Prevent RTV Mold Storage Damage: Heat, Humidity, UV

The mold that sat on a windowsill for one summer often fails before the mold that ran two thousand careful cycles. Storage is a second production environment. Treat it that way, or the library you paid for becomes a box of brittle cavities.

Tin-cure RTV, platinum RTV, and the steel LSR/HCR tools some buyers also own do not age on the same clock. Mixing them on one rack with one “keep dry” sign is how you lose the tin blocks first and then wonder why the platinum skins still look fine. This page is the RTV library. Steel rust SOP belongs on a parts-tool crate.

What actually degrades in the rack

Heat. Raises the rate of post-cure, oil migration in filled industrial grades, and compression set if the mold is stored under its own weight or in a stack. A mezzanine above an oven is a heat source. So is a black plastic crate in a sunlit van.

UV and sunlight. Attacks the polymer surface. You see chalking, a white stretch mark that does not recover, and early tear at the first undercut. Window glass does not make storage “indoor.”

Humidity. For tin-cure (condensation) RTV, moisture is part of the chemistry. The network can keep tightening; shrinkage continues for months. For platinum RTV, humidity is less of a crosslinking problem and more of a mold-growth, cardboard, and rust-on-the-mother-mold problem.

Ozone and shop chemicals. Motors and UV-ozone lamps crack some elastomers. Silicone is better than natural rubber here, but stored molds still pick up amine, sulfur, and solvent vapors from a nearby adhesive bench. Platinum libraries stored next to tin catalyst, sulfur clay, or freshly printed SLA parts pick up inhibition on the next repair pour.

Mechanical set. A glove mold hung on a nail takes a crease. A block mold stored on a corner takes a permanent dent. That is compression set in storage, not a “bad batch.”

Temperature: pick a boring room

Aim for a stable, cool space. A practical shop target is roughly 15–25 °C, away from ovens, autoclaves, and south glass. You do not need a laboratory chamber for craft RTV. You do need to stop treating the top of a 180 °C press as a shelf.

Avoid freeze–thaw of water-filled mother molds and of any jacket that still has rinse water in it. Ice in a plaster shell wrecks the jacket that keeps a 10A skin in spec.

If you must store in a hot climate without HVAC, shorten the review interval. Tin-cure blocks in 35 °C storage should be dimension-checked before any job that still needs the original size. Platinum will hold size better; heat will still age the surface.

Humidity: dry enough for jackets, not desert-dry for tin

Keep relative humidity in a mid range (about 40–60 percent is a workable shop band). Desiccant cabinets are for steel inserts you might own and for platinum tools you have fully dried. Parking a freshly washed food mold in a sealed bag while it is still damp grows odor that no later wash fully removes.

RTV rubber molds: dry the surface, then bag. Do not store wet-in-wet nested chocolate molds. Do not oil RTV “to keep it supple.” Oil is contamination.

Steel production molds (if they share the warehouse): dry every water line, then a thin, *removable* rust preventive. That SOP is not for the pour face of a gummy tray.

UV: darkness is a spec

Opaque bins, closed cabinets, or covers. Clear crates on a window wall are a UV oven. If you need to see which mold is which, use a tag on the outside of an opaque box, not a transparent stack in the light.

LED shop lights are kinder than a sunlit loading dock. They are not an excuse to leave a glove mold draped over a chair for six weeks.

How to put the mold away (this is the control)

1. Clean to the process. Food tools: wash, sanitize if required, dry. Resin tools: remove uncured resin, wipe, dry.
2. Support the geometry. Soft RTV goes back into the mother mold or onto a mandrel. Do not hang. Do not stack block molds more than the rubber can carry; use shelves, not a pile.
3. Close the bag, but after the mold is dry. Polyethylene bags keep dust and ozone off. For rubber, a dust-free bag is enough; over-desiccating tin-cure is not a cure for shrinkage.
4. Keep chemistries apart. Platinum RTV away from sulfur, tin soaps, amines, and uncured photopolymer. Food molds away from industrial release sprays.
5. Label date, compound, and last dimension check. A box that says “dragon, 10A” is not a record. Tooling charge ≠ a shop that will remember which master is whose.
6. Review on a calendar. Tin-cure library: measure a witness dimension every few months if the part still has to fit. Platinum: inspect for tears, tack, and UV chalking on a longer interval.

What not to do

Do not store a stretched glove inside-out for convenience. Do not stack baking pans in a damp dishwasher and call that storage. Do not assume a freezer extends tin-cure life in a useful way; you are more likely to crack a plaster shell. Do not leave an LSR cold-runner tool with water in the deck on the same rack and think one “keep dry” card covers both.

When you pull a mold for a job, the first act is not a full production pour. It is a visual and a witness-cast. Storage damage is cheaper to find on one test shot than on a customer lot.

Common mix-up: oiling RTV so it “stays supple”

Oil is contamination. Dry, then bag, seated in the jacket. Steel rust preventive is a different object.

What to ask when you're buying storage with the mold

• Platinum or tin, and who owns the master in storage?
• Jacket or mandrel included so the skin is not stacked?
• Opaque bin spec, or a clear crate on a window wall?
• Witness-cast after a long rest, in the price or extra?
• Food molds stored away from industrial sprays?

Worked example: seasonal 12-cavity gummy, six months on the shelf

12-cavity, 18 mm, platinum 10A–20A, jacket. Wash, dry, seat in jacket, opaque bin, ~15–25 °C. Witness-weigh vs 4.0 g before the season. If this had been tin, expect size drift and a recast. Outer-row 3–6% heavy after storage is often a warped jacket, not “the rubber went soft.”

When this article is the wrong quote

If you are storing heated LSR/HCR *parts* tools, use the steel rust/VCI crate SOP. A tiny shallow ice tray can bag without a heavy jacket; a metre-long architectural skin cannot. Geometry can override HVAC: support beats a fancy chamber.

Related reading: shipping so storage starts intact, culinary wash before you bag, compression set in a stacked cavity, custom RTV molds.

Tape a three-line card on the rack — and put it on the PO

Cool, dark, unstressed. Add the cure family. Seat in jacket. That card prevents more scrap than a new wrap of plastic.

Send compound family, jacket yes/no, and how long the SKU sleeps. We will quote platinum, a jacket that is also the storage fixture, and a witness pour after rest.

We would not recommend a windowsill, a nail, or oil in the bag

We would not recommend mixing tin and platinum on one “keep dry” rack. We would not raise Shore A first to survive stacking. We would not freeze a plaster jacket to “extend tin life.” We would not store food skins next to solvent sprays.

RUUIPON Mold Structure Decision: the jacket that holds a wet dump is the same fixture that prevents storage set. Flat-back trays still want the face unstressed. Deep undercut gloves want a mandrel, not a hook.

Typical shop values

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

| Item | Typical shop value | | --- | --- | | Storage temperature | About 15–25 °C, stable, away from ovens | | Relative humidity | About 40–60% shop band | | Platinum shrink after full cure | About 0.1–0.3%; holds on the shelf better than tin | | Tin on the shelf | Keeps moving; measure before the next job | | Cavity Shore | 10A–20A seated in jacket so it does not take a set | | Loaded-tray sag if stored unstressed vs stacked | Stacking reprints the crease; sag band ~0.5–1.5 mm on an unsupported tray |

FAQ

Why do tin-cure RTV blocks often die on the shelf before platinum skins? Moisture is part of tin condensation chemistry. The network can keep tightening and shrink for months. Platinum holds size better; humidity for platinum is more mold-growth, cardboard, and rust on the mother mold. Do not mix families on one rack with one "keep dry" sign.

Is a windowsill or a crate on a loading dock "indoor" storage? Window glass does not cancel UV. You see chalking, unrecovered white stretch marks, and early tear at the first undercut. Opaque bins are a spec. The top of a hot press and a black crate in a sunlit van are heat sources that age rubber and take compression set in a stack.

Should I oil RTV molds so they stay supple in the bag? No. Oil is contamination. Dry the surface, then bag. Do not store wet nested food molds. Steel tools get a thin removable rust preventive after channels are dry — that is a different object and a different face.

Where do platinum libraries pick up inhibition while stored? Next to tin catalyst, sulfur clay, amines, solvent vapors, or freshly printed SLA parts. Keep chemistries apart. Ozone from motors is kinder to silicone than to natural rubber, but stored molds still pick up shop vapors.

Why a witness-cast after a long rest before a customer lot? Storage damage is cheaper on one test shot. Support geometry in the mother mold or on a mandrel; do not hang gloves or pile blocks. Tin libraries need a witness dimension on a calendar if size still matters. Do not assume a freezer usefully extends tin-cure life; you are more likely to crack a plaster shell.