Rib and Boss Design to Prevent Silicone Sink Marks

If you're pouring a custom RTV silicone mold, sink in the *casting* is usually the cavity that moved, not thermoplastic-style shrink opposite a rib.

LSR marketing likes to say sink is almost nonexistent. That is a molded-*part* conversation. On a flexible RTV tool the same word means the opposite physics: a boss on the master becomes a pit in the mold; if the 10A–20A wall balloons or traps air, the dump prints a fat boss or a dull pit. Vacuum, a last-fill vent, and a rigid jacket fix those. Rib ratios copied from ABS CAD do not.

Keep ~the same DFM honesty you would use on a part tool — even walls, filleted roots, no surprise lumps — but apply it to the master and the jacket, not to heat-cured LSR mass.

Where the dent actually comes from

RTV cavity (this site). A boss in the master becomes a pit in the mold and a boss on the dump. “Sink” on the dump is often air in the boss, a soft mold wall that ballooned, or edge sag of 0.5–1.5 mm that makes the outer row 3–6% heavy. Vacuum degas of the A/B mix, a last-fill vent when you pour the mold and again when you cast, and a mother mold fix those.

Heat-cured LSR in metal (wrong RFQ). The rubber expands in a hot cavity, then shrinks ~2–3% as it cools. Thick sections can dish a show face opposite a rib. That is mass and shrink of a *part*. Don't import those rib rules into a glove mold, and do not use an RTV pour of a 1.5 mm wall to predict LSR sink on a 6 mm pad.

HCR compression (wrong RFQ). Charge knit and millable shrink (~1.5–2.5%) show as flats and knit shadows. Different quote.

If you ignore the process, you will add thermoplastic-style rib ratios to an RTV glove that did not need them, and you will skip vacuum on a tall boss that did.

Common mix-up: thickening the CAD boss to “fix sink”

A confectioner sees a dimple in the chocolate opposite a decorative rib and asks the shop to fatten the rib on the master. The next pour copies a bigger bump. The real cause was a jacket that sagged and an unvented rib intersection that trapped air. Photograph the closed empty mold with a straightedge across the back. If the sheet dishes, you do not have a rib problem. You have a missing mother mold.

ISO 3302-1 might class the *cast article*. It does not make a ballooned cavity “in spec,” and it is not another word for precision.

Ribs on the master: fillet, vent, support

A working RTV rule that matches what mold shops already pour:

• Rib thickness on the master can stay near the visual you want. The mold rubber will copy it. Don't thin the rib to 0.5× “because ABS” if the chocolate needs that section.
• Height: keep ribs from becoming walls the dump cannot fill, and from becoming grooves in the RTV that rip on demold. A 10 mm-tall knife rib in a 10A skin is a tear, not a decoration.
• Fillet the rib root on the master. A sharp inside corner in the master is a sharp groove that rips when you peel the dump. Inside fillets on the order of ~0.25–0.4× the local rib, or at least a visible break, are the usual ask.
• Draft the ribs on the master so the dump and the mold both release. 0.5–1° on untextured food ribs is cheap. More with grain.

If you ignore the fillet, the defect is a split in the RTV at the rib root after a few dozen pulls, blamed on “cheap rubber.”

Bosses: vent them like they had air, not like they had a screw

A tall boss on a master is an air pocket waiting to happen when you pour the mold, and again when you dump the food or resin.

• Vent the last fill of the boss when you pour the RTV, and again when you cast. Tilt, vacuum, or a pin vent. That is air, not shrink.
• Core out decorative lumps on the master if the dump does not need the mass — not to prevent LSR sink, but so the mold pour fills and the jacket can back the thin remaining wall.
• Keep the floor under a cosmetic face even. A 2 mm RTV floor next to an 8 mm unsupported pit is a balloon invitation.
• Inside radius at the boss root so the pour does not trap gas in a square pocket.

If you ignore this: a pale, dull patch at a rib intersection that never vented. Designers call it sink. The moldmaker calls it a dead end. Do not add more rubber to “pack it out.” Extra pack on a glove is how you balloon the jacket, not how you fill a blind rib.

Cosmetic faces vs structural faces

If the camera sees the face opposite the rib, you are in discipline: even jacket support, maybe a texture that hides a 0.05 mm dish. If the face is the back of a gummy tray, a fat rib is fine.

Transparent resin casts show everything. A dish you would ignore in opaque chocolate will look like a lens defect in clear PU.

Flexible RTV molds of a cosmetic master: polish the master, support the mold. Do not “fix sink” by thickening the CAD boss. You will copy a bigger bump.

What to ask when you're buying

• Process named: platinum or tin RTV cavity, not “silicone tooling”?
• Which faces are cosmetic, and is the jacket rigid behind them?
• Last-fill vents on tall bosses, on the mold pour and on the dump?
• Rib roots filleted on the master, or leftover CAD knives?
• Water-volume or fill-weight map on first article, every pocket, not the hero?

Worked example: 8-cavity bar with a decorative spine

An 8-cavity bar, 120 × 60 × 12 mm, food-contact, about 500 pours a month, a 4 mm decorative spine down the show face. Platinum 10A–20A skin, mix 1:1, vacuum degas, rigid jacket. We filleted the spine root on the CNC master, vented the last fill of the mold pour, and weighed every pocket. Outer-row bars that ran 3–6% heavy were sag, not “sink from the spine.” A parallel HCR rib-ratio quote for a silicone bumper does not dump chocolate.

When this article is the wrong quote

If you actually need vulcanized silicone parts with cosmetic faces in the thousands, rib ratios and cored bosses belong on an HCR or LSR DFM pack — different quote, different shrink. A tiny, shallow one-piece tray can skip the jacket. Geometry can override quantity: one 40 mm hollow boss still needs a vent and a core path at 80 dumps.

Related reading: custom RTV molds, DFM checklist, radii for longevity, gummy trays.

Master and jacket lines to copy onto the RFQ

Copy this. A rib lecture without these lines is not a mold quote.

• Process: platinum or tin RTV (ban HCR/LSR substitution)
• Cosmetic faces; jacket material and how the sheet stays flat
• Bosses: last-fill vents on mold pour and on dump
• Rib roots filleted on the master; draft on ribs
• First article: weigh every pocket; straightedge on the jacket back
• Recast of the RTV skin if a rib root tears

Send the STEP, which face is the camera face, and the dump material. We will mark vents, jacket backing, and whether thickening a boss would only copy a bigger bump.

We would not recommend copying ABS rib ratios onto a 10A glove

We would not recommend raising Shore A first to “stop sink.” Add the jacket. We would not thicken a CAD boss to hide a balloon. We would not use an RTV pour of a 1.5 mm wall as proof of LSR sink on a 6 mm pad. We would not skip vacuum on a tall boss and then blame the platinum.

RUUIPON Mold Structure Decision: tall boss or wet dump? vent + mother mold. Flat back, shallow ribs? open-back. Deep undercuts that will not peel? split or core — extra rib thickness does not remove a lock.

flowchart TD
  A[Dent on the dump?] --> B{Jacket flat on a straightedge?}
  B -->|no| C[Mother mold / support first]
  B -->|yes| D{Air at last fill of the boss?}
  D -->|yes| E[Vent + vacuum; do not fatten CAD]
  D -->|no| F{Rib root sharp in the master?}
  F -->|yes| G[Fillet; watch for RTV tear]
  F -->|no| H[Check pour head and mix]
  A --> I{Need LSR/HCR parts?}
  I -->|yes| J[Stop: different part RFQ]

Typical shop values

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

| Item | Typical shop value | | --- | --- | | Cavity Shore | 10A–20A platinum RTV, jacket behind it | | Mix | Platinum 1:1 or 10:1; vacuum degas until the foam collapses | | Linear shrink | ~0.1–0.3% (platinum); tin keeps moving | | Loaded-tray edge sag | About 0.5–1.5 mm without a jacket | | Outer-row vs center if tray sags | Edge often 3–6% heavy | | 8-cavity 120 × 60 × 12 mm bar | About 500 pours/month is a common first band |

FAQ

Does RTV “sink” the way LSR sinks opposite a thick rib? No. Heat-cured LSR can dish a show face because of mass and ~2–3% shrink. On a poured RTV part, the dent is often air in the boss or a soft mold wall that ballooned. Vacuum and a mother mold fix those. Thickening a CAD boss on a flexible tool copies a bigger bump.

Why is a pale dimple at a rib intersection often not shrink? That patch is often a gas trap at a dead end. Vent the last fill or rotate the rib so it feeds toward a pour vent. Extra pack on a glove makes a balloon; it does not fill a blind rib.

Should I copy thermoplastic rib ratios into an RTV glove? No. Keep even walls if the dump needs them, fillet roots so the mold does not tear, and support the sheet. Do not thin a chocolate spine to 0.5× because ABS CAD said so.

Will an RTV pour of a 1.5 mm wall predict LSR sink on a 6 mm pad? No. Prototype in the process you will ship if the show face is the risk. That LSR job is a different quote.

How do I confirm the jacket, not the rib, caused the dish? Straightedge on the back of the loaded tray, and a fill-weight map of every pocket. Edge sag of about 0.5–1.5 mm with outer-row 3–6% heavy is a missing mother mold, not a spine that needed more rubber.