Shore A Hardness Drift in Stored RTV Silicone Molds

Shore A on a stored silicone mold is not a permanent stamp. Condensation-cure (tin) RTV keeps hardening as leftovers leave. Under-cured platinum keeps moving as the network finishes. Heat, UV, and a compressed fold add a mechanical set that a durometer reads as “harder” even when the chemistry only aged.

If you're archiving custom RTV molds — gummy sheets, chocolate trays, resin gloves — you need a witness tab, a storage climate, and an honest chemistry choice. You cannot “set” Shore A once with a 3-second press on day one and trust it in month eighteen. ASTM D2240 is the hardness method. It is sensitive to thickness, dwell, temperature, and the operator's hurry.

Platinum RTV, stored clean, cool, and dark, is the archive rubber. Tin is a short library. LSR/HCR spare parts are a different warehouse problem on a parts OEM (cold-runner, never a LIM hot tool).

Two chemistries, two aging clocks

Tin-cure RTV. The condensation reaction leaves small molecules (alcohol in many systems). They keep leaving in a drawer. The network tightens, the tool shrinks, tear drops, Shore A rises. Library life is measured in months to a few years, not in a decade. A hot, dry loft accelerates it. This is the default hardness drift complaint in mold shops. Do not archive a tin block as a production master cavity.

Platinum RTV. No condensation byproduct. Stored clean, cool, and dark, hardness is far more stable. Residual cure (if you demolded early) will still creep Shore A up in the first days to weeks. After a full cure, platinum is the archive rubber. Inhibition leftovers (a tacky face) are not “soft platinum”; they are an unfinished face that will contaminate and never stabilize.

Do not mix a tin mold's year-two hardness into a platinum capability study. ISO 3302-1 on the casting will also drift if the cavity shrank in storage.

Common mix-up: cavity feels stiffer, tab is stable

If the tab is stable and the cavity feels stiffer, you are seeing geometry, dirt, or a compressed fold, not a new compound. Soft 10A–20A tools slump. A mother mold or a stuffed cavity keeps the shape you will later call “hardness change” when it is really a thicker wall.

False drift is more common than chemical drift: thin flash measured against a hard bench, cold tool in winter versus a 23 °C first article, one-second poke versus standard dwell, measuring on a curved pocket, oil or powder on the face.

What to ask when you're buying

• Platinum production skin vs tin copy — which one must live in the library?
• Witness tab thickness, ASTM D2240, re-measure interval?
• Storage: temperature band, dark, unloaded, who can pull the mold?
• Jacket support so 10A–20A doesn't slump?
• Who owns the master for a recast when tin has aged out?

Worked example: seasonal chocolate tray

A 16-cavity chocolate tray, platinum 20A, jacketed, parked May–October in a loft. Day-one pocket reading 20A; October 24A on the pocket, 20A on the flat witness. The jacket had been stacked; the pocket folded. Unload, support, re-measure the tab. A tin twin from a dirty-master copy would have risen on the tab too. Recast in platinum if the library must last years. Tooling charge did not include unrestricted loft storage.

Storage that actually slows the clock

• Cool, dark, dry, unstressed. Heat is the universal accelerator. UV is real near a window. Do not hang a glove mold on a nail.
• Support the cavity. Soft 10A–20A tools slump.
• No plasticizer neighbors. Store away from sulfur rubber, greasy wood, and open tin-cure stock.
• Clean before bagging. Residual resin, acid, or amine on a cavity keeps reacting.
• Tin: plan replacement. Recast in platinum. Do not fight tin aging with a harder grade; you only start from a higher Shore A.

Demold-to-ship in the same afternoon is the usual platinum trap. The TDS demold time is a tearable network, not a finished modulus. A large RTV block without a full cure will gain Shore A in the first week on the shelf. Extra heat finishes the network; hardness stays or rises. If you needed a softer tool, you needed a softer grade, not a corrective bake. Do not “stabilize” by baking a tin mold like an LSR part.

Self-lubricating parts can read slightly firmer as fluid blooms out — keep that oil out of mold rubber.

When this article is the wrong quote

Spare LSR/HCR parts in a warehouse follow compression-set (ASTM D395) as much as Shore A. That's a parts OEM. A tray you use every week doesn't need an 18-month protocol. Geometry can override: a tiny jewelry mold stored in its box is easier than a 600 mm unsupported sheet.

Related reading: silicone mold storage, custom RTV molds, platinum vs tin, what tooling means.

What to send for an RTV mold quote

• Archive chemistry: platinum RTV (tin is short-library)
• Shore A [value] per ASTM D2240 on a flat witness of [thickness] at [T]
• Production cure as stated; re-measure interval
• Store unloaded, dark, [T band], clean, jacket supported
• Recast of the skin when the witness leaves [band]
• Who keeps the master

Send how long the mold must sit unused and whether the first pour was tin. We'll specify platinum archive and a witness tab.

We would not recommend treating a day-one pocket poke as lifetime hardness

We would not recommend mixing tin and platinum archives on one shelf without labels. We would not freeze tin tools expecting platinum-like library life. We would not raise Shore A first so “drift won't matter.” We would not bake tin like LSR.

RUUIPON Mold Structure Decision: storage doesn't change split logic, but a floppy 10A–20A without a jacket will slump in the library. Flat back? open-back. Cannot peel? two-part / core. Large tray? mother mold — also the storage fixture.

Typical shop values: RTV library hardness

| Item | Typical shop value | | --- | --- | | Archive chemistry | Platinum RTV; tin is months-to-few-years | | Cavity Shore (undercut) | 10A–20A with jacket | | Mix / shrink | 1:1 or 10:1; platinum ~0.1–0.3%; tin keeps moving | | Durometer | ASTM D2240 on a flat witness; don't chase 1 Shore A | | Storage | Cool, dark, unloaded, jacket supported | | ISO 3302-1 | M2 common on the casting; a shrunken tin cavity will miss it |

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

FAQ

Why did my tin-cure mold go from 20A to the high 20s after a year in a loft? Condensation byproducts kept leaving and the network tightened. Heat and dryness speed that clock. Platinum RTV is the archive chemistry. Recast if the cavity must stay on size and on hardness.

Is a 2-point Shore A change after storage a failed lot? Often it is method, temperature, or thickness. Use a flat witness, condition to lab T, and follow ASTM D2240 dwell. Many drawings already allow a hardness band. Chase chemistry only when the witness moved outside that band.

Will post-curing a stored platinum tool bring hardness back down? No. Extra heat finishes the network or drives volatiles; hardness stays or rises. If you needed a softer tool, you needed a softer grade or a full cure before the first measurement, not a corrective bake.

Do self-lubricating LSR parts get harder in the warehouse? They can read slightly firmer in the bulk as fluid blooms out, while the surface feels slicker. That's a parts issue. Keep bloom fluids out of RTV mold rubber so food and resin cavities don't contaminate.

Can I freeze silicone molds to stop aging? Cold slows chemistry, but moisture on thaw and mechanical stress on a frozen elastomer are new defects. A cool, dry, dark shelf is the usual control. Do not freeze tin tools expecting platinum-like library life when they return to the shop.