High-Tear RTV vs LSR: When the Cavity Is Still Enough

High-tear LSR and high-tear RTV are not two grades of the same tool. On this site the silicone is the cavity: platinum RTV poured against a master, keyed, and jacketed so you can cast chocolate, gummies, resin, or a short industrial run. LSR is a heat-cured, injection-molded part. That tool is steel or aluminum with a cold-runner, not a LIM hot tool, and it will not dump pectin.

Shops upgrade at the wrong moment because a datasheet printed a large ASTM D624 number next to the word silicone. Tear on an LSR keypad is why the webbing survives a nick in service. Tear on a platinum RTV glove mold is why the cavity survives a resin pull. Those numbers are not interchangeable, and neither one is a cycle-life guarantee.

Stay on platinum when the job can cure. Tin-cure RTV can be high-tear on paper and still shrink in the library. Peroxide HCR is a millable factory path, not a poured mold.

Two tear numbers, two objects

ASTM D624 (or ISO 34-1) measures how a nicked specimen resists unzipping. Die B and Die C are not one scale. Read the die before you compare a “high-tear” LSR to a mold rubber.

RTV tear is a property of the cavity you pour into resin, wax, plaster, chocolate, or a low-melt. A high-tear platinum 20A skin inside a mother mold is a classic undercut tool. A stiff 40A with modest tear will split at the first negative draft even if tensile looks fine.

LSR tear is a property of the article you sell: bellows, baby-bottle nipples, face-mask skirts, connector seals. Heat cure and a tight network are why production LSR often sits above a poured RTV of the same Shore A on tear and tensile. That is a part decision on a different PO.

If the RFQ says “high-tear silicone mold” and the deliverable is 10,000 LSR gaskets, you asked for the wrong object. If the deliverable is a block mold for twenty urethane masters, LSR injection is the wrong object.

Common mix-up: raising Shore A to “make it stronger”

Raise hardness and you often lose the stretch that made the undercut possible. Tear and elongation are the stretch story. Hardness is the cavity-support story. The jacket holds size; the skin peels. Swapping a 20A high-tear platinum for a 40A general grade to “last longer” is how a letter with negative draft unzips on dump five.

What to ask when you're buying

• Is the deliverable a poured cavity I'll cast into, or a molded silicone *part*?
• Shore A of the cavity, ASTM D624 die (B or C), and is there a rigid jacket?
• Platinum or tin, and why (size hold vs dirty-master forgiveness)?
• One-piece peel, keyed split, or inner core for the undercut?
• Who keeps the master, and is a recast of the RTV skin in the price?

Worked example: 12-cavity bear, 18 mm, undercut ears

A confectionery line wanted a 12-cavity bear, 18 mm tall, food-contact, about 2,000 dumps a month, ears that lock. The RTV job is a CNC or fully post-cured SLA master, a high-tear platinum 10A–20A skin, keyed two-piece if the ears cannot peel, and a rigid jacket so the tray doesn't balloon. Recast the face when a tear reaches a logo.

A parallel PDF quoted high-tear LSR, heated steel, and a cold-runner. That's a molded silicone *part*. It doesn't ship as a tray you pour into. Both documents said “high-tear mold.”

When poured platinum RTV remains the right tool

RTV-2 platinum high-tear grades exist because undercut masters and resin (or food) casting are real work. They win when:

• The “tool” must stretch off a locked master, then rebound.
• You need the cavity this week, not after steel.
• The production article is not silicone. You are molding something else in a silicone cavity.
• Lot size is prototypes, bridge, texture library, or food dumps in the hundreds-to-low-thousands band on a tray.

Stay with platinum RTV if the master is clean enough to cure. If sulfur clay, amines, or undercured SLA poison the face, high-tear platinum has zero life. Switch to tin for a copy mold, then platinum for the production cavity. Tear rating does not cure inhibition.

Tin-cure high-tear is a short-run and dirty-master rubber. Library shrinkage still happens. Do not archive it as if it were platinum. Do not quote its tear as an LSR part spec.

A hybrid shops actually run: RTV high-tear glove for the first geometry freeze, then — if you later need vulcanized silicone *parts* — a new RFQ to a parts OEM. That's an upgrade of object, not a swap of tear numbers on the same PO.

The upgrade triggers that are not “more Shore A”

Upgrade out of RTV tooling when any of these show up:

• You are replacing block molds faster than you are shipping silicone parts (not castings).
• Flash, gate, and knit-line requirements belong on a steel tool, not a poured split.
• The customer's incoming inspection is ISO 3302-1 on a molded elastomer part, not “looks like the master.”
• Cleanroom, extractables, or a named ISO 10993 / USP Class VI compound are on the drawing. Those are LSR or HCR articles.

Stay on RTV when the master will change twice more, when the casting resin (or pectin, chocolate, wax) is the product, or when twenty parts is the entire job.

ASTM D624 is not a cycle count

Vendors print tear because undercuts nick rubber. They do not print “this mold survives 200 urethane pours” as a standard. Resin exotherm, release chemistry, and how you lever the part out decide the production clock. A high-tear RTV skin can still split if the mother mold does not support it.

Compare grades only:

• Same cure family (platinum RTV vs platinum RTV)
• Same die (B or C) and same unit (kN/m or ppi)
• Same hardness band, or admit you changed two variables

Do not paste an LSR tear onto an RTV RFQ to “make the mold stronger.” If the mold is tearing, change the RTV grade, the thickness of the skin, the mother mold, or the pull.

When this article is the wrong quote

If you actually need vulcanized silicone parts in the thousands, this RTV stack is the wrong asset. Quote HCR compression or LSR injection with a cold-runner. Geometry can override quantity: a one-off sculpture with a locked undercut is still high-tear RTV; 50,000 nipples are not. Tooling charge ≠ unrestricted ownership of the master.

Related reading: custom RTV silicone molds, what tooling means, gummy trays, custom mold design.

What to send for an RTV mold quote

• Object: poured platinum RTV cavity, high-tear grade
• Master: CNC, SLA/DLP, or sample; who keeps it
• Undercuts, bails, two-sided logos; peel vs split vs core
• Shore A, D624 die if you already care
• Casting material, temperature, monthly dumps
• Jacket / mother mold; recast of the skin included or extra
• Food-contact path if needed: 21 CFR 177.2600 and/or LFGB / BfR XV

Send master photos (or the STL), the undercut, and what you will cast. We'll name peel vs split, jacket, and whether high-tear platinum is the grade or you actually needed a parts quote.

We would not recommend raising Shore A first

We would not recommend a one-piece open-back mold if the part has a bail, a letter with negative draft, or a two-sided logo. Raising Shore A first is usually the wrong fix for a tearing cavity; buy tear and a jacket. We would not cut LSR steel “so we can scale later” while you still need a cavity to dump pectin into.

RUUIPON Mold Structure Decision: flat back? open-back RTV. Else deep undercuts the rubber can peel? stay one-piece. If it cannot peel, two-part / plug / inner core. Large wet casting or Shore 10A–20A? mother mold.

Typical shop values: high-tear RTV cavity

| Item | Typical shop value | | --- | --- | | Cavity Shore for undercut food/resin | 10A–20A high-tear platinum, jacket behind it | | Mix | Platinum 1:1 or 10:1 by grade; vacuum degas until foam collapses | | Linear shrink | ~0.1–0.3% platinum; tin keeps moving in storage | | Tear comparison | Same family, same ASTM D624 die (B or C) | | Outer-row gummy vs 4.0 g target | Edge often 3–6% heavy if the loaded tray sags | | Loaded-tray edge sag | About 0.5–1.5 mm on an unsupported large food tray |

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

FAQ

Is high-tear LSR just a stronger version of platinum RTV mold rubber? No. LSR is injection-molded and heat-cured in metal tooling, usually with a cold-runner. RTV is poured at shop temperature and *is* the tooling. Both can be high-tear on ASTM D624. They are different objects on the purchase order.

When does a shop outgrow RTV block molds? When the deliverable is vulcanized silicone *parts*, geometry is frozen, and the drawing wants production flash, gates, and a named LSR compound. Bridge runs, changing masters, and food/resin castings still belong in RTV.

Can I raise Shore A instead of buying a high-tear grade? Usually not for undercuts. Higher hardness fights stretch. Tear and elongation decide whether a nick at a negative draft unzips. Buy hardness for cavity support or sealing force, and tear for nicked geometry. Add the jacket; don't just go stiffer.

Does a high ASTM D624 number guarantee long mold life in urethane? No. Tear is a nicked-specimen test. Urethane exotherm, release, and how the part is levered out set the production clock. Use the number to compare grades in the same family and die, not as a cycle quote.

Should medical silicone parts be prototyped in tin-cure high-tear RTV? Not as the device material. Tin-cure is a mold rubber for dirty masters and short copies. Medical articles almost always start as platinum LSR or platinum HCR. You can still use RTV as *tooling* to cast a stand-in, if the stand-in is clearly not the device compound.