If you put a plastic part into an electrical enclosure, an automotive interior, or an aircraft cabin, someone is going to ask about its UL94 rating. Probably someone from compliance, probably right before the design review. Here's what the ratings actually mean, how they're tested, and what to spec for your application.
The tests behind the ratings
UL94 isn't one test — it's a family of test methods. The most common ones:
HB (Horizontal Burn). The specimen is held horizontally. A flame is applied to the free end for 30 seconds. To pass HB, the material must burn at less than 75mm/minute for thicknesses under 3mm, or less than 40mm/minute for 3-13mm. This is the lowest bar — if your material can't pass HB, it's basically kindling.
V-0, V-1, V-2 (Vertical Burn). The specimen is held vertically. A 20mm flame is applied to the bottom for 10 seconds, removed, then reapplied for another 10 seconds after flaming stops. The differences:
- V-0: Flaming stops within 10 seconds after each flame application. Total flaming time across 10 applications (5 specimens x 2) must not exceed 50 seconds. No flaming drips allowed. Cotton indicator under the specimen must not ignite from drips.
- V-1: Same as V-0, but flaming can continue up to 30 seconds per application. Total flaming time up to 250 seconds. No flaming drips.
- V-2: Same as V-1, but flaming drips ARE allowed (and the cotton indicator may ignite).
The flaming drip distinction is the one that actually matters in practice. V-2 material dripping flaming plastic onto something below it is a fire spread mechanism. V-0 material self-extinguishes before it can spread fire to adjacent components.
5VA and 5VB (500W, 125mm flame). The big-boy test. A 125mm flame is applied to the specimen for 5 seconds, removed for 5 seconds, repeated 5 times. Much more severe than the 20mm V-test flame. 5VA allows no burn-through (no hole in the specimen). 5VB allows burn-through but still must self-extinguish within 60 seconds. These ratings matter for enclosures and large surfaces — anything that might face a sustained flame rather than a momentary ignition source.
What engineering plastics actually achieve
| Material | Best UL94 Rating | Thickness to Achieve |
|---|---|---|
| PEI (Ultem) | V-0 | 0.25mm |
| Polyimide | V-0 | 0.3mm |
| PEEK | V-0 | 0.8mm |
| PPS | V-0 | 0.8mm |
| PVDF | V-0 | 1.5mm |
| PTFE | V-0 | 1.5mm |
| PAI (Torlon) | V-0 | 0.8mm |
PEI wins on thin-wall V-0 performance — 0.25mm is remarkable. That's why it dominates aircraft interiors where every gram counts. PEEK and PPS achieve V-0 at a still-respectable 0.8mm.
The thing the table doesn't show: PEI, PEEK, PPS, and polyimide are inherently flame retardant. They don't need halogenated additives. The flame retardancy comes from the polymer chemistry itself — aromatic rings that char rather than burn, high carbon content, thermally stable backbone structures. This matters because:
- No halogen off-gassing during processing
- No additive migration over time
- Performance doesn't degrade as the part ages
- Smoke toxicity is lower (important for aerospace and transit)
Compare this to something like flame-retardant ABS or polycarbonate, which typically uses brominated additives. The additives work, but they make the smoke nastier and can leach out over time.
When each rating is required
HB: General consumer products, non-critical housings, some automotive interior trim. If it just needs to not be kindling, HB is fine.
V-2: Consumer electronics where fire risk is low and a few flaming drips won't cascade into a bigger fire. Less common than V-0 — most manufacturers jump straight to V-0.
V-0: The standard for electrical and electronic equipment under IEC 62368-1. Power supplies, connectors, switch components, circuit breaker housings. If your part goes into anything that plugs into a wall or carries significant current, you probably need V-0.
5VA/5VB: Large enclosures that might face sustained flame. Server racks, industrial control cabinets, battery housings. Also required for certain medical equipment housings.
What to put on your drawing
Don't just write "UL94 V-0." The rating is meaningless without the thickness. Write: "UL94 V-0 at 0.8mm thickness" or whatever your part's minimum wall section is. A material that achieves V-0 at 3mm thickness might fail at 0.5mm, and if your part has a 0.5mm wall, you have a problem.
Also: UL94 is a material certification, not a part certification. The test is done on standardized bars under lab conditions. The UL Yellow Card lists the rating, thickness, and color. If your part geometry is unusual or your process introduces variables (molded-in stress, weld lines), the part-level fire performance might differ from the material certification. For safety-critical applications, part-level testing is the only way to be sure.
We stock PEEK, PEI, PPS, PVDF, PTFE, PAI, and PI — all V-0 rated at their listed thicknesses or better. If you need UL Yellow Card documentation for your material certification package, ask when you submit your RFQ and we'll include it with the quote.