RTV Mold Box vs Open Pour: When the Containment Box Actually Matters

If you're pouring platinum RTV over a master on the bench, the mold box is not packaging. It sets pour head, keeps the block square, and stops the A/B mix from running onto the table. An open pour is the right call on a small, flat-backed master. It is the wrong call the moment you need vacuum, a vertical wall, or a two-part split that has to close on keys.

This is an RTV decision: master, box, A/B mix, vacuum or bench pour, optional jacket. Steel LSR/HCR tools do not use a wooden containment box. A flood-and-scrape gummy tray is an open-back tool on purpose. Do not copy this article onto a parts OEM RFQ.

What a mold box actually is

A mold box is four walls (and often a base) around the master so the liquid silicone has somewhere to sit until it gels. Clay dams, Lego, melamine, 3D-printed walls, and a sealed vacuum-chamber tub are all boxes. The job is the same: contain the pour, set the wall thickness, and give you a square block you can split or jacket later.

The box is not the jacket. The box is there for the pour. The jacket (mother mold) is the rigid shell that holds a soft 10A–20A skin in service. You can need both. You can need neither. Mixing them on the quote is how you pay for a support shell when four strips of melamine would have done it.

RUUIPON has been pouring custom RTV in Shenzhen / Bao'an since 2018. MOQ is 20 pieces per design. A simple 3D model or design fee is USD $50 when that scope actually applies. Prototype is commonly 3–7 business days after CAD lock — that clock starts when the master and the box plan are agreed, not when the email arrives.

When an open pour is honest

Open pour means you coat or dam the master with almost no walls and let the silicone find its own edge. It works when:

• The master is small and sits stable (hand-size or less, flat back).
• You want a glove / brush-on skin, not a block.
• You are not going into a vacuum chamber (open pour plus vacuum is a mess unless the whole setup is already in a tub).
• Wall thickness can vary; you are not selling a calibrated block.

If you ignore this: the mix runs off the high side, the low side is starved, and you recast because you tried to save a box.

When the box is the job

Build a box when any of these are true:

• You need a known wall thickness around the master (block mold, not a glove).
• The pour will be vacuum degassed as a filled box, or pressure-cured in a pot — the tub is the box.
• The master has a flange, a tall wall, or a split that must come out as a square block for keying. See keying two-part molds.
• You will cut the block on a parting line. A shapeless puddle does not split clean.

Seal the box. RTV finds every gap. Clay the base of the master so it does not float. Leave pour head above the highest cavity — typically 8–15 mm of extra height on a shop block, not a named lot. Too little head and the last corner is short. Too much and you waste compound and make a brick that is hard to split.

Mix platinum 1:1 or 10:1 as the datasheet says. Vacuum degas the mix, then pour in a thin stream down a wall, not onto the master face. That is box work. It is not an open puddle.

Common mix-up: paying for a jacket when you needed four walls

Buyers send a photo of a resin master and get quoted a full mother-mold jacket because the shop assumed 10A skin in production. If you are making one block to dump epoxy twenty times a month, four walls and a 20A–30A block are enough. A jacket is for a soft skin that will not stand up by itself, or for a production dump that must sit in a rigid shell.

The other mix-up is skipping the box, flashing the bench, then asking why the silicone “won't fill.” The compound filled. It filled the table.

What to ask when you're buying

• Open pour, 4-wall box, or jacket — which one is on the quote, in those words?
• Target wall thickness and pour head above the master?
• Vacuum or pressure after the pour? Then the box has to fit the chamber.
• One-piece block or two-part split, and are keys in the rubber or the jacket?
• Who keeps the master and the box if you recast the skin?

Worked example: 180 mm resin plaque

A 180 mm decorative resin plaque, 12 mm flange, platinum RTV, about 40 dumps a month. Open pour would have run off the flange. We boxed it: 10 mm nominal walls, about 10 mm pour head, mix 1:1, vacuum degas, Shore ~20A block. No jacket. We would not have sold a mother mold for that cadence. A parallel LSR steel quote is a different parts RFQ.

When this article is the wrong quote

Steel LSR/HCR tools, cold runners, and leader pins belong on a parts OEM RFQ, not here. A tiny coin-size open-back tray can skip the box. A flood-and-scrape gummy sheet is supposed to be open on top. If the skin is 10A and you dump daily, you want a jacket, not a taller box.

Box lines to copy onto the RFQ

Copy this. If the quote is silent on box vs open vs jacket, the first pour writes that section in scrap.

• RTV platinum or tin; mix 1:1 or 10:1; vacuum yes/no
• Open pour / 4-wall box / jacket — pick one primary, note if you need both box and jacket
• Master size, flange, tallest wall; target wall thickness; pour head
• Chamber envelope if the filled box must go in vacuum or pressure
• One-piece or two-part; keys in rubber or jacket
• Recast of the skin; who keeps master and box

Send master photos or STEP, dump material, and monthly dumps. We will mark box vs open vs jacket and whether a simple 3D/$50 design fee even applies.

We would not recommend skipping the box and blaming the silicone

We would not recommend a jacket as the default add-on on a 20A block that already stands. We would not open-pour a flanged plaque into vacuum. Raising Shore A is the wrong fix for a run-out; add walls. We would not buy LSR leader pins “so we can scale later” while you still need a cavity to dump into.

RUUIPON Mold Structure Decision: small flat master, no vacuum, glove OK? open pour. Need a square block, known walls, or vacuum? 4-wall box. Soft 10A–20A skin in daily dumps? jacket. LSR/HCR steel? stop — different RFQ.

Typical shop values

Caption these as typical shop values, not a named lot.

Item	Typical shop value
Cavity Shore for a stand-alone block	~20A–30A platinum RTV
Cavity Shore if a jacket will hold it	10A–20A skin
Mix	Platinum 1:1 or 10:1; vacuum degas
Linear shrink	~0.1–0.3% (platinum)
Nominal box wall	~8–15 mm around the master
Pour head above highest point	~8–15 mm
MOQ	20 pieces per design
Prototype	3–7 business days after CAD lock

FAQ

Is a mold box the same as a jacket?

No. The box contains the pour so you get a block. The jacket is the rigid mother mold that holds a soft skin in service. You can need a box to make the skin, then a jacket to use it. Asking for “a box” and getting a full support shell is a quoting error, not a upgrade.

Can I skip the box if I vacuum degas the cup only?

You can degas the mix in the cup and still open-pour a small master. If the filled mold itself goes in the chamber, it needs walls. An open puddle in vacuum boils over the edge.

How tall should the walls be?

Tall enough for the master plus the wall thickness you want plus pour head. Typical shop blocks leave about 8–15 mm of head. Extra height wastes compound and makes a brick that is harder to split. Too little head starves the last corner.

Does tin-cure vs platinum change the box?

Not the walls. Hardness and whether you vacuum do. Tin vs platinum changes shrink and inhibition risk on the master; it does not replace four sealed walls.

Why did my quote include leader pins and a cold-runner?

That is a steel LSR or HCR part tool. Ask the supplier to requote a custom RTV mold if what you need is a cavity to cast resin, wax, soap, or chocolate. Box vs open pour is that quote.