LSR Cold Runner Systems: Cut Waste & Cost

What is an LSR cold runner system?

In short: a cold runner is a temperature-controlled manifold (often called a cold deck) that holds LSR in its uncured liquid state right up to the gate of each cavity. Because liquid silicone cures with heat, the runner is cooled with water channels while the mold cavity is heated to cure the part. Only the part vulcanizes; the material in the runner stays liquid and is injected into the next shot.

This is the opposite logic of thermoplastic molding, where a hot runner keeps plastic molten. In LSR, the runner must stay cold and the cavity hot. The result is a runnerless process: no sprue, no cold slug, no runner to cut off, recycle, or scrap.

How does a cold runner cut LSR waste and cost?

Directly: it removes the runner from the equation. Liquid silicone is an expensive two-part material, and on small parts the cured runner can weigh more than the part itself. A cold runner system can eliminate close to 100% of runner waste, which on a high-cavity tool often translates into 10-30% lower material consumption per part.

The savings compound. With no runners to remove there is no deflashing or trimming station, which lowers labor and removes a source of cosmetic rejects. Because the gate is positioned precisely and shut cleanly, parts come off cleaner and ready for automated removal. Cold runners also draw far less electricity than a heated manifold would, since there is no need to constantly warm the runner.

What is a valve-gated cold runner, and why does it matter?

A valve-gated cold runner adds a mechanically actuated shut-off pin (needle valve) at each gate. The pin opens during injection and closes before the mold opens. This matters for three reasons: it stops material drool and stringing between shots, it gives clean gate vestige with no manual trimming, and it lets you balance fill across cavities by controlling each gate independently.

For multi-cavity medical, infant-care, and micro parts, valve gating is what makes fully automated, lights-out LSR production realistic. Open (thermal-gate) cold runners are simpler and cheaper but can leave a small gate mark and are better suited to less cosmetic parts.

Cold runner vs hot runner vs conventional sprue: what is the difference?

The table compares the three approaches for LSR. Note that a true hot runner is a thermoplastic concept; in silicone the practical choice is cold runner versus a conventional cold-sprue (open) tool.

When is a cold runner worth the higher tooling cost?

The rule of thumb: a cold runner pays back fastest on high-cavity, high-volume parts where runner mass is significant relative to part mass. A 16- or 32-cavity micro-valve or duckbill tool running millions of shots a year can recover the added tooling cost in months purely on saved silicone and eliminated trimming labor.

It is harder to justify for a single large part run at low volume, where the runner is a small fraction of part mass and trimming is occasional. A quick payback estimate: multiply runner weight per shot by silicone cost and annual shot count, then add the labor of any trimming station you remove, and compare that yearly saving against the cold-runner tooling premium.

What are the design essentials of a reliable cold runner?

A cold runner only delivers if the thermal separation between the cold deck and the hot cavity is well engineered. Four essentials:

· Thermal isolation: insulation plates and air gaps keep heat from the 170-200 C cavity from creeping into the cooled deck and pre-curing the runner.

· Cooling channels: water cooling holds the deck near 15-25 C so the LSR stays liquid and stable between shots.

· Balanced flow and needle valves: equal flow length and individually controlled gates ensure every cavity fills evenly, which is critical for tight tolerance parts.

· Precise metering upstream: the machine's dosing pump must hold the A:B ratio (typically 1:1) so the well-designed runner is fed consistent, bubble-free material.

How TYM engineers cold-runner LSR systems

TYM designs LSR injection molding machines, cold-runner silicone molds, and turnkey automated production systems for medical, automotive, infant-care, and industrial customers. TYM matches the cold-runner type (open or valve-gated), cavity layout, and machine metering to the part's cosmetic and volume requirements, then integrates automated part removal for clean, runnerless output. The aim is precise, efficient, intelligent production with the lowest sustainable cost per part.

FAQs

Q: Does a cold runner really eliminate all silicone waste?

A: It eliminates the runner itself, which is the largest source of LSR scrap. A well-designed valve-gated cold runner can remove close to 100% of runner waste, leaving only the cured part. Minor losses can still occur from purging, color changes, or rejects, but per-part material consumption typically drops 10-30% on small multi-cavity parts.

Q: What is the difference between an open and valve-gated cold runner?

A: An open (thermal) cold runner has no shut-off pin and is simpler and cheaper, but may leave a small gate mark and can drool between shots. A valve-gated cold runner uses a needle valve at each gate to give clean gate vestige, prevent stringing, and allow independent cavity balancing, which is preferred for cosmetic and high-volume parts.

Q: Is a cold runner worth it for low-volume LSR production?

A: Often not. The added tooling cost is hardest to justify when volumes are low or the runner is a small fraction of part mass. For prototypes, large single parts, or short runs, a conventional sprue tool is usually more economical. Cold runners shine on high-cavity, high-volume programs.

Q: Why must an LSR runner be cooled instead of heated?

A: Because liquid silicone cures with heat, the reverse of thermoplastics. To keep the runner material reusable for the next shot, the manifold is cooled (about 15-25 C) so the LSR stays liquid, while only the heated cavity (170-200 C) cures the part.