How 1.5 to 4 Percent Shrink Hits Silicone Mold Design

If you're cutting steel for a silicone part, ignore the title's 1.5% to 4% range until you know which silicone you're shrinking.

That band is a planning number for heat-cured millable HCR and liquid LSR. It is not the shrinkage of platinum RTV prototype mold rubber. Platinum addition-cure RTV typically lands around 0.1-0.3% after a proper cure. Tin-cure RTV shrinks more than platinum and continues to move as condensation byproducts leave. If you cut a master 3% oversize for a platinum glove prototype mold, you designed a wrong part.

Unmix the four, then apply one factor. Shop-normal bands we actually cut: HCR about 1.5-2.5%, LSR about 2-3%, ISO 3302-1 M2 as a common drawing default.

One shrink percent is not a mixed-RFQ answer

A supplier quotes a single “silicone shrink 3%” on a mixed RFQ (a platinum prototype mold and an LSR part). They have not read the job. Paying a tooling charge on a cover photo does not tell you which factor was cut into the plates. Don't call the LSR stack a “LIM hot tool.” The cavity is heated; a cold-runner is kept cool. Shrink still belongs in the steel.

How a 100 mm ID gasket gets three shrink factors

Say you need a 100 mm ID gasket, 3.2 mm section, Shore 50A, 50 validation pieces and about 9,000 a year.

For the 50, an aluminum one-cavity HCR tool run cooler than production steel will not print the same size unless you say so.

For a few thousand millable gaskets, you'll usually cut a heated two-plate HCR tool with a coupon-derived ~1.5-2.5% factor.

If annual volume justifies LSR, the production tool needs cavity heat, gates, vents, and usually a cold-runner, with ~2-3% from a coupon after production post-cure. Those three PDFs can all say “mold cost.” You're not buying the same freeze of size.

Shrink questions that belong on the drawing

Before you compare prices, get answers on:

• Process: HCR compression vs LSR injection (cold-runner vs dump-runner) vs RTV prototype mold only for geometry?
• Shrink factor from a coupon of this compound after this post-cure-not a catalog midpoint?
• Measure at 23 °C after the named post-cure? ISO 3302-1 class (M2 is a common default)?
• Aluminum or steel, and will the prototype press run cooler than production?
• Who owns the plates and inserts after the tooling charge, and can you move the tool?

If those lines are missing, the cheaper quote is usually the incomplete one.

The title's 1.5-4% band is mostly heat-cured HCR and LSR

Heat-cured silicone shrinks because the network forms hot and then cools, and because volatiles leave during cure and post-cure. Catalog midpoints often sit somewhere near 2-3.5% for general LSR, with the wider 1.5-4% (sometimes quoted out to ~5%) covering grade, temperature, pressure, geometry, and post-cure. Those figures are planning only. The compound's own shrink, measured on your coupon after your bake, is the CAD number.

Use this band when:

• You are cutting steel or aluminum to injection-mold LSR parts
• You are cutting a compression/transfer tool for HCR
• You are estimating a first-article size before coupons exist

Don't use this band when you are pouring platinum RTV around a master to make a flexible prototype mold. the prototype mold rubber is not LSR just because both are silicone.

Vacuum-cast stacks (master → RTV prototype mold → polyurethane or similar part, sometimes with heat) can accumulate shrink from each layer. A 1.5-4% story on that stack is a process total, not a property of the silicone gum. Budget each layer or you will double-compensate.

Platinum RTV sits near 0.1-0.3%

Addition-cure RTV has no condensation byproduct. Dimensional change after a complete cure is typically in the near-zero class. That's why platinum prototype mold rubber is the default when the next casting must fit.

Design implications:

• Masters for platinum prototype molds are cut (or printed) at nominal, plus whatever the casting resin will shrink, plus any thermal effect while the resin is hot. Don't also add 2.5% “for silicone.”
• Library life is the other half of the dimensional story. Platinum holds. Tin does not.
• Accelerated oven cure of platinum RTV can add a small extra offset versus a room-temperature cure. If the tolerance is real, cure the production way, then measure.

Inhibition (sticky face on a UV master) is a failed prototype mold, not a shrink error. Fix the master. Don't “compensate” a wet cavity.

Tin-cure RTV keeps moving after demold

Condensation-cure loses a small molecule as it crosslinks. Shrink starts on the bench and continues for months. Forum percentages (0.5%, 1%, 3%) are not a CAD standard. Treat tin as a time-dependent, percent-class mover.

Design implications:

• Short-run texture, dirty masters, inhibition bypass: tin is honest.
• Snap fits and next-season recasts: tin is a remold, not a library tool.
• If you must use tin for a size-bearing prototype mold, pour, wait a defined time, measure, then freeze the process. Don't mix those measurements with a 3% LSR tool factor.

A hybrid remains valid: tin copy off a poisoned master, platinum production prototype mold off that copy. Only the platinum tool gets the near-zero shrink assumption.

Pressure, temperature, and post-cure on millable and liquid systems

For LSR and HCR, shrink is a process variable, not a material constant.

• Temperature. Hotter cure and hotter post-cure generally increase the offset you will measure cold. Changing post-cure after the steel is cut moves the part.
• Pressure and charge. HCR compression: underfill and overfill change density and shrink. LSR: packing and flash control change it less than HCR, still not zero.
• Geometry. Thick sections and thin membranes do not freeze alike. A single isotropic 2.7% in CAD is a lie on a part with a 1 mm lip and a 12 mm boss. Expect more argument on the thick boss.
• Orientation and constraint during post-cure. Rack versus fixture. Warpage looks like “wrong shrink.”
• Filler. Thermally conductive or high-silica grades can shrink differently from an unfilled optical LSR. Use the grade's data.

CTE (about 250-300 ppm/°C for typical PDMS) is reversible motion at temperature. Shrink is the room-temperature offset after crosslinking. Don't add CTE×ΔT on top of 3% as a second shrink factor for steel cut. The shrink factor already includes cool-down from the cure temperature in a proper coupon. CTE still matters for a hot rubber prototype mold used as a cavity.

Apply one shrink factor per process, then verify on steel-safe tools

CAD rules:

1. 1. Identify the process: platinum RTV prototype mold / tin RTV prototype mold / LSR part / HCR part / multi-step vacuum cast.
2. 2. Assign one factor from a coupon of that process. Platinum RTV: near zero, measure if the fit is critical. LSR/HCR: start in the 1.5-4% planning band only until the coupon lands (shop-typical HCR 1.5-2.5%, LSR 2-3%).
3. 3. Add the casting shrink separately if the silicone is the prototype mold, not the part (PU, epoxy, wax, pewter each have their own).
4. 4. Cut metal steel-safe on critical bores. Recast rubber prototype molds cheaper than recutting steel, but only after you measured.
5. 5. Freeze post-cure. Write it on the drawing: measure at 23 °C after post-cure X. ISO 3302-1 M2 is a common default-not a synonym for “precision.”

When this article is the wrong quote

If you need a reusable RTV cavity (gummy, chocolate, resin) rather than a vulcanized silicone part, don't apply an LSR factor to the jacket. Quote a master + box + jacket job. A cheap aluminum one-cavity can beat both a full LSR cold-runner and a large RTV stack when the gasket is tiny and annual quantity is only a few hundred. Geometry and what actually ships override the shrink slogan on the RFQ.

Related reading: thermal expansion vs shrink, silicone mold tooling explained, LSR vs HCR prototype tooling, design-for-manufacture (DFM) for silicone parts.

What to send for a shrink quote

Copy this into the RFQ.

• STEP/STL and 2D with critical dimensions and a rubber tolerance class (often ISO 3302-1 M2)
• What ships: molded silicone part, or a flexible cavity
• Process: platinum RTV prototype mold near-zero (not 1.5-4%); tin RTV time-dependent; LSR/HCR from coupon after production post-cure
• Target Shore A, polymer family, post-cure T/t/thickness
• First quantity and annual quantity
• Process preference: HCR compression vs LSR injection (cold-runner vs dump-runner)
• Who should own plates/inserts after the tooling charge

Send the STEP file, the process family, annual quantity, and who should own the tool. That is enough for a plate-stack sketch before anyone mills steel.

One factor per process, then measure

Do not apply an LSR factor to RTV, or an RTV factor to LSR steel, or cutting a steel LSR tool with a cold-runner before a 100 mm gasket has run as a one-cavity HCR prototype mold-unless annual volume and automation are already on the PO. Do not add CTE×ΔT on top of shrink for steel cut. Do not treat a tooling charge as ownership.

WYD Prototype → Compression → LSR (use this on the RFQ, then ignore quantity if geometry disagrees):

Route the RFQ from what actually ships: if Reusable RTV cavity, go to an RTV master-and-jacket quote on the WYD quote form; if Molded silicone part, go to Freeze of size and annual qty; if Tens to low hundreds / validation, go to Aluminum one-cavity prototype mold; size is a witness; if Thousands, millable HCR, simple shut-off, go to HCR compression: mill, preform, flash land, deflash; if Higher annual, LSR grade, automation, go to LSR injection: cavity heat, gates, vents, cold-runner.

Geometry and what actually ships override the process name. A tiny gasket at 400 pcs/year is still a compression job. A deep undercut at 50,000 pcs still needs side actions no matter which rubber family you pick.

RFQ comparison bands: 100 mm ID gasket

Route	Typical first qty	Hardware	Typical shrink allowance	Typical drawing class
RTV prototype mold / jacket	20-50	Master + box + jacket	Platinum RTV ~0.1-0.3%	Cavity, not a rubber-article class
HCR compression	Low thousands	Two-plate, flash land, platen or cartridge heat	~1.5-2.5%	ISO 3302-1 M2 unless the 2D says M1/M3
LSR + cold-runner	High annual, automation	Steel, cavity heat, gates/vents, cold-runner	~2-3% (grade/temp)	Same drawing class + first-article inspection (FAI) remeasure

These are shop-normal bands for RFQ comparison, not a named lot.

FAQ

Should I cut a platinum glove-prototype mold master 3% oversize because silicone shrinks 1.5-4%? No. That band is a planning number for heat-cured HCR and LSR. Platinum addition-cure RTV typically lands around 0.1-0.3% after a proper cure. If you add 2.5% for silicone on a platinum prototype mold, you designed a wrong part.

Does tin-cure use the same CAD factor as LSR? No. Tin-cure shrinks more than platinum and continues to move as condensation byproducts leave. Forum percentages are not a CAD standard. Treat tin as a time-dependent, percent-class mover, and do not mix those measurements with a 3% LSR tool factor.

Why cannot one isotropic 2.7% in CAD cover a 1 mm lip and a 12 mm boss? Thick sections and thin membranes do not freeze alike. For LSR and HCR, shrink is a process variable: temperature, pressure and charge, geometry, post-cure orientation, and filler all move the offset you measure cold. Changing post-cure after the steel is cut moves the part.

Should I add CTE times delta-T on top of a 3% shrink factor for steel cut? No. The shrink factor already includes cool-down from the cure temperature in a proper coupon. CTE (about 250-300 ppm/°C for typical PDMS) is reversible motion at temperature. It still matters for a hot rubber prototype mold used as a cavity; that is a different article.

How do vacuum-cast stacks accumulate shrink? Master to RTV prototype mold to polyurethane or similar part, sometimes with heat, can accumulate shrink from each layer. A 1.5-4% story on that stack is a process total, not a property of the silicone gum. Budget each layer or you will double-compensate.