Vacuum Degas for RTV Molds versus Compression Vacuum

A deep core is an air pocket with a deadline. Once rubber knits across the last path, that pocket has nowhere to go except into the part — or into the cavity face you're pouring.

If you're buying a custom RTV mold, vacuum assist is not a fancier vent. It is two shop moves: degas the mixed platinum until the foam rises and collapses, then, when the master is a blind well or a locked undercut, pour under a bell jar so bubbles leave before gel. HCR compression vacuum on a hot press is a cousin with a different sequence: load preform, bump, pull vacuum, close, cure. LSR injection can vacuum a closed steel tool, then shoot. Do not copy those press recipes onto a mold box, and never specify a LIM hot tool on this PO.

Those three air problems look similar on a cut-up part and are tooled differently.

Why deep cores lose the venting race

Air is displaced by rubber, not absorbed by it. On a shallow gummy tray the pour has an open face and bubbles can rise during degas. On a deep core, boot, bellows convolution, hollow chocolate, or a master with a long mandrel, the geometry closes a trap:

• The rubber front meets around the core before the bottom of the well has emptied.
• A blind pocket at the top of a core compresses remaining air into a bubble that prints as a void or a shiny unfilled patch.
• A pour from the wrong side seals the far end first.

Perimeter vents at a flash land might still work on a thin gasket tool. They do not empty a closed well in a poured jacket. Overflow grooves help only if they connect to the actual trap. Most drawings put overflow where it is easy to cut, not where the air is.

If you ignore the trap location, the defect is a good-looking OD and a porous ID — or a mold pocket with a bubble printed into every dump.

Common mix-up: shop degas versus cavity vacuum versus HCR press vacuum

Shop degas (~−0.095 MPa, minutes) pulls air from mixed RTV in the cup. It does not evacuate a sealed well on the master.

Bell-jar / chamber pour evacuates the mold box while the rubber is still liquid so bubbles leave the pocket.

HCR compression vacuum is a press plus a sealed envelope plus a sequence. LSR closed-tool vacuum is another sequence. Asking a compression shop to “just vacuum it like LIM” is how you evacuate the factory.

What to ask when you're buying

• Is this an RTV cavity with a deep master, or an HCR/LSR part with a deep core?
• Blind well depth, draft, last-fill pocket?
• Cup degas plus chamber pour, or press vacuum sequence?
• Pour path / preform concept so rubber doesn't cap the well?
• Who keeps the master if the first skin prints bubbles and we recast?

Worked example: hollow chocolate sphere, inner core

A 2-cavity platinum 15A glove, inner core for a 40 mm hollow chocolate, jacketed. Open-cup pour capped the core and printed a bubble at the pole. Fix: ring pour around the core, vacuum degas the mix, chamber pour, vents at the pole, keyed split so the core extracts without tearing. An HCR compression boot with bump-and-pull vacuum would have been the right story for a millable part, not for this tray.

RTV practice, then the compression cousin

For poured molds:

• Degas mixed rubber until the foam collapses.
• Pour path that does not re-entrain air (down a stick, from one corner).
• Chamber vacuum when the master is a well.
• Patch-test platinum; inhibition tack holds bubbles at the face.
• Mother mold so a 10A–20A skin doesn't balloon and create new traps.
• Put the vent on the trap, not only on the box rim.

Charge placement is still the first vacuum on an HCR press: a ring charge around a core, not a pancake on top of it. A pump on the press with a leaking shroud is theatre. Watch vacuum *in the cavity*. If rubber can reach the vacuum port, it will, and then you have a cured plug in a hose.

Platinum HCR gives off less cure gas than peroxide HCR; it does not remove the air that was in the cavity. Knit lines in a multi-piece preform are a charge-layout problem vacuum will not weld. A deep core with mirror polish and zero draft vacuum-locks the *part* on the way out — a demold problem. Same for an RTV glove on a barb: peel path or split.

ISO 3302-1 on a porous ID is theatre. Solid rubber at the bottom of the well is the acceptance test.

When this article is the wrong quote

Thousands of deep-core HCR boots belong on a compression press with vacuum. LSR parts with blind pockets belong on a vacuum-capable injection cell with a cold-runner. A tiny shallow tray can skip the bell jar if cup degas and an open face are enough. Geometry can override quantity.

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

What to send for an RTV mold quote

• Object: poured platinum RTV cavity with deep master / core
• Core depth, draft, whether the well is blind
• Pour path (ring, one-side fill); chamber vacuum yes/no
• Jacket, split, recast
• If it is actually HCR compression: say so — bump, vacuum time, close, cure, leak-test of the core
• Cut-up or dump-test criteria for the pocket, not only a pretty OD

Send the master section and where air will hide. We'll degas, chamber-pour, and split the core rather than copy a press vacuum recipe.

We would not recommend skipping chamber pour on a 40 mm blind well

We would not recommend a pancake pour that caps the core. We would not treat a gauge on the pump as proof the pocket is solid. We would not raise Shore A first to “push air out.” We would not copy an LSR closed-tool vacuum recipe onto an RTV box.

RUUIPON Mold Structure Decision: deep hollow that cannot peel? inner core / two-part, plus vacuum pour. Flat back? open-back. Large wet casting? mother mold. Put the vent on the trap.

Typical shop values: RTV vacuum vs press vacuum

| Item | Typical shop value | | --- | --- | | Shop degas | About −0.095 MPa until foam rises and collapses | | Cavity Shore | 10A–20A platinum with jacket | | Mix | 1:1 or 10:1 | | Platinum shrink | ~0.1–0.3% | | Blind well | Chamber pour; vent on the last-fill pocket | | HCR cousin | Load, bump, pull vacuum, close — not an RTV box sequence |

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

FAQ

Can I skip vacuum if I add more overflow grooves around a deep core? Only if the overflow actually connects to the trapped well before the rubber seals. Grooves on the box rim do nothing for air sitting at the bottom of a blind core. Vacuum plus a path on the trap is the usual fix. Extra overflow without a path just makes more flash.

Should vacuum start before the mold touches or after a bump close? That's HCR compression language: pull after seals can hold vacuum and before the land is fully closed. For RTV, degas the cup, then chamber-pour while the rubber is liquid. Do not copy an LSR closed-tool vacuum recipe onto a mold box.

Does platinum need vacuum less than peroxide HCR? Platinum systems give off less cure gas, so they are kinder on porosity from chemistry. They do not remove the air that was in the cavity or the mix. Deep cores still trap that air. Treat vacuum as an air-evacuation tool.

Will vacuum assist stop knit lines in a multi-piece preform? It helps the voids that are air. It will not weld two cold biscuits that never saw pressure and heat together. On RTV, two lifts that gelled separately can also print a seam. Pour as one, or spec a face coat plus backfill.

How do I know the vacuum is reaching the core, not just the shroud? For RTV, cut the first skin or dump a witness casting and look at the pole. For compression, instrument a port that sees the cavity. A gauge on the pump with a leaking box will look fine. Solid rubber at the bottom of the well is the acceptance test.