Converting 2D Drawings and STEP into RTV Mold Geometry

If you're converting files into a custom RTV mold, a STEP file is a shape. It is not a master until someone decides parting, jacket offset, dump shrink, and what the 2D drawing still owns.

Buyers send a STEP and a PDF that disagree, or a STEP with no PDF, or a 2D from 2011 and a “similar” model from a reseller. Shops then pour a cavity of the wrong authority. This is the intake order that turns mixed files into mold-ready geometry for an RTV master, box, and jacket. It is not a click-path through a named CAM package, and it is not an LSR steel cavity file.

Decide which file is the contract

Before you “clean up CAD,” write the winner:

• 2D PDF/DWG owns: tolerances, hardness of the cavity, flash at the split, finish, notes, revision, ISO 3302-1 class if the *article* is rubber, inspection.
• STEP (or native) owns: faces, fillets that actually exist, positions, the 3D that will be offset for skin and split.
• If they disagree: stop. A 2D that says R2 and a STEP that is sharp is two parts. So is a STEP in mm and a 2D in inches with a converted-looking 25.4 error.

No STEP: the 2D is a remodeling job, not a conversion. Budget it. A pretty scan of a competitor tray is not a STEP.

No 2D: the STEP has no class, no flash, no Shore. You can split a model. You cannot honestly pour a production mold.

Native CAD plus STEP plus 2D is the adult package. STEP-only is a prototype conversation.

Common mix-up: offset the STEP 2% and split it

Shops that “just offset the STEP 2% and split it” on a platinum glove with a metal core will misplace the nest. Platinum RTV is ~0.1–0.3% after full cure; the dump's shrink is what you compensate. Metal cores stay at 1:1. Conversion includes not doing a uniform LSR scale.

ISO 3302-1 is a drawing class on the 2D, not a modeled second surface, and not another word for precision.

What “mold-ready” means on the master

The product model becomes a master + box + jacket only after these edits exist as explicit features or as a derived body, not as tribal memory:

1. Watertight, single body (or a defined multi-body: dump + core). Open surfaces and naked edges are a failed knit.
2. Cores at 1:1, dump as a separate body. Hollows are not one lump.
3. Pull direction as a named axis.
4. Parting surfaces that actually shut, including steps for a hidden split.
5. Draft on long walls of the master so the dump and the skin release.
6. Dump shrink on the master only, from a named factor pending coupon. Never scale the 2D notes. Never paste LSR 2–3%.
7. Cores, plugs, loose pieces as separate bodies with extraction paths.
8. Pour and last-fill vents reserved — show faces marked untouchable.
9. Texture and logo as real depth or as a keep-out + plaque call, not a bitmap wrap.
10. Skin OD / jacket offset if a mother mold is planned. A product STEP is not a master until you offset for skin thickness or accept brush variation.
11. Fillets the master can cut or print, matching the 2D’s radius notes.

If steps 4–7 are “we’ll do it in CAM,” you don't have mold-ready geometry. You have a product CAD in a CAM folder.

RTV masters add: printability, drain holes for SLA, full UV post-cure and barrier coat if platinum will see resin, and a designed box that sets pour height.

2D drawings that still have to carry data STEP cannot

Importing a PDF into the model does not transfer:

• ISO 3302-1 M-class, F vs C (if the article is rubber)
• Measurement wait and method
• Flash remnant by zone at the rubber split
• Cavity Shore and platinum vs tin
• Dump compound and food-contact path
• Which dimensions are critical
• Who keeps the master
• Revision history

Those stay on the 2D and on the RFQ. Model nominal (plus dump shrink). Inspect to the 2D.

A 2D from a metal designer will often show ISO 2768, knife corners, and no draft. Translating that sheet into an RTV mold is redesign, not conversion.

Geometry that is usually not mold-ready even when the STEP is “clean”

Fix these in the product body (with the buyer) before master print:

• Zero-thickness sheets and surface leftovers
• Micro-faces from bad imports
• Logos as decals
• Threads copied from a bolt into a dump
• Internal voids with no core exit
• Assembly STEP of 40 components when you needed one pocket
• Fillets that are G0-sharp at the end of a blend
• A mirror body that is not actually symmetric, used as “2 cavities”

What to ask when you're buying

• Which rev wins — 2D or STEP — written in the filename and the title block?
• Process platinum or tin RTV, not “silicone”?
• Derived master STEP to approve before print or CNC?
• Jacket offset and skin thickness named?
• Cores at 1:1?

Worked example: 12-cavity bear, 2D vs STEP fight

A 12-cavity 18 mm bear, 4.0 g target, about 2,000 dumps a month. PDF said R1 at the chin; STEP was sharp. We stopped. After freeze: watertight master, open-back, jacket offset for a 10A–20A skin (mix 1:1, vacuum degas), dump shrink only, no LSR percent. Recast priced, master custody named. A parallel steel cavity conversion is a different RFQ.

When this article is the wrong quote

If you actually need LSR/HCR steel cavity geometry, shrink scale and gate faces belong on a parts OEM pack. A one-off artisan tray from a physical sample can skip STEP. Geometry can override quantity: a hollow with no core exit at 80 dumps is still a remodel.

Related reading: custom RTV molds, DFM checklist, shrinkage allowance, RFQ seven details.

File package that does not bounce

| File | Role | | --- | --- | | Native CAD (if you have it) | Feature history, not just dumb solids | | STEP (AP214/242), units stated | The shape | | 2D PDF (and DWG if it exists) | The law | | Checklist / notes | Process, Shore, split, jacket, dump | | Vector logos | Art | | Core STEP + finish note | Hollows | | Which rev wins | The sentence that prevents the 2 a.m. pour |

Don't send five STEP revs in email with “use the latest.” Put the rev in the filename and on the 2D title block together.

Patch when: one open surface, a missing fillet that the 2D already shows, a tiny gap from export.

Remodel when: the 2D is the only authority, the STEP is a scan, hollows have no separate bodies, or the “conversion” needs new draft/parting that would trash every face.

When those derived bodies exist — master, cores at 1:1, box, jacket offset, dump shrink tagged as pending coupon — you have mold-ready geometry. Until then you have files. The RFQ should say *authority [2D rev / STEP rev], process platinum or tin RTV, derived master to be approved as a STEP before print or pour.* That approval is the conversion. Exporting AP214 is not.

Send the package and the dump compound. We will mark which body is the master and which factor is pending coupon.

We would not recommend a uniform 2% offset on a platinum master

We would not recommend cutting or printing while the PDF and STEP disagree. We would not skip the jacket offset on a 10A–20A wet dump. We would not treat STEP-only as a production freeze. We would not convert this file into an LSR steel cavity “so we can scale later” while you still need a glove.

RUUIPON Mold Structure Decision: derived master first. Flat back? open-back. Deep undercuts that will not peel? two-part / plug / core. Soft Shore or wet dump? mother mold offset on the file, not as a shop surprise.

flowchart TD
  A[Files in] --> B{2D and STEP agree?}
  B -->|no| C[Stop: pick the authority]
  B -->|yes| D[Watertight master + cores at 1:1]
  D --> E{Pt RTV ~0.1-0.3% + dump shrink?}
  E -->|LSR 2% on the file| F[Strip that factor]
  E -->|ok| G[Split, jacket offset, vents]
  G --> H[Approve derived STEP before pour]
  A --> I{Need LSR/HCR steel?}
  I -->|yes| J[Stop: different part RFQ]

Typical shop values

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

| Item | Typical shop value | | --- | --- | | Mix | Platinum 1:1 or 10:1; vacuum degas | | Cavity Shore | 10A–20A skin; jacket offset on the file | | Platinum shrink | About 0.1–0.3% — don't paste LSR 2–3% | | Drawing class | ISO 3302-1 M2 common *if* the article is rubber | | Loaded-tray edge sag | About 0.5–1.5 mm if jacket offset was skipped | | Skin / jacket | Skin often 8–15 mm behind the face |

FAQ

If the PDF and the STEP disagree, which file is the contract? Stop. The 2D owns tolerances, Shore, flash, finish, class, and inspection. The STEP owns faces that will be offset and split. A 2D that says R2 and a STEP that is sharp is two parts, as is a millimetre STEP against an inch drawing with a converted-looking 25.4 error.

What turns a clean product STEP into a master? Watertight bodies, cores at 1:1, a named pull, parting that actually shuts, process-correct draft, dump shrink on the master only, jacket offset, vents, and real texture depth. If those live only in CAM tribal memory, you still have product CAD in a CAM folder.

Should I offset a hybrid dump STEP uniformly 2% and split it? No. Metal stays 1:1; platinum RTV is near-zero. Uniform LSR scale misplaces the core nest. Conversion includes refusing that offset.

When is a remodel cheaper than patching export gaps? Patch an open surface or a missing fillet the 2D already shows. Remodel when the 2D is the only authority, the STEP is a scan, core bodies are missing, or new draft and parting would trash every face. A metal designer's ISO 2768 sheet with knife corners is redesign, not conversion.

Why can't I pour a production RTV mold from STEP-only? You can split a model. You cannot honestly pour without Shore, dump compound, split, jacket, and criticals. Native CAD plus STEP plus 2D is the adult package; put the winning revision in the filename and on the title block together.