Liquid Silicone Injection Machine

Overview

A liquid silicone injection machine molds parts from two-part addition-cure liquid silicone rubber (LSR). Unlike thermoplastics that melt and freeze, LSR arrives as two low-viscosity liquids, compound A carrying the platinum catalyst and compound B carrying the cross-linker. The machine keeps both cold while metering and mixing them, injects the blend into a hot mold, and the heat vulcanizes the silicone into an elastic solid. The logic is inverted from thermoplastic molding: the Injection Unit stays cold and the Mold Heating System stays hot.

LSR parts are used where flexibility, biocompatibility and heat resistance matter: medical seals, baby bottle nipples, electrical insulators, gaskets, valves and overmolded grips. The Liquid Silicone Injection Machine runs as an unattended cell with automatic dosing from drums, valve-gated cold runners that leave no sprue, and full part traceability through the Control System.

Metering and mixing

The two compounds are drawn from 200 L drums by a pair of Drum Follower Pump units. Each uses a Follower Plate pressed down by a Ram Cylinder, wiping the high-viscosity material toward a Piston Transfer Pump so the drum empties cleanly. From there the A/B Metering Block holds the A to B ratio at 1:1 within 1 percent, because an off-ratio shot leaves the part either uncured and tacky or over-cross-linked and brittle.

If colored parts are needed, a Pigment Dosing Pump doses pigment paste at 0-5 percent into the stream. All three feeds meet at a Static Mixer Element, a disposable helical element that blends them with no moving parts and no dead spots where material could begin to cure. A Degassing Block pulls vacuum on the line to remove entrained air that would otherwise show as voids in the molded part. The mixed material then enters the Injection Barrel.

Injection unit

The Injection Unit is a reciprocating screw, but every surface that touches LSR is held below 40 C by the water-jacketed Injection Barrel. The Injection Screw has a low compression ratio and hard-chromed flights, because the silica filler in LSR is abrasive. During recovery the Screw Servo Motor turns the screw, drawing mixed material forward past the Non-Return Valve check ring. During injection the Injection Cylinder drives the screw axially, and the check ring seals so material flows out rather than back over the flights.

At the front the Cold Shutoff Nozzle is a needle shutoff design. The Nozzle Needle closes the melt channel between shots so the cold material cannot drool, and the Nozzle Cooling Block block keeps the Nozzle Body cold right up to the mold face. Injection pressures reach 2000 bar to fill thin medical-grade sections before any premature cure.

Cold runner and the heated mold

The biggest waste saving in LSR molding comes from the Cold Runner Block. The Cold Runner Manifold is insulated and held below 40 C by the Runner Cooling Circuit circuit, so material in the runner never cures. Each cavity is fed through a Valve Gate driven by a Gate Actuator: the gate opens to fill, then the needle closes and shears the material at the part surface. There is no sprue or runner to trim, scrap or recycle, and silicone scrap cannot be reground anyway.

Once the LSR reaches the cavity it must vulcanize. The Mold Heating System system embeds twelve or more Cartridge Heater elements in the mold plates and reads each zone with a Mold Thermocouple. A multi-loop Zone Temperature Controller holds the cavity at 160-220 C, and an Insulation Plate on each side stops that heat draining into the cold platens. Cure time scales with part wall thickness, typically 5-30 seconds.

Clamping and ejection

The Mold Clamping Unit holds the mold shut against injection pressure. A Toggle Linkage multiplies the stroke of the Clamp Cylinder into 50-300 tonne of clamp force across four Tie Bar columns. The Moving Platen travels on the tie bars between the Stationary Platen and the toggle, and the Crosshead drives the Toggle Link members through their over-center lock.

Because cured LSR is soft and tacky, ejection is gentle. The Ejector Assembly uses an Ejector Cylinder to push the Ejector Bar and Ejector Plate, often combined with air blow-off or a brush, so the flexible part peels off the core without tearing.

Hydraulics and control

A servo-driven Hydraulic Power Unit supplies the clamp, injection and ejector cylinders. The Hydraulic Pump is a variable-displacement unit driven by the Pump Drive Motor, drawing from the Oil Reservoir and feeding the actuators through a Valve Manifold of proportional and directional valves. An Hydraulic Accumulator buffers the brief high-flow demand of injection, an Oil Cooler holds oil at 45 C, and the Hydraulic Filter elements keep the oil clean.

The Control System ties it together. An PLC Controller with motion capability sequences dosing, injection, cure timing, clamp and ejection, while two Servo Drive units close the loop on the screw and metering servos. The operator works from the HMI Touch Panel touchscreen, storing recipes per mold. Everything sits in the Machine Frame and Guarding, whose Base Frame carries the loads and whose interlocked Safety Door guards stop the cycle if opened.

Variants

Smaller machines run from a 20 L pail with a single drum pump and a one or two cavity mold for prototyping. Production cells use 200 L drums, 16 or 32 cavity molds, and a robot to remove and sort parts. Full-electric machines replace the Hydraulic Power Unit with servo presses on clamp and injection, cutting energy use and oil contamination risk, which matters for cleanroom medical work. Self-lubricating LSR grades and two-shot machines that overmold LSR onto a thermoplastic substrate are common in connector and wearable production.