LSR Part Molding vs RTV Cavities: Different Quotes

If you're reading about liquid silicone rubber molding on this site, pause on the object. LSR molding is platinum LSR pumped through a mixer into a hot steel or aluminum tool on a LIM (liquid injection molding) cell. The cavity is hot on purpose. The rubber arrives cold and vulcanizes in the tool. Cycles are short compared with many HCR compression shots. Flash is a shut-off problem. That process isn't a 1:1 craft cup, not tin-cure RTV, and not millable HCR on a press. If a quote says "we pour LSR into the mold," the quote is describing RTV or a misunderstanding.

On RUUIPON, the silicone is the cavity: master, box, degassed pour, keys, jacket. If you need LIM *parts*, that's a different WYD quote. This article exists so a cold-runner PDF and a chocolate-tray PDF don't share a line.

This is a factory-process decoder for buyers who landed here with the wrong object. It isn't a machine manual.

LIM is a machine cell, not a poured kit

A LIM cell is pumps (often 1:1 LSR A/B), a static mixer, an injection unit, a heated tool, and a control system. Color can be a third stream. The operator isn't stirring a bucket. Pot life in a cup is the wrong mental model. If the shop's photo is a mixing stick, you aren't looking at LSR production — you may be looking at RTV, which is this site's actual product.

LSR grades are addition-cure platinum systems. Inhibition (sulfur, some amines, some tin residues, some dirty prints) is a real cell problem when inserts or overmolded substrates are dirty. It isn't the same as tin-cure RTV's condensation chemistry. Don't store LIM discipline next to a tin RTV bench without a contamination plan.

Baby care, many medical components, keypad webs, and high-volume consumer *parts* live on LIM because the liquid can fill thin walls and the shut-off can be tight. Those applications still need named files: FDA 21 CFR 177.2600, ISO 10993, USP Class VI, ISO 13485 as a quality system when the device file needs it. LSR isn't automatically medical.

Chocolate trays, gummy sheets, resin figurine molds, and other cavities you dump into live on platinum or tin RTV. Same word "platinum." Different factory.

Cold runner, dump runner, and waste you will pay for

Cold-runner manifolds keep LSR from curing in the feed. Hardware costs more in the tool. Waste rubber per shot can drop. Valve gates and balancing are part of the job. Never call this a "LIM hot tool" as if the runner were heated; the *cavity* is hot, the runner is often cold.

Dump-runner (or similar open runners that cure and are thrown) is cheaper tooling and more scrap rubber. Fine for some volumes. Expensive if you pretended the runner was free.

Gate vestige will exist unless you hide it. Put the gate on a hidden face. Vents at last-to-fill prevent air traps and burn-like defects. "Bad silicone" is usually a vent or a balance problem.

Multi-cavity LSR tools only pay if the runner actually balances. Cavity 8 is the qualification, not cavity 1 next to the sprue.

On an RTV tray, there is no cold-runner. Balance is pour or deposit plus a jacket that stays flat. Don't paste runner waste math onto a flexible sheet.

Heat, cure, and why the tool is hot

Typical LSR tools run hot (shop practice often in a 160–200 °C conversation, grade-dependent). The liquid fills, then crosslinks. Cooling the steel as if it were thermoplastic injection is the wrong instinct. Heaters and thermocouples are part of making the *part* mold. Uneven heat makes sticky pockets and undercured lips.

Post-cure, when the file asks for it, is an oven step after demold. Measure ISO 3302-1 dimensions after that step if the lot is post-cured. ISO 3302-1 is a drawing class, not "precision." Shrink for LIM is in the heat-cure band (about 2–3%), not in the 0.1–0.3% band of many platinum RTV mold rubbers. Cut the steel with the shrink policy you will keep. Don't paste that 2–3% onto an RTV master.

Aluminum prototype LSR tools exist. They're still injection tools. They aren't RTV jackets. Steel is a duty and shut-off choice for volume and texture (VDI 3400 / MT 11000).

What LSR does poorly — including being a chocolate tray

LSR is a poor default when:

• You needed five gaskets yesterday and you don't have a LIM cell (HCR aluminum may win — still a parts quote).
• The geometry is a large simple slab that compression likes.
• The silicone is supposed to *be* a flexible mold for chocolate or resin (that is RTV, this site).
• The budget is a poured sample and the expectation is LIM flash.

Overmolding onto plastics or metals is a real LSR strength and a real inhibition and bond-primer problem. It isn't "just inject." Substrates need a process (plasma, primer, dryness) named on the RFQ. Two-shot cells that mold plastic then LSR are a further machine class. Don't assume a single-shot LIM press can do that job without the rotary or transfer hardware.

Draft, polish, and vacuum still control strip. Soft LSR (low Shore A) sticks on a mirror cavity even when the chemistry is correct. Release spray is a last step, and it can contaminate medical and food-contact files. Design the strip. Don't perfume it.

RTV peels. A 10A–20A skin against a master doesn't need a hot cavity. It needs degas, a jacket, and a peel path.

Specifying an LSR job so the quote is for LIM — and an RTV job so it isn't

For LIM, write: platinum LSR, Shore A, LIM, cavity metal, runner type (cold-runner vs dump-runner), quantity band, named standard if any, shrink owner, gate face. Explicitly ban RTV substitution and HCR substitution. Ask for a machine list (injection volume, clamp) and a first-article pack with flash photos.

For this site, write: platinum or tin RTV, mix 1:1 or 10:1, Shore of the skin (often 10A–20A), jacket, master type, recast. Explicitly ban LIM substitution.

A practical China evaluation for LSR is a live look at pumps and a hot tool, not a warehouse of compression bakeware. Plenty of Shenzhen-area shops run real LIM. Plenty of traders will still send a poured tray. The RFQ sentence is how you sort them. If you wanted the tray, you wanted the vacuum chamber.

First article on LSR should include weight, flash photos of the worst cavity, and the dimensions you actually fixture. A pretty single pocket isn't a cell capability. ISO 3302-1 classes still apply to the rubber article; the steel can be better than the part.

LSR molding is a cell. Buy a cell on a parts OEM. Don't buy a drum and a hope, and don't buy it on an RTV PO.

Common mix-up: a cup kit labeled LSR

A lot of RFQs treat a 1:1 kit stamped "LSR" as production liquid silicone. In the shop that usually means RTV poured into a box — which may be the right product, wrongly named. Production LSR is injected on a LIM cell. If you need a flexible cavity, specify platinum or tin RTV. If you need LSR *parts*, specify LIM on a WYD-style quote.

What to ask when you're buying

• Is the silicone the cavity (RTV) or the article (LSR/HCR)?
• If LSR: pumps, hot cavity, cold-runner or dump-runner — parts OEM?
• If RTV: vacuum degas, master, jacket, platinum or tin?
• Will anyone paste a 2–3% LIM factor onto the RTV master?
• ISO 3302-1 on the article drawing, not "precision"?

Worked example: baby-care membrane vs gummy tray

A buyer wanted a 0.4 mm 30A membrane, about 50,000 pieces a month, platinum LSR, cold-runner, ISO 3302-1 on the lip. That's a LIM cell — different WYD quote. A sister PO wanted a 16-cavity gummy bear, 22 mm, 4.0 g, food-contact, about 8,000 dumps a month. That's platinum RTV, 10A–20A skin, rigid jacket so the outer row doesn't sag 0.5–1.5 mm. Same "liquid silicone" email. Two objects. The LIM PDF doesn't deposit pectin.

When this decoder is the wrong page

If you already know you need LIM parts, don't linger here — send the parts OEM the cell pack. If you need a flexible cavity, don't add heaters to the RTV 2D. A tiny shallow one-piece ice tray can skip a heavy jacket. Geometry can override quantity: a one-off deep-undercut figurine is still RTV at 20 casts, while 50,000 membranes are still LSR.

Related reading: what a custom RTV mold is, compression contrast, how molds are made, custom RTV molds.

Object check before anyone quotes a LIM cell

Copy this. An LSR line without these splits isn't comparable to an RTV bid.

• Object A (this site): poured RTV cavity — master, mix 1:1 or 10:1, jacket, recast
• Object B (WYD-style parts quote): platinum LSR LIM — hot cavity, cold-runner or dump-runner, flash class
• Explicit ban on substituting A for B or B for A
• Shrink: RTV ~0.1–0.3% after full cure; do not paste LSR 2–3% on the master
• ISO 3302-1 as drawing class on the rubber article

Send the STEP and one sentence: cavity you dump into, or vulcanized LSR part. We'll keep this PO on platinum vs tin and the jacket, and we'll tell you when the cell belongs on a different RFQ.

We would not recommend pouring a cup kit and calling it LIM

We would not recommend a heated-cavity drawing on a chocolate tray. We would not paste a 2–3% LSR factor onto a platinum master. We would not raise Shore A first to "make it injection." We would not treat a mixing-stick photo as a LIM cell, or a LIM hall tour as proof of vacuum degas.

RUUIPON Mold Structure Decision: if you dump or pour into silicone, stay RTV. Flat back? open-back. Deep undercuts that peel? one-piece. If they don't, two-part / plug / core. Soft 10A–20A or wet dump? mother mold. If the silicone *leaves* a hot tool, stop — that's LSR on a parts OEM, often with a cold-runner.

flowchart TD
  A[What is the silicone?] --> B{Cavity you dump into?}
  B -->|yes| C[RTV: master box degas jacket]
  B -->|no, injected article| D[LSR LIM: different WYD quote]
  D --> E[Hot cavity; cold-runner or dump-runner]
  C --> F{Photo is a mixing stick?}
  F -->|yes| G[That's RTV, name platinum or tin]
  A --> H{Cup kit labeled LSR?}
  H -->|yes| G

Typical shop values

Caption: Typical shop values (not a named lot).

| Item | Typical shop value | | --- | --- | | RTV mix (this site) | Platinum 1:1 or 10:1; vacuum degas | | RTV cavity Shore | 10A–20A skin with jacket | | Platinum RTV shrink | About 0.1–0.3% after full cure | | Loaded-tray edge sag without a jacket | About 0.5–1.5 mm | | Contrast: LSR *part* shrink | About 2–3% heat-cure class — do not put this on the master | | Contrast: HCR *part* shrink | About 1.5–2.5% — different RFQ | | LIM tool temperature (parts) | Often a 160–200 °C conversation, grade-dependent |

FAQ

Is liquid silicone rubber the same as platinum RTV? Both can be platinum addition-cure. RTV is poured to make a flexible mold (or a room-temperature part). LSR production is injected into a hot LIM tool. Shrink, flash, and paperwork won't match. A cup kit labeled LSR is almost always RTV.

Can I compression mold LSR on an HCR press to save a LIM cell? Some special cases exist. They aren't the default and they won't copy LIM flash and cycle. If you wanted compression, specify millable HCR on a parts OEM. If you wanted a tray, specify RTV.

Why is the LSR mold hot if the rubber is "liquid"? The liquid must vulcanize in the cavity. Heat is the cure, typically in a hot-tool band around 160–200 °C depending on grade. Thermoplastic-style cold tools are the wrong picture for LIM. RTV cavities aren't heated that way.

Does LSR automatically meet FDA or medical files? No. Food-contact and medical claims are grade and process files (FDA 21 CFR 177.2600, ISO 10993, USP Class VI, ISO 13485 as a system). LSR is a process family, not a certificate. Same rule for platinum RTV trays.

What's the fastest way to get a fake LSR quote? Send a photo and accept "we pour LSR." Demand pumps, a hot tool, and a ban on RTV substitution — then the quote is for molding, or it disappears. If you actually needed the pour, name RTV on this site instead.