RTV Undercuts: Peel, Split, Core — Not a Robot Wrist

A technician can roll a 20 Shore A boot off a barb. A robot cannot “just peel it.”

If you're buying a custom RTV mold, undercuts are a cavity problem: can the platinum skin peel off the master, and can the casting peel out of the pocket, without tearing a logo or a key? High-volume LSR lives or dies on a repeatable exit from hot steel, usually with a valve-gated cold-runner — never a LIM hot tool. Those are different drawings. HCR compression with an operator pulling parts is a third rule book. Do not mix them on the same PO.

The rubber's elongation is real. It is not a license to leave a geometry that only a human wrist can solve. Design the undercut as an ejection — or a dump — specification: direction, stretch, and what happens on the shot or pour that is stickier than the mean.

Name the undercut by how it must leave

Not all undercuts are equal:

• Local lip / sealing barb that must stretch over a core. Something has to balloon it off (air, stripper, or a peel).
• Side hole or window that needs a lifter, a side core, a plug, or a split because the rubber cannot stretch that far without tearing a thin wall.
• Internal groove on a bore that a collapsible core, a split core, or a removable RTV plug was born for.
• Cosmetic wrap that a human peels and a vacuum cup will fold into a crease — and that a gummy dump will tear if you insisted on a one-piece glove.

Write the strip vector on the drawing. If the honest demold is a rolling peel around an axis that is not the open of a press — or not the dump direction of a tray — you need a tool action, a keyed split, or a gripper/operator plan.

If you ignore the vector, the defect is a torn ear, a core that never sheds, or an operator standing next to a “lights-out” cell.

Common mix-up: robot cell versus gummy dump

A 16-cavity LSR boot cell needs stripper rings, timed core air, and a pad the gripper can hit. A 12-cavity gummy bear with locked ears needs a high-tear platinum 10A–20A skin, a jacket, and usually a keyed two-piece if the ears cannot peel. Raising Shore A so “the robot can grip” wrecks the dump. Asking an RTV shop for robotic ejection hardware is the wrong factory.

What to ask when you're buying

• Is the undercut on the master (mold making), on the casting dump, or on an LSR part?
• Peel allowed, or must we split / plug / core?
• Shore A, tear grade, jacket?
• If LSR: gripper faces, strip direction, tool actions you will pay for?
• Who keeps the master if we recast after a tear at the logo?

Worked example: 12-cavity bear, locked ears, “robot OK”

A nutraceutical wanted 2,000 dumps/month of an 18 mm bear with locked ears and a note that the line might automate later. The RTV job: CNC master, high-tear platinum 15A, keyed split through the ears, rigid jacket so the sheet doesn't sag 0.5–1.5 mm and throw outer-row weight 3–6% heavy. A robot was irrelevant. A parallel LSR quote with stripper plates would have molded silicone bears — not pectin.

What RTV will actually stretch — then the LSR cousin

Elongation and tear (ASTM D412 / D624 on the *grade*) set the budget. Soft 10A–20A high-tear platinum can come off surprising barbs. 40A with a knife-edge lip cannot. Fillers, inhibition tack, and a mirror master (vacuum lock) all spend that budget.

Useful checks:

• How much hoop stretch is required, and is that a small fraction of elongation at break?
• Is the lip thick enough to take the stretch without cut-growth?
• Is there a lead-in so the lip walks off, or a square shoulder that must snap?
• Will a textured master act as a thousand extra undercuts?
• Zero draft plus an undercut is two problems.

RUUIPON stack: one-piece glove if it peels safely; keyed two-part / removable plug / inner core if it cannot; mother mold for large wet dumps and soft Shore. Air assist through a core works on some boots if the part can balloon without wrinkling. Stripper plates are LSR/HCR hardware.

On LSR cells, valve-gated cold runners mean there is no sprue to grab — you must grip the part. Vacuum cups on 10 Shore A collapse the feature. Cure is not binary: an undercut that strips clean at 25 s may tear at 18 s. Flash on a barb turns a stretch-off into a tear-off. ISO 3302-1 on a nicked lip is meaningless. Self-bonding LSR can stick to cores; mask the core. None of that belongs on a chocolate-tray RFQ except as a warning that you asked the wrong shop.

When this article is the wrong quote

Lights-out LSR with robotic demold belongs on a parts OEM. Custom RTV food and resin molds belong here. A tiny shallow tray can skip the split. Geometry can override quantity: one locked sculpture is still a two-part RTV job.

Related reading: custom mold design, gummy trays, custom RTV molds, what tooling means.

What to send for an RTV mold quote

• Every undercut with strip / dump direction and whether stretch is allowed
• Shore, high-tear grade, jacket
• One-piece vs keyed split vs plug vs inner core
• Casting material, monthly dumps, recast of the skin
• If you actually need LSR robotic demold: separate RFQ with gripper concept and tool actions

Send photos of the undercut and a dump video if you have one. We'll name peel vs split and refuse a robot cell on a pectin tray.

We would not recommend a one-piece glove for a bail or negative-draft letter

We would not recommend raising Shore A first so a robot can grip. We would not count on a six-axis arm to “figure out” a peel. We would not cut LSR steel while you still need a cavity to dump into. Tooling charge ≠ a robot program.

RUUIPON Mold Structure Decision: flat back? open-back. Deep undercuts the rubber can peel? stay one-piece. If it cannot peel, two-part / removable plug / inner core. Large wet casting or Shore 10A–20A? mother mold.

Typical shop values: RTV peel vs LSR robot cell

| Item | Typical shop value | | --- | --- | | Undercut food/resin cavity | 10A–20A high-tear platinum, jacket | | Mix / shrink | 1:1 or 10:1; platinum ~0.1–0.3% | | Locked ears / bails | Keyed split or plug, not hope | | Loaded-tray sag | ~0.5–1.5 mm unsupported; outer row 3–6% heavy | | LSR robot | Known face at a known time; not a wrist peel | | ISO 3302-1 | M2 common default; a nicked lip fails the dump first |

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

FAQ

Can a robot peel a deep LSR undercut the way an operator does? Usually not: operators roll and invert, while robots pull along programmed axes. If the honest exit is a peel around a barb, add air, a stripper, a lifter, or change the geometry. On an RTV mold, write peel vs split instead of hoping a future robot will save a one-piece glove.

Does a softer durometer always make ejection or dump easier? It makes stretch easier and gripping harder. 10A–20A comes off a barb and then collapses in a vacuum cup — and sags as a tray without a jacket. 40A grips well and tears at a sharp lip. Pick durometer for the product, then design the split and the jacket to that rubber.

Where should the robot grip if there is no sprue on a valve-gated part? On a designed pad, rim, or thick wall you declared as a handling surface. That is LSR cell DFM. For RTV trays, the “grip” is an operator dump or a frame; keep keys and jackets in register.

Will air-blast ejection damage a medical sealing lip — or a chocolate detail? It can fold, invert, or contaminate. Aimed core air with a timed pulse is a tool on a steel LSR core. A shop air wand on an RTV pocket can tear a logo. For food molds, peel or split; don't blast sugar dust into the next cavity.

Do self-bonding LSR grades stick to cores more in automated cells? They can. Mask or plate the core. On RTV, inhibition tack (undercured platinum) is the analog: a sticky face turns a peel into a tear. Patch-test the master; don't confuse tack with a designed undercut.