How Custom RTV Silicone Molds Are Made in the Shop

If you're sourcing a custom RTV silicone mold, manufacturing is the conversion of a specified master into a specified flexible cavity — named chemistry, mix records, a jacket that actually holds the pour. The first sentence of the RFQ should say whether you need platinum or tin-cure for food, gummy, chocolate, bakeware, wax, or industrial casting. Those jobs share pour tables and jackets. They don't share presses, shrink factors, or certificates with LSR injection or HCR compression of silicone *parts*.

Once the chemistry is named, the shop's real work is master control, mix control, and a document pack that can survive a recast six months later. If the STEP file is a phone-case gasket plus a chocolate mold, you've written two RFQs, and only one of them belongs on this product line.

Name the mold family before you name the factory

Platinum addition-cure RTV. Two-part A/B, mixed, degassed, poured against the master in a mold box. Best when the cavity has to hold size in storage and you expect recasts. Food, gummy, and most production chocolate tools live here.

Tin condensation-cure RTV. More forgiving of dirty or sulfur-bearing masters, time-dependent shrink. Best for short runs and dirty patterns. Not a library tool unless the PO admits the drift.

Split, glove, and cored builds. One-piece gloves for simple trays. Two- or three-piece keyed splits for bails, two-sided logos, and negative draft. Inner cores / mandrels for hollow chocolates or ice spheres, with a pull path that doesn't tear the cavity.

Jacket / mother mold. Plaster, resin, fiberglass, or a printed shell so Shore 10A–20A rubber doesn't balloon under a dump. The jacket is manufacturing, not an accessory.

HCR compression plates and LSR tools with cold-runners belong on a separate quotation for molded *articles*. A shop that only pours RTV isn't an LSR factory, and the reverse is also true.

From master lot to a poured cavity

A competent custom run has a frozen BOM: master revision, RTV grade (or equivalent window), mix ratio, lot identity, and a cure recipe. Incoming drums get identity checks. Mixed A/B gets pot-life discipline and a degas record.

The tool is a second BOM: mold box or flask, registration keys, inner core, jacket, pour gates, and vents in the box. First-article dumps are the meeting of those two BOMs. Production dumping is repeating that meeting. When either BOM changes (new RTV lot family, recut master, new jacket, new dump compound), you're not in the same process.

Secondary operations are still manufacturing: trimming flash at the split, punching a dump, bonding a logo slug, packing the mold so the jacket doesn't warp in transit. A cheap one-piece glove with an expensive daily recast can lose to a dearer keyed platinum tool. Quote both.

Capacity is pour-table hours, degas-chamber hours, oven or rack hours, and QC. A 24-cavity gummy tray on one table isn't the same capacity as an 8-cavity chocolate with a mandrel if mix, degas, and jacket work differ. Ask for the constraint: master lead time (CNC versus SLA finish), RTV drum lead time, or table time. Custom colors of the *dump* and certified lots (FDA 21 CFR 177.2600, LFGB) add compound lead time that a generic 20 Shore A mold rubber doesn't.

Common mix-up: “custom silicone manufacturing” as one factory tour

A lot of RFQs treat any shop that says silicone as qualified to pour your mold. In the shop that usually means you're looking at LIM presses and a traveler written for parts, while you needed a pour table, a degas chamber, and a master plate. Ask which object ships: a flexible cavity you pour into, or a vulcanized component.

Paying a tooling charge doesn't, by itself, give unrestricted transfer rights. Customer-owned masters with shop-owned jackets, shared boxes with customer inserts, amortized RTV (less common than in steel, but it exists), and supplier-financed first pours are all live options. If it isn't in the agreement, don't assume it. IP is file control, visitor control, and a master plate with *your* number — overseas programs need that in the PO, not in a WeChat promise.

HS codes for finished silicone *articles* often sit in 3926.90 or 4016.99. A flexible mold shipped as a tool may sit in a different conversation with your broker. Describe the *object* (silicone mold / mother mold / master) on the invoice, not “plastic stuff.”

What to ask when you're buying

Before you pick a shop:

• Platinum or tin, mix ratio, and do they log degas and lot numbers on the traveler?
• Who owns the master, who owns the jacket, and may the shop keep a scan?
• Is a recast of the RTV skin on this PO after wear or a logo change?
• One-piece tray, keyed split, inner core — which build, and what's the pull path?
• Food-contact path: 21 CFR 177.2600 / LFGB on the article, or only a grade letter?
• What's the bottleneck they actually schedule — master finishing, drums, or table time?

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

Documents, scrap, and a worked example

At minimum, a serious program wants a drawing with process and measurement state for the *casting*, a material spec for the RTV, a first-article pack (castings plus mold photos), a mix sheet (ratio, degas, cure), and the certificates the end market requires. ISO 9001 is a quality-system claim, not a silicone grade. Don't ask for a “full cert pack” without naming tests — ASTM D412 tensile, ASTM D624 tear, durometer, and a food-contact letter are different pages.

Scrap belongs on a Pareto: bubbles at the split, inhibition sticky faces, torn keys, incomplete copy of the master. A custom program that never discusses scrap is either very simple or not measuring.

Worked example. An 8-cavity muffin, 70 mm diameter, food-contact platinum RTV, fiberglass jacket, about 1,500 bakes in the first season. The shop froze the SLA master finish (wash, UV, barrier), mixed A/B on a logged scale, degassed, poured in a registered box, and issued a mix sheet with lot numbers. The buyer owns the master; the shop stores the jacket and may recast only against a written PO. A parallel LSR *part* quote for silicone bakeware *articles* was declined as the wrong object.

When this manufacturing path is the wrong factory

If you need molded silicone *parts* (gaskets, infant products, overmolds) in the thousands, a pour table isn't your press — quote HCR compression or LSR injection with a cold-runner. Extruded cords and tubes are dies, not RTV cavities. A local art-casting shop with one master on the bench can beat a China pour line when the drawing still changes weekly. Geometry can override quantity: a hollow three-piece chocolate with mandrels can cost more to manufacture than a simple 24-cavity gummy tray.

Related: project workflow, gate by gate, what tooling means, food-grade molds.

Manufacturing RFQ the pour shop can run

• Platinum or tin, Shore target, mix-sheet requirement (ratio, degas, lot)
• Master type and who owns it; jacket ownership; recast on this PO or not
• One-piece, keyed split, or cored — pull path if hollow
• Dump compound, cavity count, first season dumps
• Food-contact path: 21 CFR 177.2600 / LFGB on the article vs grade letter
• Constraint they should schedule: master finish, drums, or table time

Send the STEP, dump material, and the ownership clause you want on the master. We'll answer with a pour sequence, jacket type, and whether a recast belongs in the price — not a tour of the wrong press.

We would not recommend a LIM tour as proof you can pour RTV

We would not recommend treating any shop that says silicone as qualified to pour your mold, or assuming a tooling charge gives unrestricted master transfer. We would not mix a phone-case gasket STEP and a chocolate tray on one PO. We would not raise Shore A first to “make it production”; name platinum or tin, mix records, and a jacket that actually holds the pour.

RUUIPON Mold Structure Decision: Flat back? → open-back glove. Else deep undercuts + peel? If no → two-part / plug / inner core. Large wet casting or Shore 10A–20A? → mother mold. HCR plates and LSR cold-runners are a different factory.

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

Typical shop values

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

| Item | Typical shop value | | --- | --- | | Cavity Shore | 10A–20A platinum RTV skin, rigid jacket behind it | | Mix | Platinum 1:1 or 10:1 by grade; vacuum degas until the foam collapses | | Platinum shrink after full cure | About 0.1–0.3%; tin keeps moving | | RTV skin wall | Often 8–15 mm; thinner 3–6 mm skins slump without a jacket | | Loaded-tray edge sag without a jacket | About 0.5–1.5 mm | | Food / gummy cavity count | 8–24 pockets is a common first production band | | Outer-row gummy vs center if the tray sags | Edge often 3–6% heavy |

FAQ

Is a factory that molds LSR parts also qualified to pour RTV molds? Not automatically. RTV mold-making is a room-temperature (or mild oven) pour against a master. LSR is a metered, hot-cavity injection process with cold-runners and a different shrink story. Ask which pour tables, which degas, and which first-article method they will use for *your* mold.

Who should own the master and the jacket? The party named in the PO, with storage, recast, and return terms. Catalog OEM trays are often factory-owned masters. If you need exclusive geometry, pay for the master and write the plate number. A handshake isn't ownership.

Can I change RTV grade without a new first article? A new lot family can move shrink, tear, and food-contact status. A new grade is at least a delta first article (size of the dump, hardness of the skin, appearance) and sometimes a new extraction file. Treat grade as a BOM change.

Do ISO 9001 certificates mean the mold meets FDA 21 CFR 177.2600? ISO 9001 is a quality-management system. Food-contact is a compound and extraction claim on an article or representative sample. You can have one without the other. Specify both if you need both.

Where do extruded silicone profiles fit? They're custom *articles* made with dies, not RTV cavities. Don't send an extrusion RFQ to a shop that only quotes RTV masters, or the reverse, and then compare piece prices. If you need both a flexible mold and an extruded gasket, you have two production systems.