The plastic horizontal burning test is a fundamental flammability assessment that measures the burning rate, afterflame time, and combustion behavior of plastic specimens ignited in a horizontal orientation. Governed by internationally recognized standards such as UL 94, ASTM D635, IEC 60695-11-10, and GB/T 2408, this test is essential for material qualification, quality control, and regulatory compliance across the electronics, appliance, and automotive industries. This article explains the applicable standards, sample requirements, test procedures, classification criteria, and typical applications of horizontal burning testing for plastics.
Technical Overview
The plastic horizontal burning test is a small-scale laboratory flame test used to evaluate the relative flammability of polymeric materials. A bar-shaped specimen is clamped in a horizontal position, and a standard laboratory burner flame is applied to one end for a defined duration. Technicians then record whether the specimen ignites, how far the flame front propagates, the time required for the flame to travel between reference marks, and whether flaming drips are produced. The results allow materials to be classified with an HB (Horizontal Burning) rating, which is the most basic flammability classification within the UL 94 system and is widely referenced in product safety specifications worldwide.
Several standardization organizations publish horizontally oriented burning test methods, and the most commonly applied standards include:
- UL 94, the Standard for Safety of Flammability of Plastic Materials for Parts in Devices and Appliances, which defines the HB classification.
- ASTM D635, which measures the rate of burning and extent and time of burning of self-supporting plastics in a horizontal position.
- IEC 60695-11-10, which specifies 50 W horizontal and vertical flame test methods for fire hazard testing of electrotechnical products.
- ISO 9772, which determines the horizontal burning characteristics of cellular (foamed) plastics.
- GB/T 2408, the Chinese national standard covering horizontal and vertical burning tests for plastics, methodologically aligned with IEC 60695-11-10.
Because test conditions, specimen geometry, and pass criteria differ slightly between these standards, the specific method must always be identified when reporting results. Manufacturers typically engage accredited third-party laboratories to perform these tests so that the resulting flammability ratings are accepted by regulators, certification bodies, and downstream customers.
Testing Samples
Rigid and Self-Supporting Plastic Specimens
For tests conducted in accordance with UL 94, IEC 60695-11-10, and GB/T 2408, specimens are typically cut as rectangular bars measuring 125 mm in length by 13 mm wide, with the thickness representative of the end-use application, generally up to 13 mm. Edges should be smooth and free of burrs, flash, or defects that could influence ignition behavior. A common test program uses at least three specimens per thickness, with five specimens recommended for formal classification.
Cellular Plastic Specimens
For foamed or cellular materials tested under ISO 9772 or the equivalent HB provisions for foams, larger specimens are used, typically 150 mm to 200 mm in length and 50 mm in width, cut at the nominal density and thickness of the commercial product. Because cell structure strongly affects combustion behavior, specimens must be cleanly cut without crushing or deforming the foam cells.
Specimen Conditioning
Prior to testing, specimens are conditioned in a controlled atmosphere of 23 °C ± 2 °C and 50 % ± 5 % relative humidity for a minimum of 48 hours, unless the governing standard or a product specification dictates otherwise. Proper conditioning ensures that moisture content and temperature do not introduce uncontrolled variability into ignition and burning behavior.
Testing Items
During a horizontal burning test, the following parameters and observations are recorded for each specimen:
- Ignition behavior, including whether the specimen ignites during the flame application period.
- Afterflame time, defined as the duration that the specimen continues to burn after the ignition source is removed.
- Linear burning rate, calculated in millimeters per minute from the time required for the flame front to travel between the 25 mm and 100 mm reference marks.
- Extent of burning or damaged length, indicating how far along the specimen the flame propagated or charring occurred.
- Dripping behavior, noting whether flaming or non-flaming particles fall and whether they ignite cotton indicator material placed beneath the specimen.
- Classification result, confirming whether the material satisfies the criteria for the HB or HB 40 mm/min designation.
Testing Methods
The general test procedure, illustrated here using the UL 94 HB and IEC 60695-11-10 methodology, is as follows:
- The specimen is clamped horizontally at one end so that the free end is exposed, and reference marks are scribed 25 mm and 100 mm from the exposed end.
- A laboratory burner fueled by methane or natural gas is adjusted to produce a blue flame of the height specified by the standard, typically 20 mm to 25 mm.
- The burner is inclined at approximately 45 degrees and applied to the free end of the specimen for a defined period, commonly up to 30 seconds or until the flame front reaches the 25 mm mark, whichever occurs first.
- The burner is withdrawn, and the afterflame time, flame propagation between the reference marks, dripping, and extent of burning are observed and recorded.
- The linear burning rate is calculated, and the observations from all specimens are evaluated against the classification criteria.
To achieve an HB classification under UL 94, the material must meet at least one of the following conditions: the specimen does not exhibit a burning rate exceeding 40 mm/min for thicknesses between 3.0 mm and 13 mm; the specimen does not exhibit a burning rate exceeding 75 mm/min for thicknesses below 3.0 mm; or burning ceases before the 100 mm reference mark. Materials that fail these conditions are considered unclassified for the horizontal burning test and typically require reformulation or must be evaluated under more stringent vertical burning regimes.
All testing should be performed inside a fume hood or ventilated enclosure with controlled ambient conditions, using calibrated timing devices and verified gas supply parameters. Reproducibility depends heavily on strict adherence to flame height, application angle, and specimen conditioning requirements.
Applications
Horizontal burning test data supports material selection, regulatory compliance, and safety certification in numerous sectors, including:
- Consumer electronics and information technology equipment, where non-metallic enclosures and internal components must demonstrate basic flame resistance.
- Household appliances and electrical accessories, including switch housings, connector bodies, cable management products, and insulating parts.
- Automotive interior and electrical components, where materials are screened for flammability alongside OEM-specific engineering specifications.
- Building and construction products, particularly decorative trim, panels, and fittings where horizontal orientation reflects the installed configuration.
- Toys, lighting fixtures, power tools, and general consumer goods that incorporate molded thermoplastic or thermoset parts.
- Material research and development, where comparative HB data guides flame-retardant formulation studies and resin grade selection.
Many end-product safety standards reference UL 94 HB as a minimum requirement for parts that are not in proximity to ignition sources, making the horizontal burning test a routine element of supply chain material qualification.
FAQ
What is the difference between horizontal and vertical burning tests?
Horizontal tests assess materials in a less severe orientation, suitable for the basic HB rating, whereas vertical burning tests, such as the UL 94 V-0, V-1, and V-2 methods, subject a vertically mounted specimen to repeated flame applications and impose much stricter afterflame and dripping limits. Vertical ratings indicate higher flame resistance and are required for parts located near potential ignition sources.
What does an HB rating mean?
HB stands for Horizontal Burning. It indicates that the material burned slowly in the horizontal orientation and extinguished or propagated at a rate within prescribed limits, for example not exceeding 40 mm/min for thicker specimens. It is the lowest flammability classification within the UL 94 plastic material scheme.
How many specimens are required for a valid test?
Formal classification generally requires a minimum of three and preferably five specimens per thickness, all conditioned in accordance with the standard. If results are inconsistent between specimens, additional sets may be tested to confirm the classification.
Can the horizontal burning test be used for foamed plastics?
Yes. Cellular plastics are evaluated with dedicated horizontal burning methods, such as ISO 9772, which use larger specimens and modified support fixtures suited to the low density and rapid flame spread characteristics of foam materials.
Are test results transferable between standards such as UL 94, ASTM D635, and GB/T 2408?
The procedures are closely aligned, but subtle differences in burner configuration, flame application, and criteria mean results should be reported against the specific standard performed. Accredited third-party laboratories can advise on which method a target market, regulator, or product standard requires.