A UL94 flame rating is the first thing an electrical, appliance or automotive buyer checks on a plastic data sheet, and the most frequently misread. Two suppliers can both claim "V-0" and deliver parts with very different fire behaviour, because the rating depends on specimen thickness, colour, and the exact formulation tested. This guide explains what each UL94 rating measures, how the test is run, and how to verify a claim before it reaches a purchase order.
What UL94 Measures
UL94 is a small-scale laboratory test, not a full fire-safety assessment. A plastic bar is held in a specified orientation and exposed to a calibrated flame for a defined time. The rating records how long the specimen continues to burn after the flame is removed, whether it drips, and whether the drips ignite a cotton pad placed below. It says nothing about heat release rate, smoke toxicity or real-scale fire growth.
The test is defined in IEC 60695-11-10 and, in North American practice, ASTM D3801 for vertical burning and ASTM D635 for horizontal burning. A supplier's certificate should name the standard, the specimen thickness and the third-party laboratory.
The Rating Ladder
From weakest to strongest, the common ratings are HB, V-2, V-1, V-0, 5VB and 5VA. HB is a horizontal test; everything from V-2 upward is a vertical test, which is far more demanding.
| Rating | Orientation | Afterflame per specimen | Dripping |
|---|---|---|---|
| HB | Horizontal | Burn rate < 40 mm/min (or self-extinguishes before 75 mm) | Not assessed |
| V-2 | Vertical | ≤ 30 s, total ≤ 250 s for 5 specimens | Allowed, may ignite cotton |
| V-1 | Vertical | ≤ 30 s, total ≤ 250 s | Not allowed to ignite cotton |
| V-0 | Vertical | ≤ 10 s each, total ≤ 50 s | Not allowed to ignite cotton |
| 5VB / 5VA | Vertical, plaque | No burn-through within 60 s after 5 applications | 5VA: no hole in the plaque |
The step from V-1 to V-0 is where most engineering effort goes, because it changes both the afterflame budget and the total burn time for the specimen set. Many flame-retardant compounds are formulated specifically to clear V-0 at 1.5 mm and then re-tested at 0.8 mm or 3.0 mm, which are separate ratings entirely.
The Detail That Breaks Real Projects: Thickness
A V-0 rating at 3.0 mm does not transfer to a 1.2 mm wall. Thinner sections heat faster, warp and drip more readily, so the same compound often drops to V-2 or fails when tested thin. When a data sheet states "V-0 (1.5 mm)" the number in brackets is part of the rating, not a footnote.
Colour matters too. Some pigments, particularly certain titanium dioxide and carbon black loadings, change burning behaviour enough to require a separate listing. If a part is moulded in a colour that was never tested, the V-0 claim is unsupported even though the base resin grade is listed.
What UL94 Does Not Tell You
- It is not a construction-code classification — that requires full-scale testing of the finished article.
- It does not address smoke density, toxicity or heat release, which matter in rail and tunnel applications.
- It does not cover thin films, foams or textiles, which have their own test methods.
- It is not a substitute for glow-wire testing (IEC 60695-2-11/2-12) required for many appliance parts.
How to Verify a UL94 Claim
Three checks settle most questions:
- Ask for the yellow-card listing reference and confirm the exact grade, colour and thickness range appear on it.
- Confirm the testing laboratory is accredited for the standard, not just an internal report.
- Request the test report for the thickness you actually intend to mould.
For flame-retardant compounds that also carry restricted-substance obligations, the two dossiers should be checked together — our breakdown of RoHS and REACH compliance for modified PPS shows how flame retardants affect both. Compound selection for these grades is covered under engineering plastic materials, and the full set of certification walk-throughs sits in the certification guide section.
Specifying UL94 in a Purchase Order
Write the rating, the thickness and the colour into the specification line, then require the supplier to state the same three values on the certificate of analysis. A purchase order that says only "V-0 material" leaves the thickness open, and the dispute that follows is expensive for both sides. Buyers sourcing flame-retardant parts from verified Chinese manufacturers should treat that wording as a non-negotiable line item.
