RTV Vacuum Chamber: Headspace vs Shot Size | RUUIPON

Size the vacuum chamber to the mixed shot, not to the pump listing. Platinum RTV 1:1 or 10:1 by weight will climb into a foam under vacuum; if the cup plus that foam cannot fit under the lid with spare headspace, the mix hits the lid, rains back as bubbles, and you pour a cratered cavity. A 2 L Amazon pot can degas a 200 g cup and still overflow a 1.5 kg block mix. A tall industrial chamber with a tired gasket can starve a 400 g food tray because the foam never collapses. Chamber ID, cup fill, and unused height above the mix are the three numbers. Pump CFM is fourth.

RUUIPON vacuum-degases platinum in Bao'an, Xinhe Blvd, Shajing, since 2018. Prototype is 3–7 business days after CAD lock. Production is MOQ 20 pieces per design. Simple 3D is USD $50 when that CAD scope applies. Platinum shrink after a full cure is typically about 0.1–0.3%. Steel/LSR *parts* from a heated cavity are a different plant: wydsilicone.com. How long you hold once the foam falls is a cousin page: how long to vacuum degas.

Does not apply when you are pressure-potting *resin into a finished mold*, when the craters are inhibition (tacky face, cured back), or when the mix is already climbing gel and a bigger pot will not save it. A hobby skip-vacuum video is not a food-logo path. LSR injection with a cold-runner is not this shop.

Three nested vessels, one overflow

Think of three nested vessels: the mixed cup, the vacuum pot, and the unused air above the cup rim.

The cup holds A/B plus the air you whipped in. Fill it more than about half and the foam has nowhere to go except over the rim. That overflow is wasted rubber and a starved flask, not a "stronger vacuum."

The pot has to swallow the cup *and* the rising dome. A short, wide chamber with a 200 mm ID can still fail a tall 1 kg cup if the lid sits 40 mm above the rim. A tall, narrow chamber can fail a wide mixing bucket that wedges against the wall and never sees even pull.

Headspace is the unused height from mix surface to lid. Foam often triples the apparent volume on a freshly whipped 1:1 before it collapses. We would not mix a production food tray in a cup that already sits 20 mm from the lid.

The photo that would prove a chamber spec: cup on the platen with a millimetre stick showing unused height, gasket face clean, gauge at the pull you actually hold — not a lifestyle shot of a shiny pot on a workbench.

flowchart TD
  A[Weigh 1:1 or 10:1] --> B[Cup fill under half]
  B --> C{Unused height to lid?}
  C -->|too little| D[Split the shot or use a taller pot]
  C -->|enough| E[Pull until foam rises]
  E --> F{Foam hits the lid?}
  F -->|yes| D
  F -->|no| G[Wait for collapse, short hold]
  G --> H[Break vacuum slowly, pour]
  D --> B

Foam rise eats litres you thought you bought

Listings sell internal litres. Foam does not care. A cold winter mix is thicker and climbs slower; a 28 °C Hong Kong bench mix climbs faster and overflows sooner. Viscosity, whip, and remaining pot life change the dome more than a 5 CFM vs 7 CFM pump.

If the foam never rises, the gasket leaks, the pump is weak, or the grade is already thick from a cold room. A bigger chamber does not fix a leaking lid. If the foam rises and never falls inside pot life, you overfilled, over-whipped, or started too late. Stop. Do not pour a still-foaming cup onto a logo.

Split large block pours: degas cup A, pour a first layer, degas cup B. Layer pour for heat is a different article; here the split is only so each cup fits the pot you own. We would not recommend buying a 20 L chamber to save a 3 kg mix you will not finish inside pot life anyway.

Cup, bucket, and flask are not interchangeable

Buyers send "we have a 5 gallon vacuum chamber" and a photo of a paint bucket sitting in a thin-wall pot. The bucket walls collapse toward the mix under pull, the surface never breaks, and they blame platinum. Rigid mixing cups, a platen that stays flat, and a gasket you can see around the whole lid matter more than the gallon number.

The flask is not the chamber. Degassing *in the box* around a master is a different, slower pull and we would not do it as the only degas on a food serif. Degas the cup first. Pour in a thin stream onto a detail coat. If you then pull the whole flask, that is a second, optional step with even more headspace demand because the box is the "cup."

ISO 3302-1 M2 is a rubber drawing class on the dump, not a vacuum-pot grade. CAD lock still freezes millimetres on the cavity, not litres on the Amazon cart.

When a bigger chamber still fails

Symptom: pinholes on the cavity face after a "new bigger pot."

Causes we actually see: cup still filled past half; lid still 30 mm from the rim; gasket nicked; mix started after a long weigh-out so pot life was already half gone; detail coat skipped so the serif never wetted.

Confirm: watch the dome. If it kisses the lid, the chamber is too small *for that shot*, even if the listing says 8 L. If it never climbs, the pump/gasket is the problem, not size.

Fix: split the shot, drop the cup fill, replace the gasket, or recast. Do not torch the face and call it degassed.

A tooling charge on the mold is not ownership of our chamber protocol. You still send the dump chemistry so we size the shot.

Typical shop values

Caption: typical shop values, not a named PO.

Chamber sizes we would not recommend

We would not recommend a 2 L kit-pot for a 1 kg block pour.

We would not recommend filling the mixing cup to the rim "to use the whole kit."

We would not recommend skipping vacuum on a food logo because the new chamber "looks big enough."

We would not recommend degassing already-gelling rubber in a larger pot as a save.

We would not recommend a paint-bucket liner that can collapse under pull.

Worked example: 1.2 kg block, 5 L pot, lid kiss

Craft figurine, two-part block, about 1.2 kg platinum 1:1. Buyer degassed in a 5 L chamber with a 2 L cup filled to 80%. Foam hit the lid at minute two; they poured anyway. Face was a crater field on the show side. We split into two 600 g cups, each under half fill in the same 5 L pot, collapsed, poured as two layers with a wet bond window. Prototype 3–7 business days after CAD lock of the split and flask. $50 CAD applied because the master arrived as photos plus a caliper. Production MOQ 20 of that design. The 5 L pot stayed; the shot size changed.

Mix-up: bigger pump equals bigger chamber

CFM and microns are not headspace. A strong pump on a short pot still slaps foam into the lid. The other mix-up is treating chamber size as a substitute for a detail coat. Vacuum does not wet a serif; a brush coat does — pour-on vs brush-on.

What to ask when you're buying a chamber — or a mold that needs one

• Mixed shot mass, and the cup OD × height you will actually use?
• Unused height from mix surface to lid, in millimetres?
• Food logo cavity, or a craft block that can tolerate a few face bubbles?
• One cup or a split layer pour because the pot is what it is?
• After CAD lock, 3–7 business days, then MOQ 20 / design?

Related: vacuum-degassed platinum trays, contact.

RFQ copy-pack — chamber vs shot

Paste into [email protected]:

Object: RTV mold (not an LSR/HCR part) Dump chemistry and whether the face is a food logo Shot estimate: master L×W×H mm, flask clearance, or your gram target Do you degas in-house? Chamber ID and unused height if yes Structure: open-back / two-part / glove + jacket File: STEP / STL / photos with a millimetre scale CAD: mold-ready STEP OR USD $50 3D if that scope applies Prototype 3–7 business days after CAD lock, then MOQ 20 per design Who keeps the master

Send the cup height and the dump, not "do you vacuum." We size the shot to a pot that will collapse — not a blank quote request. Phone +86 135 3006 2969. Licheng Technology Industry Park, Xinhe Blvd, Shajing, Bao'an. Shop since 2018.

FAQ

How big a vacuum chamber do I need for RTV silicone? Big enough that the mixed cup sits under half full and the foam can triple without hitting the lid. Litres on the listing are not that number.

Can I degas a 2 kg pour in a 5 L pot? Usually not in one cup. Split the shot so each cup has headspace, or use a taller chamber. Overflow is wasted rubber, not a complete degas.

Why did a bigger chamber still leave pinholes? Fill was still too high, the gasket leaked, pot life was already short, or the serif never got a detail coat. Watch the dome; do not trust the litre sticker.

Should I vacuum the whole mold box instead of the cup? Degas the cup first. A whole-flask pull is optional extra and needs even more headspace. We would not skip the cup pull on a food logo.

Does chamber size change your 3–7 day prototype clock? No. The clock starts at CAD lock. A shop without a pot that fits the shot is not locked. MOQ 20 per design after the witness.