The plastic horizontal burning test is a small-scale flammability test used to evaluate the ignition behavior, flame spread rate, and self-extinguishing properties of plastic materials mounted in a horizontal orientation. Conducted to internationally recognized standards such as UL 94 (HB), IEC 60695-11-10, ASTM D635 and GB/T 2408, it is one of the most widely requested tests for polymeric components in electrical devices, household appliances, automotive interiors and consumer products. This article explains the test principle, specimen requirements, test items, procedure, classification criteria and typical applications of the plastic horizontal burning test.
Technical Overview
The plastic horizontal burning test, commonly known as the HB test, is a laboratory-scale fire hazard screening method in which a bar-shaped plastic specimen is clamped in a horizontal position and its free end is exposed to a small calibrated gas flame. The test determines whether the material ignites, how far the flame front travels along the specimen, how fast it propagates, and whether the material self-extinguishes or continues to burn after removal of the ignition source. Because the horizontal orientation represents the least severe flame-spread geometry, this test provides the baseline level of flammability performance among the common plastics flammability ratings.
The test is specified in several internationally harmonized standards, the most frequently applied being:
- UL 94, Tests for Flammability of Plastic Materials for Parts in Devices and Appliances – Horizontal Burning (HB, HB40, HB75 classifications)
- IEC 60695-11-10, Fire hazard testing – Test flames – 50 W horizontal and vertical flame test methods
- GB/T 2408, Plastics – Determination of burning behaviour – Horizontal and vertical test (FH classifications)
- ASTM D635, Standard Test Method for Rate of Burning and/or Extent and Time of Burning of Plastics in a Horizontal Position
- ISO 9772 for cellular plastics (HBF classification) and ISO 3795 for automotive interior materials
Although each standard has its own reporting format, the underlying apparatus, specimen geometry and measurement principle are very similar, allowing broad comparability of results. The test is routinely performed by accredited third-party laboratories as well as by in-house quality control and research departments.
Testing Samples
Standard Bar Specimens
For UL 94, IEC 60695-11-10 and GB/T 2408 horizontal testing, the standard specimen is a flat bar of 125 mm length and 13 mm width, cut or molded at the thickness of interest. Specimens are normally tested at both the minimum and maximum commercial thickness, with a maximum thickness of 13 mm; thicker sections are reduced by machining. At least three specimens per thickness are tested. Prior to testing, specimens are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 48 hours, and edges are smoothed to remove burrs or flash that could influence flame behavior.
Cellular and Foam Materials
Rigid foams and other cellular plastics lack the self-supporting stiffness of solid bars and are therefore evaluated using the dedicated horizontal method of ISO 9772 and the corresponding UL 94 foam provisions. Larger rectangular specimens are supported on a horizontal wire support, and performance is expressed through the HBF classification together with observations of afterflame, afterglow and flaming drips.
Specimens from Finished Parts
Bars may be cut from molded plaques, extruded profiles or finished products, provided the material can be mounted flat and the reported thickness corresponds to the actual part. Results are valid only for the color, thickness and formulation tested, since pigments, fillers, release agents and regrind content can measurably alter burning behavior.
Testing Items
During a horizontal burning test, the laboratory records and evaluates the following parameters for each specimen:
- Ignition of the specimen under the applied pilot flame
- Time of application of the burner flame (typically 30 seconds or until the flame front reaches the 25 mm reference mark)
- Whether the flame front passes the 25 mm and 100 mm reference marks
- Burning rate, calculated as the time for the flame front to travel the 75 mm span between the 25 mm and 100 mm marks
- Afterflame time and afterglow time following withdrawal of the burner
- Presence of flaming droplets or incandescent particles
- Extent of damaged or charred length
- Final classification, for example HB, HB40 or HB75 under UL 94 and IEC 60695-11-10, or FH-1, FH-2 and FH-3 under the GB/T 2408 designations
Testing Methods
Apparatus and Test Environment
The test is performed inside a draft-free enclosure or fume chamber at laboratory ambient temperature. A laboratory Bunsen-type burner with adjustable air intake is fueled by methane or natural gas of specified calorific value and adjusted to produce a stable blue flame of 20 ± 1 mm height for UL 94, IEC 60695-11-10 and GB/T 2408, or approximately 25 mm for ASTM D635. The specimen holder clamps the bar horizontally with reference marks applied at 25 mm and 100 mm from the exposed end, and a timing device is used to record all flame events.
Specimen Mounting and Flame Application
The specimen is secured with its long axis horizontal and its free end projecting toward the burner. The burner tube is inclined at approximately 45 degrees, and the blue flame cone is applied to the free end of the specimen for 30 seconds, or until the flame front reaches the 25 mm mark, whichever occurs first. The burner is then removed or turned away without disturbing the burning specimen.
Measurement and Calculation
Timing of the burning rate starts when the flame front reaches the 25 mm mark and stops when it reaches the 100 mm mark or when flaming combustion ceases, whichever comes first. The burning rate is calculated by dividing the 75 mm span by the measured burning time and expressed in mm/min. If the flame extinguishes before the 100 mm mark, the laboratory instead records the extinction point, the afterflame time, the afterglow time and any dripping observations. The procedure is repeated for every specimen in the set, and individual results are reported along with the overall classification.
Classification Criteria
Under UL 94 and IEC 60695-11-10, a material is classified HB when, over the 75 mm measured span, it either extinguishes before the 100 mm mark, or burns at a rate not exceeding 40 mm/min for thicknesses from 3.0 mm to 13 mm and not exceeding 75 mm/min for thicknesses below 3.0 mm. Newer editions of these standards subdivide the category into HB40 and HB75 according to which rate limit applies. Under the GB/T 2408 designations, horizontal performance is graded from FH-1, representing self-extinguishing behavior before the 100 mm mark, through FH-2, permitting limited afterflame effects, to FH-3, allowing a measurable burning rate within the same thickness-dependent 40 mm/min and 75 mm/min limits. Foam materials are separately classified as HBF per ISO 9772. The classification applies strictly to the thickness, color and formulation tested, so any material modification requires retesting.
Applications
Horizontal burning test results are used across many industries for material selection, quality control and regulatory compliance:
- Electrical and electronic equipment: enclosures, connectors, switches, terminal blocks, cable management products and internal structural components where at least an HB grade is the baseline requirement
- Household appliances and consumer goods: housings, control panels, decorative trims and hand-held devices
- Automotive engineering: interior trim, headliners and cabin materials, frequently assessed alongside the horizontal burning rate requirements of ISO 3795, which limits the burning rate to 102 mm/min
- Building and construction: initial screening of pipes, ducting, panels and other polymeric building components ahead of larger-scale fire performance testing
- Research, development and incoming inspection: comparing flame-retardant formulations, verifying supplier lots and supporting certification projects handled by accredited third-party laboratories
- Product safety compliance: demonstrating conformity with end-product standards that specify minimum flammability grades for polymeric parts
FAQ
What is the difference between the horizontal and vertical burning tests?
The horizontal test exposes the free end of a horizontally clamped bar to a small flame and measures flame spread, making it the least severe configuration. The vertical test applies the same type of flame twice to the lower end of a vertically suspended specimen and evaluates afterflame time, afterglow and flaming drips, producing the more stringent V-0, V-1 and V-2 classifications. Materials intended for higher-risk locations are normally qualified to vertical grades rather than HB.
Does an HB rating mean the plastic is flame retardant?
An HB rating indicates that the material either self-extinguishes in the horizontal position or burns slowly within the prescribed rate limits. It is the entry-level flammability classification and demonstrates basic slow-burning behavior, but it does not indicate the self-extinguishing performance associated with V-0 or the more severe 5VA and 5VB classifications.
Which standards are considered equivalent to the UL 94 HB test?
IEC 60695-11-10 and its European adoption EN 60695-11-10 define a technically aligned 50 W horizontal flame method, and GB/T 2408 follows the same principle with its own FH designations. ASTM D635 and the legacy ISO 1210 method measure the same burning behavior and are widely accepted as comparable horizontal methods, although reporting formats differ.
How many specimens are needed and at which thicknesses should testing be performed?
A minimum of three specimens per set is typical, with some protocols requiring additional specimens for confirmation. Testing should be performed at the thinnest and thickest thicknesses to be marketed, up to 13 mm, because thin sections burn faster and usually govern the classification.
Can finished products be submitted for horizontal burning testing?
Yes, provided flat bars of the required dimensions can be obtained from the part or from plaques molded from the same material, color and process. Accredited third-party laboratories can advise on specimen preparation and will report the results together with the exact thickness tested so the classification can be correctly applied to production parts.
What factors can influence the test result?
Specimen thickness, colorants, fillers and reinforcing agents, flame-retardant loading, regrind content, surface contamination and conditioning history can all shift burning behavior. Consistent molding conditions and correct conditioning are therefore essential for reproducible and representative results.