Fluorosilicone (FSR) Molds: Designing for Extreme Fuel, Oil, and Chemical Resistance

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 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. It is not FKM (fluoroelastomer / “Viton-class”) and it is not a drop-in for every petroleum product on the shelf. Design the fluid, the temperature, and the swell limit. Then pick FVMQ, VMQ, or FKM.

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 (or related fluoroalkyl) 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 and power-steering fluids (grade-specific)
• Diaphragms that must flex in fuel vapor and still seal at cold start
• Specs that already name AMS / MIL fluorosilicone families (the M25988 / AMS-R-25988 conversation in aerospace)

What FSR is not:

• A universal oil-proof rubber. Aromatic content, additives, and biodiesel blends change swell. Test the actual fluid (ASTM D471 immersion) at service temperature.
• A ketone / low-molecular-weight ester / ether champion. Acetone and some polar solvents that FKM or other elastomers handle can still move FVMQ.
• A superheated-steam gasket. Hot water and steam above modest temperatures are a known weak zone versus some other elastomers.
• 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.

VMQ (ordinary methyl silicone) 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.

Swell, hardness, and the drawing

Fuel resistance in the shop is volume swell, hardness change, and whether the part still seals after soak. ASTM D471 is the immersion method. ASTM D2240 tracks hardness. Compression set (ASTM D395) after fuel aging is what a gasket actually is.

Write limits, not adjectives:

• Fluid name and spec (Jet A, a named gasoline blend, a named engine oil), not “oil”
• Temperature and time
• Maximum volume swell and hardness change
• Functional leak or burst after soak if the part is a diaphragm

A 20% swell that a static face gasket can live with will ruin a precision valve lip. FSR’s advantage is slight swell in many fuels compared with VMQ’s large swell. “Slight” is still a number you owe the designer.

Blends of FVMQ with VMQ exist to land intermediate fuel resistance at intermediate cost. They are compounds, not a 50/50 shop mix. If the RFQ allows a blend, lock the swell test. If it does not, do not substitute.

Processing: millable FSR is not LSR and not RTV

Most FSR in the field is millable HCR (high-consistency fluorosilicone), peroxide or addition cured, compression or transfer molded, sometimes extruded. Fluoro-LSR exists in specialist lines for injection of small fuel-system parts. Poured tin or platinum RTV that happens to say “silicone” will not become FSR because you needed oil resistance on a mold.

Design consequences:

• Shrink and flash rules follow HCR / fluoro-LSR, not platinum RTV’s near-zero shrink.
• Bonding to metal often needs a primer system rated for fluorosilicone, not a general silicone primer.
• Color range is narrower. Fuel-system parts are often rust, blue, or black for identification, not a Pantone consumer match.
• Tear and tensile are typically below a good VMQ of the same hardness. Do not design a thin, highly stretched FSR glove and expect VMQ tear.
• Hot-air aging can be harsher on FSR than on VMQ. If the part lives in a hot bay without fuel, VMQ may last longer. FSR is the fuel/oil tool.

Post-cure is normal for peroxide FSR and for many aerospace callouts. Skip it and compression set and extractables move. Follow the spec (AMS, MIL, or the OEM’s bake), not a generic silicone oven tale.

FKM versus FSR is a temperature-and-cold fork

FKM wins many hot fuel and hot oil seals on volume swell and chemical breadth. It loses when the seal must remain flexible in serious cold, where silicone-class Tg and modulus are the point. FSR is the overlap: silicone cold plus fluoro fuel resistance.

Choose:

• VMQ if no hydrocarbon fuel/oil
• FSR if fuel/oil plus cold flex or a named fluorosilicone spec
• FKM if hot hydrocarbon and cold is not the limiter
• Neither if the fluid is a ketone, amine, or strong acid/base the datasheet already flags

Do not “upgrade” a failed VMQ oil seal to FSR without naming the oil. Some gear oils, ester-based synthetics, and additive packages still swell FVMQ. D471 on the service fluid is cheaper than a second tool.

RFQ sentence that keeps chemistry honest

“Elastomer: fluorosilicone (FVMQ / FSR), not VMQ and not FKM unless a dual-path is approved. Process: [millable HCR / fluoro-LSR]. Cure family [peroxide / addition] with post-cure [T / t] per [AMS / MIL / OEM]. Fluid resistance: ASTM D471 in [named fluid] at [T / t], volume swell ≤ [x] %, hardness change per drawing. Compression set after [condition]. Cold function at [T] as [flex / leak test]. Do not substitute methyl silicone, tin-cure RTV, or an unnamed ‘chemical resistant silicone.’”

Refuse:

• “Oil-proof silicone” with no fluid name
• Using FSR as a mold rubber for resin casting
• Treating FSR as food-contact or implant material without that file
• Copying an FKM swell table onto an FSR drawing

Fuel systems also have permeability and explosion-safety rules beyond swell. The rubber grade is necessary, not sufficient.

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.

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, especially with blended fuels.

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.