Custom RTV Silicone Mold Design Before Anyone Pours

If you're designing a custom RTV silicone mold, freeze the file for a pour, not for a LIM cell. Draft, shrink, parting, and logos are process decisions. A platinum RTV cavity that peels a figurine doesn't teach an LSR steel tool how to eject a 20A membrane, and a compression HCR gasket doesn't need a cold-runner. Those last two are different quotations for molded *parts*. Write platinum or tin RTV on the title block, then draw the master, the split, the keys, and the jacket.

This is a drawing-review sequence for getting a pocket ready to pour. It isn't every undercut trick in one checklist.

Draw the cavity for a peel, then decide if production draft still belongs

On RTV, the silicone is the cavity. The master's surface is the finish. Draft on the *mold* can be low because the rubber flexes. Draft on a *future production part* should still exist if this RTV step is a prototype of an LSR or HCR article you'll quote later as steel.

Shrink is process-specific. Heat-cured LSR and HCR live in a larger shrink band than many platinum RTV grades. Tin-cure RTV keeps moving. Don't copy a 1.5% note from an LSR TDS onto a poured mold, or a near-zero RTV note onto LIM steel.

Pick the split where the seam will do the least harm, not where the CAD looks pretty. On RTV the seam is a jacket line and it prints into every casting. Hide it or accept it. A multi-piece RTV mold is extra seams on purpose, to release an undercut. Keys (pips, bushes, or CNC bosses) are how the halves return to the same shut. If two engineers can't point to the parting on a screenshot, the design isn't ready.

Walls, pour, undercuts, cores

RTV mold rubber arrives as a pour. Vacuum degas and a thin stream beat a dumped cup. That's how the *mold* is made, not how the later casting is filled. Don't paste LSR gate rules onto a poured jacket. Put a target wall of rubber on the drawing; don't write "as thin as possible."

Silicone elongates. That isn't a license for a barb that will tear a 10A lip. Shop order: add draft and a fillet so the feature strips; test a shallow undercut on the grade; split the mold or pick a core; for RTV, design a multi-piece flexible mold; or change the article. Don't skip to "it will peel" because a video showed a glove mold of a miniature. Internal voids need cores or mandrels with a pull path. If the core can't be pulled, you designed a cut mold.

Texture and logo before you freeze CAD

The art lives on the master. Print quality, sanding, and inhibition decide the cavity. A sticky SLA master that inhibits platinum gives you a sticky logo pocket. VDI 3400 / SPI-style polish callouts belong on *metal part tools*; on RTV, call out master finish (layer lines vs machined vs blasted). A logo needs depth, draft on serifs, and a home that isn't the shut. Pad-print is a different process and shouldn't be drawn as molded-in detail.

Freeze means the next change is a revision, not a WeChat voice note. After freeze: pour, sample, then recast or refinish the master only for a named miss.

Common mix-up: one DFM checklist for RTV, LSR, and HCR

A lot of RFQs treat "silicone DFM" as one PDF. In the shop that usually means LSR draft and gate notes on a chocolate tray, or an RTV glove's zero draft copied onto steel. RTV peels. LSR injects into a heated cavity with a cold-runner. HCR compresses a millable charge. Draft, shrink, flash, and gates don't transfer.

What to ask when you're buying

• Is the process on the title block platinum RTV or tin RTV (not "silicone")?
• Where is the split, and are keys drawn, not "shop to advise"?
• Jacket / mother mold: yes/no, material, and which Shore skin sits against the master?
• Who applies shrink, and is the CAD already compensated?
• Logo depth and which face; undercuts: peel, split, or core?

Worked example: 40 mm lollipop letter, negative draft

A confectionery buyer needed a 40 mm lollipop with a letter that had negative draft, about 800 pours a month, food-contact, single-cavity first tool then a 6-cavity sheet. We moved the split off the letter face, put CNC bosses on the jacket, and kept a high-tear platinum skin against the master so the serif wouldn't tear on peel. Copying the zero-draft RTV peel onto a future LSR part tool would have been a recut of steel; we left that as a separate conversation.

When this DFM does not apply

If production is molded LSR or HCR parts, don't freeze an RTV glove as the production toolpath. Open a parts RFQ with strip direction and runner notes. A tiny shallow one-piece tray can skip keys. Geometry can override quantity: one undercut-heavy 80 mm figurine at 30 resin casts still needs a multi-piece jacket.

Related reading: common design mistakes, how molds are made, custom logo on the master, custom RTV molds.

Freeze package the pourer can use

Copy this onto the 2D:

• Process: platinum RTV / tin RTV
• Shrink owner (shop or customer); CAD compensated yes/no
• Parting screenshot; key style; core pull path
• Jacket material and Shore of the skin
• Master finish: CNC / post-cured SLA plus barrier / supplied sample
• Logo: raised or recessed in the pocket, depth, face
• Freeze date and revision letter

Send the STEP, a marked split screenshot, and the dump material. We will mark keys, jacket, and whether the undercut is a peel, a split, or a core before anyone mixes A/B.

We would not recommend freezing zero draft because a glove mold peeled

We would not recommend copying RTV peel rules onto an LSR part tool. We would not raise Shore A first to hide a missing split. We would not leave shrink with two owners.

RUUIPON Mold Structure Decision: flat back? open-back. Deep undercuts that will not peel? two-part, plug, or inner core. Soft 10A-20A or large wet casting? mother mold.

flowchart TD
  A[Title block says platinum or tin RTV?] -->|no| Z[Wrong DFM pack]
  A -->|yes| B{Can it peel without tearing 10A-20A?}
  B -->|yes, shallow| C[One-piece plus jacket if soft]
  B -->|no| D[Keyed split or inner core]
  D --> E[Freeze keys, pour direction, logo face]

Typical shop values

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

| Item | Typical shop value | | --- | --- | | Cavity Shore | 10A-20A skin | | Mix | Platinum 1:1 or 10:1; vacuum degas | | Platinum shrink | About 0.1-0.3 percent; do not paste LSR 2-3 percent | | HCR part contrast | About 1.5-2.5 percent on a different drawing | | 40 mm lollipop letter, negative draft | Single cavity first, then 6-cavity sheet at about 800 pours/month |

FAQ

Can I use one DFM checklist for RTV, LSR, and HCR? No. RTV peels. LSR injects into heated steel, often with a cold-runner. HCR compresses a millable charge. Draft, shrink, flash, and gates do not transfer. Write the process on the title block first.

How much draft should a custom silicone part have? On rigid LSR or HCR part tools, treat 0.5-2 deg on vertical faces as the conversation starter, more with texture. Flexible RTV cavities can peel with less draft on the mold, but production conversions still want the metal-tool draft.

When should shrink be applied in CAD? After the process and grade family are named. Either the shop compensates the toolpath or you supply a compensated model. Never leave two people each thinking the other added it.

Do logos and textures belong in the first design review? Yes. On RTV they live on the master and they change split, peel, and whether a recast can change art later. Adding a fine serif after the master is finished is a refinish, not a surface tweak.

What drawing notes stop an RTV mold design from drifting? Process name, shrink policy, parting, pour direction, jacket, logo depth, and a freeze date. Shop-to-advise on all of those is how the pour starts twice.