How to Build a Leak-Tight RTV Mold Flask | RUUIPON

A mold box (flask) is the wall that sets how much platinum you pour, how square the block comes out, and where a leak will dump your mix on the bench. It is tooling, not packaging. A warped plywood carton prints as a tapered block. A clay dam that opens at minute six wastes a 1:1 cup you cannot recoup. On a production RTV job the box is part of the quote stack: clearance around the master, pour head, key space, and a leak plan.

RUUIPON builds or reprints flasks in Bao'an / Shajing since 2018. Prototype 3–7 business days after CAD lock. Production MOQ 20 pieces per design. Simple 3D is USD $50 when that CAD scope applies. Mix 1:1 or 10:1 by weight; platinum shrink after a full cure typically about 0.1–0.3%. Steel flasks for LSR/HCR *parts* are a different plant — wydsilicone.com.

Does not apply when the mold is a brush-on glove on a vertical panel with no flask. That sequence lives on pour-on vs brush-on. It also does not apply to a heated steel cavity.

The box sets wall, head, and leak path

Clearance around the master becomes the rubber wall. Tight clearance saves gum and risks a thin island at a logo. Excess clearance burns platinum and still sags if you skipped the jacket. Pour head — the height of rubber above the highest master feature — is how bubbles have room to rise after vacuum. Starve the head and you copy the foam.

Leak paths we actually see:

• Butt joints in foam board with no clay coil. Platinum finds the gap after the mix thins in a warm room.
• Hot glue on dusty Lego. First pour holds. Second pour, the glue peels, and you lose the cup.
• Cardboard. Wicks, swells, prints corrugation into the block back.
• Unsealed wood. Porous wall drinks A/B and can inhibit platinum if the sealer was the wrong chemistry.

Dams: a clay coil at the inside corner, or a gasketed CNC box, or tape plus a fillet on a printed flask. The dam is cheaper than a remake.

Keys can live in the box: bosses glued to the lid of a two-part pour, or pockets in a printed shell. If the box is the jacket, treat it as a mother mold — jacket trigger.

flowchart TD
  A[Master on a board] --> B{Flask leak-tight?}
  B -->|no| C[Clay coil / gasket / reprint the box]
  B -->|yes| D[Set clearance and pour head]
  D --> E{Two-part pour?}
  E -->|yes| F[Keys in lid or jacket]
  E -->|no| G[Open-back tray or block]
  C --> D
  F --> H[Weigh 1:1 or 10:1, vacuum, pour]
  G --> H
  H --> I[Watch the dam for the first minutes of pot life]

Dams belong on a water test before A/B. Fill the empty flask, wait through a typical pot-life window, and mark every dark line on the outside. That stain is where platinum will go. We would rather reprint a gasketed box than chase a leak with more tape while the cup is already mixed. A tooling flask that you will recast next season also needs a lid you can store without crushing the keys — the same library-life problem as the rubber itself.

The photo that would prove a box spec: inside dimension marked in millimetres, clay coil in the corner, master centered, pour-head gauge at the high point. A lifestyle photo of colorful brick walls does not prove the flask was square.

Why the shipping carton is not a flask

Buyers ship us a master in its retail box and write "use this as the mold box." Corrugated fibreboard moves with humidity and leaks at the score. We will not pour production platinum into a carton. We will reprint or mill a flask, and that line can sit next to the USD $50 3D charge when we have to rebuild the outline from photos.

ISO 3302-1 M2 is a class on the rubber article, not a plywood grade.

Shop numbers for flask clearance

Caption: typical shop values, not a named PO.

Flasks we would not pour into

We would not recommend a cardboard carton as a production flask.

We would not recommend hot glue as the only dam on a second pour of the same box.

We would not recommend a clearance so tight the logo island is the wall. Thicken that island or recut the master.

We would not recommend an unsealed sulfur-clay dam against platinum. Inhibition looks like a tacky face against a cured back.

A tooling charge is not unrestricted ownership of a printed flask we designed. Write who keeps the box if you need a recast next season.

Worked example: foam-board flask that opened at minute four

Resin badge, 80 × 50 × 12 mm, buyer built a foam-board box with hot glue, asked us to pour their remaining 1:1 kit on-site photos. Glue opened at minute four; mix on the table. We reprinted a gasketed flask with 8 mm clearance and pour head above the highest serif, vacuumed 1:1 platinum, poured. Prototype 3–7 business days after CAD lock of the flask. $50 3D applied because the badge arrived as photos plus a caliper. Production MOQ 20 of that design.

Mix-up: the box is "free" because it is plywood

The flask sets rubber volume, which is the expensive line. A sloppy box is a hidden gum surcharge. The other mix-up is sending a leaky brick flask and then blaming mix ratio when the cup ran out. Ratio is weight. Volume is the box. See mix-ratio mistakes.

Questions that size the flask

• Inside length × width × pour height, in millimetres?
• Clearance you want around the master, and any logo islands that cannot go thin?
• Two-part lid with keys, or open-back?
• What dam material already failed (glue, tape, cardboard)?
• Who keeps the flask after the tooling charge?

Related: how much silicone, custom mold from photos, contact.

RFQ copy-pack — mold box

Paste into [email protected]:

Object: RTV mold I pour into, not an LSR/HCR part Master size mm, and whether we must build the flask Clearance and pour head if you already spec them Split: open-back / two-part lid with keys Dump material and peak °C 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 flask and the master

Send inside dimensions, not "a box around it." We size clearance and dam — not a blank "free quote." Phone +86 135 3006 2969. Licheng Technology Industry Park, Xinhe Blvd, Shajing, Bao'an. Shop since 2018.

FAQ

Can you pour into the brick box I already built? If it is square, gasketed, and does not leak on a water test, maybe. Most toy-brick flasks leak on the second pour. We would rather reprint.

Why did my box leak after the mix looked thick? Platinum thins as it warms in the cup. A glue dam that held at minute one can open at minute six. Test with water first.

Does a bigger box make a stronger mold? It makes a heavier block. Strength on a wet dump still wants a jacket, not extra gum.

Do I pay the $50 3D fee for a flask? USD $50 when that CAD scope applies — for example when we rebuild the outline from photos so the flask fits. A mold-ready STEP often skips it.

Is the flask included in tooling ownership? A tooling charge is not unrestricted ownership of flask, master, or CAD. Write custody on the PO.