What Tooling Means for a Custom RTV Silicone Mold

If you're ordering a custom RTV silicone mold, “tooling” usually does not mean a steel LSR injection mold.

It means the hardware that lets a shop pour platinum- or tin-cure rubber against a master and pull a usable cavity: the master itself, a mold box, registration keys, any inner core or split faces, and a rigid support jacket (mother mold) so the soft cavity holds shape while you cast. If the PDF in your inbox is full of cold-runners, valve gates, and heated manifolds, you're looking at molded silicone *parts*, not a custom RTV mold.

This is the RTV side of that split, for buyers who need a food, gummy, chocolate, or industrial casting mold rather than a compression or LSR part tool.

What's actually on an RTV mold PO

A custom RTV job typically includes some mix of:

• Master (pattern): CNC, SLA/DLP, machined metal, or a finished sample. Surface and size start here.
• Mold box / flask: walls that set pour height and keep the jacket square.
• RTV cavity: platinum or tin rubber that copies the master. That's the working face.
• Split faces and keys: how a two- or three-piece mold finds itself every cycle.
• Inner core / mandrel: for hollow pours; it has to extract without tearing the cavity.
• Support jacket / mother mold: plaster, resin, fiberglass, or a printed shell so Shore 10A–20A rubber doesn't balloon under a pour.

Steel HCR plates and LSR cold-runners don't belong on this PO unless you also commissioned a molded *article* in parallel. Mixing them on one line item is how lead times and ownership clauses get scrambled.

Common mix-up: two quotes, both say “mold”

A brand wants a 12-cavity bear, 18 mm tall, food-contact, about 2,000 dumps a month.

RTV route: CNC or SLA master, keyed two-piece platinum RTV, rigid jacket, maybe a second recast in the price. Change a logo by recasting the face or dropping in a slug. This is a custom silicone *mold*.

Production-part route: if the factory is molding silicone *parts* (not candy), they'll quote heated steel or aluminum, and for LSR a cold-runner. That quote will not dump pectin.

Both PDFs can say mold cost. Ask which object ships: a flexible cavity you pour into, or a vulcanized silicone component.

What to ask when you're buying a custom RTV mold

Send these before you pick a supplier:

• Is the master CNC, printed, or a physical sample, and who keeps it after the job?
• One-piece tray or a keyed split? If there's a bail, a letter with negative draft, or a two-sided logo, you almost always need a split.
• What's the Shore of the cavity, and is there a rigid jacket behind it?
• Platinum or tin, and why (size hold vs dirty-master forgiveness)?
• Is a recast of the RTV skin included after wear or a logo change?
• Food contact: 21 CFR 177.2600 extraction on the finished article, and/or LFGB / BfR XV if that's the market, not a logo that says “FDA approved.”

If the quote only says “silicone mold” and a price, you're not comparing the same job.

Masters, boxes, and why the jacket usually isn't optional

The master's finish is the cavity finish. Layer lines, sealer blush, and undercured SLA inhibition show up as sticky faces or printed texture. Full UV post-cure, wash, and a barrier coat are part of tooling, not a courtesy.

The box sets wall thickness and pour head. A warped plywood flask prints as a tapered block. A registered, rigid box is the cheap half of accuracy.

Soft RTV without a jacket slumps. For undercut-heavy food or gummy tools, the usual stack is a thin, high-tear skin against the master and a stiff jacket behind it. The rubber does the stretch; the jacket holds the cavity.

Splits, keys, and cores

One-piece glove molds exist for simple trays. Anything with a bail, a letter with negative draft, or a two-sided logo needs a split. Keys (pips, bushes, or CNC bosses) are how the halves return to the same shut. Cheap molds that rely on “the flash line will show me” drift after a few hot cycles.

Inner cores for hollow chocolates or ice spheres need a pull path. If the core can't extract, you designed a cut mold, not a production mold.

Tin-cure versus platinum-cure is chemistry on this same hardware. Platinum holds size in storage; tin is more forgiving of dirty masters and keeps shrinking. Neither choice turns an RTV jacket into LSR steel.

What you should own (and what a recast means)

Write on the PO:

• Who owns the master (and whether the shop may keep a copy)
• Who owns the jacket and keys
• Whether a recast of the RTV cavity is included after wear or a logo change
• Storage: temperature, UV, and who can pull the mold

Paying a tooling charge does not, by itself, give you unrestricted transfer rights. Customer-owned masters with shop-owned jackets, or the reverse, are common. If it isn't in the agreement, don't assume it.

A replacement RTV cavity isn't a new steel tool. It's a recast against the same master, with the same shrink and keys, provided the master still exists and hasn't been sanded “to clean it up.”

Maintenance is still tooling

Don't scour the cavity with abrasives that cut the face. Food and gummy soils want chemistry that doesn't swell the rubber. Retire a mold when a tear reaches a logo or a key, not when it first looks tired. Store RTV out of UV, not stacked so jackets warp.

If you later need thousands of molded silicone *parts* instead of a casting cavity, that's a new RFQ: HCR compression or LSR injection with a cold-runner, new shrink, new steel. Keep that quote on a separate line.

When this article is the wrong quote

If you actually need vulcanized silicone parts (gaskets, covers, overmolds) in the thousands, this RTV stack is the wrong asset. Quote HCR compression or LSR injection with a cold-runner on a parts OEM instead. A tiny, shallow one-piece tray can also skip the mother mold; a very large multi-core RTV with a rigid jacket can cost more than a small compression tool. Geometry can override quantity.

Related reading: custom RTV silicone molds, gummy trays, food-grade molds, how we recast a worn cavity.

What to send for an RTV mold quote

Copy this list. A price without these lines is not comparable.

• Master type: CNC, SLA/DLP, or physical sample (and who keeps it)
• Finished dimensions, Shore target, platinum vs tin if you already care
• One-piece vs keyed split; photos of undercuts, bails, two-sided logos
• Casting material and working temperature (pectin gummy, chocolate, resin, soap)
• Cavity count, dump/pour height, first quantity and monthly dumps
• Food-contact path: 21 CFR 177.2600 extraction and/or LFGB / BfR XV — not a logo that says “FDA approved”
• Recast of the RTV skin: included or extra
• Current failure, if any: sag, tear at the split, sticky face, weight drift

Send master photos (or the STL), finished dimensions, and the material you will cast. We will review the split line, jacket, and whether a recast belongs in the price.

We would not recommend a one-piece glove for this geometry

We would not recommend a one-piece open-back mold if the part has a bail, a letter with negative draft, or a two-sided logo. Raising Shore A first is usually the wrong fix for a floppy cavity; add the jacket. We would not cut an LSR steel tool “so we can scale later” while you still need a cavity to dump pectin into.

RUUIPON Mold Structure Decision:

flowchart TD
  F[Flat back?] -->|yes| O[Open-back RTV]
  F -->|no| U[Deep undercuts?]
  U -->|silicone can peel safely| O
  U -->|cannot peel| S[Two-part / removable plug / inner core]
  W[Large wet casting or soft Shore?] -->|yes| M[Mother mold / support jacket required]
  W -->|tiny shallow tray| O

If the quote skipped keys, jacket, and who keeps the master, you are not comparing the same mold.

Typical shop values: 12-cavity 18 mm gummy bear

| Item | Typical shop value | | --- | --- | | Cavity Shore | 10A–20A platinum RTV, jacket behind it | | Mix | Platinum 1:1 or 10:1 by grade; vacuum degas until the foam collapses | | Linear shrink | ~0.1–0.3% (platinum); tin keeps moving in storage | | Monthly dumps | ~2,000 on a 12-cavity tray is a common first production band | | Outer-row weight vs 4.0 g target | Center ~4.0 g; edge often 4.12–4.24 g (about 3–6% heavy) if the loaded tray sags | | Loaded-tray edge sag | About 0.5–1.5 mm on an unsupported large food tray |

Caption these as typical shop values. Swap in a water-volume map from the actual tray when you have one.

FAQ

Is RTV mold tooling the same as LSR injection tooling? No. RTV tooling is master, box, cavity, keys, and jacket. LSR injection tooling is heated steel or aluminum with gates, vents, and usually a cold-runner. They don't run on the same press.

Do I need a mother mold with soft RTV? For anything but a tiny, shallow tray, yes. The jacket is how a 10A–20A cavity holds size during the pour.

Who owns the master if I paid for the mold? Only if the PO says so. Shops often keep a copy for recasts. If you need exclusive custody, write it down and ask for the file and the physical pattern.

Can you recast a worn gummy or chocolate mold? If the master and jacket still register, a new RTV skin is the normal repair. If the master was sanded or lost, you're back to CAM.

Why did my quote include cold-runner and heaters? That's an LSR or HCR *part* tool. Ask the supplier to requote a custom RTV mold if what you need is a cavity to cast food, wax, or resin.