Tear Strength vs Shore A Hardness in RTV | RUUIPON

Shore A is how far a needle goes into the rubber. Tear strength is how a nick grows when you peel a hooked dump out of the cavity. They are different tests (durometer vs a tear specimen, often discussed as ASTM D2240 vs D624 in TDS language). A 10A–20A platinum skin with a high tear grade and a stiff jacket routinely outlasts a 40A cavity that feels "strong" in your hand and then splits at a logo serif. If your mold tore, raising Shore is usually the wrong first recast.

RUUIPON pours the cavity and, when the pour is wet or the sheet is loaded, a mother mold around it. Bao'an since 2018. Prototype 3–7 business days after CAD lock. Production MOQ 20 pieces per design. Simple 3D is USD $50 when that CAD scope applies.

Does not apply when the tear is in the *dump* (chocolate serifs, gummy ears) rather than in the rubber — that is geometry vs contraction, not a D624 problem. It also does not apply to HCR/LSR *part* tear on a steel tool; those drawings go to wydsilicone.com.

Two specs, two places on the tool

Cavity skin. Soft enough to peel (often 10A–20A for gloves and undercut food/resin; 20A–30A when the wall is already thin and jacketed). Tear grade is the TDS line that predicts whether a nick at a logo becomes a split. You cannot read tear off the Shore number.

Jacket / mother mold. This is where stiffness lives for a loaded tray or a wet concrete pour. Edge sag of about 0.5–1.5 mm on an unsupported sheet is a missing jacket, not a "too soft" cavity. Hardening the skin to stop sag makes peel worse and does not hold the outer row.

When a logo wall is hair-thin, the rubber island has nothing to tear *through* — it just rips. That is a logo / thin-wall tear and a wall thickness problem. Shore will not grow an island.

flowchart TD
  A[Mold tore] --> B{Tear in the rubber or in the dump?}
  B -->|Dump| C[Geometry / contraction — not a Shore bump]
  B -->|Rubber| D{Where?}
  D -->|Logo serif / thin island| E[Thicken island, add draft, recast]
  D -->|Hook / undercut| F[Split, plug, or core — structure]
  D -->|Whole wall ripped on a floppy sheet| G[Jacket; keep the soft skin]
  E --> H[Keep or raise tear grade; do not start at 40A]
  F --> H
  G --> H

Typical shop values

Caption these as typical shop values.

We do not invent a lot-specific kN/m tear number here. Ask for the grade's TDS tear line if you need it on the PO, and treat it as typical-grade data, not a named SGS report.

We would not recommend

We would not recommend raising cavity Shore as the first response to a tear. Send the photo of the split. Island, hook, or floppy sheet — three different recasts.

We would not recommend a 40A one-piece on a hooked figurine "so it will not tear." You will tear it sooner.

We would not recommend skipping the jacket on a half-sheet gummy tool because the TDS tear number looked high. Tear grade does not hold 0.5–1.5 mm of edge sag.

We would not recommend using ISO 3302-1 M2 as a stand-in for tear. M2 is a drawing class for rubber articles.

Worked example (anonymized)

A 16-cavity resin badge, 2 mm letters, platinum 12A skin, no jacket. Tear at the letter "A" crossbar on demold 8. Buyer asked for 30A. We recast the same Shore, thicker island on the crossbar (about 0.4 mm recessed), added a light draft, and poured a thin rigid jacket. CAD needed a letter tweak; simple 3D at USD $50 applied. Prototype after that CAD lock in 3–7 business days. Letters survived the next dumps. 30A was never poured. MOQ 20 per design on the recast. Master stayed unsanded except for the letter edit — tooling charge still is not unrestricted ownership; the PO named custody.

The photo that would prove it: torn crossbar still in the cavity, calipers on the island thickness, vs the recast with the thicker bar. A hand-flex hero shot of "soft silicone" proves neither tear nor Shore.

Common mix-up: the pinch-white test as tear QC

Pinching a mold until it turns white says something about fillers and cheap gum. It does not measure D624 tear, and it does not tell you whether a 0.3 mm serif will survive. Buyers fail a high-tear 15A skin because it pinch-whites, then accept a filled 40A that splits at the logo. If you care about food-doc vs fillers, that is a different page. If you care about tears, send the split photo and the island width.

What to ask when you're buying

• Cavity Shore and jacket specified separately?
• Tear grade: are we staying on a high-tear platinum skin?
• Thinnest rubber island on demold (logo, horn root, score line) in millimetres?
• Loaded sheet or wet pour — jacket yes/no?
• Recast from the unsanded master if a nick grows later?

Related: custom RTV molds, wall thickness, 10A vs 30A resin, contact.

RFQ copy-pack — tear vs Shore

Copy to [email protected]:

• Photo of the tear in the cavity, raking light
• Dump and how you demold (flex, air, split)
• Current cavity Shore if known; jacket yes/no
• Logo / letter file or the locked CAD
• What you want next: island fix / split / jacket — not "just harder"
• Quantity: recast prototype, then MOQ 20 per design
• Simple 3D: USD $50 only if letters or CAD still need work

Matched next step: send the tear photo and the island dimension, not a Shore vote. Phone +86 135 3006 2969. Licheng Technology Industry Park, Xinhe Blvd, Shajing, Baoan.

Jacket Shore vs cavity Shore on the same PO

You can — and often should — spec two durometers. The skin stays in the 10A–20A peel band. The jacket is a rigid plaster, resin, fiberglass, or printed shell; it does not get a Shore A line because it is not the cavity rubber. If someone quotes a "40A mold" as a single number, ask whether they hardened the skin or actually built a mother mold. Those are different line items on the PI.

A recast from the unsanded master can change tear grade without touching Shore. That is the cheaper lever when the geometry is already right and the first gum nicked at a sprue. If the nick is at a serif, change the island; if it is at a hook, change the split. Keep a short note on the RFQ: "cavity Shore stays, jacket stays, tear grade up" so purchasing does not "simplify" it back to 30A.

If you are comparing two suppliers, ask both for cavity Shore, jacket yes/no, and the TDS tear line. A quote that only lists Shore is not a tear spec.

FAQ

Can a 10A mold really last longer than a 30A mold? On a hooked or logo-heavy part, yes — if the 10A is a high-tear skin in a jacket and the 30A is a stiff one-piece. Life follows tear plus structure, not the bigger Shore number.

Is tear strength on the TDS enough to skip a jacket? No. Jackets hold sag and wet mass. Tear grade holds nicks. Different jobs.

Should I order the highest Shore the catalog lists for "heavy use"? Order a high-tear platinum skin and a jacket if the pour is loaded. Catalog max Shore is often a poor peel.

Does platinum vs tin change tear more than Shore does? Chemistry changes odor, shrink drift, and inhibition. Tear is grade-specific inside each family. Do not assume tin is tougher because it feels stiffer in a cup kit.

If you recast after a tear, is that a new MOQ 20? Production quantities stay MOQ 20 pieces per design. A paid recast prototype can sit in front of that. CAD lock restarts the 3–7 day clock only if the model changed. The $50 line applies only when we still need simple 3D.