UL94 Flammability Test: A Complete Technical Guide to Flame Testing of Plastic Materials

Prior to testing, all specimens are conditioned in a controlled atmosphere of 23 ± 2 °C and 50 ± 10 % relative humidity for a minimum of 48 hours to ensure reproducible moisture content and dimensional stability.

Testing Items

A UL94 evaluation records a defined set of quantitative and observational parameters for every specimen. The primary measured items include:

• Afterflame time — the period during which the specimen continues to flame after the test flame is removed.
• Afterglow time — the period during which glowing combustion without flaming persists after flaming ceases.
• Total afterflame time — the cumulative afterflame time across all specimens in a test set.
• Flaming drips — whether burning particles fall from the specimen and whether they ignite the dry surgical cotton indicator placed 300 mm below.
• Linear burning rate (HB) — the speed of flame travel, in mm per minute, measured over the 75 mm span between the 25 mm and 100 mm marks.
• Flame spread to the clamp — whether combustion reaches the upper clamped end of the specimen.
• Burn-through (5V plaque) — whether the flame creates a hole through the plaque specimen.

These observations are compared against the acceptance criteria for each classification to determine the final rating.

Testing Methods

Horizontal Burning Test (HB)

The bar specimen is clamped horizontally, and a 25 mm blue flame from a laboratory burner is applied at a 45° angle to the free end for 30 seconds or until ignition occurs, whichever comes first. If burning continues past the 100 mm mark, the time to traverse the 75 mm gauge length is recorded and converted to a burning rate. A material meets HB when the rate does not exceed 40 mm/min for thicknesses of 3.0 to 13 mm, does not exceed 75 mm/min for thicknesses below 3.0 mm, or when combustion stops before the 100 mm mark.

Vertical Burning Test (V-0, V-1, V-2)

The specimen is suspended vertically with its lower end 10 mm above the burner tip. A 20 mm high blue flame is applied to the bottom edge for 10 seconds and withdrawn. If afterflame stops within 30 seconds, the flame is reapplied for a second 10-second interval. Timing of afterflame and afterglow, dripping behavior, and ignition of the cotton bed are recorded for all five specimens. The distinguishing criteria are summarized as follows:

• V-0: maximum afterflame of 10 seconds per application; total afterflame not exceeding 50 seconds for the set; no flaming drips that ignite cotton.
• V-1: maximum afterflame of 30 seconds per application; total afterflame not exceeding 250 seconds; no flaming drips that ignite cotton.
• V-2: identical timing limits to V-1, but brief flaming drips that ignite the cotton indicator are permitted.

500 W Vertical Burning Test (5VA, 5VB)

This severe procedure uses a burner producing an approximately 125 mm flame with a 40 mm blue inner cone, corresponding to a nominal 500 W energy output. The flame is applied to the bottom of bar and plaque specimens five times, 5 seconds on and 5 seconds off. To pass, the afterflame plus afterglow must not exceed 60 seconds after the fifth application, no flaming drips are allowed, and the flame must not consume the specimen to the clamp. A material rated 5VA additionally must show no burn-through hole in the plaque; 5VB permits hole formation.

Thin Material Vertical Test (VTM-0, VTM-1, VTM-2)

Mandrel-wrapped film specimens are tested in a manner analogous to the V-series, using the same 20 mm flame, two 10-second applications, and matching timing limits. VTM-0, VTM-1, and VTM-2 correspond to the performance requirements of V-0, V-1, and V-2 respectively, including the treatment of flaming drips.

Applications

UL94 ratings are a fundamental compliance requirement for polymeric parts in nearly every electrical and electronic product category. Product safety standards such as IEC 62368-1, IEC 60335, IEC 60950-1, and UL 60950 frequently mandate minimum flammability ratings for enclosures, PCBs, and internal components. Typical applications include:

• Enclosures and housings for consumer electronics, computers, routers, and displays.
• External power supplies, chargers, and adapters.
• Printed circuit board laminates, most commonly FR-4 rated V-0.
• Connectors, sockets, terminal blocks, coil bobbins, and wiring devices.
• Switches, relays, circuit breaker components, and breaker housings.
• Lithium-ion battery enclosures, module holders, and insulation barriers.
• LED lighting housings, lenses, and driver housings.
• Household appliance parts and power tool housings.
• Automotive interior electrical components and charging infrastructure.
• Medical device enclosures and diagnostic equipment housings.

Because ratings are thickness- and color-dependent, manufacturers document validated ratings on a UL Yellow Card, allowing OEMs to verify compliance during material selection and certification audits.

FAQ

What is the difference between V-0 and 5VA?

V-0 is evaluated with a small 20 mm burner flame applied twice for 10 seconds each to bar specimens. 5VA uses a much more energetic 500 W flame applied five times to both bar and plaque specimens, and additionally requires that the plaque resist burn-through. 5VA is therefore the highest and most demanding UL94 classification.

Are flaming drips ever acceptable?

Yes. V-2, VTM-2, and HF-2 ratings explicitly allow brief flaming drips that may ignite the cotton indicator, provided the afterflame timing limits are met. V-0, V-1, VTM-0, VTM-1, 5VA, and 5VB do not permit flaming drips.

Does a UL94 rating apply to every thickness and color of a material?

No. Ratings are granted for a specific thickness range and formulation. Pigments, fillers, and additives can alter burning behavior, so a grade certified in natural color at 1.6 mm is not automatically certified in black at 3.0 mm. Always confirm the exact combination on the UL Yellow Card or the supplier's certification documents.

How does UL94 differ from the glow-wire test?

UL94 assesses how a material reacts to a small open gas flame, focusing on self-extinguishment and dripping. The glow-wire test (IEC 60695-2 series) simulates thermal stress from an electrically heated element and is often applied to finished parts as well as materials. The two methods are complementary and are frequently both required by end-product safety standards.

Which rating should I specify for my product?

Product safety standards dictate the minimum rating based on the part's function, proximity to potential ignition sources, and whether the part is an enclosure or internal component. V-0 is the most commonly specified minimum for PCBs and critical electrical parts, while 5VA is often required for enclosures near high-energy sources. Consult the applicable end-product standard and a qualified test laboratory to confirm requirements.

How long does UL94 testing take?

A single-condition laboratory assessment on one material at one thickness can typically be completed within one to two weeks, including specimen conditioning. Full UL recognition with multiple thicknesses and colors, along with follow-up service auditing, requires additional time and should be planned during project scheduling.