UL 94 Flammability Ratings for Plastics: V-0, V-1, V-2, 5VA, HF-1 & Yellow Card Guide

UL 94 (Standard for Tests for Flammability of Plastic Materials for Parts in Devices and Appliances, harmonized with IEC 60695-11-10 / -20 and ISO 9772 / 9773) is the baseline flammability compliance standard for injection-molded electrical enclosures, EV battery modules, appliance housings, and industrial controls.

For engineering and procurement teams, the most dangerous misconception is treating “UL 94 V-0” as an intrinsic property of a plastic family. A UL 94 rating is valid only for the exact commercial resin grade, at or above the certified minimum wall thickness (e.g., 1.5 mm), and in the certified color listed on the manufacturer’s UL Yellow Card. If your CAD design steps a housing wall down to 1.0 mm to save weight—or if your molder blends 30% shop regrind or an uncertified color masterbatch into a V-0 resin—the finished part loses its UL compliance instantly. Use this guide alongside the material selection guide to lock down flammability specifications before releasing tooling.

Complete UL 94 Flammability Rating & Test Criteria Table

UL 94 classifies plastics by burner orientation (horizontal vs vertical), burner power (50W / 20 mm flame for HB and V-ratings vs 500W / 125 mm flame for 5V enclosures), self-extinguishing time, and whether flaming drips ignite surgical cotton placed 300 mm below the specimen.

UL 94 Rating	Test Orientation & Burner Power	Max Single Afterflame ($t_1$ or $t_2$)	Total Afterflame (5 Specimens, 10 Flames)	Max Afterglow ($t_3$ After 2nd Flame)	Flaming Drips Allowed to Ignite Cotton?	Plaque Burn-Through Hole Allowed?
UL 94 HB (Horizontal Burn)	Horizontal bar; 50W (20 mm) flame applied for 30 s	Burn rate ≤ 40 mm/min (3–13 mm thick) or ≤ 75 mm/min (< 3.0 mm thick), or self-extinguishes before 100 mm mark	N/A	N/A	Yes	N/A
UL 94 V-2 (Vertical Burn)	Vertical bar; 50W (20 mm) flame applied 2 × 10 s	≤ 30 seconds per flame application	≤ 250 seconds total across 10 flame applications	≤ 60 seconds	Yes (flaming drops may ignite cotton below)	N/A (must not burn up to holding clamp)
UL 94 V-1 (Vertical Burn)	Vertical bar; 50W (20 mm) flame applied 2 × 10 s	≤ 30 seconds per flame application	≤ 250 seconds total across 10 flame applications	≤ 60 seconds	No (drips may not ignite surgical cotton)	N/A (must not burn up to holding clamp)
UL 94 V-0 (Vertical Burn)	Vertical bar; 50W (20 mm) flame applied 2 × 10 s	≤ 10 seconds per flame application	≤ 50 seconds total across 10 flame applications	≤ 30 seconds	No (drips may not ignite surgical cotton)	N/A (must not burn up to holding clamp)
UL 94 5VB (500W Enclosure)	Vertical bar + Horizontal plaque; 500W (125 mm) flame applied 5 × 5 s (15 s intervals)	≤ 60 seconds (afterflame + afterglow after 5th flame)	N/A	Included in ≤ 60 s limit	No (zero flaming drips permitted)	Yes (burn-through hole in horizontal plaque is permitted)
UL 94 5VA (500W Highest)	Vertical bar + Horizontal plaque; 500W (125 mm) flame applied 5 × 5 s (15 s intervals)	≤ 60 seconds (afterflame + afterglow after 5th flame)	N/A	Included in ≤ 60 s limit	No (zero flaming drips permitted)	No (plaque must not exhibit a burn-through hole)
UL 94 HF-1 (Foamed / Gasket)	Horizontal low-density foam (< 250 kg/m³); 35 mm wing burner for 60 s	≤ 2 seconds on 4 of 5 specimens (≤ 10 s on 1 specimen); char length < 60 mm	N/A	≤ 30 seconds	No (zero flaming drips ignite cotton)	N/A
UL 94 HF-2 (Foamed / Gasket)	Horizontal low-density foam; 35 mm wing burner for 60 s	≤ 2 seconds on 4 of 5 specimens (≤ 10 s on 1 specimen); char length < 60 mm	N/A	≤ 30 seconds	Yes (flaming drops may ignite cotton)	N/A
UL 94 HBF (Foamed Horizontal)	Horizontal low-density foam; 35 mm wing burner for 60 s	Burn rate ≤ 40 mm/min over 100 mm span (or ceases burning before 125 mm mark)	N/A	N/A	Permitted	N/A

(Note: Thin flexible films that distort or shrink away from the jaw during a regular vertical bar test are evaluated under the separate UL 94 VTM-0, VTM-1, and VTM-2 vertical thin-material classifications.)

Key Engineering Distinctions: 5VA vs V-0 and HF-1 vs V-0

1. UL 94 5VA / 5VB vs V-0 (Why 5VA Is 10× More Severe)

Buyers frequently assume V-0 is the highest flammability rating in UL 94. While V-0 is the highest rating in the 50W (20 mm flame) vertical bar test, stationary power-supply enclosures, EV chargers, and industrial inverter housings subject to UL 62368-1 or UL 746C often require 5VB or 5VA.

• 10× Burner Energy: The 5VA/5VB test hits the plastic with a 500W (125 mm tall) Tirrill burner flame—ten times the thermal power of the 50W V-0 burner—applied five separate times for 5 seconds each.
• Horizontal Plaque Burn-Through Test: In addition to testing a vertical bar, 5VA and 5VB aim the 500W flame upward at the underside of a horizontal molded plaque. To pass 5VA, the material must neither drip flaming particles nor allow a burn-through hole to open in the ceiling plaque, preventing fire from escaping an electrical enclosure. Usually, a flame-retardant PC or PC/ABS grade certified to V-0 at 1.5 mm requires a wall thickness of 2.5 mm to 3.0 mm to achieve 5VB or 5VA.

2. UL 94 HF-1, HF-2, and HBF (Foamed & Microcellular Plastics)

Search queries for UL 94 HF-1 spike when procurement teams source acoustic insulation pads, EMI gaskets, microcellular polyurethane foams, or foamed silicone/TPE seals inside electronics.

• Solid injection-molded walls (> 250 kg/m³ density) use HB / V-2 / V-1 / V-0 / 5V.
• Low-density cellular/foamed materials (< 250 kg/m³ per ISO 9772) cannot be tested on the standard vertical rig and are certified to HF-1 (self-extinguishing within ≤ 2 s, no flaming drips), HF-2 (flaming drips allowed), or HBF (slow horizontal burn ≤ 40 mm/min). Never write UL 94 V-0 on a drawing for a low-density foam gasket, or HF-1 on a solid injection-molded housing.

How to Audit a UL “Yellow Card” (UL iQ Record)

Every UL-recognized plastic compound has an official certificate in the UL Product iQ / Prospector database known as the Yellow Card. Before approving a supplier’s resin quote, verify six fields on the Yellow Card against your part drawing:

Yellow Card Field	Example Listing on Card	Buyer & Engineering Verification Rule
1. Minimum Thickness (Min. Thk)	0.75 mm -> V-2 1.50 mm -> V-0 3.00 mm -> 5VA	The rating changes with thickness. If your housing has a 1.2 mm thinned boss or rib pocket and the grade is only V-0 down to 1.50 mm, the part is strictly rated V-2, not V-0.
2. Color Code (Color)	ALL (All Colors) vs NC (Natural), BK (Black), GY (Grey)	Organic red, orange, and yellow pigments frequently degrade flame performance. If the card lists V-0 only for NC and BK, molding the part in bright red voids the V-0 rating unless "ALL" is certified.
3. Relative Thermal Index (RTI Elec / Imp / Str)	RTI Elec: 125 °C RTI Imp: 115 °C RTI Str: 125 °C	Maximum continuous operating temperature (°C) before the plastic loses 50% of its dielectric (Elec), impact (Imp), or tensile (Str) properties over long-term thermal aging.
4. Comparative Tracking Index (CTI)	PLC 0 (≥ 600V) to PLC 5 (< 100V)	Measures voltage resistance to electrical arc tracking across a wet/contaminated surface; critical when live high-voltage terminals mount directly onto a nylon or PBT bobbin.
5. Glow-Wire Testing (GWFI / GWIT)	GWFI: 960 °C @ 1.5 mm GWIT: 775 °C @ 1.5 mm	Mandatory under IEC 60335-1 for unattended household appliances (coffee makers, dishwashers, HVAC relays): materials within 3 mm of connections carrying > 0.2 A must pass 750 °C–850 °C glow-wire ignition.
6. UL File Number (E-Number)	E121562 (SABIC) / E41613 (Covestro)	Require the molder’s packing slip and Certificate of Analysis (COA) to state the exact UL File E-Number and commercial grade designation on every production lot.

Tooling, Regrind, and Masterbatch Rules for Molding UL 94 V-0 Resins

Flame-retardant (FR) additives—whether halogenated brominated packages with antimony trioxide or halogen-free phosphorus/melamine systems—alter both mold steel selection and shop-floor processing:

1. Specify Corrosion-Resistant 420SS Tool Steel: If an FR resin is overheated beyond its residence-time limit (> 5–6 minutes in the barrel) or sheared through a tiny submarine gate, it decomposes and releases acidic gases that rapidly pit standard P20 or H13 cavities. For high-volume UL 94 V-0 production tools, specify hardened AISI 420SS (Stavax / 1.2083 at 48–52 HRC) and deep perimeter venting (0.02–0.03 mm with relief channels) so FR plate-out residue can be wiped clean.
2. Enforce the 25% Maximum Regrind Rule (UL 746D): Under UL 746D, a molder may blend up to 25% clean, segregated in-house runner regrind of the identical grade and color without losing the UL 94 flame rating—because higher regrind percentages suffer thermal chain scission that causes flaming drips during vertical burn testing. If regrind exceeds 25%, or if the part is a thin-wall 0.75 mm V-0 connector, mandate 100% virgin resin in your RFQ specification sheet.
3. Prohibit Uncertified Color Masterbatch: If a molder buys natural (NC) V-0 resin and blends 3% of a generic polyethylene-carrier color masterbatch at the press hopper, the polyethylene carrier acts as fuel and causes the part to fail the V-0 burn test. Either buy pre-colored factory compound covered under an "ALL" Yellow Card, or require a UL-recognized color concentrate (QMQS2) specifically listed for that base resin and let-down ratio.

Buyer FAQs

What is the difference between UL 94 HB, V-2, V-1, and V-0?

UL 94 HB is the lowest flammability rating, tested horizontally (burn rate ≤ 40–75 mm/min), which nearly all un-modified plastics (ABS, PP, general-purpose PS) pass at standard thickness. V-2, V-1, and V-0 are much stricter vertical burn tests (50W flame applied twice for 10 s): V-0 requires flames to self-extinguish within ≤ 10 seconds (≤ 50 s total for 5 bars) with zero flaming drips, V-1 allows up to ≤ 30 seconds (≤ 250 s total) with zero flaming drips, and V-2 allows ≤ 30 seconds burn time but permits flaming plastic drops that ignite cotton below.

What is the difference between UL 94 5VA and V-0?

UL 94 5VA is the most stringent flammability rating in the UL 94 standard and is ten times more severe in burner power (500W / 125 mm flame applied 5 × 5 seconds) than the UL 94 V-0 test (50W / 20 mm flame applied 2 × 10 seconds). In addition to testing a vertical bar with zero flaming drips allowed, 5VA tests a horizontal plaque from below and prohibits any burn-through hole, making it mandatory for large stationary electrical enclosures and power distribution housings.

Does wall thickness change a plastic’s UL 94 flame rating?

Yes—every UL 94 flame rating is strictly bound to the minimum wall thickness (Min. Thk) certified on the resin’s UL Yellow Card. A flame-retardant polycarbonate grade may be certified to 5VA at 3.0 mm, V-0 at 1.5 mm, and only V-2 at 0.75 mm. UL compliance is governed by the thinnest structural wall section on your molded part, so thinning a housing wall from 1.6 mm to 1.1 mm during DFM can legally downgrade the enclosure from V-0 to V-2.

What is a UL 94 HF-1 rating?

UL 94 HF-1 is the highest horizontal flammability rating for low-density foamed, cellular, and elastomeric materials (density < 250 kg/m³), such as acoustic foam liners, microcellular urethane pads, and foamed silicone gaskets. To pass HF-1, 4 of 5 foam specimens must stop burning within ≤ 2 seconds (≤ 10 s max on the 5th specimen) after a 60-second wing-burner exposure, damage length must remain < 60 mm, and no flaming drips may ignite surgical cotton below.

Disclaimer

PlasticsTechnologyAlliance.com is an independent buyer resource. It does not manufacture parts, sell materials, or conduct UL certification testing. Always verify official UL Yellow Card listings in the UL Product iQ database (iq.ulprospector.com) and confirm final end-product compliance with your safety certification laboratory.