DFM (Design for Manufacturability) Checklist Before Ordering Custom Silicone Molds

Send the STEP. Then send the answers the STEP cannot give.

A custom silicone mold 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 cuts metal or pours a glove.

Work top to bottom. Stop and fix anything that is still “TBD” on a feature that ships.

1. Name the process, not “silicone”

Circle one primary path:

• Platinum RTV prototype or low-volume flexible mold (near-zero shrink, inhibition risk on SLA).
• Tin RTV when the master is dirty or you accept time-dependent shrink.
• LSR injection into hot steel or aluminum (typical shrink ~2.5-4% after cool-down, more with post-cure).
• HCR compression or transfer (typical shrink ~1.5-3%, longer cycles).

If the STEP will be an RTV sample *and* later an LSR tool, say so. Draft, parting, and shrink must already be production-shaped. A 0° glove of a figurine is not an LSR DFM pack.

2. Hardness and grade family

Shore A is a demold and a function number. 10A skins invert; 70A industrial caps do not. Self-bonding LSR is a different SKU from a general 50A. Colorant and filler change shrink. Write hardness, food/medical/industrial, and whether post-cure is required. “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 / side action. Flexible RTV can invert a lock that LSR steel cannot. If you skip this, the quote is a two-plate tool for a three-lock part.

4. Draft

LSR/HCR metal: 0.5-1° on free walls, more with texture and shut-off (3-5° once grain is real). RTV cavities can be 0°; the *part* still needs production draft if steel is the destination. Zero draft on a 40 mm core is a vacuum lock, not a flex feature.

5. Walls, ribs, bosses

Nominal wall. Local thins (membranes, lips). Local thicks (pads). LSR can run ~0.25 mm over short spans; HCR and RTV products sit thicker. Ribs 0.5-1.0× wall. Core out bosses. Thick LSR lumps are cycle time, then green cores. If you ignore mass, you buy sink, shorts, or a 90-second cycle you did not quote.

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 SPI / VDI / artwork depth. Extra draft for grain. Keep logos off the split. RTV copies the master, including layer lines. LSR copies steel grain. Do not send “make it look like the render.”

8. Tolerances as rubber, not as milled steel

ISO 3302-1 (DIN ISO 3302-1) classes M1-M4, F vs C. Default M3. Promote sealing F dimensions. O-rings are outside 3302-1. Measuring soft rubber with a crushing caliper is not M1.

9. Shrink and measurement state

State whether dimensions are as-molded or after post-cure, at what temperature. LSR and HCR need cavity shrink. Platinum RTV does not cancel an oversized SLA master.

10. Cores, hollows, inserts

Name the exit path: straight pull, air, collapsible, lost core, or a seam. Insert alloy and finish. Primer vs interlock. PA vs PC vs ABS if overmolding. Through-holes beat a prayer.

11. Gates, vents, flash

Show faces that cannot take a gate vestige. LSR will flash a 0.005 mm gap. HCR will flash because that is how the press closes. Plan trim. RTV needs a pour and a last-fill vent, not a cold runner.

12. Cavitation and volume

One cavity to freeze shrink, then duplicate. A 16-cavity M1 family tool is how quotes go to die. MOQ and annual volume belong next to the class, not in a separate email.

13. Files that are actually mold-ready

Watertight solid, STEP (or IGES/Parasolid as agreed), 2D with the classes and notes above. No construction surfaces. No “reference” logos that are also functional. If the only file is a 2D PDF, the shop is modeling, not just tooling. 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 one-page RFQ block you can paste

*Process [RTV Pt / RTV Sn / LSR / HCR]; Shore A []; post-cure [Y/N]; pull direction []; no-split faces []; undercuts [stretch/split/core]; draft []; nominal wall []; ISO 3302-1 M3 general, M2 on [F dims]; measure after []; texture []; overmold substrate []; insert []; annual qty [].*

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

FAQ

Why isn't a STEP enough to order a custom silicone mold?

The solid cannot name process, Shore A, pull, flash, class, or post-cure. 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 cuts metal or pours a glove.

Can one STEP serve an RTV sample and a later LSR tool?

Only if you say so and already shape draft, parting, and shrink for production. 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 M3 as a common default and tighter F dimensions on seals. O-rings sit outside 3302-1. 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; 70A caps do not. Colorant, filler, food versus medical versus industrial, and post-cure 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.