Steel Mold Classifications (SPI Class 101 to 105) for Liquid Silicone Molding

SPI Class 103 is not a steel grade. It is a construction intent tied to cycle life.

The SPI/SPMA mold classifications (Class 101 through 105) exist so two quotes describe the same *kind* of tool. They come from thermoplastic injection practice. They still work as a language for LSR tools, with extra hardware the class number will not mention. If you RFQ “steel mold” with no class, you invited one shop to bid a prototype and another to bid a million-cycle press tool, then you compared the numbers like a fool.

The published cycle counts are design targets, not warranties. Abrasive fillers, neglected shutoffs, and bad storage will miss them. Easy grades and real PM will beat them. Buy the intent. Maintain the tool.

What each class is trying to be

Numbers below are the industry’s usual cycle-life *intent*. Construction comments are the spirit of the class, not a shopping list of alloys. Do not treat “P20 vs H13” as a class. Hardness ranges on bases and cavities are specified in the SPI notes; shops still pick steels that meet the *life and duty*, including corrosion from steam, cooling water, or LSR’s peroxide-free but still hot environment.

Class 101. Extremely high production. Cycle-life intent: 1,000,000+ cycles. Hardened cavities and cores, robust guidance, serious heating/cooling, wear parts thought through. This is continuous production. Most custom silicone programs never need it. If you ask for 101 on a 40,000-lifetime gasket, you are buying a monument.

Class 102. High production. Intent: up to about 1,000,000 cycles. Still a hardened, guided, production tool. Slightly more optional items than 101 depending on how the quote is written. Use when the product is a long-life SKU and downtime is expensive, but you are not running a million-shot commodity.

Class 103. Medium production. Intent: under about 500,000 cycles. This is the class that shows up on competent custom LSR quotes. Detailed design expected. Cavities/cores heat-treated to a production hardness (SPI’s notes start cavities around 28 Rc). Extras (plating, guided ejection, corrosion-proof channels) are choices you should *name*, not assume.

Class 104. Low production. Intent: under about 100,000 cycles. Milder construction. Bases can be milder steel or even aluminum in the original SPI framing; cavities can be aluminum or milder steels. For LSR, aluminum 104-ish tools are common bridge tools. They are not 103. Do not be surprised when the shutoff wants a refresh well before 100,000 if you clamp them like production.

Class 105. Prototype only. Intent: not exceeding about 500 cycles. Built the cheap way to make a few parts that look like the CAD. Printed inserts and crude aluminum plates live here in spirit even when nobody stamps “105” on the folder. A 105 should not be asked to hold a medical flash spec for a year.

If a vendor says “Class 103” and shows unguided plates, no heater plan, and a garden-hose cold sprue on a 4-cavity LSR tool, they used the number as decoration.

What the class does not buy (LSR extras)

LSR is a thermoset. The cavity is hot. The feed stays cold so the shot does not cure in the manifold. That inversion is not in the 101-105 table.

Name these on the RFQ even after you pick a class:

Cold runner / cold sprue. Open cold sprue wastes a slug every shot. A cold-runner manifold with valves costs more in the tool and saves compound and trim at volume. Class 105 often skips it. Class 103 at volume should not skip the conversation.

Heaters and control. Cartridge heaters, insulation from the cold half, thermocouples in locations that represent the cavity. A class number does not place them.

Vacuum. LSR prints air as voids and will flash instead of packing a blind rib. Vacuum grooves, seals, and a timed pull are process equipment sitting in the tool.

Clamp and shutoff design. Low viscosity means the land geometry *is* the flash spec. Hardened, well-spotted shutoffs matter more for LSR than the class’s thermoplastic heritage suggests. An LSR 103 without a shutoff discussion is under-specified.

Corrosion and water. Heated tools plus cooling/heating channels. SPI 101 language already worries about rust in channels on thermoplastic tools. LSR’s heat makes that worry legitimate even at 103.

Parting-line load. High clamp on a small projected area will brinell a soft land. That is why “104 in aluminum” and “103 in hardened inserts” are different flash stories at the same tonnage.

How to choose a class without a volume fantasy

Use lifetime shots, not this quarter’s PO.

• Need 200 parts to learn: 105, or a printed/aluminum proto that you will scrap.
• Need a few thousand, design almost frozen: 104 / aluminum bridge, maybe steel inserts on the shutoff.
• Need tens to low hundreds of thousands, frozen: 103 is the default ask.
• Need years of continuous supply, service parts, high cost of downtime: 102, sometimes 101, and only with a PM plan.

Bump *up* a class (or add hardened inserts) if the grade is filled, the cosmetic standard is brutal, or you cannot tolerate a mid-life recut. Bump *down* if the product dies with a season.

Cavitation multiplies shots on the *tool* but not equally on every cavity if you run unbalanced. An 8-cavity 103 at 50,000 shots of the tool is 400,000 cavity-impressions of wear *if* all cavities run. Class is still about construction. Cavitation is about piece price. Do not use cavities to “fake” a higher class.

Quoting so Class 103 means Class 103

Put the class on the RFQ. Then add a short construction sheet: cavity count, insert vs full block, cold runner yes/no, vacuum yes/no, heater zones, spare inserts, expected lifetime shots, flash spec.

Ask the shop to restate the class and the extras. If their restatement drops vacuum and spare shutoffs, the price is not a 103 LSR tool. It is a 103 thermoplastic skeleton.

What not to buy: a class number as a substitute for DFM. 101 will not save a part that needs three slides and has no draft. 105 will not become capable because you called it “pre-production.”

One RFQ line that pays for itself: “SPI/SPMA Class 103 intent, under 500,000 cycles; LSR cold runner, cavity heat, vacuum provision, shutoff inserts; extras listed, not assumed.”

FAQ

Is SPI Class 103 a steel grade such as P20 or H13?

No. It is a construction intent tied to cycle-life target, imported from thermoplastic practice. Hardness ranges exist in the SPI notes, but shops pick steels for life and duty, including corrosion from steam or hot channels. "Steel mold" with no class invites a prototype bid next to a million-cycle bid.

What LSR hardware does the class number not buy?

Cold runner or cold sprue, cavity heaters and thermocouples, vacuum grooves, and a shutoff discussion for low-viscosity flash. The cavity is hot and the feed stays cold; that inversion is not in the 101-105 table. An LSR 103 without those extras is a thermoplastic skeleton.

When is Class 101 a monument?

Extremely high production intent, on the order of a million-plus cycles, with hardened cavities and serious wear planning. Most custom silicone programs never need it. Asking 101 on a short-life gasket buys a monument. Class 105 is the opposite: a few hundred cycles, not a year of medical flash.

Can more cavities fake a higher class?

No. Cavitation multiplies impressions if every pocket runs; class is still construction. An 8-cavity 103 at modest tool shots is not a 102. Do not use cavities to fake durability, and do not stamp 103 on unguided plates with a garden-hose sprue.

How do I make "Class 103" mean an LSR tool on the quote?

Put class on the RFQ, then a construction sheet: cavities, insert versus block, cold runner, vacuum, heater zones, spare inserts, lifetime shots, flash spec. Ask the shop to restate class and extras. If vacuum and spare shutoffs disappear, the price is not a 103 LSR tool.