Vertical and Horizontal Burning Tests: Principles, Methods, and Flammability Classification of Plast

Vertical and horizontal burning tests are essential flammability evaluations used to assess the fire resistance of plastic materials and components in electronics, appliances, and industrial products. Conducted in accordance with internationally recognized standards such as UL 94, IEC 60695-11-10, GB/T 2408, and ASTM D635, these tests classify materials into ratings such as V-0, V-1, V-2, and HB, providing critical data for product safety certification and regulatory compliance.

Technical Overview

The vertical and horizontal burning test is a small-scale laboratory flammability test designed to measure the ignition characteristics, flame propagation behavior, and self-extinguishing properties of polymeric materials when exposed to a standardized flame source. It is one of the most widely performed fire hazard tests in the electrical and electronics industry and serves as a fundamental basis for material selection and product safety certification.

In the vertical burning configuration, a bar-shaped specimen is clamped with its longitudinal axis oriented vertically, and a Bunsen burner flame is applied to the lower end of the specimen. In the horizontal burning configuration, the specimen is mounted horizontally, and the flame is applied to one free end. The orientation of the specimen significantly influences burning behavior: vertical orientation promotes flame spread due to upward convection, making it a more severe test condition than the horizontal orientation.

The principal standards governing these tests include UL 94 (Standard for Safety of Flammability of Plastic Materials for Parts in Devices and Appliances), IEC 60695-11-10 and IEC 60695-11-3 (fire hazard testing for electrotechnical products), ASTM D635 and ASTM D3801, ISO 1210 and ISO 9773, GB/T 2408 and GB/T 5169.16 (Chinese national standards), and JIS standards for the Japanese market. Although the technical requirements of these standards are largely harmonized, subtle differences exist in specimen preparation, conditioning, and classification criteria, so the applicable standard must be confirmed before testing.

Testing Samples

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

Standard bar specimens measure 125 mm in length by 13 mm in width, with the thickness matching that of the intended end-use application. Common thicknesses include 0.8 mm, 1.6 mm, 3.2 mm, and 6.4 mm, since flammability ratings are thickness-dependent. A minimum of two sets of five specimens is required for a complete test series, and materials that melt or shrink away from the flame may additionally require plate specimens tested under IEC 60695-11-3 (5VA and 5VB procedures).

Horizontal Burning Specimens (HB)

Horizontal specimens also measure 125 mm by 13 mm and are cut to the nominal thickness of the material. Reference marks are placed 25 mm and 100 mm from the ignited end to enable measurement of the linear burning rate. Three specimens are typically tested initially, with additional specimens evaluated if inconsistent results are observed.

Thin Film and Foam Materials

Flexible films and foamed plastics require modified procedures because they deform or retract during burning. Thin films are generally tested using a mandrel-supported method in accordance with ISO 9773 or UL 94 VTM (vertical thin material) procedures, while cellular plastics are evaluated under ISO 9772 or equivalent methods that yield an HBF classification.

Specimen Conditioning

Before testing, specimens must be conditioned for a defined period (typically 48 hours at 23 °C and 50% relative humidity) in accordance with the governing standard. An additional set of specimens is often conditioned at elevated temperature and low humidity to evaluate performance under dry, warm conditions, particularly for materials suspected of being hygroscopic.

Testing Items

Depending on the selected method and orientation, the following parameters are measured and recorded:

• Afterflame time following the first flame application (t1) and after the second application (t2)
• Total afterflame time (t1 + t2) for the complete specimen set
• Afterglow time following extinction of flaming combustion
• Whether flaming or flaming drips ignite a surgical cotton indicator placed below the specimen
• Linear burning rate in millimeters per minute for horizontal specimens that burn past the 25 mm gauge mark
• Whether burning extinguishes before reaching the 100 mm reference mark
• Damaged length and afterflame plus afterglow behavior for 500 W (5VA/5VB) plate tests
• Flame application behavior, including whether the flame contacts the specimen continuously or the specimen retracts from the flame

Testing Methods

Vertical Burning Test (20 mm Flame, V-Rating)

A specimen is suspended vertically with its lower edge approximately 10 mm above the top of a Bunsen burner tube. A 20 mm-high blue flame is applied to the center of the lower edge for 10 seconds and then withdrawn. The afterflame time t1 is recorded. After flaming ceases, the flame is reapplied for a second 10-second interval, and the afterflame time t2 and afterglow time t3 are recorded, along with any ignition of the cotton indicator by burning drips. Five specimens are tested in this manner, and the results are classified according to strict timing limits.

• V-0: each afterflame time does not exceed 10 seconds; the total afterflame time for the set does not exceed 50 seconds; no flaming drips ignite the cotton; and no specimen burns to the clamp.
• V-1: each afterflame time does not exceed 30 seconds; the total afterflame time does not exceed 250 seconds; no flaming drips ignite the cotton; and no specimen burns to the clamp.
• V-2: identical timing limits to V-1, except that flaming drips are permitted if they ignite the cotton indicator.

Vertical Burning Test (500 W Flame, 5VA and 5VB)

For more demanding applications, such as enclosures of large information technology equipment, bar and plate specimens are tested using a 500 W premixed flame applied five times for 5 seconds each. A 5VB classification requires that no specimen burn with flaming or glowing combustion for more than 60 seconds after the final flame application and that plates not exhibit burn-through (hole formation). A 5VA classification additionally prohibits any hole formation, making it the most severe vertical rating under UL 94.

Horizontal Burning Test (HB Rating)

The specimen is clamped horizontally with its transverse axis inclined at 45 degrees, and marks at 25 mm and 100 mm from the free end are used as timing references. A 25 mm blue flame is applied to the free end for 30 seconds and then removed. The time at which the flame front crosses the 25 mm and 100 mm marks is recorded, and the linear burning rate is calculated. An HB classification is assigned when the material either ceases to burn before the 100 mm mark or exhibits a burning rate not exceeding 40 mm/min for thicknesses between 3.0 mm and 13 mm, or not exceeding 75 mm/min for thicknesses below 3.0 mm, and no flaming drips ignite the cotton when specified.

Equipment and Environment

Testing is performed inside a fume hood or enclosed chamber that supplies reproducible air movement while allowing clear observation of the specimen. The apparatus consists of a laboratory Bunsen or Tirrill burner with a defined tube diameter, a gas supply producing a calibrated flame, specimen clamps, timing devices accurate to at least 0.1 seconds, dry surgical cotton, and a calibrated ruler. Testing should be conducted by accredited third-party laboratories or qualified in-house facilities operating under strict quality management to guarantee the repeatability and acceptance of results.

Applications

Vertical and horizontal burning test results directly influence material selection and regulatory acceptance across many industries:

• Enclosures and internal structural parts of consumer electronics, chargers, and power adapters, which typically require V-0 or 5VA rated materials
• Household appliance components, switchgear housings, and terminal blocks covered by product safety standards referencing UL 94 or IEC 60695-11-10
• Insulating materials, connectors, and printed circuit board substrates used in information technology and communication equipment
• Automotive interior and electrical components, complementing ISO 3795 and related fire performance requirements
• Wire and cable insulation and jacket compounds, supporting compliance with flame retardancy requirements
• Engineering plastics such as flame-retarded polycarbonate, ABS, polyamide, PBT, and polyester compounds, for which the flammability rating is a primary datasheet parameter
• Regulatory submissions and certification files where material certificates listing flammability ratings at specific thicknesses are mandatory

FAQ

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

The classifications differ mainly in permitted afterflame times and drip behavior. V-0 requires afterflame times of no more than 10 seconds per application and a total of 50 seconds for five specimens. V-1 extends these limits to 30 seconds per application and 250 seconds in total. V-2 shares the timing limits of V-1 but permits flaming drips that ignite the cotton indicator, whereas V-0 and V-1 do not allow cotton ignition.

How does the vertical test differ from the horizontal test?

The vertical test is more severe because buoyant convection carries heat upward along the specimen, promoting sustained burning and dripping. It is used for materials that must self-extinguish quickly, such as those near live electrical circuits. The horizontal test evaluates slower, creeping flame propagation and expresses results as a linear burning rate or as an HB rating, making it suitable for less critical components.

Why is specimen thickness so important?

Flammability ratings are assigned at defined thicknesses because thicker sections dissipate heat more effectively and extinguish faster than thin sections. A material rated V-0 at 3.2 mm may only achieve V-2 or no rating at all at 0.8 mm. Certification documents therefore always state the thickness at which the rating is valid, and the end-use wall thickness must not be thinner than the tested thickness.

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

The two standards are technically aligned for the V and HB procedures, and results are generally accepted across both frameworks. However, small differences in wording, conditioning requirements, and reporting details exist, so manufacturers targeting specific end-product standards should verify which material flammability standard is referenced in the applicable requirement.

What does a failed or unrated result mean for a product?

If a material fails the vertical test or only achieves an HB rating, the design may need to incorporate additional safeguards, such as increased spacing from ignition sources, flame barriers, metal enclosures, or selection of a more flame-retardant compound. Many end-product standards prescribe the minimum rating and thickness required for parts based on their proximity to potential ignition sources.