Silicone & Rubber Molding
Compression molding, LSR injection, and transfer molding — the three ways to turn raw silicone and rubber into finished parts. Each method has a different volume, cost, and quality profile.
Silicone and rubber parts — gaskets, seals, keypads, medical components, kitchenware — are produced by fundamentally different processes than rigid plastic parts. Thermoset materials cure irreversibly under heat and pressure. Once cured, they cannot be remelted. This means the molding process must bring the material to its final chemical state inside the mold — a different engineering challenge than injection molding thermoplastics.
For hardware founders, silicone and rubber molding is often the last process they learn about and the first one they need. A waterproof seal, a shock-absorbing bumper, a flexible keyboard cover — these parts make or break a product’s functionality. Understanding the three core molding processes and when each applies ensures you spec the right process for your volume and quality requirements.
Compression molding is the simplest and most established method. A pre-measured slug of uncured rubber or silicone is placed into a heated mold cavity. The mold closes under pressure (typically 500–2,000 psi), forcing the material to flow and fill the cavity while heat cures it. Cycle time is 3–10 minutes depending on part thickness and cure chemistry. Tooling cost is the lowest of the three methods — $1,000–5,000 for a simple mold. Compression molding is ideal for prototypes, low-to-medium volumes, and thick parts.
Liquid Silicone Rubber (LSR) injection molding is the high-volume precision method. Two liquid components (A and B) are mixed and injected into a heated mold, where they cure rapidly — typically in 15–90 seconds per cycle. LSR produces parts with excellent detail, consistency, and cleanliness. It is the standard for medical devices, baby care products, and high-volume consumer goods. Mold cost is higher ($5,000–25,000+) and molds run in dedicated LSR injection machines with cooled barrels to prevent premature curing.
Transfer molding is a middle ground. Uncured material is placed in a chamber (the pot) above the mold. A plunger forces it through channels (sprues and runners) into the closed cavity. Transfer molding produces more consistent parts than compression molding and handles more complex geometries, but generates more waste (the cured runner system). It is used for rubber-to-metal bonding — where a metal insert is placed in the cavity and rubber is molded around it — and for parts too complex for compression but too low in volume to justify LSR tooling.
Silicone molding failures
Incomplete cure
Parts that feel soft or tacky after molding were under-cured. The material has not fully cross-linked, compromising mechanical properties and potentially leaching uncured chemicals. Verify cure state with a post-cure oven step when required.
Flash at the parting line
Silicone flows into gaps more readily than rigid plastics. Expect some flash around the parting line. Specify acceptable flash in your quality requirements — typically under 0.1 mm for precision parts, under 0.3 mm for general consumer goods.
Choosing compression molding for complex geometries
Compression molding cannot reliably fill thin ribs, tall bosses, or deep undercuts. If your silicone part has sharp details or complex features, spec LSR or transfer molding.
Air traps causing voids
Compression-molded parts can trap air, creating internal bubbles that weaken the part. Mold design must include vent channels at high points where air collects. First-article parts should be cut and inspected for internal voids.
What founders should remember
Compression molding for low volume, LSR for high volume
Under ~5,000 units, compression molding wins on tooling cost. Above ~10,000, LSR wins on cycle time and consistency. Transfer molding fills the gap for medium-complexity parts.
LSR is the default for medical and food-contact products
LSR’s closed-loop injection process minimizes contamination risk. Combined with medical-grade or food-grade silicone, it is the standard for regulated consumer products.
Post-cure processing is not optional for some applications
Medical and food-contact silicone parts often require a post-cure oven cycle (200°C for 2–4 hours) to remove volatile compounds and complete cross-linking. Include this in your production timeline.