IllustrationUL 94 rates how a plastic burns under a small lab flame. HB is the lowest class, then V-2, V-1 and V-0, and 5VB and 5VA use a larger flame. A V-0 bar stops burning within 10 seconds of each flame, with no drips that ignite cotton. A rating holds only at the tested thickness and formulation.
A datasheet line that says "UL 94 V-0" tells you less than it seems. The rating belongs to one grade, at one thickness, in the formulation tested. This guide explains each UL 94 class, how to check a rating on a Yellow Card, and when a device enclosure needs a flame-rated material.
This is an engineering summary built from public pages by UL Solutions, the IEC and material suppliers, not from the full text of the standards. Confirm what applies to your product with your test lab.
What is UL 94, and what do the flammability ratings mean?
UL 94 is a set of small-scale flame tests for plastics. A resin supplier describes it as measuring how polymeric materials in devices and appliances react to a small open flame under controlled lab conditions. It adds that the test is not intended to reflect the hazards of a material under actual fire conditions (Plaskolite TEC103, as of November 2026).
UL Solutions publishes the pass criteria for each class (UL Solutions, combustion tests for plastics, as of November 2026). The table runs from the lowest class to the highest.
| Class | Specimen | Flame height and applications | Pass criteria, summarized |
|---|---|---|---|
| HB | Horizontal bar | 20 mm, once for 30 s | Burns 40 mm/min or slower at 3 to 13 mm, or 75 mm/min or slower under 3 mm, or goes out before the first mark. |
| V-2 | Vertical bar | 20 mm, twice for 10 s | Afterflame 30 s or less per application. Burning drips that ignite cotton are allowed. |
| V-1 | Vertical bar | 20 mm, twice for 10 s | Afterflame 30 s or less per application. No drips that ignite cotton. |
| V-0 | Vertical bar | 20 mm, twice for 10 s | Afterflame 10 s or less per application. No drips that ignite cotton. |
| 5VB | Vertical bar and plaque | 125 mm, up to five times | Burning and afterglow 60 s or less after the fifth application. No burning drips. A hole in the plaque is allowed. |
| 5VA | Vertical bar and plaque | 125 mm, up to five times | Same as 5VB, and no hole may form. |
UL says the 125 mm flame in the 5V tests is about five times more severe than the 20 mm flame. Plaskolite adds that it is applied for 5 s and removed for 5 s, five times (as of November 2026).
Plaskolite also gives typical uses. HB would typically be acceptable for portable, attended, intermittent-duty household appliance enclosures such as hair dryers. The V classes would typically be acceptable for portable, unattended, intermittent-duty household appliances such as coffee makers. 5V is particularly useful for enclosures of fixed electrical equipment (as of November 2026). Treat these as examples. The class a part needs comes from the standard for the finished product.
What does a UL 94 V-0 rating require compared with V-1 and V-2?
V-0, V-1 and V-2 come from one vertical test. A 20 mm flame is applied to a vertical bar for 10 s, removed, then applied for another 10 s as soon as the bar stops burning. UL lists two conditioning treatments: 2 days at 23 °C and 50% relative humidity, and 7 days at 70 °C in a hot air oven. The class depends on how long the bar burns and glows, and on whether drips ignite cotton below it (UL Solutions, as of November 2026).
| Criterion | V-0 | V-1 | V-2 |
|---|---|---|---|
| Afterflame after each flame application | 10 s or less | 30 s or less | 30 s or less |
| Total burning time, 10 flame applications | 50 s or less | 250 s or less | 250 s or less |
| Burning and afterglow after the second application | 30 s or less | 60 s or less | 60 s or less |
| Burning drips that ignite the cotton below | Not allowed | Not allowed | Allowed |
| Specimen burns up to the holding clamp | Not allowed | Not allowed | Not allowed |
Source: UL Solutions, as of November 2026.
V-1 and V-0 both forbid drips that ignite the cotton. V-0 allows a third of the afterflame time per application. V-2 has the same burn limits as V-1 and accepts burning drips. For a part that sits above other components, the drip rule can matter as much as the burn time.
Where VTM and HB fit
Thin films get VTM-0, VTM-1 or VTM-2. UL says failing the V test is a precondition for a VTM rating, the flame is applied twice for 3 s, and VTM-0 cannot be considered equivalent to V-0 (as of November 2026). HB is a horizontal burn rate test, and its limit changes with specimen thickness.
Why does a flammability rating depend on thickness and color?
A rating describes a test specimen. Change the specimen and the result can change. Protolabs says UL 94 ratings are thickness dependent, and a resin that passes at 1.5 mm may fail at 0.74 mm. Some materials perform consistently across a wide range of wall thicknesses, but Protolabs says this is not guaranteed (Protolabs, as of November 2026).
Formulation matters too. Protolabs says most flame-retardant grades rely on specific additives, and even small changes such as switching colorants can void certification. It advises asking the supplier for a Yellow Card that covers the full formulation and your thickness range (as of November 2026).
Small devices feel this most. A wearable shell, a button cap or a battery door is often thinner than the specimens a supplier tested. A custom color may also fall outside the card. Find the thinnest wall in every part that must carry a rating, and check the card at that wall and in that color.
How do you check a resin's UL 94 rating on a Yellow Card?
UL issues a Yellow Card when a polymeric material has passed its testing procedures. The card lists multiple safety and performance properties, and the program's tests include ignition and burning characteristics. A digital card is available through UL Product iQ. UL says the material is monitored at regular intervals by an independent lab and kept through periodic surveillance (UL Solutions Yellow Card program, as of November 2026). Product iQ can be searched by file number, among other identifiers (UL Product iQ, as of November 2026).
A datasheet line is a claim. The card and its listing are the record. Run these checks when a supplier proposes a grade.
| Check | Why it matters | What to do |
|---|---|---|
| Exact grade name | Protolabs notes that the same material can have different ratings based on a variety of factors. | Match the grade on the card, the datasheet and the drawing character for character. |
| Thickness and class | The rating holds at the thickness tested, and one grade can carry different classes at different thicknesses. | Find the thinnest wall that must carry the rating and read the class the card gives at that thickness. |
| Color and additives | Protolabs says switching colorants can void certification. | Confirm the card covers your color. |
| File number | Product iQ can be searched by file number. | Get the number from the supplier and look it up yourself. |
| Active status | UL says recognized materials are kept through periodic surveillance. | Check the listing again before you order production resin. |
A Yellow Card material is a recognized component, and the card is evidence for the lab that evaluates your product. Listed products and recognized components are compared in UL certification for connected devices, which also covers whether you need a listing at all.
When does a device enclosure need a flame-rated material?
UL 94 grades materials. The safety standard for the finished product decides which parts need which class. For audio, video, information and communication technology equipment, that standard is IEC 62368-1, now at edition 4.0 of May 26, 2023 (IEC webstore, as of November 2026). Its scope, its UL counterpart and whether you need a listing are covered in UL certification for connected devices. The text is paywalled, and we have not read the current clauses.
UL's technical briefs from 2010 and 2011, written when the standard was new, show the logic. Each circuit is classed by the maximum power its source can deliver and for how long. The power is measured into a worst-case load fault and with a worst-case fault inside the source, and the higher result sets the class (UL brief, July 3, 2011, as of November 2026).
| Power source class | Maximum power | What the 2010 brief says about materials |
|---|---|---|
| PS1 | 500 W maximum in the first 3 s, then 15 W maximum | Not considered able to ignite materials. Under the fire spread route, needs no supplementary safeguard unless it shares a fire enclosure with PS2 or PS3 circuits. |
| PS2 | Above PS1, and 100 W or less after 5 s | Parts in a circuit that is a potential ignition source generally need V-1 or better. Other parts are separated from it, mounted on V-1 material or made of V-2 material. |
| PS3 | Above PS2, or not classified | Same as PS2, plus a fire enclosure. |
Sources: UL brief on PS1, PS2 and PS3 (2011) and UL brief on two fire safeguard options (2010), as of November 2026. Both predate edition 4.0, so details may differ in the current edition.
The 2010 brief describes two routes. One reduces the likelihood of ignition through single fault testing, where flaming must not last beyond 5 s, plus keeping combustible material away from ignition sources or behind a fire barrier. The brief limits it to circuits with mains energy up to 4,000 W. The other route controls fire spread. It preselects materials with suitable flammability, including printed boards of V-1 or better, and adds a fire enclosure where required. One product can use both routes in different circuits (UL brief, September 3, 2010, as of November 2026).
For a battery device, do not guess the class from the cell size. The lab measures available power under fault conditions. Ask early which route it expects to apply, because the answer can decide your enclosure resin before tooling starts.
How do you prototype and build with flame-rated plastics?
Prototype parts can use rated print materials. Formlabs lists its Flame Retardant Resin as UL 94 Blue Card certified at V-0, tested at 3 mm (Formlabs, as of November 2026). Its resource page says the 1.0 mm minimum wall is for printability, while the certification was tested at 3 mm (Formlabs, as of November 2026). Protolabs lists Arkema FR512, a photopolymer for its HPS process, as V-0 at 0.79 mm. It also offers dozens of UL 94 rated materials for injection molding (Protolabs, as of November 2026).
A printed V-0 part lets you check fit and wall thickness with a rated material. How the print processes compare is in SLA vs SLS 3D printing. The parts you ship are molded from the production resin, and that resin's card is the one the lab reads. Trade-offs between common enclosure resins are in PC vs ABS vs PC/ABS.
For production, put grade, color, required class and minimum thickness on the part drawing, and ask the molder to confirm each lot against it. If someone proposes a new colorant, a second-source resin or a thinner wall, check the card again first.
What should you ask your hardware team?
Bring this list to the enclosure design review, before tooling is ordered.
- Which end-product safety standard and edition apply to this device?
- Which parts must carry a flame rating, and what class does each need?
- Is the thinnest wall of each rated part at or above the thickness on the card?
- Does the Yellow Card cover our exact grade and color?
- Who checked the file number in Product iQ, and when?
- Has anyone measured the worst-case available power of each circuit?
- Are grade, color, class and minimum thickness on the part drawing?
Resin selection, wall thickness and drawing callouts sit inside the mechanical work. For how that work is organized, see engineering.
Questions about UL 94 ratings
Is UL 94 5VA a higher rating than V-0?
Yes. Plaskolite and Protolabs both call 5VA the highest UL 94 rating, and Plaskolite says the 5V test is for materials already classified V-0 (Plaskolite, Protolabs, as of November 2026). The 5V tests use a 125 mm flame that UL describes as about five times more severe than the V and HB flame, applied up to five times (UL Solutions, as of November 2026). The class a part needs comes from the end-product standard, so check that before you choose a 5VA grade.
If my resin is V-0, is my enclosure V-0?
The rating describes the material at the tested thickness and formulation. The finished product is evaluated against its own standard. UL says a Yellow Card material can save time when certifying end products (UL Solutions, as of November 2026).
Is VTM-0 the same as V-0?
No. UL says a VTM-0 rating cannot be considered equivalent to a V-0 rating, because the test methods are quite different (UL Solutions, as of November 2026).
How can Duobay help with enclosure materials?
Duobay develops custom devices, including the enclosure, with a Shenzhen engineering team. Resin grade, color and wall thickness are part of the mechanical design and go on the part drawings. We build samples and small batches, and source files are assigned to you at every paid gate. Initial calls are free. Tell us about your device.
If you want resin grade, color and wall thickness designed in and called out on the part drawings, tell us about your device.
Sources. All pages accessed November 6, 2026. Standards and listings change, so confirm current values with the material supplier and your test lab.
- UL Solutions, Combustion (Fire) Tests for Plastics (link)
- UL Solutions, Yellow Card Plastics Recognition Program (link)
- UL Solutions, Material Substitution and Modification for Plastics (link)
- UL Solutions, Product iQ (link)
- UL, IEC 62368-1 Technical Brief: Two Options for Safeguarding Against Fire Under Fault Conditions (September 3, 2010) (link)
- UL, IEC 62368-1 Technical Brief: Classification and Electrical Measurement of PS1, PS2 and PS3 Power Sources (July 3, 2011) (link)
- IEC webstore, IEC 62368-1:2023, edition 4.0 (link)
- Protolabs, UL 94 Classification and Flame-Retardant Materials (June 12, 2025) (link)
- Formlabs, Flame Retardant Resin product page (link) and resource page (link)
- Plaskolite, TEC103 UL 94 Flame Ratings (link)
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