The plastic horizontal and vertical burning test is a standardized laboratory method for evaluating the flammability, self-extinguishing behavior, and flaming-drip characteristics 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, this test assigns critical classifications including HB, V-0, V-1, V-2, 5VA, and 5VB that govern material selection in electronics, appliances, and electrical equipment. This article provides a comprehensive technical overview of the test principles, sample requirements, procedures, classification criteria, and typical applications.
Technical Overview
The plastic horizontal and vertical burning test is a small-scale laboratory procedure used to determine the relative burning characteristics and extinguishing capability of plastic materials when exposed to a standardized ignition source. The method measures parameters such as afterflame time, afterglow time, linear burning rate, and the tendency to produce flaming particles, and it assigns widely recognized flammability classifications that serve as the foundation for material qualification in the electrical and electronics industries.
The test is governed by a family of technically harmonized standards. UL 94 describes the flammability testing of plastic materials for parts in devices and appliances, IEC 60695-11-10 specifies the 50 W horizontal and vertical flame test methods, and IEC 60695-11-3 covers the more severe 500 W flame test. Additional equivalent or related standards include ISO 1210, ASTM D635, ASTM D3801, and the Chinese national standard GB/T 2408. Accredited third-party laboratories routinely perform these tests for product certification, regulatory compliance, quality control, and flame-retardant material development.
The core apparatus consists of a laboratory burner producing a small premixed methane-air flame, a ventilated test chamber with exhaust hood, specimen clamps and holders, dry absorbent surgical cotton used as an ignition indicator, and a timing device. Flame height, gas flow, and burner geometry are strictly controlled and verified before testing to ensure repeatability and comparability of results between laboratories.
Testing Samples
Specimen Dimensions and Preparation
Standard bar specimens measure 125 mm in length by 13.0 mm in width, with the thickness normally corresponding to the intended end use, typically 1.5 mm or 3.0 mm. Specimens must be cut cleanly and free from flash, burrs, cracks, bubbles, and visible surface defects, as edge quality can significantly influence ignition and burning behavior.
Specimen Quantity and Grouping
The horizontal test generally requires three bar specimens per set, while the vertical test requires five specimens per condition, with an additional set of five specimens tested in certain borderline cases. Under the requirements of UL 94, materials should be evaluated in their most and least flammable states, and separate test series are required for different colors, densities, melt indices, and thicknesses of the same polymer family.
Conditioning Prior to Testing
Specimens are conditioned for a minimum of 48 hours at 23 ± 2 °C and 50 ± 5% relative humidity before testing. For certain vertical test applications, an additional preconditioning of 168 hours at 70 ± 2 °C followed by cooling in a desiccator may be required. Proper conditioning ensures that moisture content and thermal history do not distort the measured flammability performance.
Testing Items
The horizontal and vertical burning test evaluates the following key parameters:
- Afterflame time after the first flame application (t1) and after the second application (t2), recorded in seconds.
- Afterglow time (t3), defined as the duration of glowing combustion without a visible flame after removal of the ignition source.
- Total afterflame time across all specimens and flame applications within a test set.
- Linear burning rate in mm/min for horizontal specimens, calculated from the time required for the flame front to travel between reference marks.
- Ignition of the cotton indicator by flaming or glowing particles dropped from the specimen.
- Burn-through behavior of plaque specimens in the more severe 500 W test.
- Final classification: HB, V-0, V-1, V-2, 5VA, or 5VB.
Testing Methods
Horizontal Burning Test (HB Classification)
In the horizontal configuration, the bar specimen is clamped with its longitudinal axis horizontal and its transverse axis inclined at 45°, and reference marks are placed 25 mm and 100 mm from the free end. A small premixed blue flame, typically 20 mm to 25 mm high depending on the standard applied, is directed at the free end for 30 seconds, or withdrawn earlier if the flame front reaches the 25 mm mark. The operator records the times at which the flame passes the 25 mm and 100 mm marks and calculates the burning rate over the 75 mm gauge distance.
A material is classified HB if it meets one of the following criteria:
- No specimen exhibits flaming combustion reaching the 100 mm mark.
- Burning ceases before the 100 mm mark for each specimen.
- For thicknesses from 3.0 mm to 13 mm, the burning rate does not exceed 75 mm/min.
- For thicknesses below 3.0 mm, the burning rate does not exceed 40 mm/min.
Vertical Burning Test at 50 W (V-0, V-1, V-2 Classifications)
In the vertical configuration, the specimen is suspended so that its lower edge is approximately 10 mm above the top of the burner, and a pad of dry cotton is positioned 300 mm below the specimen. A 20 mm blue flame is applied to the center of the lower edge for 10 seconds, then removed while the afterflame time t1 is recorded. After flaming stops, the flame is applied a second time for 10 seconds, and the afterflame time t2 and afterglow time t3 are recorded along with whether flaming drips ignite the cotton.
The classification criteria are summarized as follows:
- V-0: afterflame time does not exceed 10 s for each application, the total afterflame time for all ten applications on five specimens does not exceed 50 s, the cotton is not ignited, and afterglow does not persist beyond 30 s.
- V-1: afterflame time does not exceed 30 s for each application, the total afterflame time does not exceed 250 s, and the cotton is not ignited by flaming particles.
- V-2: identical timing limits to V-1, but flaming particles that ignite the cotton are permitted.
- For all vertical grades, no specimen may exhibit combustion propagating to the clamp, and afterflame or afterglow must not exceed 60 s after the second flame application.
500 W Vertical Test (5VA and 5VB Classifications)
For applications demanding the highest level of flame resistance, such as enclosure materials in close proximity to live electrical parts, the 500 W test described in IEC 60695-11-3 and UL 94 is applied. A premixed flame approximately 125 mm high with a 40 mm blue inner cone is applied to the lower edge of the bar specimen five times for 5 seconds each, separated by 5-second intervals. In addition to bar specimens, plaque specimens are tested to evaluate burn-through resistance. The 5VA classification, the most stringent plastic flammability rating, prohibits flaming drips and burn-through of the plaque, while 5VB permits burn-through provided that flaming does not persist excessively and flaming drips do not occur.
Applications
Horizontal and vertical burning test data are used extensively across multiple industries and regulatory frameworks:
- Enclosures and internal parts of information technology and audiovisual equipment, where flammability ratings support compliance with product safety standards.
- Household appliances and power tools, including non-metallic parts carrying live components or located near potential ignition sources.
- Electrical wiring devices such as switches, sockets, connectors, terminal blocks, and circuit breaker housings.
- Printed circuit board laminates, battery housings, charger casings, and power adapter components.
- Research and development of flame-retardant polymer formulations, including halogen-free compounds, and incoming material quality control.
- Regulatory market-access documentation, supplier declarations, and material data sheets supported by reports from accredited third-party laboratories.
FAQ
What is the difference between V-0, V-1, and V-2?
The three vertical grades differ mainly in permitted afterflame time and flaming-drip behavior. V-0 requires the fastest self-extinguishing performance with a total afterflame time of no more than 50 s and no flaming drips, V-1 allows up to 250 s total afterflame time with no ignition of cotton, and V-2 matches the V-1 timing requirements but permits flaming drips that ignite the cotton indicator.
Are UL 94, IEC 60695-11-10, ISO 1210, and GB/T 2408 equivalent?
These standards are technically aligned and share the same 50 W flame principle, specimen geometry, and classification logic. However, minor differences exist in apparatus details, conditioning, and reporting requirements, so certification programs generally require testing to be conducted against the specific standard and scheme referenced in the applicable product safety regulation.
Does the flammability rating depend on specimen thickness?
Yes. Burning behavior is strongly thickness-dependent, and a rating is only valid for the thickness at which it was tested. Materials are normally qualified at their minimum wall thickness, and any design change that reduces thickness below the tested value requires re-evaluation.
Why is cotton placed below the specimen?
The cotton pad detects flaming particles or drips that fall from the burning specimen. This is safety-critical because burning drops can ignite secondary materials located beneath a component inside real equipment, potentially propagating fire beyond the original part.
Can the test results predict behavior in a real fire?
No. The horizontal and vertical burning test is a small-scale screening method intended for quality control, comparative evaluation, and compliance verification of materials. It does not by itself describe the fire hazard of end products under actual fire conditions, which may require additional large-scale fire performance assessments.
How many specimens are needed and how long does testing take?
A typical evaluation uses three specimens for the horizontal method and five or ten specimens for the vertical method, per color and thickness. After conditioning is complete, the flame exposure itself takes only minutes per specimen, and accredited third-party laboratories commonly issue full test reports within about one to two weeks.