Determination of Horizontal Burning Flame Propagation Speed of Plastics: Principles, Standards, and

The horizontal burning flame propagation speed test measures how quickly a flame front travels along a horizontally mounted plastic specimen after ignition with a standardized low-energy flame, providing the linear burning rate data essential for flammability classification. Performed in accordance with standards such as GB/T 2408, IEC 60695-11-10, ISO 1210, UL 94, and ASTM D635, this method is widely used to qualify plastic materials as HB, HB40, or HB75 for electrical, electronic, and consumer product applications. This article explains the test principle, specimen requirements, apparatus, step-by-step procedure, calculation method, classification criteria, and typical applications of horizontal flame spread testing.

Technical Overview

The horizontal burning flame propagation speed, also referred to as the linear burning rate, is the velocity at which the flame front advances along a plastic test bar mounted in a horizontal orientation after it has been ignited by a standardized small flame. The result is expressed in millimeters per minute (mm/min) and serves as a key indicator of how readily a polymeric material sustains combustion once ignited by a minor ignition source such as an electrical fault or a small open flame.

In this test, a bar-shaped specimen is clamped with its long axis horizontal, a nominally 50 W premixed methane flame is applied to the free end of the specimen at an angle of 45 degrees, and the time required for the flame front to travel between two reference gauge marks is recorded. From this measured time, the linear burning rate is calculated, and the material is assigned a horizontal flammability classification. Because the specimen burns horizontally, gravity-driven melt flow and dripping behavior differ from vertical orientations, making the horizontal method particularly suitable for evaluating thin-wall sections, sheets, and components where flame spreads along a surface rather than upward.

It should be emphasized that the horizontal burning test is a small-scale, comparative laboratory procedure. It provides repeatable data for material ranking, quality control, and regulatory compliance, but it does not by itself characterize the fire hazard of a finished product under real-scale fire conditions. Accredited third-party laboratories routinely perform this test to support material qualification and product certification programs.

• GB/T 2408 - Burning characteristics of plastics: horizontal and vertical burning test (Chinese national standard, technically aligned with IEC 60695-11-10)
• IEC 60695-11-10 - Fire hazard testing: 50 W horizontal and vertical flame test methods
• ISO 1210 - Plastics: determination of flammability characteristics by horizontal and vertical burning
• UL 94 - Standard for Tests for Flammability of Plastic Materials for Parts in Devices and Applications (HB method)
• ASTM D635 - Rate of burning, extent and time of burning of plastics in a horizontal position
• ISO 9772 - Cellular plastics: determination of horizontal burning characteristics (applicable to foamed materials)

Testing Samples

Standard Bar Specimens

The standard specimen geometry for horizontal burning tests is a rectangular bar measuring 125 mm ± 5 mm in length and 13.0 mm ± 0.5 mm in width, with the thickness taken as supplied or as specified by the relevant product standard. A thickness of 3.0 mm is the commonly referenced baseline for comparative evaluation. Because burning behavior is strongly thickness-dependent, materials intended for classification across a thickness range should be tested at both the minimum and maximum thicknesses of commercial use, and each thickness is reported as a separate classification.

Specimen Preparation and Marking

Specimens are typically cut from compression-molded or injection-molded sheets, or directly from finished parts where permitted. The ignition end must be flat and perpendicular to the long axis, and all edges should be smooth, free of burrs, flash, and surface contamination. Two reference gauge lines are marked 25 mm and 100 mm from the end to be ignited, using a marking medium that does not influence the burning behavior of the material.

Conditioning and Quantity

Unless otherwise specified, specimens are conditioned at 23 °C ± 2 °C and 50 % ± 10 % relative humidity for a minimum of 48 hours prior to testing. A complete test normally uses a set of at least three specimens, with five specimens preferred for classification purposes. Additional specimen sets are required for each thickness, color, or density variant being evaluated. Certain regulatory programs may also require a second set tested after accelerated aging or dry conditioning.

Testing Items

The horizontal burning determination yields a defined set of measured and derived parameters, each of which is documented in the final test report:

• Linear burning rate (mm/min) between the 25 mm and 100 mm gauge marks
• Time for the flame front to traverse the 75 mm measured distance (s)
• Afterflame time and afterglow time for specimens that self-extinguish before the 100 mm mark
• Damaged or charred length along the specimen axis (mm)
• Whether ignition occurred within the maximum 30 s flame application period
• Observations of flaming molten drips, sample distortion, or collapse during burning
• Final horizontal flammability classification, for example HB, HB40, or HB75

Testing Methods

Apparatus and Test Environment

The test is conducted inside a draft-free enclosure or fume cabinet with a volume of at least 0.5 m³, fitted with a darkened observation window and an extraction system that is switched off during the actual measurement to avoid disturbing the flame. The core apparatus comprises a Bunsen or Tirrill-type burner with a barrel inner diameter of approximately 9.5 mm, a regulated supply of methane or natural gas of at least 98 % purity, a precision gas flow control system, a specimen clamp mounted on an adjustable support stand, and timers readable to better than one second. A calibrated rule or scale plate behind the specimen allows the operator to track the flame front position relative to the gauge marks.

Burner Flame Adjustment

Before testing, the burner is adjusted to produce a stable blue flame of the height specified by the method, typically 20 mm ± 1 mm for the 50 W flame defined in GB/T 2408, IEC 60695-11-10, and UL 94. Older editions of ASTM D635 reference a blue flame of approximately 25 mm. Flame height is verified with a ruler immediately before specimen ignition, and the gas and air controls are locked to ensure repeatability across all specimens in the set.

Specimen Mounting

The specimen is clamped at one end so that its long axis lies in a horizontal plane and its transverse axis is inclined at 45 degrees to the horizontal, presenting the lower edge to the burner. The marks at 25 mm and 100 mm are oriented away from the clamped end so that the free end receives the ignition flame. For thin or flexible specimens that sag or distort during heating, a support such as a fine wire mesh may be positioned beneath the specimen, provided it does not interfere with the flame front or drip path.

Ignition and Timing Procedure

The burner barrel is tilted at 45 degrees and advanced so that the flame contacts the free end of the specimen. The flame is applied for a maximum of 30 seconds, or until the flame front reaches the 25 mm gauge mark, whichever occurs first, after which the burner is withdrawn without extinguishing the specimen flame. Timing begins the moment the flame front crosses the 25 mm mark, and the elapsed time is recorded when the front reaches the 100 mm mark or when the specimen extinguishes. If the specimen does not ignite during the 30 second application, the result is recorded accordingly. Each specimen in the set is tested in the same manner.

Calculation of Burning Rate

For specimens in which the flame front travels the full 75 mm distance between the gauge marks, the linear burning rate is calculated as: v = 60 × 75 / t, where v is the linear burning rate in mm/min and t is the measured time in seconds. For example, if the flame front requires 90 seconds to travel from the 25 mm mark to the 100 mm mark, the reported burning rate is 50 mm/min. If the flame self-extinguishes before the 100 mm mark, no burning rate is calculated; instead, the afterflame time, afterglow time, and damaged length are reported.

Classification Criteria

Based on the measured behavior, the material is assigned a horizontal burning classification in accordance with the criteria of the applied standard. Typical criteria under UL 94 and IEC 60695-11-10 are:

• The flame front extinguishes before reaching the 100 mm gauge mark, or
• For specimens with thickness from 3.0 mm to 13 mm, the linear burning rate does not exceed 40 mm/min, corresponding to class HB40, or
• For specimens less than 3.0 mm thick, the linear burning rate does not exceed 75 mm/min, corresponding to class HB75
• Materials meeting the applicable combination of criteria are designated HB; earlier editions of GB/T 2408 used the designations FH-1, FH-2, and FH-3, and legacy reports may still reference these classes

Applications

Horizontal burning flame propagation data underpin material selection and compliance verification across many industries. In the electrical and electronics sector, product safety standards for information technology equipment, audio-video apparatus, household appliances, and luminaires require plastic parts located away from high-energy sources to meet at least an HB-class level, with higher-rated materials mandated near potential ignition sources. Manufacturers of connectors, switches, battery housings, cable accessories, and decorative trims use horizontal burning results to demonstrate conformity in certification files.

Beyond regulatory compliance, the test is a core tool in flame-retardant compound development. Polymer producers and compounders rely on comparative burning rates to screen halogenated, phosphorus-based, mineral-filled, and intumescent formulations, to optimize loading levels, and to verify batch-to-batch consistency during production. Incoming inspection departments at component manufacturers apply the method as an acceptance check on supplied resins and fabricated parts.

Additional application areas include automotive interior components and trim materials, household consumer products, construction accessory parts, textile-coated polymers, and rigid cellular plastics evaluated under the related ISO 9772 foam-specific method. Test reports issued by accredited third-party laboratories are accepted in market access dossiers, supplier qualifications, and research publications.

FAQ

What is the difference between the horizontal and vertical burning tests?

The horizontal test (HB method) measures flame propagation along a horizontally oriented specimen and yields a linear burning rate, making it suitable for parts where flame would spread laterally across a surface. The vertical test (V-0, V-1, V-2 methods) applies the same 50 W flame to the bottom edge of a vertically suspended specimen, measures afterflame and afterglow times, evaluates flaming drips against cotton indicator, and addresses more demanding applications where upward flame spread is a concern. Vertical classifications represent stricter performance than horizontal classifications.

Does an HB classification mean the plastic is flame retardant?

Not necessarily. HB is the baseline classification within the UL 94 system and indicates that the material burns slowly along a horizontal surface, with a rate limited to 40 mm/min or 75 mm/min depending on thickness, or self-extinguishes before the 100 mm mark. Many general-purpose, non-flame-retarded engineering plastics achieve HB. Applications exposed to higher ignition energy typically require vertical ratings such as V-2, V-1, or V-0, or the more severe 5VA and 5VB classifications.

Which thickness should be tested for classification purposes?

Classification applies to the specific thickness tested. Since thin sections usually burn faster than thick sections, a material intended for use between 1.5 mm and 6.0 mm should be tested at both extremes so that the reported classification, such as HB40 at 3.0 mm or HB75 at 1.6 mm, covers the entire commercial thickness range claimed.

Are GB/T 2408, IEC 60695-11-10, ISO 1210, and UL 94 results interchangeable?

The current editions of these standards are technically harmonized in specimen geometry, flame power, application time, and classification logic, so results obtained under one method are generally recognized against the others in international trade. Nevertheless, certification bodies typically require that the test be cited against the exact standard named in the applicable product safety specification, and small procedural differences, such as flame height in certain ASTM D635 editions, should be confirmed before drawing equivalences.

How many specimens are required and what causes invalid tests?

A minimum of three valid specimens per set is required for a classification, and five are commonly tested to improve statistical confidence. A run may be invalidated if the enclosure draft disturbs the flame, the specimen dislodges from the clamp, molten material flows in a way that alters the flame path, or the burner flame drifts out of specification. Invalidated runs are repeated with replacement specimens.

What is reported at the end of testing?

The final report documents the standard applied, full specimen identification including material, thickness, and color, conditioning details, the adjusted flame parameters, individual results for each specimen including burning rates, afterflame and afterglow times, and damaged lengths, any observations of dripping or distortion, and the resulting horizontal burning classification issued by the testing laboratory.