Prevent Compression Set in Stored RTV Mold Cavities

The gasket still looks intact. The joint leaks. That is compression set on a *part*. On this site the same physics shows up as a 10A–20A cavity that sat under its own jacket, took a crease, and now dumps a fat wall.

Compression set is not a mystery property and it is not a single catalog number. ASTM D395 (Method B is the usual rubber callout) compresses a specimen a defined amount, holds it at a defined temperature for a defined time, then measures how much of the squeeze never comes back. A low number at 70 °C for 22 hours does not describe a cover that lives at 180 °C — and it does not describe a tray that slept in a stack all summer. Do not copy a percentage off a general HCR bulletin into an RTV storage SOP.

If you need low-set industrial *gaskets*, that is a parts quote: grade, squeeze, post-cure. If you need the cavity to hold shape between dumps, that is jacket, mandrel, and unstressed storage.

What the test is telling you (and what it is not)

Method B is constant deflection: buttons (or a cut O-ring) squeezed, typically 25 percent, against spacers, in an oven, then released and measured after a recovery period. Set is the unrecovered fraction of that deflection.

Method A is constant force. Most silicone seal drawings that mention D395 mean Method B. ISO 815 is the cousin. If the drawing says “compression set < 20 percent” with no method, time, or temperature, it is not a requirement yet.

Post-cure of the *article* before the test changes the number. A ring tested as-molded and a ring tested after a 200 °C post-cure are different articles. Specify the state.

Do not invent a passing value here. Ask the gum supplier for D395 Method B at your temperature and time, on your hardness and post-cure — for the *article* if this is a seal. For the *mold*, treat set as a storage and support problem first.

The three design knobs that cause shop failures

Squeeze. Static silicone faces often work in a moderate squeeze band (many glands are designed around the same neighborhood as the 25 percent used in Method B, commonly discussed as roughly 15–30 percent for O-rings and similar static seals). Too little squeeze, no line pressure, leak at rest. Too much squeeze, you have already spent the recovery the rubber owns, and heat finishes the job.

On an RTV tray, “squeeze” is the jacket sitting on the skin, a stack of molds, or a clamp that “makes the split close.” A gasket that is “a bit thicker so it seals” at the same bolt length is a set generator on a parts PO. A skin stored under a crate is the mold analog.

Gland fill for O-rings (parts): an overfilled gland at high temperature has nowhere to expand. Follow a gland design. Do not freehand a groove from the ring’s ID only — and do not freehand a floppy RTV block as if it were that gland.

Temperature. Heat accelerates set. A silicone that is honest at 150 °C can still be the wrong *part* grade at 200 °C continuous. For RTV libraries, a mezzanine above an oven and a sunlit van are the heat sources that take a crease.

Time. D395’s 22 hours is a standard, not your warranty. A joint clamped for years at temperature will set more than a 22-hour button. A tray stacked for a season will take a set you will see on the first dump.

Those three knobs sit on top of grade: polymer, filler, and cure. Platinum RTV holds size on the shelf better than tin. A random 50A shop HCR is not “silicone, therefore low set.” A random 10A skin is not “soft, therefore immune to set.”

Mold-design mistakes that look like compound failure

• Jacket pinch / stacking. The RTV analog of a dished compression plate. The thick region is over-squeezed. Fix support, not the datasheet.
• Keys that crush a split land. You just changed the dump locally. Flash at the split follows.
• Oval cavities from storage. Skins dumped in a hot pile. Seat in the jacket or on a mandrel.
• Flash that is sanded into the pour face. You just changed the local thickness of the copy.
• Wrong hardness for the job. Hard rubber on a coarse architectural master needs more peel strain. Soft rubber without a jacket sags 0.5–1.5 mm on a loaded tray. Either finish the master or add the jacket, not unlimited crush.

Call ISO 3302-1 on the *casting* thickness (M2 is a common default). Inspect with a method that does not squash the rubber.

Process: cure and post-cure are part of set

Undercured HCR and LSR *parts* have more unset chains to rearrange under load. They set. A “faster cycle” that still looks filled is a set problem six weeks later — on that OEM.

On RTV, undercured skins are tacky and take a crease. Post-cure the food article per the gum schedule, in the jacket so a 10A tray does not banana. Skip support and you will not match the cavity you poured.

How to stop the leak — or the fat dump — before you recast

1. Write the joint *or* the storage: media, temperature profile, clamp load or jacket mass, service life.
2. Choose squeeze from the gland or from the jacket design, not from leftover rubber thickness.
3. For *parts*, call ASTM D395 Method B at a temperature at least as hot as the soak. For *molds*, call unstressed storage and a witness dump after rest.
4. Pick the grade that has data there. Fluorosilicone if the media demands it on a parts quote; do not use it just because it sounds tougher.
5. Tool thickness and bead to that squeeze — or tool the jacket to hold the skin.
6. First-article: measure thickness without crushing, then a heat-soak of the assembled joint (parts) or a witness dump after storage (molds). Cool-down leak check is the one that catches set on a cover.

If the part already leaked in the field, do not first “mold it thicker.” Measure remaining thickness versus original. If the tray already dumps fat, do not first raise Shore A. Seat it in the jacket.

Common mix-up: D395 on a gummy tray RFQ

A low-set gasket bulletin does not keep a stacked 10A skin round. Jacket and calendar do.

What to ask when you're buying

• Are we talking mold set in storage, or gasket set in a flange?
• Jacket / mandrel included so the skin is unstressed?
• For parts: D395 Method B at which T and time, post-cured state?
• ISO 3302-1 class on the casting, measured without crush?
• Platinum vs tin library?

Worked example: 12-cavity gummy stored a season vs a 150 °C cover gasket

Tray: platinum 10A–20A, jacket, opaque bin, 15–25 °C, witness-weigh vs 4.0 g after rest. Outer-row 3–6% heavy is sag, not D395. Cover gasket at 150 °C: different quote — Method B at soak, squeeze 15–30% band, post-cure, do not thicken at the same bolt length.

When this article is the wrong quote

If you need low-set industrial seals at volume, send the parts OEM. A tiny ice tray can bag without a crate; a metre-long skin cannot. Geometry can override: a locked undercut still wants a split, not a harder Shore to “resist set.”

Related reading: storage SOP, gasket/O-ring cavities, measuring without crush, custom RTV molds.

RFQ lines that stop a thicker-gasket hope

For molds: seat in jacket, unstressed storage, witness dump after rest, platinum vs tin. For parts (if that is really the PO): D395 Method B at soak, squeeze from the gland, post-cure state, ISO 3302-1 thickness.

Send whether the silicone is the cavity or the gasket. We will quote a jacket, not a harder Shore, unless you actually needed the seal OEM.

We would not recommend raising Shore A first to “stop set”

We would not recommend stacking 10A skins. We would not paste a 70 °C bulletin onto a 180 °C cover or onto a summer warehouse. We would not make the gasket thicker at the same bolt length. We would not skip the jacket on a wet dump.

RUUIPON Mold Structure Decision: the jacket that holds a wet dump is the fixture that prevents storage set. Flat back? open-back, still unstressed. Deep undercuts? two-part, still seated. Large wet cast? mother mold required.

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 | | --- | --- | | RTV skin | Platinum 1:1 or 10:1, 10A–20A, jacket | | Platinum shrink | About 0.1–0.3% after full cure | | Method B squeeze (article test) | Often 25% specimen deflection | | Static seal squeeze band (article) | Often discussed ~15–30% | | Storage | About 15–25 °C, opaque, unstressed | | Loaded-tray sag without jacket | About 0.5–1.5 mm |

FAQ

Why does a gasket that still looks intact leak after a heat cycle? The rubber took the flange shape and did not push back when temperature or load changed. That is compression set, not a missing bolt. ASTM D395 Method B (ISO 815 is the cousin) names the test. A low number at 70 C for 22 hours does not describe a 180 C cover — and a stacked RTV tray is a different object.

Is "compression set less than 20 percent" a requirement by itself? Not without method, time, temperature, and as-molded versus post-cured state. Ask the gum supplier for Method B at your soak and time, on your hardness and post-cure. A random 50A shop HCR is not "silicone, therefore low set." A random 10A skin is not immune to a crease.

Why is "make the gasket thicker" a set generator? At the same bolt length, extra thickness is extra squeeze. You have already spent the recovery the rubber owns; heat finishes the job. On an RTV tray, extra "support" by stacking is the same mistake. Follow a gland design for O-rings; seat skins in a jacket.

How do mold mistakes impersonate a bad compound? A pinched jacket or a stack makes thickness vary so one region is over-squeezed. Oval cavities from a hot pile, flash sanded into the pour face, and hard rubber on a coarse master all look like datasheet failure. Call thickness class and inspect without crushing.

What first-article check catches set the oven button test can miss? For parts: measure thickness without crush, heat-soak the assembled joint, leak-check after cool-down. For molds: witness dump after storage, seated in the jacket. If a field part already leaked, measure remaining thickness versus original before molding it thicker.