RTV Micro Detail versus 0.05 mm LSR Part Tolerance

0.05 mm is not a silicone superpower. It is a stack of tool, grade, heat, and metrology that only some micro parts can hold.

If you're buying a custom RTV mold, the cavity copies the master. Platinum RTV linear shrink is typically about 0.1–0.3% after a complete cure. A 2.5 mm pocket at 0.2% is already 0.005 mm of rubber shrink — plus master error, plus how you measure a soft wall. A print that says “±0.05 mm everywhere” on a 12 mm gummy is a different conversation from ±0.05 mm on a 0.8 mm LSR valve slit.

ISO 3302-1 already exists for rubber dimensions. Class M2 is a common RFQ default. Class M1 is tight. Sub-0.05 mm on a micro feature can sit inside or outside that class depending on the nominal. Treating 0.05 mm as a shop default is how an RFQ gets quoted as “yes” and measured as “no.”

HCR compression and RTV pours can make small parts and small pockets. They are not the process that holds this band in volume. Volume micro LSR is injection in steel with a cold-runner, never a LIM hot tool.

The 0.05 mm budget is spent before you open the box

Four movers eat the same hundredths:

The master. SLA layer lines, sealer blush, and CMM of a printed pattern already spend 0.05 mm. CNC or a buffed metal master is the honest start if you care about this band. The master's finish is the cavity finish.

Shrinkage of the grade. Platinum RTV ~0.1–0.3%; tin keeps moving in storage. LSR linear shrink is typically in a roughly 1.5–3% band, moving with cavity temperature, hold, post-cure, and filler. A 2% shrink on a 2.5 mm OD is 0.05 mm by itself. Compensate from *your* process window.

Hot steel versus cold rubber (LSR path). The cavity is at about 160–200 °C. The part is measured at room temperature. A dimension cut to print at 20 °C on a CMM of the *tool* is not the dimension of the part. RTV doesn't have that heat, but it has master error and a soft wall.

Flash, vestige, and metrology. LSR leaks through gaps on the order of 0.005 mm. Gate vestige on a micro part can be the whole tolerance. A contact CMM probe will indent 10A–20A and report a small dimension. Optical / vision / CT are kinder, and they still argue about edge detection on a flash line. If you ignore how the part — or the pocket — is measured, you and the customer are not talking about the same 0.05 mm.

If you ignore post-cure on LSR, the defect is a first article that passed green and drifted after the oven. If you ignore tin aging on RTV, the library pocket moves.

Common mix-up: rubber class versus a blanket ±0.05 mm

DIN ISO 3302-1 / ISO 3302-1 should be on the drawing as the language of rubber tolerances. If you need tighter than the class for a named feature, name that feature. A blanket ±0.05 mm on every radius and flash land is not a micro-molding spec. It is a fight. An RTV chocolate tray should not inherit an LSR micro-valve callout.

What to ask when you're buying

• Is ±0.05 mm on an RTV pocket (copy the master) or on an LSR part (steel, process lock)?
• Which few features, measurement method, after which cure?
• Master process: CNC vs SLA? Who keeps it?
• ISO 3302-1 class on the rest of the drawing?
• Recast: same master, same shrink story?

Worked example: 0.4 mm logo in a gummy pocket vs a micro slit

A confectionery wanted a 0.4 mm recessed brand in an 18 mm bear, 12 cavities, platinum 15A, jacketed, “±0.05 mm.” Honest RTV: CNC master, logo depth as a visual/fill-weight feature, ISO 3302-1 M2 on the pocket envelope, jacket so edge sag doesn't throw weight 3–6%. The logo will copy the master; it will not hold a Cpk study at 0.05 mm from a DLP print.

A device OEM wanted ±0.05 mm on a 0.8 mm LSR slit. That's steel micro inserts, valve-gated cold deck, optical metrology, process freeze. Different factory.

Tooling that can even try — and what RTV can honestly copy

Micro LSR tooling is not a scaled-down plastic toy mold: hardened, well-guided inserts; shut-offs and vents in the micron-to-tens-of-microns band; valve-gated cold deck for milligram shots; heater control on the *insert*; cavity count you can balance. Aluminum micro inserts teach flow; they will not hold this band for a production life.

RTV tooling that can even try: a CNC or metal master, platinum (not tin) production skin, vacuum degas, keys that actually register, a jacket so a 10A–20A wall doesn't balloon, measurement agreed (optical on a dump or on a plaster witness, not a hard probe in a 15A pocket). Walls and slits below 0.2 mm are a fill problem first. Holding ±0.05 mm *on* a 0.2 mm wall is harder than holding it on a 1.2 mm boss. State both the nominal and the tolerance.

Honest for RTV: copying a good master, holding food-tray function, named logo depth as a visual. Not honest: every dimension on a 40 mm part at ±0.05 mm; HCR with hand deflash; RTV from a printed master sold as micro LSR capability.

Medical micro parts add ISO 13485 and, if patient-contact, ISO 10993 on the *grade*. Neither standard creates 0.05 mm.

When this article is the wrong quote

If you need capability-held micro LSR parts, quote a parts OEM. If you need a custom gummy or resin mold with fine texture, stay on RTV and tell the truth about the master. A tiny jewelry mold can copy surprising detail without a 0.05 mm title block. Geometry can override quantity.

Related reading: custom mold design, custom RTV molds, what tooling means, how we make the mold.

What to send for an RTV mold quote

• The critical few dimensions, not a blanket ±0.05 mm
• Measurement method (prefer non-contact on soft rubber)
• Master process and who keeps it
• Platinum RTV vs tin; jacket; recast
• ISO 3302-1 class on the envelope; extras marked on named features
• If you need LSR micro capability: separate RFQ, steel, grade, post-cure, optical method

Send the print with three circled features and how you'll measure them. We'll copy a CNC master honestly or send the micro-LSR job to a parts quote.

We would not recommend ±0.05 mm from a printed master and a tin jacket

We would not recommend a blanket hundredths callout on every radius. We would not raise Shore A first to “hold tighter.” We would not probe 15A pockets with a CMM and argue about 0.03 mm of indent. We would not cut LSR steel while you still need a tray to dump into. Tooling charge ≠ a Cpk promise.

RUUIPON Mold Structure Decision: micro texture doesn't change peel logic. Flat back? open-back. Fine letter with negative draft? two-part so you don't tear the detail. Soft 10A–20A? mother mold — sag spends more than shrink.

Typical shop values: RTV copy vs micro LSR

| Item | Typical shop value | | --- | --- | | Platinum RTV shrink | ~0.1–0.3%; tin keeps moving | | LSR shrink | Often ~2–3% (grade- and temp-dependent) | | Rubber drawing class | ISO 3302-1 M2 common default unless the drawing says M1/M3 | | Cavity Shore (food/resin) | 10A–20A with jacket | | Mix | 1:1 or 10:1; vacuum degas until foam collapses | | Probe on 15A | Indent can spend a large fraction of 0.05 mm |

Reference values only. Confirm material-specific limits against the supplier’s technical data and agree project acceptance criteria before production.

FAQ

Is ±0.05 mm a standard LSR — or RTV — tolerance for small parts? No. ISO 3302-1 classes are the rubber language, and 0.05 mm may be tighter than the class for that nominal. Some micro LSR features can hold it after the steel is compensated and the process is frozen. An RTV mold copies the master; a blanket ±0.05 mm is a request to be priced and then walked back.

Why did the tool CMM pass but the parts miss 0.05 mm? On LSR, the tool was measured cold and the part shrinks from a hot cavity, then maybe a post-cure oven. Probe indent on soft rubber also lies. On RTV, a master CMM doesn't include rubber shrink, jacket sag, or a probe in a 15A pocket. Agree an optical method.

Can aluminum micro molds or printed masters hold this tolerance in production? Aluminum LSR inserts can teach fill and vestige; they will not hold a sub-0.05 mm band for a production life. Printed RTV masters will not either. Cut steel for LSR capability; cut CNC masters for RTV detail.

Does a valve-gated cold runner improve micro tolerances? It improves vestige and shot-to-shot fill on LSR, which is often where a micro tolerance is actually spent. It does not cancel shrink, and it does not exist in a poured RTV jacket. For milligram LSR shots, the deck is part of the tolerance stack. For a gummy tray, the jacket and the master are.

How should we measure a 0.05 mm spec on 20 Shore A silicone? Prefer non-contact optical or CT with a written edge rule, on parts or dumps that have reached dimensional equilibrium. Contact probes indent. If a customer CMM must be used, agree force, tip, and fixturing, or you will argue about 0.03 mm of rubber deflection forever.