Cryogenic RTV Molds: Tg vs Crystallization at -100 C

Minus 100 °C is possible for some silicone elastomers. It is not a TDS “low temperature” line that was measured at −55 °C. Polydimethylsiloxane (PDMS) has a glass transition typically around −120 °C, which sounds like a free pass. In practice, crystallization of regular dimethyl sequences stiffens many VMQ grades in the −40 to −80 °C band, long before Tg. The mold that flexed in a freezer is a board in LN2 vapor.

If you're buying a custom RTV mold, most chocolate, gummy, and resin jobs never see −100 °C. The honest split: a cavity that must still peel in a cold-cast box versus a part (bladder, O-ring, bellows) in a cryostat. Phenyl-containing silicone (PVMQ-class) exists to disrupt crystallinity. Too much phenyl raises Tg and can steal the cold advantage. Then you flex the real article at −100 °C, not a plaque at room temperature after a dunk.

This is not an invitation to treat every 40A RTV mold as cryogenic. Production cryogenic seals are often LSR/HCR on a parts OEM (cold-runner, never a LIM hot tool).

Two transitions, one failure

Glass transition (Tg). Chains freeze into a glass. Below Tg the elastomer is a hard plastic. PDMS Tg is very low compared with organic rubbers.

Crystallization / melting. Regular –Me2SiO– sequences pack. DSC shows a melt endotherm in the neighborhood of roughly −40 °C for unfilled PDMS gums in many studies; isothermal crystallization can be fast at still colder holds. Once crystallites form, modulus jumps. Time at temperature matters.

A drawing that says “flexible to −100 °C” without a hold time is incomplete. Write the soak, the flex or leak, and whether the part is allowed to warm between cycles.

Fluorosilicone (FVMQ) keeps silicone-class cold better than FKM, but it is not automatically the −100 °C champion versus a phenyl VMQ, and fuel resistance is a different spec.

Common mix-up: freezer mold versus cryostat seal

A shop stores RTV trays in a walk-in and calls them “cryogenic.” That's storage (see Shore A drift), not −100 °C service.

A lab wants a platinum glove that still releases a cold-cast wax at −80 °C after a one-hour hold. That's a low-temp / phenyl RTV conversation plus a jacket, because a crystallized 10A skin will tear at the first peel.

What to ask when you're buying

• Is the silicone the cavity in a cold box, or a part in a cryostat?
• Soak temperature, hold time, flex/leak/actuation while cold?
• Phenyl / PVMQ-class grade vs general VMQ “−55 °C” TDS line?
• Jacket and split — a frozen glove still needs a peel path?
• Who keeps the master if the first skin tears cold and we recast?

Worked example: cold-cast wax glove, −80 °C hold

A 1-cavity platinum glove, 6 mm skin, 20A, jacketed, to cast a wax pattern after a 60 min soak at −80 °C. General platinum RTV felt board-like after the hold; peel tore at a letter undercut. A crystallization-suppressed (phenyl) platinum RTV of similar Shore, same jacket, same keyed split on the letter, peeled. Room-temperature Shore A was a weak predictor. ISO 3302-1 on the wax was still “looks like the master,” not M1 at −80 °C.

Phenyl grades, tests, and molds versus parts

A few percent of phenyl is a known way to kill PDMS crystallization so the rubbery plateau extends down toward Tg. Higher phenyl raises Tg. There is a window, not a monotone “more is colder.”

Practical buying:

• Standard VMQ RTV for ordinary molds and −50 to −60 °C class if crystallization at the real hold is acceptable
• Low-temperature / phenyl VMQ when holds live in the crystallization band or you need −70 to −100 °C class flexibility
• Confirm with DSC plus a Gehman or TR-10 / ASTM D1329-style retraction, or a part-level cold flex
• Fillers raise modulus. A high-tear filled grade can feel “frozen” earlier than a gum-rich low-temp grade of the same Shore A at 23 °C.

Failure modes at −100 °C: seal leak, brittle fracture at a nick, crystallization after a long hold, differential contraction versus metal (silicone CTE is large), LN2 splash as thermal shock.

If the silicone is the mold, crystallization will change cavity size and release. Most “cold” resin shops are not −100 °C. If the silicone is a bladder in a cryostat, gland design matters as much as gum. Silicone is permeable; cryogenic helium service may want a different barrier. Avoid oil-bleed grades and high mineral fill unless you re-measured cold flex. Tin-cure RTV is a poor archive and a poor scientific-service rubber.

When this article is the wrong quote

Chocolate and gummy trays don't need phenyl cryogenic grades. Frozen-dessert molds that see a blast freezer may, if peel fails after a hold — still an RTV job. Thousands of cryostat O-rings belong on a parts OEM. Geometry can override quantity.

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

What to send for an RTV mold quote

• Object: cold-cast RTV cavity vs cryogenic *part*
• Soak [t] at [T], then flex/peel/leak in the cold state, [n] cycles
• Grade family: phenyl / PVMQ-class or named low-temp VMQ
• Jacket, split, Shore; tin-cure excluded
• Room-temperature Shore A is not the acceptance test

Send the cold box temperature, hold, and whether you need a cavity or a seal. We'll refuse a −55 °C TDS line standing in for −100 °C.

We would not recommend every 40A RTV mold as cryogenic

We would not recommend more phenyl as automatically colder. We would not qualify with a room-temperature tug after a nitrogen dunk. We would not raise Shore A first to “feel stiffer in the cold.” We would not pay FSR prices unless the fluid needs fluoro side groups.

RUUIPON Mold Structure Decision: cold peel is still peel. Flat back? open-back. Letter/undercut that cannot peel when stiff? two-part / core — even more so at −80 °C. Soft skin? mother mold.

Typical shop values: cold RTV vs catalog “low temp”

| Item | Typical shop value | | --- | --- | | Ordinary RTV mold | Platinum VMQ, 10A–20A, jacket; not a −100 °C rating | | Mix / shrink | 1:1 or 10:1; platinum ~0.1–0.3% | | PDMS Tg vs crystallization | Tg often ~−120 °C; crystallites stiffen ~−40 to −80 °C | | Catalog “low temp” | Often TR-10 / brittle nearer −50 to −70 °C | | Acceptance | Soak then flex/peel while cold | | ISO 3302-1 | M2 common at room T; cold size is a different check |

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

FAQ

If PDMS Tg is about −120 °C, why won't every silicone work at −100 °C? Because many dimethyl grades crystallize well above Tg. Crystallites stiffen the rubber in the −40 to −80 °C region, especially after a hold. Phenyl copolymers are used to suppress that packing.

Is −100 °C a standard rating I can pick from a catalog? No. Catalog “low temperature” is often TR-10 or a brittle point nearer to −50 to −70 °C. −100 °C is a program: compound choice plus a soak-and-flex of the article.

Does more phenyl always make the rubber colder? No. A little phenyl can erase crystallization and extend the rubbery range. A lot of phenyl raises Tg. There is a window, not a monotone “more is colder.”

Can I qualify with a room-temperature tensile after a nitrogen dunk? That misses isothermal crystallization and the actual cold modulus. Flex, peel, leak, or actuate at temperature after the service soak. DSC on the gum supports the compound choice; it does not replace the part or mold test.

Is fluorosilicone automatically better at cryogenic temperatures? FVMQ is chosen for fuel plus cold, versus FKM. For a dry −100 °C flex, a phenyl-containing methyl silicone is often the more direct conversation. Do not pay for fluoro side groups unless the fluid needs them, and do not pour FSR as a chocolate mold.