Compression HCR vs RTV Molds: Two Different Quotes

If you're reading about compression molded silicone products on this site, pause on the object. Compression here is usually HCR (high consistency rubber, millable, sometimes called HTV) formed in a heated press: an operator places a weighed preform, the mold closes, heat and pressure vulcanize the gum, and flash leaves at a designed land. That's a molded silicone *part*. It isn't a poured platinum or tin RTV cavity. Buyers who say "compression LSR" are mixing a press motion with a pumped platinum liquid. Buyers who send that RFQ to an RTV jacket shop are mixing factories.

On RUUIPON, the silicone is the cavity: master, box, keys, jacket. If you need compression-molded *parts*, that's a different WYD quote. This article exists so those two PDFs don't land in the same spreadsheet.

Some shops can compression-mold certain liquids in special tools. It isn't the default, and it isn't what this contrast describes.

The press cycle in shop language (the parts object)

The mill mixes HCR with filler, peroxide or platinum cure system, pigment, and whatever the grade file allows. Sheet or pig comes off the mill. Preforms are cut or blanked to a weight. The tool is hot. The charge goes in. The press closes. Cure time is a function of thickness, temperature, and the recipe, not a cup pot life.

Then the mold opens. The article is picked or brushed out. Flash is torn or trimmed. Some peroxide grades go to a ventilated post-cure oven. First article measures after that oven if the lot will be post-cured.

Compare that to LSR: pumps, static mixer, injection into a hot cavity, often a cold-runner, short cycles, different flash physics. Compare that to RTV (this site): mix A/B 1:1 or 10:1, vacuum degas, pour, room-temperature cure, the rubber *is* the mold. Compression HCR shares heat with LSR and shares almost nothing with a chocolate tray.

Preform, flash, and why the charge weight matters

Charge weight is the gate on a press. Too light: shorts, knit, unfilled logo pockets. Too heavy: thick flash, hard close, plate that won't shut, dimensions that walk. Placement matters as much as grams. A charge dumped on one side of a large gasket fills late on the other side.

Flash isn't a defect by default. It's the overflow the land was cut to allow. A compression drawing that demands "zero flash like LSR" is a process change request. You can tighten the land. You can't repeal overflow. Transfer molding (a pot and sprue) is the related process when you don't want the blank sitting in the cavity. It's still millable HCR. It still isn't LIM, and it still isn't an RTV pour.

Operators and loading fixtures decide whether a 4-cavity compression tool is actually four cavities or one good pocket and three excuses. Multi-cavity advantages here are handling advantages, not runner-balance slogans, and not RTV dump-weight slogans.

On an RTV tray, the analog of charge weight is dump or deposit into each pocket, plus a jacket that stays flat. Don't paste a flash-land note onto a flexible food mold.

Geometry that compression likes — and what RTV likes instead

Compression likes:

• Moderate thickness without hair-like membranes.
• Simple stacks that can take a charge.
• Parts where a trim line is acceptable.
• Bakeware and gaskets that won't pay for LIM steel.

Compression dislikes:

• Deep, sharp undercuts without splits or picks.
• Ultra-thin webs that LSR would inject.
• Optical cosmetics that hate a flash land.
• Tiny serif logos far from the charge.

RTV likes a peel, a master, and a jacket. Deep undercuts that a 10A–20A skin can peel are normal RTV. A flash land isn't. If your geometry is a chocolate mascot or a gummy sheet, you're on this site. If it's a gasket with a trim line, you're on the parts quote.

Draft still exists on metal compression tools. Elongation isn't a waiver. 0.5–2° on vertical metal faces is the same conversation as other rigid tools, more with VDI 3400 texture. Zero-draft bakeware that operators pry will flash worse on the next shot. RTV mold draft can be lower because the rubber flexes — don't copy that onto the HCR plate.

Peroxide versus platinum HCR on the same press

Peroxide HCR. Common, often economical, needs the post-cure the recipe calls for if volatiles and compression set matter. Food-contact and medical files are grade-specific, not automatic because the press is the same.

Platinum HCR. Addition-cure in millable form. Chosen when the file needs that chemistry (some food and medical paths). Inhibition and cleanliness look more like LSR discipline than like a dirty mill room. Don't mill platinum HCR in a peroxide-dirty mill and expect the file to survive.

Same press, different compound control. Same ISO 3302-1 language for dimensions — that's a drawing class, not "precision." Different ovens. Different COCs. A shop that says "we compress silicone, 50A, food grade" without peroxide versus platinum isn't specifying a product.

FDA 21 CFR 177.2600 and LFGB attach to the grade and the article, not to the word compression. UL 94, if you need it, attaches to a specimen thickness. None of those is a press setting, and none of them turns an RTV tray into HCR.

When compression loses to LIM — and when both lose to RTV

Choose LSR LIM when flash class, thin walls, high cavity-count automation, and a platinum LSR file are the product. Choose compression when the geometry and the volume match a mill and a press, including many programs that stay on aluminum tools for years. Choose RTV when the silicone is the cavity you pour or dump into.

A China evaluation for compression is a mill room, a press list, and a trim method, plus compound lot control. A LIM cell video doesn't prove HCR. A poured RTV tray doesn't prove HCR. Ask to see a preform and a flash land on a tool like yours — on the parts OEM. Ask to see vacuum, master, and jacket on this site.

ISO 3302-1 is the usual dimensional language for the rubber *article*. Don't paste a metal ±0.05 mm on a floppy gasket and call the press incapable. Don't paste it on a 10A–20A RTV tray either.

Compression molded silicone products succeed when the RFQ says HCR, names peroxide or platinum, and designs a land instead of wishing for injection scars. Write that on the WYD-style parts quote. Write master, mix, jacket, recast on this one.

Common mix-up: "compression silicone mold" on an RTV RFQ

A lot of RFQs treat compression, LIM, and poured RTV as three ways to make "the mold." In the shop that usually means one bidder mills a flash land and the other pours a glove. You can't tell until the object is named: cavity you dump into, or article that leaves a press.

What to ask when you're buying

• Is the silicone the cavity (RTV) or the article (HCR/LSR)?
• If HCR: peroxide or platinum, flash land, post-cure — and is that a parts OEM?
• If RTV: platinum or tin, mix 1:1 or 10:1, Shore of skin, jacket?
• Who keeps the master vs who keeps the steel?
• ISO 3302-1 on the article drawing, not "precision"?

Worked example: gasket on a press vs tray on a jacket

A buyer wanted a 120 x 80 x 3 mm gasket, 50A, about 5,000 pieces a month. That's millable HCR, aluminum two-plate, flash land, ISO 3302-1 on the article — a parts quote. A sister PO wanted an 8-cavity chocolate bar, 120 x 60 x 12 mm, about 500 pours a month, food-contact. That's platinum RTV, 10A–20A skin, rigid jacket, recast clause — this site. Same outline. Two factories. Lining them up as "compression vs silicone mold" is how purchasing files two objects as one.

When this contrast does not apply

If you already know you need HCR parts, don't linger here — send the parts OEM the mill-and-press pack. If you need a flexible cavity, don't add a flash land 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 resin figurine is still RTV at 20 pieces, while 10,000 identical spatulas are still HCR.

Related reading: what a custom RTV mold is, LSR molding contrast, what tooling means, custom RTV molds.

Object check before anyone quotes a press

Copy this. A compression line without these splits isn't comparable to an RTV bid.

• Object A (this site): poured RTV cavity — master, mix, jacket, recast
• Object B (WYD-style parts quote): HCR compression — mill, preform, flash land, post-cure
• Explicit ban on substituting A for B or B for A
• ISO 3302-1 as drawing class on the rubber article, not a slogan
• Food-contact path attached to the grade you will sell

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

We would not recommend compressing RTV as the production article

We would not recommend a flash-land drawing on a chocolate tray. We would not raise Shore A first to "make it compression." We would not treat a mill-room tour as proof of vacuum degas. We would not paste LSR zero-flash onto an HCR land, or HCR shrink (about 1.5–2.5%) onto a platinum master.

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 press, stop — that's HCR on a parts OEM.

flowchart TD
  A[What is the silicone?] --> B{Cavity you dump into?}
  B -->|yes| C[RTV: master box keys jacket]
  B -->|no, article from a press| D[HCR compression: different WYD quote]
  D --> E[Mill preform flash land]
  C --> F{10A-20A or wet dump?}
  F -->|yes| G[Mother mold]
  A --> H{LIM flash thin wall?}
  H -->|yes| I[LSR part RFQ, not this pour]

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: HCR *part* shrink | About 1.5–2.5% on steel/aluminum — different RFQ | | Contrast: LSR *part* shrink | About 2–3% heat-cure class — different RFQ |

FAQ

Is compression molding the same as LSR molding? No. Compression here means millable HCR, a preform, and a flash land. LSR is injected as a platinum liquid on a LIM cell, often with a cold-runner. Don't order one and expect the other's flash. Neither is a poured RTV tray.

Do compression silicone products always have flash? They have a land that was designed to overflow. You can trim and you can tighten. "No flash" as an LSR-style shut-off is a different tool. An RTV tray doesn't use that land.

Can platinum RTV be compression molded as the product? RTV is a poured mold rubber. Don't plan production articles as compressed RTV. If you need a flexible mold, pour it. If you need HCR parts, mill and press them on a parts OEM.

Why does peroxide HCR need post-cure? Many peroxide recipes use a ventilated oven to drive residuals and to stabilize compression set. Measure dimensions after that oven if the lot is post-cured. Platinum HCR follows its own file. Platinum RTV follows a room-temperature or mild-oven cure on this site.

What should I inspect on a China compression-mold supplier? Mill, press platens, preform control, trim, and compound storage, plus a first-article that includes flash photos. A showroom of trays is not a mill room. If you needed the tray, inspect vacuum and the master bench instead.