Custom LSR Molding Manufacturer: How Cold Runner Systems Improve LSR Injection Molding

Cold runner systems are the backbone of liquid silicone rubber (LSR) injection molding. Because LSR is a thermoset elastomer that crosslinks under heat inside the mold, the material must stay cold and uncured throughout the feed system until the moment it enters the cavity. A well-designed cold runner holds the rubber at a stable low temperature, prevents premature cure, cuts material waste, and stabilizes cycle times. In this guide, our engineering team explains how cold runners work, which design parameters matter most, and when a buyer should specify one for a new project.

Why LSR Requires a Cold Runner

LSR is a two-component, platinum-catalyzed system. It crosslinks at mold temperatures typically between 150°C and 200°C and stays workable at much lower temperatures. That is the opposite of thermoplastics, which need heated channels to remain molten. If an LSR runner were heated the way a plastic hot runner is, the rubber would cure inside the channels and block them within a few shots. For this reason every LSR mold uses a temperature-controlled cold runner: the runner plate carries its own cooling circuit that keeps the material below its cure onset temperature, shot after shot. The real engineering question is not whether to use a cold runner but how to design it, with the two main options being an open cold runner and a valve-gated cold runner.

Open Cold Runner vs. Valve-Gated Cold Runner

An open cold runner is simple and robust. After each shot, the material left in the channels remains uncured and is ejected as a runner scrap together with the part. The scrap is usually small, but on high-cavitation or high-volume tools it adds up, so channel cross-sections are kept as small as the filling pressure allows. A valve-gated cold runner goes further: pneumatically actuated gate pins close at the end of injection, so almost no runner waste is produced, gate vestige on the part is minimal, and gates can be made smaller and placed more freely. Valve gating also shortens cycle time, because less material has to be injected and stabilized, and it supports fully automated production with clean degating. The trade-off is higher tooling cost and more maintenance, which is why valve gates are normally reserved for medium-to-high production volumes, tight cosmetic requirements, or expensive silicone grades. When planning a project with a custom LSR injection molding partner, the expected annual volume is what decides which option pays off.

Design Parameters That Decide Runner Performance

Channel diameter is the first balance: an oversized channel increases waste and the time needed to keep the rubber conditioned, while an undersized one creates shear heating that can start a premature cure and raise injection pressure. For most LSR parts, channels are a few millimeters in diameter and are sized against shot volume and cavitation. Gate selection follows the same logic — valve gates for precision and volume, open pin-point or sub gates for simpler tooling. Temperature control deserves its own circuit: the runner plate is cooled independently from the heated cavity plates, and both zones are monitored so the rubber never approaches its cure onset temperature before injection. Because LSR has very low viscosity, good venting at the parting line is essential to avoid air traps and burns. Finally, runner surfaces are machined in hardened, polished tool steel so the uncured rubber releases cleanly and the tool reaches long service life. These decisions belong in the DFM stage, which our mold making team reviews with every customer before steel is cut.

If your project runs at medium-to-high volume, demands minimal gate vestige, or uses a costly medical or optical silicone grade, a valve-gated cold runner is usually the right specification; for prototyping and low volumes, an open cold runner keeps tooling affordable. Send us your drawing or 3D file and World Strong Rubber will confirm the runner concept, cavitation, and budget for your part.