High-Volume Cold Runner Systems for Zero-Waste LSR Production Tools

A cold runner for LSR is a cooled manifold feeding a hot cavity. The silicone in the runner stays liquid. Only the part vulcanizes. That is the opposite of a thermoplastic hot runner, and it is the reason high-volume LSR can approach zero runner waste.

“Zero-waste” is a direction, not a literal empty drum. You still purge at color change, scrap start-up shots, and lose the occasional reject. What you should not lose, on a 32-cavity baby teat or a 64-cavity O-ring family, is a cured sprue that weighs as much as the parts.

Do not specify a brand of cold deck as if it were a material. Specify the thermal split, the gate style, and the waste you will not accept.

Why LSR cannot use a plastic hot runner as-is

LSR is a platinum thermoset. Heat is the cure. A manifold that stays hot would vulcanize in the channel and block the tool. The barrel, the static mixer, and the runner stay cool (water-jacketed; typical talk is a cool circuit in the 10-25 °C neighborhood, follow the machine and deck builder). The cavity plates are hot (often on the order of 160-200 °C, grade-dependent). Insulation and a sharp thermal break at the gate keep those two worlds from averaging into a scorching orifice.

A “hot runner LSR tool” in casual speech is usually a misnamed cold-runner deck, or an older open-sprue tool that still cures a runner. Ask which.

Open cold sprues still exist: a cooled sprue bushing into a small runner that *does* cure with the parts. You throw that runner away (or, rarely, grind it for non-critical filler, which is not a quality path for medical or food). That is not a zero-waste tool. It is a cheaper tool.

Valve-gated cold decks vs. open gates

Valve-gated (needle) cold runner. Each cavity has a needle that shuts the gate. The next shot pushes liquid LSR that never cured. Gate vestige is a small, repeatable bump or a flush face, depending on set. Balancing is done with needle timing and channel design, not by throwing more rubber into an overflow. This is the high-volume, low-waste architecture.

Open gate into a hot cavity. Some tools still freeze (cure) a small gate nub you destage. Waste is lower than a full runner tree, higher than a true valve gate. Useful on larger parts where a vestige is hidden.

Cold runner with a small cured sub-runner. A compromise on very small parts or when a deck with 64 needles is not in the budget. Count the cured rubber per shot before you call it “almost zero.”

Needles, heaters on the cavity side, and cooling on the deck make the tool cost real. The payback is material plus destage labor plus the cosmetic rejects you used to get from tearing a sprue off a teat.

When the deck pays

The arithmetic is simple even without fake prices. If the cured runner plus sprue is a large fraction of shot weight, and the gum is a healthcare, optical, or pigmented LSR you will not re-use, a cold deck starts to win as cavity count and annual volume rise. Tiny O-rings and infant products are the classic case: the tree can outweigh the rings.

The deck also pays when destage would mark a cosmetic or skin-contact face. A valve gate on the back of a strap is a process. A torn sprue on the same face is a defect.

The deck does not pay on a two-cavity industrial boot you run a few days a year, in a general HCR-adjacent LSR, with a hidden gate. An open cold sprue is honest there.

What “zero waste” still requires from the tool

• Balance. Unbalanced cavities overfill the easy one and starve the hard one. You then add pressure, flash the easy cavity, and scrap both. A cold deck does not magically balance a bad layout.
• Thermal stability. A warm deck scorches in the needle. A cavity that is too cool under-cures and smears on eject. Water scale in the deck is a process drift. Treat cooling water as a quality parameter.
• Needle condition. A scored needle leaks, strings, or drools into a hot gate and cures a crust. That crust is waste and a vestige defect. PM the needles.
• Purging discipline. Color and grade changes still cost rubber. Design the mixer and hoppers so you are not purging a five-kilogram loop to change a 20-gram shot if you change colors daily.
• Part design. A valve gate needs a land. Putting it on a sealing lip recreates the destage problem you just paid to avoid.

Medical and food notes

A cold runner reduces scrap; it does not confer ISO 10993 or 21 CFR 177.2600. What it does for those files is reduce handling and destage particles, and keep unapproved regrind out of the hopper. Keep the deck out of conventional grease. Drool that cures on a needle is a particle source in Class 8.

Do not regrind cured LSR sprues into a healthcare lot. A cold runner’s job is to make that temptation unnecessary.

RFQ language that gets you a cold runner, not a slogan

“LSR valve-gated cold runner. Manifold cooled, cavities heated. No cured runner remaining on the shot. Gate on Face B, vestige flush to [limit]. Cavity count [N]. Annual volume [range]. Compound family platinum LSR, [food/medical/industrial].” If the quote returns a cold sprue with a tree, that is a different tool. Ask for the shot photo from a similar deck, not a brand name on a manifold catalog.

FAQ

Why can't LSR use a thermoplastic hot runner as-is?

LSR is a platinum thermoset; heat is the cure. A hot manifold would vulcanize in the channel. The runner stays water-cooled; the cavity plates are hot, with a sharp thermal break at the gate. Casual "hot runner LSR" is usually a misnamed cold deck or an open-sprue tool that still cures a runner.

Is "zero-waste" a literal empty drum?

It is a direction. You still purge at color change, scrap start-up, and lose rejects. What high-cavity infant or O-ring tools should not lose is a cured sprue that weighs as much as the parts. Open cold sprues that throw a runner are cheaper tools, not zero-waste tools.

When does a valve-gated cold deck pay for itself?

When cured runner is a large fraction of shot weight, the gum will not be reused (healthcare, optical, pigmented), cavity count and annual volume are high, or destage would mark a cosmetic or skin-contact face. It does not pay on a two-cavity industrial boot you run a few days a year with a hidden gate.

Can I regrind cured LSR sprues into a healthcare or food lot?

No. A cold runner's job is to make that temptation unnecessary. The deck also does not confer ISO 10993 or 21 CFR 177.2600; it reduces handling, destage particles, and unapproved regrind. Drool that cures on a needle is a Class 8 particle source.

How do I tell a cold-sprue quote from the valve-gated tool I asked for?

Specify manifold cooled, cavities heated, no cured runner remaining on the shot, gate face and vestige limit, cavity count, volume, and compound family. If the quote returns a tree, that is a different tool. Ask for a shot photo from a similar deck, not a brand name on a catalog. Unbalanced cavities still flash and starve; a deck does not magically balance a bad layout.