Key takeaways
- UL 94 is a small scale material test, not a test of the finished part in its application.
- A rating applies at a stated thickness and colour. A thinner wall or a different colour may not carry it.
- V-0, V-1 and V-2 differ mainly in afterflame time and whether burning drips are allowed.
- Flame retardant additives change impact, processing and tool deposits, so re-check the whole part when the grade changes.
What UL 94 measures
UL 94 is a laboratory test of how a plastic specimen behaves when a small flame is applied and then removed. The specimen is held in a defined orientation, the flame is applied for a defined time, and the tester records how long the material continues to burn, whether it drips, and whether the drips ignite a cotton pad below.
It is a screening test. It tells you how a standard bar or plaque of a material behaves in a repeatable laboratory condition. It does not tell you how a housing with ribs, a gate mark, a weld line and a 0.8 mm wall will behave in a fault condition inside a switchgear enclosure. Those questions are answered by the product standard and by tests such as the glow wire test.
The test is defined internationally in IEC 60695-11-10 as well as in the UL 94 standard, and the two are closely aligned for the vertical burning ratings. Results are reported with the specimen thickness, the colour and the conditioning, because all three affect the outcome.
The ratings and what separates them
The horizontal burning rating, HB, is the least demanding. A specimen held horizontally burns at a limited rate, or stops before a marked distance. It is a basic indication and is often not enough for a part near an arc or a heat source.
The vertical ratings are V-2, V-1 and V-0, in increasing order of difficulty. All three use a specimen held vertically with a flame applied twice for ten seconds, with the afterflame time recorded after each application. V-0 allows the shortest afterflame and does not permit drips that ignite the cotton. V-2 allows the same afterflame time as V-1 but permits burning drips, which is a significant difference in a real enclosure.
The 5V ratings use a larger 500 W flame applied five times and a plaque as well as a bar. 5VA and 5VB differ in whether the plaque may be pierced by the flame. They are used where the material must resist a more severe ignition source, and they are harder to achieve at thin walls.
| Rating | Orientation | Afterflame limit | Burning drips |
|---|---|---|---|
| HB | Horizontal | Limited burn rate or self extinguishing | Not assessed |
| V-2 | Vertical, two 10 s applications | 30 s each, 250 s total | Allowed if cotton ignites |
| V-1 | Vertical, two 10 s applications | 30 s each, 250 s total | Not allowed |
| V-0 | Vertical, two 10 s applications | 10 s each, 50 s total | Not allowed |
| 5VB | Vertical, 500 W flame, five applications | 60 s each | Not allowed, plaque may be pierced |
| 5VA | Vertical, 500 W flame, five applications | 60 s each | Not allowed, plaque not pierced |
Simplified summary. The standard sets additional criteria including glow time and specimen count. Confirm the exact limits with UL 94 or IEC 60695-11-10 before declaring a rating.
Thickness, colour and the small print
A UL 94 rating is attached to a thickness range, not to a material in the abstract. A grade may be listed as V-0 at 1.5 mm and 3.0 mm but only V-2 at 0.8 mm, because a thinner section has less material to absorb heat and can burn through faster. If the drawing calls for a 0.8 mm wall and quotes V-0, check the listing at 0.8 mm.
Colour matters too. Pigments and additives change the combustion behaviour, and a rating at natural colour does not automatically transfer to black or to a custom colour match. The yellow card lists the colours covered. A new colour is a new approval, even when the base polymer is the same.
The part also has features that the test specimen does not: edges, ribs, gate vestiges and weld lines. A thin rib can act as a wick, and a sharp moulded edge can ignite more readily than a smooth bar. The tested thickness should be the minimum wall in the part, not the nominal wall.
Quote the thickness, not just the rating
V-0 at 1.6 mm is a different claim from V-0 at 0.8 mm. The drawing should state both.
How flame retardants work, and what they cost you
Flame retardants interrupt the combustion cycle in different ways. Halogenated systems release species that quench the radical chain reaction in the flame, and they are effective at low loading, which keeps mechanical properties closer to the base resin. Their drawback is smoke corrosivity and the regulatory pressure that surrounds certain halogenated compounds in some markets.
Halogen free systems usually rely on phosphorus chemistry, on metal hydroxides that release water and cool the material, or on nitrogen and melamine compounds that form a char layer. They often need higher loadings, which raises viscosity, reduces impact strength and can make thin walls harder to fill. Metal hydroxides in particular need loadings that change the density and the surface finish.
Phosphorus based systems can be sensitive to moisture and can cause plate-out on the tool. Some additives migrate to the surface over time, which shows as a bloom or a sticky film and can affect laser marking and printing. Others affect the colour, making a bright white or a strong colour harder to match.
The processing window changes as well. A flame retardant grade may need a lower melt temperature to avoid degradation, a shorter residence time and more frequent purging. Those constraints belong in the moulding quote, not in a note discovered at trial.
- Higher additive loading usually means higher viscosity and lower impact.
- Some systems are moisture sensitive and need tighter drying control.
- Plate-out and migration can affect tool cleaning and surface appearance.
- Corrosivity of combustion products matters for the enclosure and the electronics inside.
UL 94 is not the whole requirement
A UL 94 rating answers a narrow question: how does a standard specimen burn under a small flame. Switchgear and appliance standards often ask a different question, which is how a part behaves when a hot element or a glowing contact touches it. That is the glow wire test, and a material with an excellent UL 94 rating can still fail a glow wire requirement.
The reverse is also true. A grade formulated for glow wire performance does not automatically achieve V-0 at a thin wall. The two tests apply different heat sources for different times and assess different outcomes, so both results are needed when both requirements apply.
The product standard also sets the distances and the barriers that decide whether a part needs a flame rated material at all. A part separated from an arc by a metal barrier or an adequate air gap may not need the same rating as an exposed barrier. Read the standard before specifying the most expensive grade.
Design details that undermine a rating
Thin sections are the first risk. A rib, a living hinge or a thin cover wall may be below the thickness at which the rating applies, and it may also be the first feature to ignite. Check the minimum wall, not the nominal wall, against the listing.
Weld lines and gate marks are the second. A weld line can create a weak, porous interface that burns differently from the bulk material, and a gate vestige is a local thick lump that can sustain a flame longer than the surrounding wall. Neither is represented in a test bar, so both deserve attention on a part that must pass a flame test.
Sharp edges and thin flanges are the third. A knife edge has a high surface area to mass ratio and heats quickly. Radiusing an edge is cheap at the design stage and can change the outcome of a test.
Finally, consider the environment. A part that is cleaned with solvents, exposed to UV or contaminated with oil may not behave like the conditioned specimen. If the end product sees those conditions, the material choice should reflect them.
Specifying and verifying the grade
Write the rating, the thickness and the colour on the drawing, and name the specific grade where the design has been tested. Add the standard, for example UL 94 V-0 at 1.5 mm in black, so the requirement cannot be read as a general aspiration.
Ask for the yellow card or the equivalent listing with the first article, and check that the grade, colour and thickness are covered. A certificate of conformity that names only the polymer family is not enough.
If the part is subject to a glow wire requirement, ask for that report too, at the production thickness and colour. Keep both reports with the first article so a later grade change can be compared against the original evidence.
- 01
Find the minimum wall
Check ribs, hinges and cover walls, not just the nominal section.
- 02
Match the listing
Confirm the grade, colour and thickness on the yellow card.
- 03
Check the other test
Add the glow wire result if the product standard requires it.
- 04
Record the evidence
Keep both reports with the first article and the approval file.
Frequently asked questions
What does UL 94 V-0 mean?
It is a vertical burning rating in which each specimen stops burning within 10 seconds after each of two flame applications, the total afterflame for the set is 50 seconds or less, and no burning drips ignite the cotton below. It applies at a stated thickness and colour.
Is V-0 better than V-2?
Yes for most applications. V-0 and V-2 allow the same afterflame time, but V-2 permits burning drips that ignite cotton. In an enclosure, a burning drip can spread fire to another part, so V-0 is usually the requirement near an arc.
Does a UL 94 rating apply at any wall thickness?
No. The rating is listed for specific thicknesses. A grade rated V-0 at 1.5 mm may only reach V-2 at 0.8 mm. Check the listing against the minimum wall in the part, including ribs and thin covers.
Do flame retardants affect moulding?
Yes. Higher additive loadings raise viscosity, reduce impact strength and can cause plate-out, moisture sensitivity and colour variation. Some grades need lower melt temperatures and tighter drying. Include those constraints in the moulding review.
Sources and further reading
- UL 94, Standard for safety of flammability of plastic materials
- IEC 60695-11-10, Test flames, 50 W horizontal and vertical flame test methods
- IEC 60695-11-20, Test flames, 500 W flame test methods
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