Magnetic RTV Silicone: Ferrite Loading vs Mixability

Magnetic silicone is polysiloxane loaded with ferrite until the cured part answers a magnet. Pull scales with how much ferrite you actually disperse. Mixability scales the other way.

If you're buying a custom RTV mold, say which object must be magnetic. A ferrite-filled part (fixture gasket, toy, sensor) is one job. An ordinary platinum cavity that casts a magnetic resin is another. Loading the jacket so the mold sticks to a press bed is a third — usually the wrong reason to wreck tear and contaminate food.

RTV cups hit the wall first: dense powder, agglomerates, settling, and a paste that will not wet a master. Two-roll HCR mills take more ferrite because shear wets. LSR magnetic grades are a specialist fill, not a bucket experiment. Do not invent a loading. Write the function and let the compounder pick a ferrite and a process that still mixes.

What “magnetic silicone” is (and is not)

Ferrites used in rubber are ceramic powders: magnetite (Fe3O4) and related iron oxides, hard ferrites (barium or strontium ferrite) for permanent-magnet compounds, and some soft MnZn / NiZn ferrites for a flexible flux path. They are not interchangeable with carbon-black conductive grades, with carbonyl iron for EMI, or with a fridge-magnet sheet that is actually filled thermoplastic.

Unfilled silicone is not magnetic. A pinch of ferrite that tints the rubber grey will not hold a tool to a press. Magnetic moment is a volume-fraction problem. Color goes dark. Density jumps. Elongation and tear drop. None of those are reasons to skip ferrite if the part must locate on a steel fixture. They are reasons not to use magnetic RTV as a general mold rubber.

This is not about magnetizing a steel mold.

Common mix-up: magnetic mold versus magnetic part

A toy RFQ wants a poured RTV mold for a magnetic character. The cavity stays unfilled platinum; the magnet is in the casting or a discrete insert. Ferrite in the jacket prints grit into every pour and can inhibit platinum.

A fixture gasket must snap to a steel table. That's a magnetic HCR or a formulated RTV part, not a chocolate tray.

What to ask when you're buying

• Must the RTV cavity, the casting, or a part respond to a magnet?
• Holding force on named steel at named gap and thickness?
• Hard ferrite vs soft ferrite; alignment field during cure yes/no?
• Process: platinum RTV prototype, magnetic HCR, or magnetic LSR?
• Food-contact forbidden if ferrite dust can reach a culinary mold?

Worked example: 4-cavity toy mold, “make the silicone magnetic”

A 4-cavity platinum 20A glove, jacketed, for urethane figures with a magnet in the base. The mold stayed dry platinum RTV. The magnet was a discrete disc in the casting. A cup of ferrite-loaded RTV settled, the bottom sludge stayed sticky (platinum inhibition), and the top wouldn't hold a paperclip. Production flexible magnets are often milled HCR. LSR magnetic is a parts OEM (cold-runner, never a LIM hot tool).

Ferrite loading versus mixability

More ferrite volume: more remanence or permeability, more weight, more viscosity, worse tear, more abrasion, more settling. Less ferrite: it mixes, it pours, it looks like silicone, and it barely holds a paperclip.

RTV is the least forgiving carrier. Low shear in a cup does not wet a dry ferrite bed. High loading makes a non-pourable paste that still settles.

Practical mixing control without a fake percent: dry-blend or masterbatch into one component only if the supplier says so; wet the powder then vacuum (not a dry dump); use a thixotropic base; do not chase loading past a homogeneous streak on a card.

Ferrite density is several times the gum. In a tall mold the bottom becomes the magnet and the top becomes rubber. Some magnetic parts are anisotropic: a field during cure orients hard-ferrite. Shop RTV will not invent that. Platinum is picky: some iron-oxide and ferrite packages interfere with addition cure. Magnetite as a black pigment does not make every ferrite platinum-safe. Test the actual powder including the heavily loaded bottom of a settled cup. Tin-cure is less catalyst-sensitive and still a poor library tool.

If the part only needs to locate on a fixture, a bonded steel insert or a discrete magnet overmolded in unfilled silicone is often cheaper and stronger than a barely loaded RTV. Magnetic rubber is for when the rubber itself must be the magnet. FDA 21 CFR 177.2600 and ISO 10993 are finished-formulation problems. Ferrite dust on a food mold is a contamination failure.

When this article is the wrong quote

Food and gummy molds stay unfilled platinum RTV. Production flexible magnets belong on a mill. A one-off fixture puck can still be formulated RTV paste if you accept grit and a pull test on top and bottom of the shot. Geometry can override quantity.

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

What to send for an RTV mold quote

• Object: unfilled platinum cavity vs magnetic *part*
• Pull or holding force on named steel at named gap and thickness
• Hard vs soft ferrite; alignment field yes/no
• Homogeneity: density or pull coupons from opposite ends
• Mechanicals on the filled compound
• Ban shop percents and food-mold ferrite

Send whether the mold or the article must stick to steel. We'll keep ferrite out of the jacket unless the rubber itself is the magnet.

We would not recommend dumping unknown ferrite into a platinum 1:1 kit

We would not recommend copying an HCR loading into an RTV cup. We would not claim EMI shielding because the part is magnetic. We would not raise Shore A first to “hold more powder.” We would not put ferrite in a culinary cavity.

RUUIPON Mold Structure Decision: if the cavity is still a mold, split as usual. Flat back? open-back. Cannot peel? two-part / core. Large wet pour? mother mold. Magnetic paste as the mold rubber is usually the wrong object.

Typical shop values: magnetic RTV vs mold rubber

| Item | Typical shop value | | --- | --- | | Default mold | Unfilled platinum RTV, 10A–20A, jacket | | Mix / degas | 1:1 or 10:1; vacuum after powder is wet | | Platinum shrink | ~0.1–0.3% | | RTV vs HCR loading | Cups accept less ferrite than a two-roll mill | | Settling | Dense ferrite falls; check pull at both ends of the shot | | Food molds | Ferrite dust is a contamination failure |

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

FAQ

How much ferrite should I add to platinum RTV? Do not invent a percent. Loading is a compounder's trade between pull and a mix you can still wet and degas. RTV cups accept less ferrite than a two-roll HCR mill. Specify holding force or a coupon flux method on the geometry, then use a formulated grade or a tested let-down.

Why did my magnetic RTV pour cure sticky on the bottom? The sludge layer is the highest ferrite and the most contamination. Some ferrite coatings and dirty iron oxides interfere with platinum addition cure. Test the actual powder in the actual base, including a settled cup. Tin-cure is not an automatic fix; it still shrinks with time.

Can I stop ferrite from settling by mixing longer? Longer mixing wets better and then puts more air in. Once you stop, dense ferrite falls through a thin RTV. Use a thixotropic base, mix then pour without a long rest, and check density or pull at both ends of the part. Vacuum after the powder is wet, not on a dry dump.

Is magnetic LSR just RTV with more powder? No. LSR needs an injectable rheology, meter-mix, and a tool that can take an abrasive, high-density compound. Weld lines can be ferrite-poor. Production flexible magnets are often HCR milled grades. Copying a mill loading into a cup is how you get a paste you cannot pour.

Does a stronger mix mean I can skip magnet orientation in the tool? Not if the drawing needs a pole face. Hard-ferrite compounds get a large share of their useful pull from alignment in a field during cure. A random RTV pour is weaker for the same powder. If orientation is required, that is a fixture and a production process, not a longer stir.