Key takeaways
- UL 94 is a material test: it rates how a specific plastic grade burns, not whether your finished product is safe.
- The ratings run from HB (slow burning) through V-2, V-1 and V-0 (self-extinguishing) to 5VB and 5VA (larger flame, used for some enclosures).
- A rating applies only at the thickness and color listed for that grade, so a thinner wall or new color can fall outside it.
- The end-product safety standard and the test lab decide which rating each plastic part in your product needs.
UL 94 is the standard test for how a plastic burns. It places a material grade into one of several classes, from HB (burns slowly in a horizontal test) through V-2, V-1 and V-0 (stops burning in a vertical test) up to 5VB and 5VA (survives a much larger flame). The rating belongs to a specific resin grade at a listed thickness and color. Your product’s own safety standard decides which rating each plastic part needs.
Flame rating is the third filter in how plastic parts get made, where it narrows the materials a process can use. This article covers the central contrast founders most often blur: a material’s flame rating versus the certification of the finished product.
The UL 94 ratings from HB to 5VA
The tests use small molded bars (and, for the 5V ratings, plaques) held over a small burner. The vertical and 5V methods are harmonized with the international IEC 60695-11 series, so equivalent ratings appear on European and Asian datasheets too.
| Rating | Test | What it takes to pass, in plain terms |
|---|---|---|
| HB | Horizontal bar, small flame | Burns slowly: no faster than 40 mm/min for 3 to 13 mm thick bars, or 75 mm/min under 3 mm, or stops before the 100 mm mark |
| V-2 | Vertical bar, two 10-second flame applications | Each burn stops within 30 seconds; burning drips are allowed |
| V-1 | Same as V-2 | Each burn stops within 30 seconds; no drips that ignite cotton below |
| V-0 | Same as V-2 | Each burn stops within 10 seconds, with a 50-second total across five bars; no igniting drips |
| 5VB | Larger flame, five 5-second applications | Burning stops within 60 seconds; the plaque may burn through |
| 5VA | Same as 5VB | Burning stops within 60 seconds; no burn-through on the plaque |
HB is the lowest class and means slow burning, not self-extinguishing. Many standard ABS, polycarbonate and polypropylene grades only reach HB. Flame-retardant (FR) grades are formulated to reach the V or 5V classes. Thin films use a parallel VTM scale, and foams use HBF, HF-1 and HF-2.
Free tool: find which US certifications your product likely needs, in 2 minutes →Material rating vs product certification
A UL 94 rating describes a test bar. It says nothing about how your enclosure behaves around your circuit board, battery or power supply.
| Question | UL 94 material rating | End-product safety certification |
|---|---|---|
| What is tested | Standard bars or plaques of one resin grade | Your finished product, under normal and fault conditions |
| Who holds it | The resin manufacturer | The product’s manufacturer or brand |
| What it decides | Which class the material falls into | Which class each part must meet, based on its role and nearby energy sources |
| Example standards | UL 94, IEC 60695-11-10 and -11-20 | UL/IEC 62368-1 for audio, video and IT equipment; appliance standards for household products |
Under IEC 62368-1, for example, fire enclosures and materials near potential ignition sources often need V-1 or better, depending on how the power source is classified and how the product is built. The test lab assessing the finished product makes that call, so treat any blanket rule (“all enclosures must be V-0”) as a starting assumption to confirm, not a requirement.
Buying a V-0 resin does not make your product compliant. It gives the lab a qualified material to assess inside your design.
Why thickness and color change the rating
A grade is rated at specific thicknesses. A thin bar has less mass to char and insulate, so many grades rated V-0 at 1.5 mm are only V-2 or HB at 0.8 mm. If your enclosure has a thin rib or a thinned wall around a button, that section may sit below the listed thickness.
- Thickness. Check the minimum listed thickness against the thinnest wall in the part, not the nominal wall.
- Color. Listings are either for “ALL” colors or for specific ones. Pigments can change flame behavior, so a new color may need its own listing.
- Process. The rating assumes the material is molded. Printed parts do not inherit it, a point covered in how 3D printing is transforming product development.
- Regrind. Ask the molder whether the listing allows reground material and at what percentage.
How to verify a flame rating before you buy resin
Datasheets summarize ratings, but the authoritative record is UL’s own listing for the grade, often called the yellow card. It can be searched in UL Product iQ.
- Match the exact grade name, including suffixes, since one letter can separate an FR grade from a standard one.
- Confirm the rating at your thinnest wall and in your color.
- Ask the molder for material certificates tied to each production lot.
- Keep the listing with your product documentation for the safety lab.
The same habit of checking the primary record applies to company claims as well as materials, covered in how to verify a development partner’s certifications.
What a flame-retardant grade costs you in design
FR grades solve one problem and can create others, so choose them deliberately rather than by default.
- Higher material cost raises the per-part price, most noticeably on large housings.
- Changed mechanical properties (some FR additives reduce impact strength) mean drop tests should run on the FR grade itself. The base-resin trade-offs are covered in ABS vs polycarbonate.
- Appearance limits can show up as reduced gloss or harder color matching, which affects your injection molding surface finish choices.
- Chemistry restrictions apply too: RoHS restricts certain brominated flame retardants (PBB and PBDE), and some buyers ask for halogen-free grades.
Frequently asked questions
Is V-0 better than 5VA?
5VA is the more severe test, with a larger flame applied five times. Products typically need a 5V rating only for certain enclosure roles, and a material usually has to meet a V rating as well before it is given a 5V rating.
Does every plastic part in my product need a flame rating?
Not always. Requirements depend on each part’s role and its distance from potential ignition sources, as assessed under your product’s safety standard. Small decorative parts away from power circuits are often treated differently from the enclosure around a power supply.
Is HB considered flame retardant?
No. HB means the material burns slowly in a horizontal test. It does not mean the material stops burning on its own.
Can a 3D printed part be UL 94 V-0?
A printed part does not automatically carry the rating of the filament or resin it was made from. Printing changes density and structure, so a printed part needs its own evaluation if a rating matters.
Who tests the finished product’s fire safety?
An accredited safety lab, such as a Nationally Recognized Testing Laboratory in the US, evaluates the finished product against its end-product standard. The resin maker’s UL 94 listing is one of the inputs to that evaluation.
Where Inventornest fits
At Inventornest, we consider applicable certification requirements early, alongside battery performance and manufacturability, because the flame class a part needs affects the material, the wall thickness and the cost. We have not experienced a certification failure that required a major redesign; occasionally, a lab requests minor changes, which we address during the certification process.
Formal certification testing is a separate activity where applicable, and external laboratory certification remains a separate professional activity from our design work. Our product certification services cover the engineering that gets a design ready for that lab, and you can book a consultation to check the flame requirements for your enclosure.
