Plastic Burning Length Testing: Standards, Methods, and Applications

Plastic burning length testing measures how far flame propagates along a polymer specimen after exposure to a standardized ignition source, providing essential flammability data for material classification. The test is widely applied under internationally recognized standards such as UL 94, IEC 60695-11-10, ISO 9772, ASTM D635, and GB/T 2408 to evaluate plastics used in electrical products, automotive interiors, and building materials. This article explains the test principle, sample requirements, measurement procedures, classification criteria, and typical applications of burning length testing for plastics.

Technical Overview

Plastic burning length testing is a small-scale laboratory flammability procedure that quantifies the extent of flame propagation along a polymer specimen after ignition by a controlled burner flame. The measured parameter, often called burned length or damaged length, is expressed in millimeters and reflects the combined effects of combustion, charring, melting, and softening along the specimen axis after the test is completed.

The test is most commonly performed in a horizontal burning configuration, in which one end of a clamped specimen is exposed to a low-energy diffusion flame. The operator records whether the flame sustains combustion, how far the burning front travels between reference marks, and whether the flame self-extinguishes. Results are used to classify materials into flammability ratings such as HB, HBF, HF-1, and HF-2, which are fundamental acceptance criteria for plastics in electrical equipment, vehicles, appliances, and construction products.

Typical standards governing burning length measurement include:

• UL 94 — Tests for Flammability of Plastic Materials for Parts in Devices and Appliances
• IEC 60695-11-10 / GB/T 5169.16 — Horizontal and vertical burning tests using a 50 W flame source
• ISO 1210 — Determination of burning behavior using a small flame source
• ASTM D635 — Rate of burning for rigid plastics in a horizontal position
• ISO 9772 — Cellular plastics, horizontal burning test (small flame)
• GB/T 2408 — Burning characteristics of plastics, horizontal and vertical test
• ISO 3795 / FMVSS 302 — Burning behavior of automotive interior materials

Testing Samples

Rigid Plastic Bars and Sheets

For horizontal burning tests on rigid thermoplastics and thermosets, bar specimens of approximately 125 mm in length and 13 mm in width are cut from molded plaques or finished sheet stock. Testing is usually performed at the minimum and maximum wall thickness intended for the end application, since burning behavior is strongly thickness-dependent. Specimen edges should be smooth and free of flash, burrs, or stress cracks that could influence ignition.

Cellular and Foam Materials

Flexible and rigid foams are tested as rectangular blocks, typically 150 mm long and 50 mm wide, with a thickness of up to 13 mm as specified in ISO 9772 and the UL 94 foam classifications. A wire gauge or support may be required for specimens that shrink away from the flame, ensuring consistent contact between the flame and the specimen edge.

Thin Films and Flexible Materials

Films below approximately 0.25 mm in thickness cannot be reliably tested in the standard bar configuration and are evaluated using dedicated procedures such as ISO 9773 or the VTM (vertical thin material) method, in which the specimen is wrapped around a mandrel and supported so that reproducible flame contact is achieved.

Automotive Interior Trim Materials

For vehicle cabin components, specimens of 356 mm by 102 mm are tested horizontally in accordance with ISO 3795 and FMVSS 302, either as cut sections of material or as representative sections taken from finished parts in the orientation installed in the vehicle.

Testing Items

Depending on the governing standard and the material type, a burning length test program may include the following measured and derived parameters:

• Burning length (damaged length) in millimeters, determined after the test by measuring the undamaged portion of the specimen
• Linear burning rate in mm/min, calculated from the time for the flame front to travel between two reference marks
• Afterflame time — the duration of continued flaming after removal of the ignition source
• Afterglow time — the duration of glowing combustion without flame
• Whether combustion ceases before the reference mark or the specimen is consumed
• Occurrence of flaming drips and whether they ignite indicator cotton placed beneath the specimen
• Flammability classification (for example HB, HF-1, HF-2, HBF) based on the combined criteria

Testing Methods

Specimen Preparation and Conditioning

Specimens are conditioned prior to testing, typically for at least 48 hours at 23 ± 2 °C and 50 ± 5 % relative humidity, unless the applicable standard specifies otherwise. Each test series normally requires a minimum of three, and often five, replicate specimens. For some classifications, an additional set of specimens is pre-conditioned at an elevated temperature (for example 70 °C) and aged for one week before testing to simulate long-term thermal exposure.

Horizontal Burning Test Procedure

The specimen is clamped horizontally with its long axis parallel to the burner. A reference mark is positioned 25 mm from the exposed end, and a second mark 100 mm from the first. A laboratory burner producing a 20 mm or 25 mm blue flame is applied to the free end of the specimen for a defined period, commonly 30 seconds, or until the flame reaches the 25 mm mark. After the ignition source is removed, the operator observes whether the specimen continues to burn and, if so, records the time taken for the flame front to travel from the first mark to the second mark, enabling the burning rate calculation.

Foam Material Test Procedure

For cellular plastics tested to ISO 9772 or UL 94 foam criteria, the flame is applied to the specimen edge for 60 seconds. After flame removal and extinction of afterflame and afterglow, the specimen is cooled, soot and loose char are carefully removed, and the undamaged length is measured with a scale or depth gauge to the nearest millimeter. The damaged length is the difference between the original length and the undamaged length.

Calculation and Classification

The linear burning rate is calculated as 60 times the burned distance between reference marks divided by the recorded elapsed time. Typical classification criteria include the following:

• HB rating: burning rate not exceeding 40 mm/min for thicknesses of 3 mm to 13 mm, or 75 mm/min for thinner materials, or combustion ceasing before the 100 mm mark
• HF-1 and HF-2 ratings: damaged length not exceeding 60 mm after 60 seconds of flame application, with defined limits on afterflame time and flaming drips
• HBF rating: burning rate not exceeding 40 mm/min, or combustion ceasing before the 127 mm mark

All observations, including drip behavior and cotton ignition, must be reported together with the measured burning length to support the final classification.

Applications

Burning length testing provides the flammability evidence required for material qualification, regulatory compliance, quality control, and product safety documentation across many industries. Typical applications include:

• Enclosures, mounting plates, and internal insulating parts of household appliances and consumer electronics
• Connectors, switch housings, circuit breaker components, and other electrical accessories
• Automotive interior materials such as door panels, headliners, seat padding, and trim parts
• Building and construction products, including decorative panels, foam insulation boards, and pipe supports
• Furniture foams, mattresses, and upholstered seating components
• Wire and cable jacketing, tubing, and protective sleeving
• Medical device housings and laboratory equipment components

Manufacturers frequently submit specimens to accredited third-party laboratories to obtain independent test reports that support product certification, market access, and supplier qualification programs.

FAQ

What is the difference between burning length and burning rate?

Burning length is the physical distance, in millimeters, over which the specimen is damaged or consumed after the test, measured once combustion has fully ceased. Burning rate is the speed of the flame front, expressed in mm/min, calculated from the time required for the flame to travel between two reference marks during the test. Both parameters may be required for a complete flammability assessment.

Why must specimens be tested at multiple thicknesses?

Burning behavior is strongly influenced by wall thickness. Thin sections ignite and propagate flame more readily than thick sections, so standards require testing at both the minimum and maximum thickness of the material as used in the end product. A material qualified at one thickness cannot be assumed to hold the same rating at another thickness.

What does an HB classification mean?

HB (Horizontal Burn) is the lowest flammability rating in the UL 94 system and indicates that the material burns slowly along a horizontal specimen, either self-extinguishing before the end mark or exhibiting a burning rate within the limits defined for its thickness. Many product safety standards accept HB materials only for specific, lower-risk applications.

How are damaged length measurements made accurately?

After the specimen cools to room temperature, soot, loose char, and degraded material that can be wiped or brushed away are removed. The unaffected length is then measured with a straightedge and scale or a depth gauge, and the damaged length is calculated by subtracting the undamaged length from the original specimen length. Measurements are reported to the nearest millimeter.

Can finished parts be tested instead of molded bars?

Yes. Many standards permit testing sections cut from finished parts, provided the sections meet the dimensional requirements and are tested in the orientation and thickness representative of the installed condition. Where cut sections are impractical, molded plaques of equivalent material and thickness are used.

How long does a typical burning length test take?

The flame exposure and observation for each specimen takes only a few minutes, but a complete program includes specimen preparation, conditioning for at least 48 hours, replicate testing, and measurement of damaged length. A standard test series for a single material and thickness is usually completed within one working day after conditioning is finished.