Silicone Living Hinges: RTV Prototype vs Fatigue

A polypropylene living hinge is a stretched, oriented film. A silicone “living hinge” is a thick or thin elastomer that bends. They share a name and almost no mechanics.

If you copy a 0.25–0.38 mm PP hinge recipe into LSR or HCR, you will get a perforation that tears on first fold or a floppy strap that never sits. Silicone fatigue is a strain, tear, and stress-concentration problem. There is no honest universal equation that spits out “12,000 cycles” from Shore A and thickness.

On this site the analog is twofold. You can prototype the fold in platinum RTV so the geometry works before anyone cuts steel. You can also tear an RTV *jacket* at a hinge-like thin section if the master has a knife crease. RTV prototypes tell you if the fold *works*. They do not certify fatigue of the production grade. Cycle-test in the chemistry you will ship.

Do not use the PP hinge formulas

PP hinges rely on molecular orientation along the flow, a very thin web, and often a first fold while warm. LSR is an amorphous network. Flow direction through a valve gate does not “orient” it into a million-cycle film. HCR is millable and filled; it is even less like PP. RTV is a poured network; it is not oriented either.

Useful mental model:

• Bending a hinge of thickness t to a radius R puts the outer fiber at a strain on the order of t / (2R + t) for a simple wrap (engineering strain from geometry, not a secret constant). Tighter fold, thicker strap, higher strain.
• Compare that strain to elongation at break from ASTM D412 on *that grade*, then apply a generous knockdown. Elongation at break is a one-pull number. Fatigue lives much lower. If calculated outer strain is a large fraction of break strain, the hinge is a tear waiting for a nick.
• Nicks are the real killers. Tear (ASTM D624) and cut-growth thinking (ASTM D813 family) matter more than a pretty average strain.

If you ignore the nick, the defect is a hinge that passed 200 slow folds in a lab and split on a dirty assembly fixture — or an RTV jacket that split on peel one because the master crease was a knife.

There is no t–R–Shore table in this article that claims cycle counts. Those tables on the internet are not your compound, not your fold angle, not your temperature, and not your cut edge.

Geometry that spreads strain

Design the strap as a bend volume, not a score line.

• Thickness: thinner flexes easier and tears easier. Thicker needs a larger bend radius or it overstrains the skin. Many production silicone folds live in a roughly millimetre-class wall, not a 0.2 mm PP web. If you need a 0.2 mm hinge, you are in thin-wall LSR fill territory *and* tear territory at once — and you are asking an RTV jacket to copy a perforation.
• Length of the flexible zone: a short, sharp crease concentrates strain. A longer convolution, a bellows, or a dual-radius strap drops peak strain for the same packing envelope. Foldable bottles often fail at a crease that was thinned “to fold easier.” Sometimes a slightly thicker, longer hinge lives longer because strain dropped.
• Radii: no knife-edge at the junction to the rigid wall. A fillet that looks large on a 30 Shore A part is still cheap insurance. Same fillet on the master protects the RTV skin.
• Width: wider hinges share load until a nick starts; then tear runs across. A hinge split into several straps can stop a single tear from opening the whole lid. It also creates edges. Pick on purpose.
• Hardness: Shore A is not fatigue life. Soft grades elongate more and creep more. Hard grades take less strain at the same fold and crack sooner if you force the same packing radius.

LSR versus HCR: LSR fills fine convolutions and is common for lids, valves, and straps in volume. HCR compression can make thick folding bodies; flash at the hinge line is a built-in nick. Flashless or well-trimmed hinge lands are a fatigue spec. RTV copies the master crease including the nick you left on the pattern.

Environment is half the life

Silicone is not immortal in every fold:

• Heat plus oil plus ozone plus UV change the network. A kitchen fold and an under-hood fold are different tests. An RTV food mold that flexes every dump is a jacket-life problem, not a PP hinge problem.
• Self-bonding grades and some filled grades tear differently than a clear 40A.
• Platinum versus peroxide cure, and post-cure, change volatiles and sometimes crack resistance. Do not assume. Tin RTV keeps shrinking and is a poor fatigue stand-in.
• A hinge that is stretched over a plastic boss sees tensile plus bending. The geometric strain formula above was only bending.

FDA 21 CFR 177.2600, LFGB, or ISO 10993 constrain the grade. They do not assign cycles. UL 94 is not a hinge test.

How to “calculate” without lying

A defensible shop sequence:

1. From CAD, extract t, bend radius at full fold, fold angle, and any extra stretch.
2. Estimate outer-fiber strain from geometry. If you have FEA with a hyperelastic model fitted to the *grade’s* tensile curve, use that instead of the napkin wrap formula. The napkin is for killing bad designs early, not for a spec sheet.
3. Reject designs where peak strain is obviously too close to break, or where a sharp inner corner exists.
4. Pour an RTV geometry mockup if you need to feel the fold this week. Then mold in the production chemistry (LSR or HCR) for the cycle test.
5. Cycle on a fixture that matches the product (angle, speed, temperature, any pinch). Record cycles to first visible crack, then to functional fail. Do not stop at a round number you wanted for marketing.
6. Put a nick-sensitivity check in the plan: a hinge with a realistic flash remnant or a dust cut.

That is the calculation: geometry screening plus a test. Anyone selling a closed-form N-cycle formula with three invented constants is writing a brochure.

RUUIPON Mold Structure Decision: Hinge-like crease on a master that the jacket must peel? If the silicone cannot peel safely → two-part so the crease is not a tear. Soft 10A–20A? → mother mold so the strap thickness you designed is the strap you poured.

Common mix-up: RTV fold count as the production life

A lot of RFQs stamp “50k cycles” from a platinum mockup onto an HCR drawing. In the shop that usually means a different network, a flash nick, and a fight at PPAP.

What to ask when you're buying

• Open and packed states: t, radii, fold angle, extra stretch?
• Grade or durometer range — and is the RTV pour a geometry check only?
• Expected cycles and environment; flash allowed on the strap?
• Test method (angle, rate, temperature, fail definition) if N is on the drawing?
• Process: LSR, HCR, or RTV prototype only?

Worked example

You're folding a 40A lid strap, t = 1.2 mm, packed R ≈ 3 mm, 90°, 8,000 cycles at 23 °C, 500 geometry samples then production LSR. Napkin outer strain is on the order of t/(2R+t) ≈ 0.17 — screen, don't publish N. Pour platinum RTV to feel the fold; cycle-test the LSR tool with a nick check. A 0.3 mm PP-style crease in 15A RTV will tear the jacket on peel.

We would not recommend copying 0.2–0.4 mm PP hinge thickness onto silicone

We would not recommend a closed-form N from Shore A, thinning a crease “so it folds” without growing R, or putting fatigue life on the drawing with no method. We would not treat an RTV mockup as the production grade.

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 | | --- | --- | | PP hinge recipe (do not copy) | About 0.25–0.38 mm oriented web | | Production silicone strap | Often millimetre-class wall, real bend radius | | RTV mockup Shore | 10A–20A copies fold *feel*, not LSR life | | Platinum shrink | About 0.1–0.3% — not a fatigue number | | LSR / HCR shrink (contrast) | About 2–3% / 1.5–2.5% | | ISO 3302-1 | M2 common; cycle life is a test, not a class |

Copy this into a hinge RFQ

• Hinge in open and packed states, thickness, radii
• Grade or durometer range; expected fold cycles and environment
• Whether flash is allowed on the strap
• Process: LSR, HCR, or RTV prototype only
• If N is on the drawing, write the *test* (angle, rate, temperature, fail definition)

Send the open/packed sections. We'll screen strain and pour a geometry mockup; we will not stamp a fake cycle count. Custom RTV molds.

Related: thin-wall 0.2 mm, deep-draw demold, high tear RTV.

FAQ

Can I use polypropylene living-hinge thickness (about 0.2–0.4 mm) on silicone? Not as a copy-paste. PP needs a thin oriented web; silicone needs a strain-tolerant strap, often thicker, with a real bend radius. A 0.2 mm LSR crease may fill and then tear at a nick. Size t and R from strain, then cycle-test the grade.

Is there a formula that gives silicone hinge cycles from Shore A? Not a trustworthy one for design sign-off. Outer-fiber strain from thickness and bend radius is a screening tool; cycle life depends on tear, nicks, temperature, and chemistry. Prove N on a fixture. Treat internet cycle-versus-Shore tables as someone else’s product.

Does a thicker silicone hinge always last longer? No. Thicker straps raise bending strain for the same packing radius. Some foldable bodies last longer when the crease is thicker *and* the radius is larger, because peak strain dropped. Thinning a crease “so it folds” is a common way to shorten life.

Should the hinge be LSR injection or HCR compression? LSR is usually better at thin, clean convolutions and flashless lands in steel. HCR can make thick folding parts but often leaves a flash nick on the strap unless the land is excellent. Prototype in platinum RTV if you must; fatigue-test in the production process.

Where do silicone hinges actually crack first? At a stress riser: flash, a sharp inner corner, a gate vestige, a dust cut, or the junction to a rigid wall. Bulk strain in a smooth strap is rarely the first crack if you kept strain reasonable. Design and inspect the edges. RTV jackets crack first at a knife crease on the master.