Preventing Mold Storage Degradation: Temperature, Humidity, and UV Controls

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, LSR spare tools, and HCR metal tools 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.

What actually degrades in the rack

Heat. Raises the rate of post-cure, plasticizer and 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 a press 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. For steel LSR/HCR tools, humidity is flash rust on unplated cavities and on ejector holes you forgot to dry.

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 LSR press as a shelf.

Avoid freeze-thaw of water-filled mother molds and of any tool that still has rinse water in it. Ice in a cooling channel splits steel; 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 steel, 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 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.

Steel production molds: dry every water line and ejector pocket, then a thin, *removable* rust preventive if the steel is not stainless. Bag with desiccant. Label the rust preventive so the next setup does not run a food or medical lot through oily cavities.

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.

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. Steel: strip residue, dry, rust-protect.
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. Leave a small desiccant pack for steel. 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.
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. Steel: inspect for rust before the next set-up.

What not to do

Do not store a stretched glove inside-out for convenience. Do not leave an LSR cold-runner tool with water in the deck. 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.

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 or a first-article. Storage damage is cheaper to find on one test shot than on a customer lot.

Tape a three-line card on the rack: cool, dark, unstressed. Add the cure family. That card prevents more scrap than a new wrap of plastic.

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 LSR 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 and ejector pockets are dry, labeled so the next lot is not a food or medical run through oily cavities.

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.