Conductive Fillers for RTV Molds: Carbon vs Silver

Electrically conductive silicone is ordinary polysiloxane loaded until filler particles touch often enough to carry current. That loading is the percolation threshold. Below it, extra powder barely moves volume resistivity. Across it, resistivity can drop orders of magnitude for a small change in loading. That is why these grades are compounded, not stirred into a platinum RTV cup on the bench.

If you're buying a custom RTV mold, say which object must conduct. An insulating platinum cavity that casts a conductive resin is one job. A dissipative mold face that bleeds dust is another. A carbon- or silver-filled part (EMI gasket, grounded pad) is a third — usually platinum LSR or HCR on a parts OEM, not a poured jacket.

Carbon black, silver (and silver-coated particles), and nickel / nickel-coated graphite are the three families buyers actually specify. They are not interchangeable “conductive filler.” Unfilled mold rubber is an insulator. Calling a tray “conductive” without a resistivity method is an adjective, not a spec.

Percolation is a network, not a percentage you invent

Conductive particles form chains. Aspect ratio helps: spheres need more contacts than flakes or high-aspect carbon. Once a path spans the specimen, current flows. Strain can break those contacts. Specify the installed resistance if the part is a seal, not only volume resistivity on a slab.

Typical language on a real drawing:

• Volume resistivity (often ASTM D257 or D991, method named) in Ω·cm
• Or surface resistance / resistivity with electrode geometry
• Or a finished-gasket resistance between flanges at a stated compression

Do not write “carbon filled” and expect 0.01 Ω·cm. Carbon grades in silicone commonly live from dissipative / antistatic bands down into semi-conductive territory. Silver-filled compounds are the ones that reach the very low resistivity used for EMI shielding gaskets. Nickel-graphite sits between.

Filler loading is the compounder's recipe. Do not invent a shop weight percent. Ask for the resistivity target and let the formulator pick the loading that still pours or mills.

Common mix-up: EMI gasket versus a dusty mold face

A buyer wants the mold not to hold lint on a chocolate or electronics-potting tray. That's a dissipative / antistatic RTV conversation (a resistivity band that bleeds charge). It is not a silver EMI O-ring.

A buyer wants a waveguide gasket. That's a compounded conductive part. Stirring a pinch of silver into a 1:1 platinum mold kit will not land a data-sheet EMI compound, and it will likely inhibit cure, settle, and sand the master.

What to ask when you're buying

• Must the RTV cavity bleed charge, or must the article conduct / shield?
• Method: volume resistivity, surface resistance, or a finished-part test?
• Filler family: carbon, nickel-graphite, or silver — and why the housing needs that decade?
• Color constraint (carbon is black; silver is grey/metallic)?
• Master, split, jacket, who keeps the pattern if this is still a poured mold?

Worked example: 4-cavity potting tray for a grounded housing

An electronics line wanted a 4-cavity platinum RTV tray, ~6 mm skin, to pot a small module next to a chromated housing. The email said “conductive silicone mold.” What they needed was an insulating platinum cavity (so the potting doesn't short pins) and a dissipative handling face so the empty tray doesn't collect dust. EMI shielding belonged on the housing gasket — a nickel-graphite part on a different PO.

Dumping carbon into the mold rubber would have marked the potting and wrecked insulation.

Carbon, silver, nickel — keep them in the compound

Carbon black is the cost lever for static bleed and semi-conductive pads. It is black. High loading raises viscosity and Shore A and cuts elongation. Surface carbon can rub off. Wrong EMI gasket when the OEM needs metal-particle conductivity.

Silver and silver-coated particles reach the low resistivity decade used for EMI / RFI enclosure gaskets and some electrodes. Cost tracks the metals market. Soft grades are limited because metal particles do not stretch. Galvanic pairing with the flange metal is a design input.

Nickel-coated graphite is the workhorse EMI gasket filler when silver is too expensive and carbon is not conductive enough.

Mixing a pinch of silver into a carbon grade in a cup is not a hybrid EMI compound. Dispersion and percolation will not land where a data sheet lands.

Every conductive particle displaces elastomer. Expect a higher Shore A floor, lower elongation, abrasive mix, and knit lines that can be resistive even when the plaque is conductive. RTV pours of conductive paste exist for form-in-place gaskets; they are not the same as a molded O-ring, and they are rarely a mold jacket.

When this article is the wrong quote

If you need production EMI gaskets in the thousands, quote HCR or LSR conductive grades on a parts OEM — cold-runner if it's LSR, never a LIM hot tool. If you only needed a chocolate tray that dumps clean, you wanted food-grade platinum RTV, not silver flake. Geometry can override: a one-off dissipative handling fixture can still be poured RTV.

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

What to send for an RTV mold quote

• Object: poured platinum RTV cavity vs a conductive *part*
• If the cavity must be dissipative: surface resistance band and method
• If the article must conduct: filler family, resistivity method, mating alloy
• Master, split, jacket, Shore, who keeps the master
• Casting/potting material — especially whether it must stay insulating
• Recast of the skin: included or extra

Send the pocket STEP, whether the rubber must bleed charge or stay insulating, and the material you'll cast. We'll refuse a cup-blend EMI story and name a real mold stack.

We would not recommend stirring silver into a 1:1 kit

We would not recommend substituting a silver EMI grade to “be safe” on a device-cover mold. We would not raise Shore A first to “hold the carbon.” We would not cut LSR steel while you still need a cavity to pot into. Paying a tooling charge does not mint a resistivity certificate.

RUUIPON Mold Structure Decision: flat back? open-back RTV. Else deep undercuts that cannot peel? two-part / plug / core. Large wet potting or Shore 10A–20A? mother mold.

Typical shop values: conductive vs mold rubber

| Item | Typical shop value | | --- | --- | | Unfilled platinum RTV mold | Electrical insulator; not an EMI gasket | | Dissipative band (dust control) | Specify method (e.g. IEC 61340-2-3 language); not silver EMI | | Platinum mix for the cavity | 1:1 or 10:1; vacuum degas until foam collapses | | Cavity Shore, undercut tools | 10A–20A with jacket; conductive fill raises the floor | | Platinum shrink | ~0.1–0.3% | | Filler families (parts) | Carbon (dissipative/semi-con); Ni-graphite (cost EMI); silver (lowest R) |

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

FAQ

Why did adding more carbon black barely change resistance, then suddenly the part conducted? You were below percolation, then you crossed it. Below the threshold, particles do not form a spanning network. Across it, resistivity can fall several decades for a small loading change. That is why these compounds are formulated, not dosed by eye in a mold-rubber cup.

Is silver-filled silicone always required for EMI gaskets? No. Nickel-coated graphite is the common cost-aware EMI filler. Silver and silver-coated particles are for the lowest resistivity and certain electrode jobs. Carbon black is usually not an EMI gasket filler, and none of these belong in a food-mold jacket by default.

Will a conductive grade pour at the same Shore A as the unfilled twin? Often not. Filler loading raises hardness and viscosity and cuts elongation. Soft conductive parts are limited, especially with metal particles. A 10A–20A peel mold plus conductive fill is usually the wrong stack.

Can I mix silver powder into platinum RTV on the bench? You can make a messy prototype. You will not control percolation, inhibition, settling, or lot resistance. Production conductive parts belong in compounded LSR or HCR. Production molds that only need to dump resin stay insulating platinum RTV.

Why is the plaque conductive but the molded gasket measures high resistance? Knit lines, particle orientation, incomplete compression, and coating wear at the contact face all break the network. Measure the finished part at the installed compression against the real flange alloy. A mold-rubber plaque is not that gasket.