RTV Silicone Mold Sampling From First Pour to Freeze

If you're sampling a custom RTV silicone mold, you're not collecting souvenirs. You're proving two things together: the cavity copied the master (keys, splits, no sticky inhibition, no trapped bubbles at the parting), and the *casting* from that cavity meets the drawing in a stated wait and temperature. Shop language like T0/T1/T2 comes from steel tools. You can borrow the idea — dry fit the jacket, first production-intent pour, corrections in — but a T1 that used a substitute RTV, a water dump, or a master that will be sanded later isn't a T1.

RTV sampling is different from LSR first-article language. You sample the castings from the rubber mold, and you also inspect the mold. Don't paste a LIM pack onto a poured chocolate cavity without translating it. If the quote is full of cold-runners and heated manifolds, that's a different RFQ for a molded *part*.

First pour, first dump, then freeze

Dry fit / jacket check. The box is square, keys locate, the core extracts, the jacket doesn't rock. This answers “will this stack pour without leaking into a gap?” It doesn't approve cosmetics.

First production-intent pour. Production RTV lot, production mix ratio, production degas, production cure. Then production-intent dump or cast (pectin, chocolate, resin, wax — whatever you actually sell). Identify cavities. This is the first time shrink, flash at the split, texture, and logo fill are evidence. If you waived color of the dump, say so in writing; a later pigment can move apparent size.

Correction pour. Steel-or-master corrections and process corrections from the first dump are in. Same measurement recipe. This is where you freeze versus recut the master or recast the skin. Endless extra pours usually mean the CAD was still moving, not that the shop is thorough.

Tin-cure adds age: record how old the mold was when the casting was measured. A “good” sample on day three is not a library approval for month six.

What belongs in the pack

Name the pack on the PO so you're not negotiating after rubber is poured:

• Cavity map and part IDs (bag by cavity; unlabeled mixed bags make arguments impossible).
• Mix sheet: RTV grade, lot, ratio, degas method, cure time, jacket ID.
• Dimensional report on the *casting* at 23 °C after a named wait. A CMM that sinks into a soft gummy lip isn't a report — use a method the dump can stand.
• Durometer of the skin (and where it was taken). Tensile or tear (ASTM D412 / D624) only if the PO paid for them.
• Appearance: flash at the split, bubbles, texture versus chip, logo fill, sticky faces.
• Food, medical-adjacent, or toy paperwork only if specified (FDA 21 CFR 177.2600, LFGB, CPSIA). A first pour isn't a secret certification event.

A WeChat album isn't a pack. FAI formats (shop equivalent of a first-article form) are fine.

Color of the dump, hardness of the skin, and dimension of the casting are three approvals. Buyers often sign a shape on Monday, a pectin color on Wednesday, and a hardness on Friday, then act as if one sample covered all three. They don't. Multi-cavity first dumps must show all cavities or a documented sampling, not the prettiest pocket.

Common mix-up: pretty dumps from one cavity as a freeze

A lot of RFQs treat a handful of hand-trimmed chocolates from cavity 2 as the sample. In the shop that usually means you've evaluated a trimmer, not the mold. Freeze when the pour recipe makes conforming dumps without heroics: no extra clamp on a warped jacket, no extra wait to chase tin shrink, no one technician who “knows how to fill cavity 7.” Recut the master when a pattern error (wrong shrink, missing draft, texture keep-out too small) is cheaper to cut than to process around. Don't recut because marketing wants a rounder corner unless you issue an ECO and pay for it.

The classic drift: first dumps were 50 pieces, lovingly demolded, from a fresh RTV lot, on the day-shift table. SOP is 2,000 dumps a month, a new pectin lot, night shift, a different rack. Catch it by using the SOP dump recipe at first article as far as possible, keeping a retain dump and a retain photo of the cavity, and requiring a delta pack on RTV lot-family changes.

What to ask when you're buying

Before you approve production dumps:

• What's in the first-article pack — mix sheet, all cavities, photos of keys, dimensions at 23 °C after wait X?
• Production RTV lot and production dump compound, or a substitute with a written waiver?
• Platinum or tin, and at what mold age are we measuring?
• Who pays if the first dump fails on a drawing error versus a pour error?
• Are food-contact tests on production-intent dumps, or only a grade letter?
• After freeze, what counts as a recast versus a new sample?

If the shop can't run first dumps on the production table and recipe, label the sample a prototype.

Worked example

A 12-cavity gummy, 16 mm tall, food-contact platinum RTV, keyed two-piece with a resin jacket, about 1,800 dumps a month. The pack was 12 identified first dumps, mix sheet (lot, 1:1, 8-minute degas, overnight cure), photos of the split keys, and size at 23 °C after a 24-hour wait. Cavity 9 was short on logo fill because of a bubble at the master; they recast that skin against the same master and repeated the pack for 9 only. A water-filled “T1” the buyer had seen from another shop would have missed the pectin shrink.

When a full first-dump pack is the wrong spend

If you're only asking "does this undercut stretch off the master," a prototype pour on a substitute dump can be enough — write the waiver. Food-contact extraction on 20 souvenir chocolates from one cavity isn't an article test. LSR T1 language (heaters, cold-runner, steel-safe bores) doesn't translate onto a poured tray; that's a parts OEM sample. Geometry can override quantity: an 8-cavity hollow chocolate still needs all mandrels identified even at 200 dumps.

Related: quality inspection on arrival, shrinkage coupons, gummy trays.

First-dump pack to copy onto the PO

• All cavities identified; mix sheet (grade, lot, ratio, degas, cure, jacket ID)
• Dump compound = production intent, or written substitute
• Dimensions of the *casting* at 23 °C after wait ____ (tin: mold age)
• Photos of keys, bubbles, sticky faces, texture vs chip
• Who pays the correction pour if CAD vs mix is guilty
• Freeze vs recast vs recut-master decision rule

Send the pack list you want and the dump compound. We'll sample the mold and the casting together and tell you freeze, recast one skin, or recut the master.

We would not recommend freezing on pretty dumps from one cavity

We would not recommend a water-filled “T1,” a substitute RTV lot, or a master that will be sanded later as a production sample. We would not sign shape, dump color, and skin hardness as one approval. We would not skip all-cavity identity on a 12-pocket tray. Extra bake will not un-poison a sticky face; that is a recast, not a sample waiver.

RUUIPON Mold Structure Decision: dry-fit the jacket first. Production-intent pour uses production gum and production dump. Correction pour freezes or recuts. Tin? write mold age. Hollow chocolate? every mandrel identified even at low dump counts.

flowchart TD
  A[Sample gate] --> B[Jacket dry fit]
  B --> C[Production RTV + production dump]
  C --> D{All cavities identified?}
  D -->|no| E[Not a freeze]
  D -->|yes, 23 C after wait X| F{Pass?}
  F -->|pattern error| G[Recut master]
  F -->|pour / bubble / sticky| H[Recast that skin]
  F -->|window holds without heroics| I[Freeze + traveler]

Typical shop values

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

| Item | Typical shop value | | --- | --- | | Mix | Platinum 1:1 or 10:1 by grade; vacuum degas until the foam collapses | | Cavity Shore | 10A–20A skin with a rigid jacket | | Platinum shrink after full cure | About 0.1–0.3% — coupon the *casting* | | Measurement state | 23 °C after a named wait (24 h is a common first-article wait) | | First-dump count | Every cavity, plus retains and any named lab pieces | | Loaded-tray edge sag | About 0.5–1.5 mm if the jacket rocks — looks like a shrink miss | | Outer-row gummy vs center if the tray sags | Edge often 3–6% heavy |

FAQ

How many first dumps should a buyer request? Enough to cover each cavity, the measurement plan, retains, and any lab tests the PO named. A handful of pretty pieces from one cavity isn't a sample. The count follows the pack. If destructive tests are required, add those pieces explicitly.

Can first dumps use a water or wax stand-in if the product will be pectin later? Only if you accept a later dimensional and appearance delta. Pectin, chocolate, and PU don't shrink like water. If fits are tight, plan a production-compound first dump or a paid delta pack when the real dump lands. Write the waiver if you skip it.

What's the difference between a prototype pour and a production sample? A prototype pour proves geometry on a substitute master, a tin glove, or a stand-in dump. A production sample proves the production-intent master, RTV, jacket, mix, and dump together. Mixing the words is how a dirty SLA glove gets treated as a freeze.

Who pays for the correction pour if the first dump failed on a drawing error? The party who owned the error. Missing draft, unstable CAD, and late logo files are buyer-side. Wrong mix against a named recipe, or a cavity that doesn't match the approved master, is shop-side. Put that split in the tooling PO before the first pour.

Should food-contact extraction be done on first dumps? If the PO requires a report on the article, yes, on production-intent compound and wait. A generic compound letter isn't an article test. If you only need a material-grade statement, say so, and don't pretend the first pour was a full FDA 21 CFR 177.2600 campaign.