Fluorosilicone vs RTV Molds for Fuel and Oil Service

Fluorosilicone rubber (FSR, ASTM D1418 FVMQ) is dimethyl silicone with fluorinated side groups. Those groups are why it survives many gasolines, jet fuels, and mineral oils that turn ordinary VMQ — including the platinum RTV in a custom mold — into a swollen sponge. They are also why FSR costs more, can run weaker in hot air than VMQ, and still fails in fluids people casually file under “chemical.”

FSR is a part compound: O-rings, diaphragms, gaskets, boots in fuel and oil service. It is not a poured RTV texture mold. Ordinary mold rubber is VMQ chemistry. It will not inherit FSR's fuel resistance because you needed oil resistance on a cavity. If you're buying a custom RTV mold, the honest question is whether the casting fluid (resin, solvent wipe, oil-based release, fuel-tank wax) will swell the jacket, and whether you actually needed a fuel-system part on a different PO.

FVMQ is still silicone. The side groups are the fuel story.

The backbone is siloxane. That is the cold flexibility and the wide temperature span. The trifluoropropyl side groups raise resistance to non-polar hydrocarbons. ASTM D2000 / SAE J200 type/class callouts for fluorosilicone exist so you can buy heat and oil resistance as a line, not as a brand.

What FSR is for:

• Aviation and automotive fuels (including many gasoline and jet-fuel families)
• Many mineral oils, some synthetic lubricants, transmission fluids (grade-specific)
• Diaphragms that must flex in fuel vapor and still seal at cold start
• Specs that already name AMS / MIL fluorosilicone families

What FSR is not:

• A universal oil-proof rubber. Aromatic content, additives, and biodiesel blends change swell. Test the actual fluid (ASTM D471) at service temperature.
• A ketone / low-molecular-weight ester / ether champion.
• A superheated-steam gasket.
• A silicone-oil seal. Silicone fluids can swell silicones, fluoro or not.
• A cheaper FKM. If you do not need silicone's cold, FKM often seals fuel better on hot hydrocarbon duty.
• A drop-in mold rubber you pour over a master at 1:1.

VMQ (ordinary methyl silicone, including platinum RTV molds) remains the correct buy when there is no fuel or oil. You pay FSR's premium only for the hydrocarbon the methyl grade cannot take.

Common mix-up: oil-proof mold versus fuel-system part

A shop pours urethane with a solvent wipe and the RTV glove swells and tears at the split. That's a mold-chemical problem: change release, shorten solvent contact, or pick a mold grade the resin supplier actually lists. It is not a reason to mill FSR into a jacket.

A fuel-rail OEM wants FVMQ O-rings. That's millable HCR or specialist fluoro-LSR. There is no cold-runner in a poured mold, and you should never specify a LIM hot tool on either RFQ.

What to ask when you're buying

• Is the silicone the cavity I'll cast into, or a fuel/oil part?
• Named fluid, temperature, time — not “oil-proof.”
• Maximum volume swell and hardness change (ASTM D471 / D2240)?
• If it's still an RTV mold: resin system, solvent wipe, exotherm?
• Who keeps the master if the first skin swells and we recast?

Worked example: 2-part resin glove next to a solvent wipe

A pattern shop ran a keyed two-piece platinum 20A glove, jacketed, ~40 pulls of filled urethane. They wiped the cavity with a ketone blend between pulls. The skin swelled, keys drifted, and ISO 3302-1 talk on the casting went sideways because the cavity had moved. Fix: stop the ketone wipe, use a release the RTV TDS allows, recast the platinum skin on the same master. FSR millable sheet would not have poured as that glove.

Swell, hardness, and the drawing

Fuel resistance in the shop is volume swell, hardness change, and whether the part still seals after soak. Write limits: fluid name and spec, temperature and time, maximum swell, functional leak after soak if it is a diaphragm.

A 20% swell that a static face gasket can live with will ruin a precision valve lip — and it will ruin a keyed RTV split. Blends of FVMQ with VMQ exist as compounds, not a 50/50 shop mix.

Most FSR in the field is millable HCR, peroxide or addition cured, compression or transfer molded. Fluoro-LSR exists in specialist lines. Poured tin or platinum RTV that happens to say “silicone” will not become FSR.

Design consequences if you *are* buying FSR parts: shrink and flash follow HCR / fluoro-LSR, not platinum RTV's ~0.1–0.3% shrink; bonding to metal needs a fluorosilicone primer; tear is typically below a good VMQ of the same hardness; post-cure is normal for peroxide FSR.

FKM wins many hot fuel seals on swell. FSR is the overlap: silicone cold plus fluoro fuel resistance. Do not “upgrade” a failed VMQ oil seal to FSR without naming the oil.

When this article is the wrong quote

Custom chocolate, gummy, and most resin molds stay platinum VMQ RTV. If you need thousands of fuel-system seals, quote FSR HCR or fluoro-LSR on a parts OEM. Geometry can override: a one-off fuel-vapor fixture gasket might still be a cut FSR sheet, not a poured mold.

Related reading: custom RTV molds, what tooling means, how we make the mold, mold shrinkage.

What to send for an RTV mold quote

• Object: platinum RTV cavity vs FSR *part*
• Named fluid / resin / solvent wipe and contact time
• Swell limit if you have one; otherwise we'll tell you when VMQ RTV is enough
• Master, split, jacket, Shore, recast
• If it is truly FSR: process (millable HCR / fluoro-LSR), D471 fluid, cold function

Send the fluid name and whether you need a cavity or a seal. We'll refuse “oil-proof silicone” with no fluid.

We would not recommend FSR as a poured mold rubber

We would not recommend using FSR as a texture mold for resin casting. We would not treat FSR as food-contact or implant material without that file. We would not copy an FKM swell table onto an FSR drawing, or onto an RTV jacket. We would not raise Shore A first to “fight swell.”

RUUIPON Mold Structure Decision: if this is still an RTV cavity — flat back? open-back. Cannot peel? two-part / core. Large wet casting? mother mold. If it is FSR, stop: different factory.

Typical shop values: VMQ mold vs FSR part

| Item | Typical shop value | | --- | --- | | Platinum RTV mold shrink | ~0.1–0.3%; tin keeps moving | | HCR / FSR part shrink | Often ~1.5–2.5% (grade-dependent) | | Swell method | ASTM D471 in the named fluid | | RTV cavity Shore | 10A–20A with jacket | | FSR vs VMQ | FSR for many fuels/oils; VMQ RTV for most molds | | ISO 3302-1 | M2 common default on rubber articles; not a fluid class |

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

FAQ

Is fluorosilicone just silicone that resists all oils? No. FVMQ resists many fuels and mineral oils that swell VMQ. It is weaker in some polar solvents, steam, and silicone oils, and some formulated lubricants still swell it. Name the fluid and run ASTM D471.

Can I pour an FSR mold in platinum RTV for a fuel-resistant cavity? Not as a drop-in. FSR is a fluorinated gum, usually processed as millable HCR or specialist fluoro-LSR. Ordinary RTV mold rubber is VMQ chemistry. It will not inherit FSR's fuel resistance.

When should I use FKM instead of FSR? When the seal sees hot hydrocarbons and does not need silicone-class cold flexibility. FKM often swells less in those fuels. Use FSR when cold flex or a fluorosilicone aerospace callout is the reason. Neither is a chocolate-mold upgrade.

Does ASTM D2000 replace a fuel soak on my geometry? No. D2000 is a callout system for heat and oil resistance of the compound. Your gasket still needs immersion and a functional leak test in the service fluid. Your RTV mold still needs a named resin/solvent, not a type/class line.

Why did an FSR O-ring swell in “just engine oil”? Additive packages, ester synthetics, and aromatic content vary. “Engine oil” is not a single test fluid. Soak in the named oil at temperature. If swell exceeds the groove design, change compound family or groove volume.