Flame Retardant Silicone Molds: UL94 V-0 Standards and Industrial Applications

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 mold. It is not a factory audit. It is not a finished-device fire rating.

A custom silicone manufacturer can mold a compound that a lab has classified V-0 on plaques of a given thickness. Your gasket, keypad, or wire boot is a different article if the wall is thinner, the pigment changed, the foam density changed, or the part drips differently than the bar. UL 94 (Tests for Flammability of Plastic Materials for Parts in Devices and Appliances) says this up front: the methods describe materials in response to a small open flame under laboratory conditions. End-use fire performance belongs to the appliance or enclosure standard.

Unmix the rubber. Flame-retardant parts are usually platinum LSR, platinum HCR, or peroxide HCR formulated for the rating. A poured RTV mold that happens to be silicone is not a V-0 part. Do not paste a mold-rubber TDS onto a UL file.

The rating lives on a bar, not on the tool

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 on plaques and (for 5VA) a burn-through criterion. Do not write “UL 94” on a drawing and mean V-0. Write the class and the thickness.

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

A UL Yellow Card (UL Product iQ listing) 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. If the OEM’s UL file needs a recognized material, buy a listed grade and keep the color and thickness inside the listing.

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 failures. The plaque was 2 mm. The part is 0.4 mm at the gate vestige. Drips, if they occur, also change with geometry: a hanging boot can drip; a bonded sheet cannot. Foam and sponge need their own ratings at density and thickness; do not transfer a solid HCR V-0 to a cellular sheet.

Specify:

• Minimum wall of the production part, 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. Pretending a 13 mm bar represents a 0.3 mm membrane is how listings get rejected.

What actually interrupts the flame

Unfilled silicone already chars to silica. Many industrial grades still only reach HB or a weak vertical class at the thickness you need. 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, compression set, 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. Light V-0 is a different formula.
• 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. Filter those separately.

Tin-cure RTV is the wrong starting chemistry for a listed electrical part. Platinum LSR or HCR is the usual production path. Peroxide HCR can be formulated for FR after a real post-cure; leftover peroxide is a separate odor and extractables problem, not a flame problem.

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 and power-tool boots, grommets, and keypad mats inside enclosures
• EV and industrial connector seals where the housing spec calls a vertical class
• Lighting gaskets near hot sources (still not a building flame-spread rating)
• Battery pack cushions and busbar boots, subject to the pack-level test, not instead of it
• Wire-entry seals and bushings in listed equipment

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

• Building construction, wall coverings, furnishings (UL 94 says it does not cover these; other fire tests apply)
• Hourly fire-resistance of a wall or door
• Aircraft cabin heat-release / smoke (FAR / CS 25.853 and related methods)
• Rail EN 45545-2, which is a set of material and product tests, not a V-0 sticker

A flame-retardant mold rubber used to cast polyurethane or wax is a shop convenience. It does not make the casting V-0. If you needed the mold itself not to burn in a resin room, say that in plain language and pick a grade. Do not call it a UL listing.

Put the test article on the purchase order

Write:

“Platinum [LSR or HCR], flame-retardant compound [code], color [code]. UL 94 V-0 (or IEC 60695-11-10 V-0) on specimens molded from the production compound at [t] mm, report [lab / date / file]. Production minimum wall [t_min] mm shall not be below the classified thickness. Pigment, sponge, and fabric changes require a new class. This is a material preselection, not an appliance certification. Tin-cure RTV is excluded.”

Refuse:

• “UL94 mold” with no class, no thickness, no specimen
• Transferring a 3 mm plaque class onto a 0.5 mm skin
• Using a craft RTV plus a mystery FR powder as a listed material
• Treating V-0 as a food, medical, or building-code stamp

If the OEM already has a Yellow Card grade, lock that grade, that color range, and that thickness. The molder’s job is to stay inside the listing, not to invent a parallel burn story.

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.

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.