If you need flexible, heat-resistant, biocompatible parts in production volumes, liquid silicone rubber (LSR) molding is likely the process you are looking for. LSR molding produces elastomeric parts that maintain their physical properties from -60 degrees C to +200 degrees C, resist UV degradation, pass FDA and ISO 10993 biocompatibility standards, and can be molded into complex geometries with wall thicknesses as thin as 0.3 mm.<\/p>\n\n\n\n
This guide explains what LSR molding is, how the process works step by step, the material science behind liquid silicone rubber, how it compares to conventional rubber molding and thermoplastic elastomers, the design rules you need to follow for successful LSR parts, and the industries and applications where LSR delivers the most value.<\/p>\n\n\n\n
Liquid silicone rubber is a two-part, platinum-catalyzed thermoset elastomer. Unlike thermoplastic resins that melt when heated and solidify when cooled, LSR undergoes an irreversible chemical crosslinking reaction (vulcanization) when exposed to heat. Once cured, the material cannot be remelted or reshaped. This thermoset nature gives LSR its exceptional long-term stability: the crosslinked molecular network resists creep, compression set, and degradation over thousands of hours of continuous use.<\/p>\n\n\n\n
The two liquid components, commonly designated Part A (containing the vinyl-functional polysiloxane base) and Part B (containing the hydride-functional crosslinker and platinum catalyst), are stored separately and mixed at a precise 1:1 ratio immediately before injection. Pigments, additives, and self-bonding agents can be added to the mix for color, radiopacity, or adhesion to substrates during overmolding.<\/p>\n\n\n\n
Key material properties of LSR: <\/strong>Hardness range of 10 to 80 Shore A. Operating temperature range of -60 degrees C to +200 degrees C (short-term excursions to +250 degrees C). Tensile strength of 5 to 12 MPa depending on grade. Elongation at break of 200 to 1,000%. Excellent compression set resistance (typically under 15% after 22 hours at 175 degrees C). Biocompatible per ISO 10993 and USP Class VI. FDA 21 CFR 177.2600 compliant for food contact. Electrically insulating (dielectric strength 15 to 25 kV\/mm). Chemically resistant to ozone, UV, dilute acids, and many solvents. Optically transparent grades available.<\/p>\n\n\n\n LSR injection molding follows a fundamentally different thermal cycle than thermoplastic molding. In thermoplastic molding, hot material is injected into a cold mold to solidify. In LSR molding, cold material is injected into a hot mold to cure. This reversed thermal logic requires specialized equipment and tooling design.<\/p>\n\n\n\n Part A and Part B are stored in separate drums or pails connected to a metering unit (pumping system). The metering unit feeds each component at a precisely controlled 1:1 volume ratio into a static mixer, where the two parts are combined into a homogeneous mixture. The mixed LSR is kept at room temperature (or slightly chilled) in the barrel to prevent premature curing before injection. If pigment or additives are required, a third metering stream introduces color paste into the mix at this stage.<\/p>\n\n\n\n The mixed LSR is injected through a cooled nozzle and runner system into a mold heated to 150 to 200 degrees C. The cold runner system keeps the material liquid until it enters the mold cavity, minimizing waste (unlike hot runner systems in thermoplastic molding, LSR uses cold runners because the material must stay below curing temperature until it reaches the cavity). Injection pressures for LSR are significantly lower than for thermoplastics, typically 30 to 70 bar compared to 500 to 1,500 bar for engineering plastics. The low viscosity of uncured LSR (similar to honey) allows it to fill thin walls, micro features, and complex geometries that would require extreme pressure in thermoplastic molding.<\/p>\n\n\n\n Once the LSR fills the mold cavity, the platinum catalyst initiates a crosslinking reaction driven by the mold temperature. Curing times are fast, typically 10 to 60 seconds depending on part thickness and mold temperature. The crosslinking reaction bonds the silicone polymer chains into a three-dimensional elastomeric network that gives the material its final mechanical properties. Higher mold temperatures accelerate curing but can increase flash if clamping force is insufficient.<\/p>\n\n\n\n After curing, the mold opens and the finished parts are removed. Due to the flexible nature of LSR, parts are typically removed by hand or with a robot-assisted pick system rather than ejector pins (which would deform the soft material). Flash is trimmed manually or with automated deflashing systems. For some applications, a post-cure oven cycle at 200 degrees C for 2 to 4 hours is used to complete crosslinking, drive off volatile byproducts, and improve compression set performance. Post-cure is standard for medical and food-contact applications where purity requirements are strict.<\/p>\n\n\n\n LSR is often compared to two other flexible material families. Understanding the differences helps you select the right material for your application:<\/p>\n\n\n\n LSR vs. HCR (High-Consistency Rubber \/ Solid Silicone Rubber): <\/strong>HCR is also a silicone elastomer but comes as a solid gum that must be compounded, rolled, and compression-molded or transfer-molded. LSR’s liquid state enables injection molding with faster cycle times (10 to 60 seconds vs. 3 to 15 minutes for HCR compression molding), tighter dimensional tolerances, and less flash. HCR is better suited for very thick parts (above 10 mm) and applications where the lowest possible compression set is required. For most precision silicone parts, LSR injection molding is the more efficient and cost-effective process.<\/p>\n\n\n\n LSR vs. TPE (Thermoplastic Elastomers): <\/strong>TPEs (TPU, TPV, SEBS) are thermoplastic materials that can be processed on standard moldagem por inje\u00e7\u00e3o<\/a> equipment without specialized metering systems. TPEs are cheaper per kilogram and faster to mold. However, TPEs cannot match LSR’s temperature resistance (TPEs soften above 100 to 130 degrees C vs. LSR at 200 degrees C+), compression set performance, biocompatibility, or chemical resistance. For automotive under-hood components, medical implants, and food-contact applications, LSR is the superior choice. For consumer product grips, soft-touch overmolds, and non-critical seals, TPE is often more cost-effective.<\/p>\n\n\n\n LSR molds differ from standard thermoplastic molds in several important ways:<\/p>\n\n\n\nHow Does the LSR Injection Molding Process Work?<\/strong><\/h2>\n\n\n\n
Step 1: Material Preparation and Metering<\/strong><\/h3>\n\n\n\n
Step 2: Injection Into the Heated Mold<\/strong><\/h3>\n\n\n\n
Step 3: Heat Curing (Vulcanization)<\/strong><\/h3>\n\n\n\n
Step 4: Demolding and Post-Cure (Optional)<\/strong><\/h3>\n\n\n\n
LSR vs. High-Consistency Rubber (HCR) vs. Thermoplastic Elastomers (TPE)<\/strong><\/h2>\n\n\n\n
LSR Mold Tooling: Design Considerations<\/strong><\/h2>\n\n\n\n