CNC Milling Tolerances: Aluminum Prototyping vs. Steel Production Silicone Molds

The mill can hold a hundredth on aluminum Monday and a hundredth on H13 Wednesday. The *mold* will not hold the same hundredth in a 180 °C LSR press.

Aluminum prototyping tools (7075, 6061, QC-10-class, and similar) exist to buy a T0 or T1 in days: flow, gate vestige, shrink ballpark, handling. Steel production tools (P20, H13, S136 / 420ESR, stainless, and their cousins) exist to keep a shut-off, a texture, and a heater map through a life you actually intend to run. CNC tolerance is one slice. Thermal growth, bruise, and how the cutter finishes a land are the rest.

SPI/SPMA class language (101-105) is a construction intent, not a mill callout. An aluminum 105-ish tryout and a hardened 103 LSR tool can both be “±0.02 mm in CAD.” Only one of them still is after 10,000 hot cycles.

What CNC is actually holding

A 3-axis or 5-axis mill plus EDM plus grinding. Typical shop realities, not a certificate:

• Open features (pockets, cores, profiles): well-run CNC can sit in a roughly 0.01-0.02 mm band on steel and aluminum if the setup is rigid, the tool is honest, and the metrology is CMM, not a worn caliper. Tighter is a grind or a jig grind or a walk-in, not a default roughing pass.
• Shut-off flats: LSR wants land gaps in the few-micron to ~0.005 mm leak band. That is spotting, grinding, and lapping after the mill, on steel. Aluminum can be cut pretty. It will not stay that pretty under clamp.
• Small ribs and logos: cutter radius and deflection lie. A 0.2 mm steel rib is a different program from a 0.2 mm aluminum rib. The aluminum rib also bends in use.
• Deep cores: stick-out, chatter, taper. Compensation in CAM is not a core-shift fix in the press.

ISO 3302-1 is a *part* rubber standard. Do not put it on the tool steel as if the mill owed you M1. Tool tolerances are metal. Part tolerances are shrink plus flash plus measurement of rubber.

If you ignore cutter deflection in a 40 mm core, the defect is a beautiful CMM on the shank and a banana in the cavity.

Aluminum tryouts: fast, soft, thermally loud

Use aluminum when:

• Cavity count is low, life is short, and you need parts that look like the CAD for a fit or a regulatory mockup.
• You are proving a 0.2 mm membrane fill or a gate location before you freeze a cold deck in steel.
• The durometer is not a self-bonding medical grade that will glue itself into every ding.

Do not expect:

• Flashless medical lands for a year. Aluminum brinells. LSR clamp to hold expansion will print the land.
• Stable pitch versus a cold deck designed for steel growth. Coefficients of thermal expansion differ (aluminum is roughly twice steel, order-of-magnitude shop memory: ~23 vs ~12 × 10⁻⁶ /K). A 200 mm layout that was mill-perfect at 20 °C will not sit on a steel-deck pitch at 180 °C the same way a steel cavity will. Offset or keep aluminum tools on simple sprues.
• Texture that matches a Mold-Tech steel plaque after 2,000 cycles of a filled HCR. You can mill or blast a grain. It wears.

HCR compression in aluminum is common for short runs. The same bruise story applies on the land. Vacuum boxes still need a seal that the plate does not dish.

Mill aluminum with the finish you can actually polish or blast in a day. Mirror LSR cavities in aluminum are possible and fragile.

Steel production: slower cutter, cheaper per shot later

Steel buys:

• Shut-offs that can be ground and maintained.
• Heater holes and thermal mass that match the cycle you sold.
• EDM VDI texture and etch grains that last.
• Inserts you can replace without throwing the shoe.

CNC on steel is slower, tool wear is real, and hard milling after heat treat is a different program from milling P20 pre-hard. Call the process on the RFQ: mill pre-hard then harden and grind lands, or mill hardened, or EDM the cavity. Each path has a different honest tolerance on a rib versus a land.

Stainless (S136-class) for clear medical LSR fights rust and can fight the cutter. Budget time. Do not demand aluminum-tryout lead time on a four-drop stainless medical tool with valve gates.

How to spec mill tolerances without a fight

Split the drawing:

• Cavity CAD surfaces that make rubber dimensions: compensate for shrink *after* you have a process, or steel-safe and recut. A ±0.01 mm mill spec on an uncompensated LSR cavity is false precision.
• Fits (pins, inserts, cold-deck pitch): metal-to-metal, at *operating* temperature. State whether dimensions are cold.
• Lands: flatness and match, often as a process (spot to a blue, grind) rather than a single ±.
• General mill on the shoe: whatever the toolroom’s default is. Do not drag the whole plate into ±0.01 mm.

Prototype aluminum and production steel should not share one tolerance block copied from a plastic Class 101 checklist. Write two intents.

RFQ notes for mill intent versus class

Name aluminum tryout versus steel production, process (LSR / HCR), cavity count, whether flashless lands are required, cold-deck yes/no, and which dimensions are steel-safe. If you need ISO 3302-1 on the part, say so on the part drawing and let the toolmaker back into metal. If you ignore CTE and clamp, the mill report will be green and T1 will flash.

FAQ

Can an aluminum LSR mold hold the same CNC tolerance as steel in production?

The mill might. The press will not. Aluminum grows more, bruises on LSR clamp, and wears on shut-offs. Use it to learn fill and shrink, then recut steel for the life and the flash spec you will ship.

Should I put ISO 3302-1 on the mold machining drawing?

No. That standard is for rubber part dimensions. Tool drawings need metal tolerances, fits at temperature, and a shrink compensation plan. Translate part classes into steel after the grade and the process window exist.

Why did a ±0.02 mm aluminum cavity miss the silicone part by more than that?

Shrink, hot growth, flash, and measuring rubber with a probe. The mill number was never the part number. Compensate from measured LSR parts in the real heat, or accept the tryout as a geometry check, not a capability study.

Is hard-milled H13 more accurate than EDM for LSR logos?

It depends on the rib. EDM is honest on deep, sharp, hard features and on VDI grain; hard milling is fast on open surfaces and can leave cutter witness. Many production tools mill the open, EDM the logos, and grind the lands. Specify by feature, not by a shop slogan.

Do SPI mold classes tell me the mill tolerance?

They tell you construction intent and life, not a hundredths table for every pocket. A Class 103 LSR tool still needs you to name land process, steel, and which faces are steel-safe. Class number plus “aluminum, flashless, 0.01 mm everywhere” is three contradictory intents.