LSR Mold Design: Venting, Gating & Shrinkage

What Makes LSR Mold Design Different?

LSR mold design differs from thermoplastic tooling because liquid silicone rubber is extremely low in viscosity and cures with heat rather than cooling. That combination means the mold must vent trapped air aggressively, seal tightly against flash, gate precisely to avoid waste, and be sized to compensate for silicone's relatively high shrinkage. Getting these four factors right is the difference between a high-yield tool and a scrap generator.

This article is for mold engineers, product designers, and tooling buyers specifying an LSR mold—covering venting, gating and cold runners, parting lines, shrinkage, steel selection, and the design rules that drive first-pass yield.

Why Is Venting the Most Critical LSR Mold Feature?

Venting is the single most important LSR mold feature because low-viscosity silicone fills fast and readily traps air, producing voids and incomplete fills. Unlike thermoplastics, LSR flashes through vents that are too deep, so vents must be fine enough to release air but shallow enough to stop silicone escaping.

Typical LSR vent depths are far smaller than for thermoplastics—often in the single-micron to low-tens-of-microns range depending on material and location. Vacuum venting is common on production tools: the cavity is evacuated before injection so air cannot become trapped at all, which is often essential for thick sections, complex geometry, and medical parts where voids are unacceptable.

How Should You Gate an LSR Mold?

Gate LSR molds with a cold-runner system wherever production volume justifies it, because it delivers silicone to the cavity uncured and eliminates sprue and runner scrap. Since LSR cures with heat, the runner is kept cool while the cavity is hot—so material in the runner stays liquid for the next shot instead of becoming waste.

Gating choices depend on part and volume:

• Cold-runner valve gates:minimal waste, clean part separation, best for medium-to-high volume and multi-cavity tools.
• Open/pin gates:simpler and cheaper, but leave a gate vestige and generate some cured waste.
• Gate location:place to fill from thick to thin, avoid air entrapment, and keep the vestige off cosmetic or sealing surfaces.

How Do You Design Parting Lines and Prevent Flash?

Design parting lines on tight, well-supported surfaces because low-viscosity LSR will flash into any gap under injection pressure. Flash is the most common LSR defect and traces directly to parting-line fit, clamp force, and steel flatness.

Practical anti-flash measures include: placing the parting line on a simple, accessible edge for easy deflashing; specifying high-precision surface grinding and lapping on shut-off faces; ensuring clamp tonnage matches projected area and cavity pressure; and using compression-style shut-offs where geometry allows. Precise steel and a well-matched machine reduce flash far more than downstream trimming ever can.

How Much Shrinkage Should the Cavity Compensate For?

Cut LSR cavities oversized to compensate for roughly 2–4% shrinkage, since silicone contracts more than most thermoplastics as it cools after cure. The exact figure depends on the specific LSR grade, durometer, part geometry, and cure conditions, so always design to the material supplier's published shrinkage and validate on the first sample trial.

Because real shrinkage varies, experienced toolmakers cut cavities "steel-safe" (slightly small on critical dimensions) so the tool can be adjusted after T1 trials rather than welded up.

Which Steel and Surface Finish Suit LSR Molds?

Choose hardened, corrosion-resistant tool steel with a finish matched to the part, because LSR reproduces the cavity surface exactly and tools run hot for millions of cycles. Stainless grades such as 1.2083/S136 resist corrosion from silicone by-products and hold fine finishes; pre-hardened steels suit lower-volume tools.

Surface finish transfers directly to the part: a high-polish cavity yields glossy silicone, while textured or matte finishes (e.g., bead-blasted) give a soft-touch appearance and can aid demolding. Match the finish to cosmetic and functional needs, and specify wear-resistant coatings on high-cycle shut-offs and gates.

What Design Rules Improve First-Pass Yield?

Improve yield by designing the part and tool together from the start—uniform walls, generous radii, adequate draft, and planned venting. The most productive LSR tools reflect design-for-manufacture decisions made before steel is cut.

Core rules: keep wall sections reasonably uniform to cure evenly and shrink predictably; add radii instead of sharp corners to ease flow and demolding; provide draft or design for the elastic pull-out that silicone allows; and integrate venting, gating, and ejection (or automated demolding) into the concept rather than adding them later.

FAQ

Q: Why do LSR molds need such shallow vents? 

Because liquid silicone rubber has very low viscosity, it escapes through vents that would be fine for thermoplastics, creating flash. LSR vents are typically only a few microns to a few tens of microns deep—enough to let trapped air out but too shallow for silicone to flow through. Many production molds add vacuum venting to remove air entirely before injection.

Q: What shrinkage rate should I use for an LSR mold cavity? 

Most LSR grades shrink roughly 2–4%, but the exact value depends on the specific material, durometer, wall thickness, and cure conditions. Always design to the material supplier's published shrinkage and cut critical dimensions steel-safe, then confirm and fine-tune on the first sample trial rather than assuming a single fixed number. 

Q: Do I need a cold-runner system in my LSR mold? 

A cold-runner system is strongly recommended for medium-to-high volumes because it keeps silicone uncured in the runner and eliminates sprue and runner scrap on every shot, lowering material cost and improving part quality. For very low volumes or prototypes, a simpler open-gate design may be acceptable despite the small amount of cured waste it produces. 

Q: How do I prevent flash in a silicone mold? 

Prevent flash by placing parting lines on tight, well-ground shut-off surfaces, matching clamp tonnage to the part's projected area and cavity pressure, and machining shut-offs to high precision. Because LSR flows into any gap under pressure, precise steel fit and correct machine sizing prevent flash far more effectively than trimming parts after molding. 

Q: What steel is best for LSR molds? 

Hardened, corrosion-resistant stainless tool steels such as 1.2083/S136 are common for LSR because they resist corrosion from cure by-products, hold fine polished or textured finishes, and endure long, hot production runs. Pre-hardened steels can serve lower-volume tools, but high-cavitation production molds generally justify hardened stainless with wear-resistant coatings on gates and shut-offs. 

Conclusion

Strong LSR mold design comes down to four disciplines: venting that releases air without flashing, gating (ideally cold-runner) that eliminates waste, parting lines and steel that resist flash, and cavities sized for real shrinkage. Decide these together with the part design, and validate on sample trials rather than assuming.

TYM designs and manufactures precision LSR molds with vacuum venting, cold-runner systems, and hardened stainless tooling, matched to the injection machine and automation that run them. Request a mold design review or quote at tymmold.com/contact with your part drawing and volume.