Shrinkage Calculations for Precision RTV Mold Cavities

If you're buying a precision custom RTV mold — a cavity that has to hold a seal profile, a gland, or a tight food pocket — paste 2% shrinkage on the master and you will cut it wrong.

The internet band about 1.5% to 4% is mostly heat-cured silicone (LSR injection and millable HCR) plus some stacked vacuum-cast *part* processes. It is not a law. Platinum RTV mold rubber is typically ~0.1–0.3% after a proper cure. Tin-cure RTV shrinks more and keeps shrinking in storage. ISO 3302-1 even warns that some silicones’ larger shrink makes classes M1 and M2 hard to hold — and that standard is a drawing class, not another word for precision.

This is how to put allowance into a master for an RTV cavity, and how not to contaminate that math with LSR steel data. If the “seal” you need is a vulcanized LSR/HCR *part*, that is a different quote.

The percentage is not a constant

Shrink here means: the dump comes out smaller than the cold cavity (after cool and after any set), so the master is cut to the size you want after stacking master error + RTV + dump shrink.

What moves the number:

• Master. CNC aluminum is usually frozen. SLA that is still post-curing is not. An oversized print stays oversized; platinum will not shrink you back to CAD.
• Mold rubber. Platinum after full cure is near-zero. Tin is time-dependent.
• Cast article. Chocolate, gummy, wax, PU each have their own freeze or cure shrink. That last layer is usually the big one.
• Constraint. A dump locked on a metal core cannot shrink with the core the way a free lip can.
• Section thickness. Thick and thin dumps don't always shrink alike.
• Jacket. A slumping glove without a mother mold prints a different size every pour, which looks like a “bad factor.”

Don't publish a fake universal table. Coupon the dump in the production RTV lot, at the production mix (1:1 or 10:1), degas, and cure.

Common mix-up: putting an LSR percent on an RTV master

A lot of RFQs treat “silicone shrink ~3%” as a cavity number for any silicone job. In the shop that usually means someone pasted a heat-cured LSR planning band onto a platinum glove mold. Platinum RTV after a full cure is near-zero; the dump's shrink is what you compensate on the master. If you apply the LSR number, the pin, the logo, or the fit goes the wrong way.

Platinum RTV, tin, and the coupon

A workable sequence for a precision cavity (gasket-shaped food pocket, custom lip, industrial seal *mold* — not a standard O-ring that belongs on an O-ring spec):

1. Freeze the chemistry. Platinum or tin, Shore 10A–20A with jacket, color of the *dump* if it moves fill.
2. Make the master near nominal plus the dump's expected shrink, not plus 2%.
3. Don't scale metal cores. Cores stay at 1:1. Only the free rubber volumes of the dump grow or shrink.
4. Pour a coupon in the real RTV, real mix, real vacuum, real jacket.
5. Measure after a named wait at 23 °C, undistorted. Soft dumps squash in calipers; use the method you will use in production.
6. Adjust the master, not the process window, for the remaining error.

Worked numbers (illustrative, not a grade promise):

• Target pocket ID 40.00 mm, free dump, platinum RTV ~0.2%, dump freeze ~0.8% → master ID ≈ 40.00 × 1.010 if both apply in the same direction — coupon, don't tattoo this.
• First-article ID after wait = 40.12 mm → recut or recast from that, not from a forum 2%.

If you ignore post-cure of a tin mold: T1 passes. A month later the cavity has moved and the gland leaks. The allowance was for day-two platinum physics on a tin skin.

Constrained dumps and bonded hardware

A dump against a stainless core:

• The rubber over the lock goes into tension or peels
• The free lip still tries to shrink toward the core
• ID may grow or shrink depending on whether the lip is inside or outside the constraint

Uniform scale of the whole CAD will put the groove in the wrong place relative to the core. Model the metal at 1:1. Grow only the free dump, then expect to tune the lip after a coupon. ISO 3302-1’s note on non-rubber parts affecting shrink is this case.

Vacuum-cast PU in an RTV tool has its own shrink stack (tool + PU). Do not use that stack as an LSR steel factor, and do not use an LSR factor as an RTV master factor.

What to ask when you're buying

• Platinum or tin, and at what age will you measure the first dump?
• What's the dump compound, and do you already have a coupon of *that*?
• Measure at what temperature, after what wait?
• Is the master getting compensation for the dump only, or did someone also add an LSR factor?
• Inner core or jacket constraint — which dimensions are locked, which are free?
• If we recast next season, is the same master still in custody, unsanded?

Worked example: 40 mm precision pocket, platinum skin

A lab-consumable tray needed a 40.00 mm pocket, food-adjacent, about 1,200 dumps a month. CNC master near nominal plus dump freeze only. Platinum 10A–20A skin, mix 1:1, vacuum degas, rigid jacket (unsupported sag 0.5–1.5 mm would have looked like shrink). Coupon at 23 °C after a named wait. We would not have applied 2.5%. A parallel LSR seal quote does not dump into this cavity.

When this article is the wrong quote

If you actually need heat-cured LSR/HCR seals as molded *parts*, cavity allowance lives in steel at ~2–3% / ~1.5–2.5% — different quote, coupon in real steel temperature. Tin-cure is illegal as a library dimension if the ID must hold a month later. Geometry can override quantity: a bonded lip at 80 dumps still needs two measurement states.

Related reading: custom RTV molds, DFM checklist, converting STEP, food-grade molds.

What to write so the shop does not pick 2% from a forum

RFQ sentence: *Process platinum or tin RTV, shrink to be established on a dump coupon; do not apply LSR 2% default. Unspecified article dims ISO 3302-1 M[n] if the article is rubber; pocket ID/OD/height are criticals. Cores at 1:1. Who keeps the unsanded master.*

If the vendor answers “we always use 2.5%,” ask for the last coupon on a similar dump. Always is how precision pockets miss by a percent and still look “in family” on a loose class.

Send the STEP, the dump compound, and whether anyone already applied an LSR percent to the master. We will mark which layer actually needs compensation.

We would not recommend an LSR percent on a platinum master

We would not recommend pasting a heat-cured LSR 2–3% class factor onto a platinum RTV glove mold. That oversizes the master in the wrong direction. We would not stack CTE×ΔT on top of a coupon that already cooled, and we would not sand the master “to clean it up” if you expect a recast next season. Raising Shore A first is the wrong fix for a size miss; fix the dump coupon and the jacket.

RUUIPON Mold Structure Decision (shrink layer): platinum RTV after full cure is near-zero — compensate the *cast* article, not the gum. Tin is time-dependent — freeze a wait, or don’t treat it as a library. Hollow dump on a mandrel? two measurement states. Soft 10A–20A without a jacket? sag looks like “bad shrink.”

flowchart TD
  A[Which layer needs compensation?] --> B{Object on the RFQ}
  B -->|LSR/HCR molded part| C["Different quote: heat-cure 2-3% class on steel"]
  B -->|RTV cavity| D{Platinum or tin?}
  D -->|platinum, full cure| E["RTV ~0.1-0.3% — coupon the DUMP"]
  D -->|tin| F["Time-dependent: name mold age"]
  E --> G{Jacket / core lock?}
  G -->|free dump| H[Near-isotropic coupon at 23 C]
  G -->|mandrel or slumping glove| I["Two states: locked vs free; add mother mold"]

Typical shop values

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

| Item | Typical shop value | | --- | --- | | Platinum RTV linear shrink after full cure | About 0.1–0.3% | | LSR *part* contrast (wrong RFQ) | About 2–3% heat-cure class — do not put this on the master | | Mix | Platinum 1:1 or 10:1 by grade; vacuum degas until the foam collapses | | Cavity Shore | 10A–20A skin with a rigid jacket | | Loaded-tray edge sag | About 0.5–1.5 mm without a jacket | | Coupon state | Dump at 23 °C after a named wait |

FAQ

Why is pasting 2% on a platinum RTV pocket the wrong master? The 1.5–4% band is mostly heat-cured LSR and HCR. Platinum RTV is typically near 0.1–0.3% after a proper cure, so the cavity tracks the master. An oversized SLA master stays oversized. Tin-cure keeps shrinking in storage and is not a source of LSR steel factors.

Do I scale the metal core with the dump? No. Cores stay at 1:1. Only free dump volumes grow. A uniform "scale part 2%" on a hybrid body puts the groove in the wrong place relative to the nest.

Why wait to measure a precision dump after a named wait? Soft dumps squash in calipers, tin is still moving, and a dump still warm from a chocolate table mixes CTE with shrink. Write “measure at 23 °C after wait X” on the drawing and on the first-article pack. ISO 3302-1 expects conditioned, undistorted measurement of a rubber *article*.

Can an RTV prototype ID become an LSR shrink factor? No. Vacuum-cast PU in an RTV tool has its own stack. Platinum RTV near-zero shrink is a different number from heat-cured LSR. That part job is a different quote.

How does a stainless core change dump shrink? Rubber over the lock goes into tension or peel; the free lip still tries to shrink toward the core. ID may grow or shrink depending on whether the lip is inside or outside the constraint. Uniform CAD scale will misplace the groove; grow only free dump, then tune the lip on a coupon.