UL 94 V-0 on RTV Molds: Specimen Rating, Not a Cert

UL 94 V-0 is a flammability class of a specimen, at a stated thickness, after a defined vertical burn. It is not a certificate hanging on a poured RTV mold, a factory audit, or a finished-device fire rating.

If you're buying a custom RTV silicone mold, say which object you actually need classified. A platinum RTV cavity that happens to be silicone is not a V-0 part. A resin, wax, or gummy you dump into that cavity is not V-0 because the jacket is silicone. A custom mold shop can pour a compound a lab has classed V-0 on plaques of a given thickness. Your tray, glove, or mother-mold skin is a different article if the wall is thinner, the pigment changed, or the part drips differently than the bar.

Unmix the rubber. Flame-retardant parts (gaskets, keypad mats, wire boots) are usually platinum LSR or HCR formulated for the rating — a different RFQ. A poured RTV mold used to cast polyurethane or chocolate is tooling. Do not paste a mold-rubber TDS onto a UL file, and never write “LIM hot tool” on this PO.

The rating lives on a bar, not on the jacket

UL 94 vertical classes (20 mm / 50 W flame) are assigned to bars, typically on the order of 125 mm × 13 mm, at the thickness you claim. Two sets of five specimens are common. Conditioning matters. The class is thickness-specific: a compound can be V-0 at 3 mm and fail V-0 at 0.8 mm.

V-0, in shop language:

• Afterflame after each 10 s flame application stays within the short V-0 window (each afterflame ≤ 10 s; sum of afterflames for the set within the V-0 total).
• No afterflame or afterglow to the holding clamp.
• No flaming drips that ignite the cotton indicator.
• Afterglow after the second flame stays within the V-0 glow limit.

V-1 allows longer afterflame. V-2 allows drips that ignite cotton. HB is a slower horizontal burn, a different and weaker claim. 5VA / 5VB use a more severe 500 W flame. Do not write “UL 94” on a mold drawing and mean V-0. Write the class and the thickness.

IEC 60695-11-10 is the closely related IEC method many non-UL files cite. UL 1694 exists for small polymeric components that cannot be fabricated into standard bars. Neither one certifies a mold cavity.

A UL Yellow Card is a material recognition at listed colors and thicknesses, with a file number. A shop COC that says “V-0 compound used” is not that card.

Common mix-up: FR mold rubber versus a V-0 casting

A resin-room buyer wants the mold not to catch if a urethane exotherm flares. That's a shop convenience: pick a platinum RTV grade that chars rather than runs, and say so in plain language. It is not a UL listing.

A device OEM wants the article that ships to carry V-0 at a minimum wall. That's a formulated LSR or HCR part. The RTV block you used to cast twenty urethane stand-ins does not transfer its class onto those stand-ins, and the stand-ins do not transfer onto production LSR.

If the RFQ says “UL94 mold” with no class, no thickness, and no specimen, you're not specifying either job.

What to ask when you're buying

• Is the test article the RTV cavity, the casting you dump into it, or a molded silicone *part* on a separate PO?
• Minimum wall of that article, not the TDS plaque.
• Solid, sponge, or fabric-reinforced? Color / pigment family?
• Do you need a Yellow Card grade locked, or a shop-screened FR mold rubber for a resin room?
• Who owns the master and jacket if we later recast the skin in a different FR grade?

Worked example: 6-cavity resin glove, 8 mm skin

A lighting vendor needed a 6-cavity platinum RTV glove, about 8 mm skin, Shore 20A, jacketed, to cast filled urethane housings. The housing drawing called UL 94 V-0 at 1.5 mm. That's a casting-compound problem, not a mold-rubber problem. We would pour a standard high-tear platinum cavity (vacuum degassed, keyed split, rigid jacket) and leave V-0 to the urethane supplier's plaque at 1.5 mm.

A second PDF quoted heated steel and a cold-runner for a silicone boot in a V-0 LSR. That's a parts OEM. It will not dump urethane. Both emails said “V-0 mold.”

Thickness is the line most RFQs omit

Silicone's silica char helps. It does not make every wall self-extinguishing. Thin membranes, keypad webs, and overmolded skins are the usual part-side failures. On the mold side, a 10A–20A food tray is often 4–8 mm in the pocket and thicker at the jacket — still not a 3 mm bar.

Specify:

• Minimum wall of the production article, not the TDS plaque.
• Whether the test article is solid, sponge, or fabric-reinforced.
• Color / pigment family, because some packages move the burn.

If the part cannot be cut into a standard bar, say so. Then the conversation is UL 1694, an end-use appliance test, or a non-UL internal screen.

What actually interrupts the flame

Unfilled silicone already chars to silica. Reaching V-0 at thin section usually means a formulated flame-retardant package: platinum-complex synergists in some addition systems, mineral hydrates, or other FR packages the compounder owns. Those packages change viscosity, tear, and sometimes platinum cure. They are not a powder you stir into craft RTV.

Consequences the drawing should expect:

• Hardness and tear move. FR loadings are fillers. Soft, high-tear V-0 is a compound, not a blend on the bench.
• Color is constrained. Carbon-heavy packages go dark.
• Electrical properties can move. Some FR minerals raise dielectric loss; some carbon packages kill insulation.
• Food-contact and implantable paths are not automatic. 21 CFR 177.2600 and ISO 10993 do not follow from a burn class.

Tin-cure RTV is the wrong starting chemistry for a listed electrical part. For a poured mold, tin is a dirty-master copy rubber, not an FR shortcut.

Industrial jobs that actually need the class

Buy V-0 when a device or appliance standard points at it, or when an OEM's UL file lists it. Typical silicone *articles*: appliance boots, EV connector seals, lighting gaskets near hot sources, battery cushions — still not a building flame-spread rating.

Jobs that are not UL 94, even if someone wrote V-0 on the email:

• Building construction, wall coverings, furnishings
• Hourly fire-resistance of a wall or door
• Aircraft cabin heat-release / smoke (FAR / CS 25.853)
• Rail EN 45545-2
• A chocolate or gummy tray that someone wanted “safer in a bakery oven”

A flame-retardant mold rubber used to cast polyurethane or wax is a shop convenience. It does not make the casting V-0.

When this article is the wrong quote

If you need thousands of vulcanized V-0 parts, quote HCR compression or LSR injection with a cold-runner on a parts OEM. Geometry can override: a one-off FR-room texture mold is still an RTV job; 50,000 V-0 keypad mats are not. Paying a tooling charge on the RTV jacket does not give you unrestricted ownership of a UL file that never existed.

Related reading: custom RTV silicone molds, what tooling means on an RTV PO, how we make the mold, food-grade molds.

What to send for an RTV mold quote

Copy this list.

• Object: poured platinum RTV cavity (not an LSR/HCR part tool)
• Test article: mold rubber, casting compound, or neither
• If a class is required: UL 94 or IEC 60695-11-10, class, thickness, solid vs sponge
• Master type; who keeps it; recast of the skin included or extra
• Casting material, exotherm, working temperature
• Cavity count, split, jacket, Shore target
• Pigment / color constraint

Send the pocket STEP (or sample photos), the material you'll cast, and whether V-0 attaches to the mold or to the article. We'll name platinum vs tin, jacket, and whether FR mold rubber even belongs on this PO.

We would not recommend pasting a mold TDS onto a UL file

We would not recommend calling a platinum RTV tray “UL94 certified.” We would not stir a mystery FR powder into tin-cure RTV and expect a Yellow Card. We would not cut an LSR steel tool “so we can scale later” while you still need a cavity to dump urethane into. We would not transfer a 3 mm plaque class onto a 0.5 mm skin.

RUUIPON Mold Structure Decision: flat back? open-back RTV. Else deep undercuts the rubber can peel? stay one-piece. If it cannot peel, two-part / plug / inner core. Large wet casting or Shore 10A–20A? mother mold / support jacket.

Typical shop values: UL 94 language vs RTV mold

| Item | Typical shop value | | --- | --- | | What UL 94 V-0 actually classes | Specimen bar at a stated thickness, not the mold | | RTV cavity Shore (undercut food/resin) | 10A–20A platinum, jacket behind it | | Platinum mix | 1:1 or 10:1 by grade; vacuum degas until foam collapses | | Platinum linear shrink | ~0.1–0.3%; tin keeps moving in storage | | Plaque vs thin wall | V-0 at 3 mm can fail at 0.8 mm | | ISO 3302-1 on a rubber article | M2 is a common drawing default; it is not a fire class |

Reference values only. Confirm material-specific limits against the supplier’s technical data and agree project acceptance criteria before production.

FAQ

Is a silicone mold UL 94 V-0 certified if the rubber is flame retardant? No. UL 94 classifies a standard specimen of a polymeric material at a stated thickness. A mold is tooling. A molded part can be made from a V-0 compound and still need its own specimens if thickness, color, or cellular structure differ from the listed plaque.

Does UL 94 V-0 mean the finished device will pass a fire test? It does not. UL 94 is a material preselection. Appliance, enclosure, battery-pack, and building standards use other methods on the end-use configuration. V-0 on a boot does not pass the housing, and V-0 language on an RTV jacket does not pass the casting.

Can I get V-0 at 0.5 mm because the TDS shows V-0 at 2 mm? Not automatically. Vertical class is thickness-specific. Thin walls, fabrics, and foams fail for reasons a thick bar never sees. Classify at or below the minimum production wall, or change the wall.

Is IEC 60695-11-10 the same as UL 94 V-0? They are closely related vertical/horizontal specimen methods and are often treated as counterparts in files, but they are not the same document. Cite the method the listing or OEM actually uses, including class and thickness.

Will a flame-retardant additive make tin-cure RTV a listed electrical grade? No. Listed electrical parts start from formulated LSR or HCR with a documented class, often a UL file. Shop-mixed FR powder in condensation RTV is not a Yellow Card, and it can wreck tear and cure. Tin remains a copy-mold rubber for dirty masters.