Can Silicone Be Used in Injection Molding?
Yes. Silicone rubber – particularly in its liquid form (LSR, Liquid Silicone Rubber) – can be injection molded using specialized equipment. In fact, liquid silicone injection molding (often called Liquid Injection Molding, LIM) is a common industrial process. It involves pumping a two-part silicone (base and catalyst) through a static mixer into a heated mold, where the material rapidly cures into a flexible part. Solid (high-consistency) silicone (HCR) can also be molded, but it usually requires alternative methods (compression or transfer molding) due to its high viscosity. The brief answer is that silicone is well-suited to injection molding processes when properly formulated and handled, as we explain in detail below.
Silicone Elastomers and Forms
Silicone rubber is a family of synthetic elastomers (polysiloxanes) that are widely used for their flexibility and stability. A typical silicone rubber is a polymer with a silicon–oxygen backbone and organic side groups (often methyl). Silicone rubbers are generally non-reactive, stable, and can withstand extreme environments: for example, many silicone rubbers remain flexible and useful from about –55 °C up to +300 °C. These broad temperature limits, plus resistance to ozone, UV light, moisture and many chemicals, make silicone a unique material for seals, gaskets, medical devices, and more.
Silicone elastomers typically come in two main processing forms:
Liquid Silicone Rubber (LSR): A two-part, flowable paste or liquid (often platinum-cured) supplied in a pre-measured “kit” (Part A and Part B) that flows under low pressure. When mixed, LSR cures rapidly (vulcanizes) under heat. LSR has a low viscosity, meaning it flows easily and can fill molds with fine detail. LSR is ideal for automated injection molding because it does not require much pre-processing and can be pumped through hoses.
High-Consistency Rubber (HCR) or Solid Silicone: A thicker, doughy (gum-like) solid rubber, sometimes called gum rubber or HTV (high-temperature vulcanizing) silicone. HCR has a high viscosity and usually comes on rolls or blocks. It often needs to be pre-formed (sheeted or chunked) before molding. HCR is traditionally processed by compression or transfer molding, though specialized hydraulic injection presses can also use it.
The key difference for injection molding is flowability. LSR’s flowable nature makes it easy to inject: “LSRs have a lower viscosity, meaning they flow more readily and are easy to process via injection molding. The automated injection molding process makes LSR preferable for high production volumes”. In contrast, “HCR’s high viscosity makes it tricky to handle” in an injection press. Thus, modern silicone injection molding typically uses LSR.
Silicone comes in various grades (medical, food-safe, flame-retardant, etc.) and hardnesses (durometers) to suit applications, but the molding process follows the same basic principles once the material is liquid.
Silicone Injection Molding Process
Silicone injection molding (LIM) is conceptually similar to plastic injection molding, but with important differences due to silicone’s chemistry. A simplified description of the process is:
Metering and Mixing: Two liquid components (“Part A” base polymer and “Part B” catalyst) are stored in separate drums. Each is pumped by a metering unit at a precise ratio (typically 1:1 by volume). The two streams are combined in a static mixer where they thoroughly mix and start curing. (Color pigments can also be injected here if needed.)
Injection: The mixed liquid silicone is then injected under pressure into a closed mold cavity. The molds are typically made of hardened steel and may be temperature-controlled (heated) to accelerate curing. Because LSR is very low viscosity, the system must seal perfectly to prevent leaks (even “burrless” valve gates are used).
Cure in Mold: Inside the heated mold, the silicone polymer cures (vulcanizes) in place, turning from a liquid into an elastic solid. Typical cure times are short (often just seconds to a couple of minutes, depending on part size and temperature).
Mold Open and Ejection: After curing, the mold opens and ejector pins or strippers push out the silicone part. (Silicone parts are elastic, so specialized ejector designs and careful cooling are needed to avoid tearing.) The part is usually nearly finished; little trimming is needed.
Automation: In practice, liquid silicone injection machines are often fully automated. Robots can remove parts and perform secondary processes, making it a low-labor process.
In a LSR injection press, the machine components include metering pumps, mixers, injection units, and heated molds. A typical LSR press (for example, an Arburg or a BORCHE machine) uses two actuators to push the A and B compounds through a mix head. The mixed silicone is forced into the mold, which is kept under pressure until cure. A flow diagram is shown here for clarity:
Process Diagram: The A and B drums feed a metering unit (plungers), then to a static mixer. The output enters a heated injection nozzle and flows into the cold-runner and mold cavity. The mold is heated (often with water jackets) to quickly cure the silicone. Because of the tight timing (LSR begins curing as soon as A and B combine), modern machines use a cold feed system (cooled runners) to avoid premature curing in the nozzle.
In contrast, conventional plastic injection molding melts solid plastic pellets in the barrel, then cools them in the mold. For silicone LSR molding, the process is almost the opposite: you start with flowable liquid, heat the mold to cure it, and use a static mixer rather than a reciprocating screw melting plastic. In short, a LSR injection press is essentially a high-precision, two-component metering system feeding a heated mold.
Advantages of Silicone Injection Molding
Injection molding offers several strong advantages for making silicone parts, due to both the process and the material’s properties:
High Production Efficiency: Injection molding (especially with LSR) is highly automated and fast. Modern LIM machines can produce thousands of parts per hour. In general, injection molding excels at high-volume, consistent production, making parts relatively inexpensive per unit once the mold is made. For example, one source notes LIM as a “high-volume” process with short cycle times and automated feeding. Another notes injection molding can produce “millions of parts per machine per year” with rapid cycle times. In practice, silicone LIM lines run 24/7 for mass-produced goods (baby nipples, medical valves, etc.).
Precise, Complex Shapes: The low viscosity of LSR allows it to flow into very fine mold features, so injection molded silicone parts can have thin sections, intricate details, and tight tolerances. LIM typically produces virtually flashless parts (very little excess at parting lines) because the material fills the cavity fully under pressure. In fact, LIM is often chosen over compression molding when high cosmetic quality and precision are needed. As one comparison notes, compression molds usually leave some flash (burrs) that must be trimmed, whereas LSR injection produces “almost zero” flash. Injection machines also precisely meter material weight per part, so dimensional consistency is excellent.
Short Cure Time: LSR cures quickly under mold heat. The curing reaction of platinum-cure silicone is rapid, so injection cycles can be much shorter than compression molding cycles. One source reports that LIM cures in roughly one-third the time of compression-vulcanized silicone. Shorter cure time means faster cycle times and greater throughput (and less chance for errors during cure).
Clean, Automated Process: LSR injection is a closed-system process. The mixing and injection happen in sealed equipment, minimizing contamination. This is especially important for medical, food, and baby products. (For example, injection nipple machines often include cleanrooms.) The automation also reduces manual handling: the raw silicone compound is never exposed to air during molding, so the parts are sanitary. In one comparison, the LSR process was noted as “absolutely environmental-friendly and non-toxic” because there is no open handling of the silicone. In practice, this has made LIM the method of choice for sensitive items like medical components, cookware, and baby care products.
Excellent Material Performance: Silicone rubber itself has outstanding properties. Parts made by LSR injection inherit them fully. For example, temperature resistance is extreme – molded silicone can work from cryogenic lows up to ~200–300 °C without losing elasticity. Silicone parts are flame-retardant and do not melt, which is a major safety benefit. They also resist ozone, UV, and weathering better than most elastomers. Mechanically, LSR parts have high tear strength and elongation, making them durable and flexible. They are also good electrical insulators.
Additionally, silicone is biocompatible and hypoallergenic in many grades. This means it can be used for implants, medical tubing, baby bottle nipples, and so on, without causing adverse reactions. Many silicone grades meet strict FDA and medical standards. The fact that silicone is chemically inert and non-toxic also makes it ideal for food-contact parts. In short, injection-molded silicone parts excel in any application that demands flexibility, heat/chemical resistance, and purity.
Color and Additives: During injection, it’s easy to introduce colorants or additives into the liquid mix. The static mixer (or an upstream color pump) can add pigments, so molded parts can be produced in custom colors or with special properties (e.g. conductive fillers, UV-stable pigments). The sealed mixing also ensures uniform dispersion of such additives.
Reduced Scrap and Material Savings: Because LIM uses a cold-runner or hot-runner system, nearly all the injected silicone goes into the parts. There is little or no waste sprue to trim. (In fact, one of the advantages of LSR injection is “no waste” in the runner – any unused mix is re-circulated by the machine.) This contrasts with compression molding, which often requires trimming flash and deals with material at room temperature.
Comparison with Other Molding Processes
Silicone can also be shaped by other molding methods, and the choice depends on part requirements and volume. Here is how injection molding (LIM) stacks up against common alternatives:
Compression Molding: This classic method involves placing a pre-measured charge of silicone (solid silicone sheets or lumps) into an open mold, then closing and heating it under pressure. Compression molding has lower tooling costs and can handle large or simple parts well. However, it is slower and less precise than injection molding. Benefits of compression molding include suitability for small production runs, low tooling complexity, and ability to mold very large parts or thick cross-sections. Its drawbacks are long cycle times (because the whole blank must cure) and more manual steps (loading/unloading). Also, compression molding typically produces flash that must be trimmed, and tight tolerances are harder to achieve. In contrast, LSR injection molding is faster (shorter cures) and produces finer details with minimal flash. For high-volume or precise parts, injection molding is generally superior, whereas for low-volume or very large simple parts, compression might be chosen.
Transfer Molding: Transfer molding is somewhat a hybrid. Silicone is pre-formed (often as blocks or pellets) and placed into a pot above the mold; a plunger then forces it through a sprue into the heated cavity. The mold closes and cures the silicone. Transfer molding allows more control (and better tolerances) than basic compression molding, since you can regulate the preform weight. However, it still shares many disadvantages of compression: manual charging, flash in the pot, and additional trimming. It’s mainly used when injection molding is not feasible due to equipment constraints. In practice, LIM (liquid injection) has largely supplanted transfer for many silicone parts, because LIM uses liquid directly and yields more consistent mixing and filling without a pot.
Extrusion Molding: Extrusion is used for silicone products with a uniform cross-section, such as tubing, cords, profiles, or sheets. In this process, silicone (either LSR or HCR) is forced through a shaping die (like a pipe or custom shape) and then vulcanized, often on a conveyor or in an oven. For example, silicone extruders make hoses, O-ring stock, sealing strips, and other long continuous shapes. The Wikipedia “Silicone rubber” entry notes that silicone can be “extruded into tubes, strips, solid cord, or custom profiles… Cord can be joined to make O-rings and extruded profiles can be joined to make seals”. In summary, extrusion is ideal for linear parts, but cannot make complex 3D shapes in one step. Injection molding, by contrast, can create intricate 3D geometries (like intricate diaphragms, valves, or enclosures) that extrusion cannot.
Blow Molding: Blow molding (used for hollow plastic bottles) is rarely applied to silicone. Silicone’s viscosity and curing process make it less suitable for traditional blow molding. There are some specialized blow-silicone techniques (e.g. for hollow tubing or bottles), but these are uncommon. If a hollow silicone part is needed, often it is made by injection over a core rod, or by dip molding, rather than true blow molding. (Plastic blow molding involves inflating molten plastic in a mold – a process not well-matched to liquid silicone’s properties.)
Rotational Molding / Calendering: These are other rubber/plastic methods. Rotational molding (rotomolding) coats the interior of a spinning mold with material and cures it; it is used for large hollow plastic tanks, not typically for silicone. Calendering rolls silicone into sheets. These processes are not common for shaped silicone parts where injection or extrusion is preferred.
3D Printing: Additive manufacturing of silicone is an emerging field. Some technologies use a pump-nozzle to deposit liquid silicone, but they require special formulations. In fact, silicone rubbers used for conventional molding usually don’t work in 3D printing, because they cure too fast or not fast enough for the printing process. The wiki notes that standard silicone formulations are “not applicable” to 3D-printed deposition (LDM) without adjustments. For now, injection molding remains far more practical for most production quantities of silicone parts. However, for prototypes or very low volumes, silicone 3D printing is becoming an option.
In summary, injection molding of LSR is generally the most efficient method for mass-producing flexible silicone parts with complex shapes and tight tolerances. Other methods (compression/transfer) serve smaller runs or special cases, and extrusion handles simple continuous profiles.
Technical Considerations
Because silicone has unique properties, its injection molding requires some special design considerations:
Mold Design and Gating: Silicone’s low viscosity helps it fill molds easily, but it can also cause flashing if gates/vents are not well-designed. Molds often use stratified flow gates or valve gates that shut off cleanly. Venting is important to let air escape because LSR fills the cavity quickly. Automated cameras and thermal sensors are sometimes used to catch any incomplete fills.
Shrinkage and Cooling: Silicone generally has higher linear shrinkage than plastics when curing. Molds must account for this in cavity dimensions. Also, the cured silicone does not cool like plastic; instead, the material essentially cures at the mold temperature. Many molds use chilled runners to keep the silicone unmelted outside the cavities.
Injection Pressure: LSR is injected at high pressure (often thousands of psi) but, unlike plastic, it does not solidify under cooling; it cures by chemical reaction. The injection pressure must be carefully controlled to avoid flash and to ensure even filling before cure.
Post-Mold Processing: One big advantage of LIM is near-zero waste, but sometimes a brief post-cure (oven or UV) is used to fully stabilize the silicone’s properties. Also, if any minor flash or trimming is needed, it is very minimal for LIM parts.
Machine Requirements: As mentioned, LSR molding needs specialized machines with static mixers and ratio pumps. These machines are more complex and expensive than standard plastic presses, which is why LIM tooling costs are high.
Despite these requirements, modern equipment makes the process quite robust and repeatable.
Applications of Silicone Injection Molding
Injection molding of silicone finds uses across many industries:
Medical and Healthcare: Because LSR parts are biocompatible and produced in closed systems, LIM is used for medical components (syringes, valves, tubing connectors, seals) and surgical tools. The ability to sterilize silicone also means many implantable and diagnostic parts are injection-molded from medical-grade LSR.
Consumer and Baby Products: Silicone baby bottle nipples, pacifiers, cookware (e.g. oven mitts, baking molds), toothbrush heads, and keypads are commonly injection molded. For baby products, the combination of purity and precise molding (no seams in the lip, for example) is crucia.
Automotive and Industrial: Silicone gaskets, O-rings, seals, and vibration-damping components (like grommets and connectors) are molded for automotive and heavy equipment. The high heat resistance of silicone makes it suitable for engine bay components. Electronic connectors and cable boots also use LSR for insulation.
Electronics and Electrical: Silicone’s insulating properties mean it’s used for flexible keypads, button covers, cable seals, and sensor housings. Its durability under heat is valued in electronics.
Miscellaneous: Because silicone resists weather and fire, it appears in building products (firestop collars, sealants) and marine hardware. LSR can be molded into custom shapes like seals for windows, protective feet for equipment, and even molded optics.
In a nutshell, if you need a custom-shaped silicone part – from a simple gasket to a complex valve – injection molding (LSR) is often the method of choice for production. As one industry write-up put it, “Just about every industry imaginable has discovered the opportunities that LSR and the liquid injection molding process offers”. And indeed, silicone parts produced by injection molding can be found in applications ranging from aerospace to food service.
Conclusion
In summary, Liquid silicone injection molding yields high-quality, durable elastomer parts with excellent flexibility, biocompatibility, and thermal stability. Compared to other molding methods, LSR injection offers superior detail, speed, and cleanliness for high-volume silicone products.
Our factory has extensive experience in silicone molding across consumer, medical, and industrial markets. We offer custom LSR injection molding services, utilizing precision molds and a range of silicone grades to meet exact specifications. Whether you need thousands of silicone seals or bespoke medical valves, our state-of-the-art injection molding capabilities can deliver consistent, reliable results. For more information on our silicone injection molding services, please get in touch.
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