UL94 Flammability Testing: Standards, Methods, and Rating Criteria for Plastic Materials

UL94 flammability testing, developed by Underwriters Laboratories (UL), is the most widely recognized standard for evaluating how plastic materials ignite, burn, and self-extinguish when exposed to a small open flame. The resulting ratings, such as V-0, V-1, V-2, and 5VA, are essential compliance benchmarks for manufacturers of electronics, appliances, automotive components, and consumer products worldwide. This article provides a comprehensive technical overview of UL94 test samples, methods, classification criteria, and practical applications.

Technical Overview

UL94, officially titled "Standard for Safety of Flammability of Plastic Materials for Parts in Devices and Appliances," is a globally accepted test method published by Underwriters Laboratories (UL Solutions). It assesses the relative flammability of polymeric materials by measuring how quickly a specimen ignites, how long it continues to burn after the ignition source is removed, and whether burning drips are produced that could ignite adjacent materials. The standard assigns hierarchical flammability ratings that range from the least demanding horizontal burn classification (HB) to the most severe vertical classifications (V-2, V-1, V-0, 5VB, and 5VA). These ratings are not standalone product certifications; rather, they are material-level qualifications that feed into end-product safety standards such as IEC 62368-1, IEC 60335, and UL 60950-1. Equivalent international methods are described in IEC 60695-11-10 (20 mm vertical and horizontal flame tests) and IEC 60695-11-20 (500 W vertical flame test).

Testing Samples

Horizontal Burn (HB) Specimens

HB test specimens are rectangular bars measuring 125 mm in length and 13 mm in width, cut at the minimum, intermediate, and maximum intended thickness of the material. A minimum of three specimens is required per thickness. Each bar is marked with reference lines at 25 mm and 100 mm from one end to allow measurement of the linear burning rate.

Vertical Burn (V-0/V-1/V-2) Specimens

Vertical burn specimens are bars of 125 mm × 13 mm × material thickness, tested in the vertical orientation. Five specimens constitute one test set, and testing is typically performed at both the minimum and maximum thickness of the finished part. If any single specimen produces borderline results, a second set of five specimens is evaluated to confirm the classification.

500 W Vertical (5VA/5VB) Specimens

The 5V test uses two specimen geometries: rectangular bars of 125 mm × 13 mm × thickness and flat plaques of 150 mm × 150 mm × thickness. Testing requires five bars and three plaques. The plaque test determines whether the flame burns through the material, which is the key differentiator between 5VA and 5VB classifications.

Thin Material (VTM) Specimens

For flexible films and thin sheeting that distort under the standard bar test, the VTM (Vertical Thin Material) method is used. Specimens are 200 mm × 50 mm strips rolled onto a mandrel and secured to create a cylindrical test article, evaluated against criteria analogous to VTM-0, VTM-1, and VTM-2.

Specimen Conditioning

Prior to testing, specimens must be conditioned for a minimum of 48 hours at 23 ± 2 °C and 50 ± 10% relative humidity. When specified by the end-product standard or the material qualification program, additional sets are conditioned in a circulating air oven at 70 °C for 168 hours, followed by cooling in a desiccator, to evaluate flammability retention after thermal aging.

Testing Items

The UL94 standard comprises several distinct test programs, each producing its own classification. The principal testing items are listed below:

• Horizontal Burning Test — classifications HB (rigid materials) and HBF (foamed materials)
• Vertical Burning Test, 20 mm (50 W) Flame — classifications V-0, V-1, and V-2
• Vertical Burning Test, 125 mm (500 W) Flame — classifications 5VA and 5VB
• Thin Material Vertical Burning Test — classifications VTM-0, VTM-1, and VTM-2
• Foamed Material Horizontal Burning Test — classifications HF-1 and HF-2
• Ignitability Assessment (comparative tracking of afterflame and afterglow behavior)

Testing Methods

Horizontal Burning (HB) Method

The specimen is clamped horizontally with its long axis aligned to the airflow. A 20 mm blue flame from a Tirrill-type burner is applied at a 45° angle to the free end for 30 seconds, or until the flame front reaches the 25 mm reference mark. The operator records the time for the flame front to travel between the 25 mm and 100 mm marks. An HB classification is granted when the burning rate does not exceed 40 mm/min for thicknesses between 3 mm and 13 mm, does not exceed 75 mm/min for thicknesses below 3 mm, or when burning ceases before the 100 mm mark is reached.

Vertical Burning 20 mm (50 W) Method — V-0, V-1, V-2

The specimen is suspended vertically above a layer of surgical cotton placed 300 mm beneath the lower edge. A 20 mm high methane flame is applied to the lower tip of the specimen for 10 seconds and then withdrawn. The afterflame time t1 is recorded. Once the afterflame extinguishes, the flame is applied a second time for 10 seconds; the second afterflame time t2 and the afterglow time t3 are then recorded. The classification criteria are as follows:

• V-0: each afterflame time does not exceed 10 seconds; the sum of afterflame times for all five specimens does not exceed 50 seconds; no flaming drips ignite the cotton; no afterflame plus afterglow persists beyond 30 seconds after the second flame application.
• V-1: each afterflame time does not exceed 30 seconds; the total afterflame time does not exceed 250 seconds; no flaming drips ignite the cotton; afterflame plus afterglow does not exceed 60 seconds after the second application.
• V-2: identical timing criteria to V-1, but flaming drips are permitted provided they ignite the cotton below.

Vertical Burning 500 W (5V) Method — 5VA and 5VB

This more severe test applies a 125 mm high, approximately 500 W premixed methane flame at an angle of 20° from vertical to the lower corner of the specimen. The flame contacts the specimen for 5 seconds and is then removed for 5 seconds, and this cycle is repeated five times. The bar specimens are evaluated for afterflame, afterglow, and flaming drips, while the 150 mm × 150 mm plaques are examined for burn-through. Classification criteria are:

• 5VA: no flaming drips; afterflame plus afterglow does not exceed 60 seconds after the fifth flame application; the plaque is not burned through (no hole).
• 5VB: no flaming drips; afterflame plus afterglow does not exceed 60 seconds; the plaque may exhibit burn-through.

Thin Material (VTM) Method

For materials thin enough to distort, warp, or shrink away from the standard burner flame, the VTM method wraps the specimen around a 12.7 mm mandrel to form a rigid cylindrical test piece. The 20 mm flame is applied in two 3-second cycles, and afterflame times, glow times, and cotton ignition are evaluated. Timing requirements mirror those of V-0, V-1, and V-2, producing VTM-0, VTM-1, and VTM-2 classifications respectively.

Applications

UL94 ratings are fundamental quality and safety credentials across many industries. Typical applications include:

• Consumer electronics and IT equipment — enclosure plastics, battery holders, connectors, and cable insulation requiring V-1 or better under IEC 62368-1
• Printed circuit boards — FR-4 laminate substrates commonly qualified to 94 V-0
• Household appliances — internal and external plastic parts governed by IEC 60335 and UL 60335
• Automotive and e-mobility — interior trim, charging connectors, high-voltage components, and enclosures where supplementary horizontal burn data (FMVSS 302 / ISO 3795) is also required
• Medical devices — equipment housings and structural plastics under IEC 60601
• Lighting products — lamp holders, diffusers, and luminaire enclosures under UL 1598 and IEC 60598
• Material research and quality control — comparing flame-retardant formulations, verifying lot-to-lot consistency, and supporting UL Yellow Card material qualifications

FAQ

What is the difference between V-0, V-1, and V-2?

The three ratings differ in permitted afterflame duration and dripping behavior. V-0 requires individual afterflame times of 10 seconds or less with a 50-second cumulative limit and no flaming drips. V-1 allows up to 30 seconds per specimen with a 250-second cumulative limit and no flaming drips. V-2 matches V-1 timing but tolerates flaming drips that ignite the indicator cotton, making it the least demanding of the three vertical classifications.

Which UL94 rating offers the highest flame resistance?

5VA is the most demanding classification in the standard. It subjects specimens to a 500 W flame applied five times and additionally requires that a flat plaque resist burn-through. Products such as fire enclosures and certain IT equipment housings frequently specify 5VA for critical barriers.

Does the rating depend on material thickness?

Yes. Flammability performance is strongly thickness-dependent, so a rating applies only to the thickness range actually tested. A material certified V-0 at 3.0 mm may achieve only V-1 or V-2 at 1.5 mm. For this reason, UL Yellow Cards list the specific thicknesses at which each rating was obtained, and parts must be designed within those limits.

Are afterflame and afterglow the same thing?

No. Afterflame is the time during which the specimen continues to burn with a visible flame after the test flame is removed. Afterglow is the time the specimen continues to glow without flaming after the afterflame extinguishes. Both are recorded because afterglow can still ignite adjacent combustible materials.

Is UL94 testing mandatory?

UL94 itself is a voluntary material test method, but it is invoked by mandatory end-product safety standards and certification schemes such as UL listing, CE marking under the Low Voltage Directive, and IEC-based national certifications. In practice, most electrical and electronic products containing plastic parts must use materials with appropriate, documented UL94 ratings.

How does UL94 relate to IEC 60695-11-10?

IEC 60695-11-10 and IEC 60695-11-20 are internationally harmonized fire hazard test methods that replicate the UL94 20 mm and 500 W vertical flame tests respectively. Results from these IEC methods are generally considered technically equivalent to the corresponding UL94 V and 5V classifications, allowing manufacturers to use a single test campaign to support multiple market approvals.