongoing certification listing of the material grade rather than a single test report.