A common problem
An engineer chose a material labeled "V-0". The part wall is 1.0 mm. After testing, the part did not reach V-0. What went wrong?
The material data shows that its V-0 was obtained at 1.5 mm or 3.0 mm. At the thinner 1.0 mm, the material may only reach V-2, or may not be classifiable at all.
What UL 94 is
UL 94 is a small-scale test standard for the burning behavior of plastics. Using standard specimens, it tests whether a material ignites easily under a small flame, whether it self-extinguishes and whether it drips flaming particles. Its methods broadly correspond to IEC 60695-11-10 and IEC 60695-11-20.
UL 94 has three main kinds of test:
Horizontal burning (HB)
The specimen is held horizontally and the burning rate is measured.
Vertical burning (V-0, V-1, V-2)
The specimen is held vertically and a 20 mm flame is applied twice, 10 seconds each time.
5V test (5VB, 5VA)
A larger 125 mm flame is applied five times, 5 seconds each time; for enclosures with higher requirements.
How each rating is judged
Vertical burning (5 specimens per set)
| Criterion | V-0 | V-1 | V-2 |
|---|---|---|---|
| Afterflame time of each specimen after each flame application | ≤ 10 s | ≤ 30 s | ≤ 30 s |
| Total afterflame time of the 5 specimens | ≤ 50 s | ≤ 250 s | ≤ 250 s |
| Afterflame + afterglow time after the second application | ≤ 30 s | ≤ 60 s | ≤ 60 s |
| Burning up to the clamp | Not allowed | Not allowed | Not allowed |
| Drips ignite the cotton below | Not allowed | Not allowed | Allowed |
Horizontal burning (HB)
Burning rate ≤ 40 mm/min for thicknesses of 3–13 mm; ≤ 75 mm/min below 3 mm; or the flame goes out before the 100 mm mark.
5V test
Both bars and plaques must meet the time requirements after five flame applications, with no drips igniting the cotton. 5VA also requires that the plaque is not burned through; 5VB allows burn-through.
Conditioning
Specimens are usually split into two sets, one kept at 23 °C and 50 % relative humidity for 48 hours, the other at 70 °C for 168 hours. Both sets must meet the requirements.
Why thickness matters
The thinner the specimen, the faster heat passes through, the faster the combustible material decomposes and the harder the flame is to put out. So the same material:
- may be V-0 at 3.0 mm;
- may be V-1 or V-2 at 1.5 mm;
- may not be classifiable at 0.8 mm.
Thickness 3.0 mm
May be V-0 at 3.0 mm
So the correct way to write a rating always includes the thickness, for example "V-0 @ 1.5 mm (UL 94)". Data that says only "V-0" is incomplete.
Selection rule: The test thickness of the material's flammability rating should be equal to or less than the thinnest wall of the part. For example, if the thinnest wall is 1.2 mm, choose a material that reaches the target rating at 1.2 mm or thinner.
Color, recycled content and processing also affect the result
Color
Pigments and carbon black change burning behavior. UL certification is usually registered by color range, such as natural, black or "all colors". A new color may need additional testing.
Recycled content
Recycled material varies in composition, which can affect flame retardancy. Versions with recycled content must be validated separately.
Processing
Over-drying, too high a temperature or too long a residence time degrades the flame retardant or the base resin and lowers flame retardancy. Too much regrind carries the same risk.
Material ratings and end-product standards
UL 94 is a material rating. The end product must meet a product standard, for example:
- Household appliances: IEC 60335-1 (often requires glow-wire testing);
- Audio/video and information technology equipment: IEC 62368-1 (sets enclosure material flammability requirements by energy class).
The product standard sets which material rating is needed, at what thickness, and whether glow-wire, needle-flame and other tests are also required. Confirm the product standard first, then work back to the material requirements.
Also, "tested to the UL 94 method" in a datasheet is not the same as "UL certified". UL-certified materials are listed in the UL materials database with rating, thickness and color (commonly called the "Yellow Card"). Many end-product certifications require certified materials.
Material selection checklist
References
- Combustion (Fire) Tests for PlasticsUL Solutions
- UL 94 test report (example)Intertek (via eksensunger.com)
- UL Yellow Card brochureUL
- Plastics Recognition Program webinar seriesUL Solutions
- UL 94 flame ratingstexasinjectionmolding.com
Accessed 2026-10-19.