DFM Checklist Before Ordering Custom Silicone Molds

If you're ordering a custom RTV silicone mold, send the STEP. Then send the answers the STEP cannot give.

An RFQ that is only a solid model is a request to guess the process. The shop will assume LSR in steel, or RTV from a printed master, or HCR compression, and one of those assumptions will be wrong. Design for manufacturability here is not a software button. It is a list you finish before someone prints a master or pours a glove.

Work top to bottom. Stop and fix anything that is still “TBD” on a feature that ships. This is the RTV checklist. Steel part tools are a different quote.

1. Name the process, not “silicone”

Circle one primary path:

• Platinum RTV custom mold (near-zero shrink after full cure, inhibition risk on undercured SLA).
• Tin RTV when the master is dirty or you accept time-dependent shrink.

If the STEP will be an RTV mold *and* later an LSR *part* tool, say so — and don't pretend the glove is production LSR data. Draft, parting, and shrink for a molded part must already be production-shaped. A 0° glove of a figurine is not an LSR DFM pack.

If the PDF is full of cold-runners and heated manifolds, you are on the wrong site.

2. Hardness and grade family

Shore A is a demold and a function number. 10A–20A skins invert with a jacket; 30A–40A blocks don't invert like 10A. Food-contact platinum is a different SKU from a general industrial tin. Write hardness, food/industrial, platinum vs tin, mix 1:1 or 10:1. “Silicone, medium” is not a spec.

3. Draw direction and parting

Mark the pull. Mark faces that cannot carry a witness. Mark undercuts as stretch / split / core / plug. Flexible RTV can invert a lock that LSR steel cannot. If you skip this, the quote is a one-piece glove for a three-lock dump.

4. Draft

RTV cavities can be 0° on the *mold* because the rubber stretches. The *master* still wants honest draft on long walls so the dump releases and so you can pull the master without tearing the skin. Zero draft on a 40 mm core is a vacuum lock, not a flex feature. If steel is a later destination for a *part*, the sample already needs production draft.

5. Walls, ribs, bosses

Nominal wall of the dump. Local thins (membranes, lips). Local thicks (pads). Tall bosses need last-fill vents when you pour the mold and when you cast. Ribs: fillet roots so the RTV does not tear. If you ignore mass in the dump, you buy air, shorts, or a sagging sheet — not LSR sink.

6. Radii

Fillet corners the rubber must climb. Specify the few true sharps (seals, logos). A default 0.2 mm break-edge will eat a valve lip. A knife in a 10A glove will eat the mold.

7. Texture, logos, gloss

Call artwork depth, raised vs recessed on the dump. Extra draft for grain. Keep logos off the split. RTV copies the master, including layer lines. Don't send “make it look like the render.” Finish the master (wash, UV post-cure, barrier coat) as part of tooling.

8. Tolerances as rubber, not as milled steel

ISO 3302-1 (DIN ISO 3302-1) classes M1–M4, F vs C, apply to a rubber *article* on a drawing. Default M2 or M3 as the drawing says. They don't make a platinum glove “precision,” and they don't replace a fill-weight map of the tray. O-rings are outside 3302-1. Measuring a soft dump with a crushing caliper is not M1.

9. Shrink and measurement state

Platinum RTV: ~0.1–0.3% after full cure — compensate the dump, not an LSR 2–3% class number. Tin keeps moving in storage. State whether dimensions are as-cast, at what temperature (23 °C after a named wait is the usual lab state). Platinum RTV does not cancel an oversized SLA master.

10. Cores, hollows, jackets

Name the exit path: straight peel, split, removable plug, inner core. Jacket / mother mold: plaster, resin, fiberglass, or printed shell. Soft 10A–20A without a jacket slumps ~0.5–1.5 mm on a large food tray.

11. Pour, vacuum, flash

RTV needs a pour path, a last-fill vent, and vacuum degas of the A/B mix until the foam collapses. It does not need a cold-runner. Show faces that cannot take a split witness. Plan how you will trim flash at the rubber split.

12. Cavitation and volume

One or two cavities to freeze peel and fill-weight, then duplicate. A 16-cavity M1 family tray is how quotes go to die. Monthly dumps belong next to the jacket design, not in a separate email.

13. Files that are actually mold-ready

Watertight solid, STEP, 2D with the classes and notes above. No construction surfaces. If the only file is a 2D PDF, the shop is modeling the master, not just pouring. Budget that.

14. What not to send as DFM

• A proprietary CAD click-path as if it were the mold method.
• “Silicone doesn’t need draft/walls/parting.”
• A PLA print as the production *cavity* for hot LSR.
• An RTV photo sample as proof of LSR bond or shrink.
• A cold-runner sketch on an RTV RFQ.

Common mix-up: one STEP for a glove and a later LSR tool

A 0° figurine that peels from 10A is not production DFM for a molded part. If you need both, say so and already shape draft, parting, and shrink for the *part* process. Platinum near-zero shrink also does not cancel an oversized SLA master.

What to ask when you're buying

• Platinum or tin, Shore, mix ratio?
• One-piece vs keyed split vs core?
• Jacket yes/no, and who keeps the master?
• Dump material and monthly dumps?
• ISO 3302-1 class on the *article*, not as a slogan?
• Recast of the RTV skin included?

Worked example: 12-cavity 18 mm bear

A 12-cavity 18 mm gummy, 4.0 g target, food-contact, about 2,000 dumps a month. Checklist freeze: platinum 10A–20A, mix 1:1, vacuum degas, keyed two-part only if the chin return cannot peel, otherwise open-back plus jacket, ISO 3302-1 M2 on the dump drawing if they sell the gummy as a rubber article — most food dumps skip that and use a fill-weight map instead. Recast priced. A parallel LSR DFM pack is a different RFQ.

When this checklist is the wrong quote

If you actually need vulcanized silicone parts in the thousands, use an HCR/LSR DFM pack on a parts OEM. A tiny artisan tray can skip half the list. Geometry can override quantity: one deep hollow still needs a core at 80 dumps.

Related reading: custom RTV molds, converting STEP to mold geometry, RFQ seven details, gummy trays.

A one-page RFQ block you can paste

*Process [RTV Pt / RTV Sn]; Shore A []; mix 1:1 or 10:1; pull direction []; no-split faces []; undercuts [stretch/split/core]; jacket [Y/N]; dump material []; ISO 3302-1 M[] on the article if it applies; measure after []; texture []; monthly dumps []; recast [Y/N]; who keeps the master [].*

If a line is blank, the mold is not ordered. It is estimated.

Send the STEP, the dump compound, and undercut photos. We will mark split, jacket, and whether a recast belongs in the price.

We would not recommend ordering from a STEP-only RFQ

We would not recommend a one-piece glove if there is a bail, a negative-draft letter, or a two-sided logo. Raising Shore A first is the wrong fix for a floppy cavity; add the jacket. We would not cut LSR steel “so we can scale later” while you still need a cavity to dump pectin into. We would not paste an LSR 2–3% shrink onto the master.

RUUIPON Mold Structure Decision: Flat back? open-back. Else deep undercuts + peel? If no → two-part / plug / core. Large wet casting or soft Shore? mother mold.

flowchart TD
  A[STEP in hand?] --> B{Process named Pt or Sn RTV?}
  B -->|no| C[Stop: guess = wrong tool]
  B -->|yes| D{Split / stretch / core marked?}
  D -->|no| E[Mark pull and locks]
  D -->|yes| F{Jacket for 10A-20A or wet dump?}
  F -->|needed and missing| G[Add mother mold]
  F -->|ok| H[Freeze Shore, dump, recast, master custody]
  A --> I{Need LSR/HCR parts?}
  I -->|yes| J[Separate 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 | | Drawing class | ISO 3302-1 M2 is a common default *if* the article is rubber | | Loaded-tray edge sag | About 0.5–1.5 mm without a jacket | | 12-cavity 18 mm bear | ~2,000 dumps/month is a common first band |

FAQ

Why isn't a STEP enough to order a custom RTV mold? The solid cannot name process, Shore A, pull, jacket, dump material, or who keeps the master. The shop will assume LSR steel, RTV from a print, or HCR compression, and one of those guesses will be wrong. Finish the list before anyone pours a glove.

Can one STEP serve an RTV mold and a later LSR part tool? Only if you say so and already shape draft, parting, and shrink for the *part*. A zero-draft glove of a figurine is not an LSR DFM pack. Platinum RTV near-zero shrink also does not cancel an oversized SLA master.

What tolerance language should unspecified rubber dims use? ISO 3302-1 classes M1–M4, F versus C, with M2 or M3 as the drawing says. It is a drawing class, not “precision.” Many food dumps use a fill-weight map instead. Measuring soft rubber with a crushing caliper is not M1.

Why does hardness belong on a DFM checklist, not later? Shore A is a demold number and a function number. 10A skins invert; 30A blocks do not. Food versus industrial, platinum versus tin, and mix ratio change shrink and strip. "Silicone, medium" is not a spec.

What should I refuse to send as if it were DFM? A proprietary CAD click-path, a claim that silicone needs no draft or walls, a PLA print as a hot LSR cavity, or an RTV photo as proof of LSR bond or shrink. If a checklist line is still blank on a feature that ships, the tool is estimated, not ordered.