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A buyer may choose LSR because a normal plastic part cannot seal well, stay flexible after heat aging, or keep soft performance in a harsh environment. This often happens with gaskets, soft buttons, medical parts, electrical insulation parts, baby product components, and overmolded silicone parts. Liquid silicone rubber is useful in these cases, but it is not just a “soft plastic.” It is a cured silicone elastomer, so material grade, tooling precision, curing, flash control, and part design all need to be reviewed before tooling starts.
What Is Liquid Silicone Rubber?
Liquid silicone rubber, often called LSR, is a two-part silicone elastomer supplied as liquid A and B components. These two components are metered, mixed, injected into a heated mold, and cured into a flexible rubber-like part. If you are asking what is liquid silicone rubber, the short answer is this: it is a liquid silicone material that becomes a thermoset silicone rubber after curing.
Liquid silicone rubber is different from common thermoplastics such as ABS, PP, PC, PA, or POM. Thermoplastics soften when heated and can usually be remelted. Liquid silicone rubber forms a crosslinked structure after curing, so it cannot be melted back into liquid form. This gives LSR good stability for sealing, heat resistance, weather resistance, and flexible parts, but it also means the molding process and tooling requirements are different from ordinary plastic injection molding.
What Is Liquid Silicone Rubber Used For?
Liquid silicone rubber is used when a part needs reliable elasticity, sealing performance, temperature resistance, clean material behavior, or stable recovery after compression. It is not usually chosen because it is cheap. In many cases, it is selected because TPE, TPU, or a standard plastic cannot handle the working conditions.
Common liquid silicone rubber applications include:
- Seals, gaskets, O-rings, and sealing lips
- Medical device components
- Baby care parts, including nipple and pacifier components
- Personal care product parts
- Automotive connectors, seals, and soft interior parts
- Electrical insulation components
- Buttons, keypads, and soft housings
- Protective parts for consumer electronics
- Industrial flexible components
- Food-contact parts when the correct grade is used
- Silicone overmolding on selected plastic or metal inserts
For a simple soft-touch grip, TPE may do the job at a lower cost. But when the part must resist heat, keep a low compression set, tolerate sterilization, perform outdoors, or meet stricter cleanliness requirements, liquid silicone rubber can be the better engineering choice.
For medical, food-contact, and baby care products, the decision should never stop at “LSR is safe.” The exact grade, compliance documents, test requirements, post-curing needs, and applicable standards must be confirmed before production.
How Does Liquid Silicone Rubber Injection Molding Work?
Liquid silicone rubber injection molding uses two liquid components and a heated mold. The material stays flowable before it enters the cavity. After injection, it cures inside the mold and becomes an elastic part.
This process is useful for precision soft parts because LSR flows well into thin walls, small sealing lips, fine details, and complex geometry. But the same behavior also makes the process less forgiving. A small problem in mixing, mold sealing, venting, or curing can turn into flash, bubbles, incomplete curing, or unstable part dimensions.
Metering and Mixing the A/B Components
Liquid silicone rubber is supplied as two liquid components, usually called Part A and Part B. The molding system meters them at a controlled ratio and sends the mixed material through a static mixer before injection.
The ratio and mixing quality have to be stable. If the two components are not mixed properly, the part may show soft spots, incomplete curing, sticky surfaces, weak mechanical performance, or inconsistent appearance.
This is where experienced injection molding suppliers should look beyond the machine screen. Material storage, feed pressure, mixing condition, the material path, and the stability of the dosing system all affect the final part. For production parts, these details are not optional checks. They are part of the process control.
Injecting LSR into a Heated Mold
After mixing, the liquid silicone rubber is injected into a heated mold cavity. Because LSR has very low viscosity, it can fill fine ribs, thin sections, small sealing features, and detailed surfaces more easily than many rubber materials.
That low viscosity is useful, but it also creates one of the biggest challenges in liquid silicone rubber molding: flash. LSR can enter gaps that would not cause problems in many plastic injection molding projects. If the parting line, insert fit, shut-off surface, or venting area is not controlled well, flash can appear quickly.
Curing and Demolding
Inside the heated mold, liquid silicone rubber cures into a flexible solid. Cure time depends on the material grade, wall thickness, mold temperature, part geometry, and catalyst system. Thin sections may cure quickly, while thick areas often need more time and can limit the cycle.
Datasheets are useful, but they do not replace mold trials. During trial molding, the team should check whether the part is fully cured, whether the surface feels right, whether the dimensions are stable, and whether the part can be demolded without tearing or stretching.
Post-Curing When Needed
Some liquid silicone rubber parts need post-curing after molding. Post-curing can help reduce residual volatiles, improve certain material properties, or meet application-specific requirements.
Not every LSR project needs this step. For general industrial parts, it may not be required. For medical, food-contact, low-volatile, or customer-specified applications, however, post-curing should be reviewed carefully. The decision depends on the material grade, end-use conditions, regulatory expectations, and customer testing requirements.
Liquid Silicone Rubber vs. Solid Silicone Rubber
Liquid silicone rubber and solid silicone rubber are both silicone materials, but they are not processed in the same way. Their physical form, flow behavior, tooling requirements, and production style are different.
| Factor | Liquid Silicone Rubber | Solid Silicone Rubber |
| Material form | Two-part liquid system | Solid or gum-like compound |
| Common process | Liquid silicone rubber injection molding | Compression molding, transfer molding, or other rubber molding |
| Flow behavior | Very low viscosity | Higher viscosity |
| Mold requirement | Tight sealing and high precision | Different mold and handling logic |
| Detail capability | Good for small, complex features | Depends on compound and process |
| Production style | Suitable for automation and repeatability | Often more batch-based, depending on process |
| Curing behavior | Cures in a heated mold after A/B mixing | Cures through heat and rubber molding process |
| Best fit | Precision soft parts, seals, medical parts, automated production | Larger rubber parts or certain custom rubber applications |
Liquid silicone rubber molding is often selected for precision soft parts, repeatable production, small details, and automated molding. Solid silicone rubber may still be a better fit for larger rubber parts, lower-volume projects, or applications where a solid compound and its molding method make more sense.
The better choice depends on the part, not on the material name alone.
Advantages of Liquid Silicone Rubber
Liquid silicone rubber makes sense when a part needs long‑term flexibility, reliable sealing, or stability in hot or harsh conditions.
- Stable sealing and recovery. LSR stays elastic across a wide temperature range and bounces back well after compression. Good for gaskets, O‑rings, sealing lips, and waterproof features.
- Heat, weather, and aging resistance. Common plastics or TPE grades harden, soften, crack, or deform under heat, UV, or outdoor exposure. LSR handles those conditions much longer.
- Electrical and medical potential. Works for electrical insulation parts and some healthcare applications. But medical or food‑contact jobs require confirmed material grades, test reports, and certifications – not just the name “LSR”.
- Fine detail and overmolding. Low viscosity fills small features and thin walls. Can be overmolded onto selected plastics or metals, provided bonding, substrate, and tooling design are reviewed first.
Disadvantages of Liquid Silicone Rubber
LSR is not the right answer for every soft part. Many times TPE or TPU does the job with less trouble.
- Higher material cost and special equipment. LSR costs more than most thermoplastics. Needs dedicated metering, mixing, and curing equipment, not a standard injection press.
- Tight tooling precision required. Low viscosity means LSR seeps into tiny gaps. Flash control is hard. The mold must seal well at parting lines, inserts, vents, and shut‑offs.
- Cannot be remelted. Once cured, no reheating and reprocessing like thermoplastics. Scrap is scrap.
- Bonding and cure need attention. Overmolding may need self‑bonding grades, primer, surface treatment, or mechanical locks. Incomplete cure leaves weak or sticky parts.
- Not needed for simple soft‑touch. If the goal is just a comfortable grip or soft feel, TPE is usually simpler and cheaper.
Choose LSR because the application demands its sealing, heat resistance, recovery, or long‑term stability – not just because soft feels nice.
Design and Tooling Notes for LSR Parts
Good liquid silicone rubber injection molding starts before the mold is cut. Part design, tooling strategy, material selection, and inspection planning all affect the final result. A design that works well for rigid plastic may not work well for LSR.
Wall Thickness and Part Geometry
LSR can fill thin areas, but wall thickness still needs attention. Very thin lips may tear during demolding. Very thick sections may need longer curing time and can affect cycle consistency.
Smooth transitions are usually better than sudden changes in thickness. Sharp corners, unsupported thin sections, and aggressive undercuts should be reviewed early, especially if the part must be demolded automatically.
Flash Control and Parting Line Design
Flash is one of the most common issues in liquid silicone rubber molding. Since LSR can flow into extremely small gaps, the parting line, shut-off surfaces, vent areas, and insert fits must be designed carefully.
Flash location should be discussed before mold manufacturing begins. On some parts, flash on a non-functional edge may be acceptable. On a sealing surface, medical contact area, or visible cosmetic surface, it can become a serious problem.
Venting and Flow
Air has to leave the cavity as liquid silicone rubber enters. Poor venting can cause trapped air, incomplete filling, burn marks, or surface defects.
Vents should be placed near flow ends, thin features, and areas where air is likely to be trapped. The challenge is to release air without creating excessive flash, which is why vent design is especially important in LSR molds.
Shrinkage and Tolerances
LSR shrinkage depends on the material grade, part design, mold temperature, curing conditions, and tool construction. Tolerances should be realistic for a flexible molded part.
Soft parts can deform during handling, assembly, or measurement. Because of that, the inspection method should be defined early. If the measurement setup is not controlled, the same part may appear to have different dimensions under different inspection conditions.
Overmolding and Insert Bonding
Liquid silicone rubber can be molded over selected plastics or metals, but bonding must be engineered. It is not automatic.
The project may need a self-bonding LSR grade, primer, surface treatment, or mechanical features such as holes, grooves, undercuts, or wraparound edges. The substrate must also tolerate the mold temperature and the molding process. If the insert warps, outgasses, or shifts during molding, bonding and dimensions can both become unstable.
Surface Finish and Demolding
Surface texture affects appearance, feel, and release from the mold. A very soft LSR part may stick to the tool or deform during ejection. Deep texture may improve feel but can also make demolding more difficult.
Mold finish, draft, part geometry, and release method should be reviewed together. Treating these as separate issues often leads to avoidable mold changes later.
Common Liquid Silicone Rubber Molding Defects
Liquid silicone rubber parts can develop defects when material handling, mixing, mold condition, curing, or demolding is not controlled. The defect may show up on the surface, but the root cause is often somewhere earlier in the process.
| Defect | Possible Cause | Prevention |
| Flash | Poor mold sealing, low viscosity, high pressure | Improve mold precision, parting surface design, and process control |
| Bubbles | Trapped air, poor venting, mixing issues | Improve venting, metering, and material handling |
| Incomplete curing | Low mold temperature, short cure time, wrong mix ratio | Check mix ratio, cure time, and mold temperature |
| Tearing during demolding | Thin areas, undercuts, poor release design | Improve geometry, draft, and demolding method |
| Poor bonding | Wrong LSR grade, dirty insert, weak mechanical lock | Review substrate, surface treatment, primer, or self-bonding grade |
| Surface defects | Contamination, poor tool surface, trapped air | Keep the material path clean, improve venting, maintain mold surfaces |
| Dimensional variation | Cure variation, shrinkage, soft part deformation | Control the process and define a suitable inspection method |
Good troubleshooting should not stop at machine settings. The team should check the material grade, A/B ratio, mixing condition, mold temperature, venting, flash areas, curing time, insert condition, demolding method, and inspection setup.
Conclusion
Liquid silicone rubber is a good choice when a molded part needs more than basic softness. It can be useful for sealing, flexibility, heat resistance, weather resistance, electrical insulation, and selected medical or food-contact applications when the right grade is used. At the same time, LSR is not a simple soft plastic. The material grade, tooling precision, curing control, flash control, bonding method, and part design all need to be reviewed before tooling starts.
If your project involves silicone parts, LSR molded parts, or soft flexible components, HingTung injection molding manufacturer can help review your drawings, material requirements, tolerance needs, overmolding conditions, and production goals. This early review can help confirm whether LSR, another silicone option, or a different molding solution is the better direction before you invest in tooling.