Plastic horizontal burning testing is a small-scale flammability test that measures the ignition behavior, burning rate, and self-extinguishing performance of polymeric materials when ignited in a horizontal orientation. The method is governed by internationally recognized standards such as UL 94 (HB), IEC 60695-11-10, ASTM D635, ISO 1210, and GB/T 2408, and is widely used for material qualification in the electronics, appliance, automotive, and construction industries. This article explains the test principle, specimen requirements, operating procedures, classification criteria, and typical applications of horizontal burning testing for plastics.
Technical Overview
Horizontal burning testing is one of the most fundamental flame-safety evaluation methods for plastic materials. In this test, a bar-shaped specimen is clamped in a horizontal position, and a small laboratory burner flame is applied to its free end. The test quantifies how quickly the material burns, whether it self-extinguishes after removal of the ignition source, and whether flaming drips or molten particles are produced. Because the horizontal orientation is the least severe of the common small-flame configurations, the resulting rating—typically HB—is the baseline classification in the UL 94 flammability hierarchy.
The test is primarily used for quality control, material comparison, flame-retardant formulation development, and demonstration of compliance with product safety requirements. Many equipment safety standards accept HB-rated materials for internal and external parts that carry a lower fire risk, while more demanding positions require the stricter vertical burning classifications (V-2, V-1, V-0) or the advanced 5VA/5VB grades.
- UL 94 — Horizontal Burning (HB) test for polymeric materials
- IEC 60695-11-10 — Fire hazard testing, 50 W horizontal and vertical burning test methods
- GB/T 2408 — Determination of burning characteristics of plastics, horizontal and vertical test
- ASTM D635 — Rate of burning of plastics in a horizontal position
- ISO 1210 — Flammability of plastics using small flame ignition sources
- ISO 3795 and GB 8410 — Horizontal burning rate of automotive interior materials
- ISO 9772 / GB/T 8332 and UL 94 HBF, HF-1, HF-2 — Horizontal burning of cellular (foamed) plastics
Testing Samples
Standard Bar Specimens for Rigid and Semi-Rigid Plastics
The standard specimen for the UL 94 HB, IEC 60695-11-10, and GB/T 2408 methods is a rectangular bar measuring 125 mm in length and 13 mm in width, with a representative thickness—commonly 3.0 mm. Because burning behavior depends strongly on thickness, it is standard practice to test the minimum and maximum thicknesses at which the material will actually be used. At least three specimens per thickness are required, and additional sets are recommended when results are marginal.
Thin Films and Sheets
Materials thinner than 3.0 mm, such as films and thin sheets, may be tested either unsupported or resting on a support (for example, wire gauze) as specified by the applicable standard. These specimens are evaluated against the less severe burning-rate limit of 75 mm/min.
Foamed (Cellular) Plastics
Cellular materials are tested on larger panels in accordance with the dedicated foamed-plastic procedures such as UL 94 HBF/HF-1/HF-2 and ISO 9772, with correspondingly longer flame application times and different gauge marks, because foam burns with fundamentally different behavior than dense plastics.
Specimen Preparation and Conditioning
Specimens are prepared by injection molding, extrusion, or careful cutting from representative sheet or finished parts, with smooth edges free of flash and cracks. Reference marks are made at 25 mm and 100 mm from the end to be ignited. Unless otherwise specified, specimens are conditioned for a minimum of 48 hours at 23 ± 2 °C and 50 ± 5 % relative humidity prior to testing, and the material is tested in the color and additive state in which it will be supplied.
Testing Items
Horizontal burning testing evaluates the following measurable characteristics of a plastic material:
- Ignition behavior during flame application
- Afterflame time—the duration of flaming combustion after removal of the burner
- Afterglow time—glowing combustion without flame after flaming ceases
- Linear burning rate between the 25 mm and 100 mm gauge marks, expressed in mm/min
- Whether the flame front reaches the 100 mm reference mark or self-extinguishes earlier
- Total damaged length and extent of combustion
- Formation of flaming drips or burning molten particles
- Final classification, such as HB, HB40, or HB75 (or HBF/HF-1/HF-2 for foamed plastics)
Testing Methods
The test is performed inside a draft-protected fume hood or enclosure with ventilation switched off during measurement. A Bunsen or Teclu-type burner fitted with a methane or natural-gas flame of approximately 20 mm visible height (blue inner cone) serves as the ignition source, together with a calibrated timer and precision measuring tools for marking the gauge length.
UL 94 HB / IEC 60695-11-10 / GB/T 2408 Procedure
- Clamp the specimen horizontally at one end so that the free end extends into the test zone, and orient the burner tube at 45° to the horizontal.
- Apply the 20 mm blue flame to the free end of the specimen for 30 seconds, or until the flame front reaches the 25 mm mark, whichever occurs first, then withdraw the burner.
- If the flame front passes the 25 mm mark, start timing and record the time at which it reaches the 100 mm mark or at which flaming combustion stops.
- Record afterflame time, afterglow, damaged length, and any flaming drips; repeat for all specimens and thicknesses.
Classification Criteria
A material is assigned the HB classification when it satisfies one of the following conditions across the specimen set:
- The specimen ceases to burn before the flame front reaches the 100 mm mark, or
- The measured burning rate does not exceed 40 mm/min for specimens 3.0 mm to 13 mm thick, or
- The measured burning rate does not exceed 75 mm/min for specimens less than 3.0 mm thick.
Under the IEC 60695-11-10 and GB/T 2408 framework, these outcomes correspond to the designations HB (self-extinguishing before 100 mm), HB40, and HB75 respectively. Materials that exceed these limits are unclassified and require improved flame-retardant formulation or the use of a different material grade.
ASTM D635 and ISO 3795 Rate-of-Burning Methods
ASTM D635 applies a similar horizontal-bar approach to determine the rate of burning and/or the extent and time of burning of rigid plastics in a horizontal position, reporting results as a linear burning rate in mm/min or as a pass/fail extent of combustion. ISO 3795 and the corresponding national automotive standard GB 8410 evaluate larger interior-trim panels by igniting one end of a horizontally mounted specimen and measuring the time for the flame front to travel between two gauge marks; the reported value is the horizontal burning rate required by vehicle interior-material regulations.
Burning Rate Calculation
The linear burning rate is calculated as v = 60 × L / t, where L is the gauge length between the two reference marks (75 mm in the UL 94 HB configuration) and t is the time in seconds required for the flame front to traverse that distance. If the specimen extinguishes before the second mark, the rate is not calculated; instead, the self-extinguishing behavior and damaged length are reported.
Applications
Horizontal burning test data are used across the plastics supply chain, from resin producers and compounders to OEM design and compliance teams. Accredited third-party laboratories routinely perform these tests as part of material datasheets, incoming inspection, and product certification programs.
- Electrical and electronic equipment — enclosures, insulating parts, connectors, and structural components where equipment safety standards accept HB-class materials
- Household appliances and consumer products — housings, knobs, brackets, and decorative polymer parts
- Automotive — interior trim, panels, and padding evaluated against GB 8410 and ISO 3795-based regulatory burning-rate limits
- Flame-retardant R&D — screening halogen-free and mineral-based flame-retardant formulations and verifying synergist loading levels
- Quality control — batch release testing, supplier comparison, and verification of flame-retardant masterbatch consistency
- Construction and lighting — auxiliary polymer components where horizontal burning performance supports overall fire-safety documentation
FAQ
What is the difference between horizontal (HB) and vertical (V) burning tests?
In the horizontal test the specimen lies flat, so the flame must travel along the material against less favorable heat-transfer conditions, making this the mildest configuration. In the vertical test the specimen stands upright, the rising flame preheats the material above the ignition point, and combustion is more easily sustained. Consequently, V-2/V-1/V-0 ratings indicate significantly better flame-retardant performance than HB, and the two ratings are not interchangeable.
How many specimens are required and which thicknesses should be tested?
A minimum of three specimens per thickness is standard for the HB test, with additional sets tested when results are borderline or when verification is needed. Because burning rate increases as thickness decreases, materials should be evaluated at both the minimum and maximum thicknesses intended for the end application, and the report should state the thickness for which the classification applies.
Is an HB rating sufficient for electrical and electronic products?
It depends on the function and location of the part. Many equipment safety standards permit HB-class materials for housings and unpowered or low-risk internal parts, but components near potential ignition sources—such as those supporting live connections or located near arcing parts—generally require vertical ratings such as V-1 or V-0. Designers must confirm the specific material flammability requirement in the applicable product standard.
What factors can influence horizontal burning test results?
Specimen thickness is the dominant factor, followed by fillers, glass fiber content and orientation, pigments, release agents, aging, and conditioning state. Ambient drafts, flame calibration, and specimen edge quality also affect repeatability, which is why testing is performed in a controlled, draft-free enclosure by trained operators.
What information does a typical test report contain?
A professional report identifies the test standard, material designation and color, specimen dimensions and thickness, conditioning details, individual and average afterflame times, burning rates or self-extinguishing distances, observations of dripping and afterglow, the number of specimens tested, and the final classification together with the laboratory's measurement conditions.