Overcoming Shore A Hardness Drift During Long-Term Silicone Tool Storage

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

If you archive molds or spare LSR/HCR tools for a season, 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. Drift you can fix and drift you only measure start with a real number.

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.

LSR and HCR parts or metal-tool inserts. The rubber article can post-cure in storage if the factory bake was short. Peroxide HCR with leftover split products is the usual slow hardener/yellower. Platinum LSR that saw a proper post-cure is stable compared with tin RTV. Self-lubricating LSR can read slightly harder as bloom depletes the bulk; the surface feels slicker, the durometer on a cleaned face may not match day one.

Do not mix a tin mold’s year-two hardness into an LSR part capability study.

How to measure so the drift is real

False drift is more common than chemical drift:

• Thin flash measured against a hard bench (the anvil reads through)
• Cold tool in winter versus a 23 °C first article
• One-second poke versus the standard dwell
• Measuring on a curved cavity instead of a flat tab
• Oil or powder on the face

Practice:

• Mold a flat witness tab of specified thickness with every tool, same batch, same cure.
• Condition at lab temperature before reading.
• ASTM D2240, Shore A, average of several points, same operator instruction.
• Log date, shop T, and whether the tab was washed.
• Never chase 1 Shore A. That is method noise on many shops’ benches. A specification band exists for a reason (ISO 48-4 / D2240 language, and ISO 3302 dimensional cousins).

If the tab is stable and the cavity feels stiffer, you are seeing geometry, dirt, or a compressed fold, not a new compound.

Storage that actually slows the clock

• Cool, dark, dry, unstressed. Heat is the universal accelerator. UV is secondary for indoor tools but real near a window. Do not hang a glove mold on a nail; you will store a set.
• Support the cavity. Soft 10-20A 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.
• No plasticizer neighbors. Some packaging films and greases migrate. Silicone likes to pick up and give off oils. 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. A platinum tool stored dirty is not an archive.
• Tin: plan replacement. If the library must last years, recast in platinum. Do not fight tin aging with a harder grade; you only start from a higher Shore A.

For LSR/HCR parts in a spare-parts warehouse, stack so they are not compressed. Compression set (ASTM D395) is the cousin of hardness drift in seals. A stored O-ring in a tight bag can take a set that a durometer on the free tab will not show.

Incomplete cure looks like storage drift

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 or a thick LSR shot without post-cure will gain Shore A in the first week on the shelf. That is not mystery aging. That is the rest of the cure.

Fixes:

• Witness coupon of the same thickness before you call the tool done
• Grade-allowed warm finish for platinum RTV after gel
• LSR/HCR post-cure as the production spec already required
• Do not “stabilize” by baking a tin mold like an LSR part; you will shrink it and still not get platinum library life

Plasticizer-like extending fluids in cheap industrial HCR can leave slowly. The part goes a little harder and a little lighter. If hardness is a sealing spec, do not buy an extended gum for a five-year spare.

Spec the library, not a hope

“Hardness: Shore A [value] per ASTM D2240 on a flat witness of [thickness] at [T], [n] points. Production cure [and post-cure] as stated. Archive: platinum-cure RTV or post-cured platinum LSR/HCR. Tin-cure tools are short-library; recast for long storage. Store unloaded, dark, [T band], clean. Re-measure witness at [interval]. Drift inside [band] is accepted method-plus-age; beyond that, recast or recure per chemistry.”

Refuse to treat a single day-one pocket reading as the lifetime hardness of a folded glove mold. Refuse to mix tin and platinum archives on one shelf without labels. The durometer will blame the rubber. The clock was the chemistry.

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. Measure a cleaned witness if hardness is a drawing dimension, and treat bloom as a separate functional spec.

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.