Process Characteristics of Chinese LSR Liquid Silicone Rubber Injection Molds

Liquid silicone rubber (LSR) features excellent high and low temperature resistance, elasticity and biocompatibility, widely used in medical components, baby products, automotive seals and electronic waterproof parts. Unlike thermoplastics such as ABS and PC, LSR is a two-component thermoset material cured by cross-linking reaction under heat. LSR keeps low viscosity at room temperature. Its special forming mechanism puts strict requirements on mold sealing, runner temperature control and vent design. Mold scheme directly determines yield and production efficiency.

Cold runner gating system design

Cold runner is the mainstream structure for LSR molds. Runner zones are kept under low temperature to avoid premature curing and blockage. LSR only starts vulcanization after entering high-temperature cavities. Cold runners eliminate sprues and waste material for mass production. Pin-point gates and film gates with small dimensions are commonly adopted to match fast flow of low-viscosity LSR. Thermal insulation is built between cold runners and hot mold plates to isolate heat transfer.

Vent and sealing management

LSR releases gaseous by-products during vulcanization. Poor ventilation causes bubbles and incomplete filling. The vent slot depth of LSR molds is only 0.01~0.02 mm, much shallower than thermoplastic molds. It exhausts air while preventing flash. Parting line, ejector pin and slider clearance are the key sealing zones. LSR penetrates tiny gaps easily, so parting surfaces require careful lapping. Vacuum assistance and vent inserts are applied for complex parts.

Mold temperature control

Vulcanization speed is highly dependent on mold temperature. Independent temperature control systems are equipped for LSR molds. Over-high temperature causes surface scorch, while low temperature extends cycle time and leads to insufficient curing. Heating rods are arranged evenly to avoid local hot spots. Parts with uneven wall thickness need special temperature adjustment to guarantee consistent curing degree.

Mold material selection, ejection and surface treatment

Cured LSR is soft and elastic, but sticks easily to cavity surfaces. Large ejector pins are adopted with tightly controlled fitting clearance. Sufficient draft angle reduces deformation during ejection. Corrosion-resistant stainless steel is used for LSR molds to resist corrosion from silicone additives. Polishing, chrome plating or PTFE coating reduces surface adhesion.

Daily maintenance for stable production

Silicone residue accumulates inside vents and cavities after continuous shots, blocking vents and causing batch defects. Regular cleaning schedule is required without scratching the coating. Residual material inside cold runners must be fully purged before long-term shutdown to avoid solidification and clogging. Mold surfaces are cleaned and protected against rust after each production run.