Exploring the Pros and Cons of Liquid Silicone Injection Molding

As we delve into the world of manufacturing, a crucial process often emerges: injection molding. This versatile technique allows for the creation of intricate parts, especially those requiring flexibility. When we talk about flexible parts, two materials frequently enter the conversation: liquid silicone rubber (LSR) and thermoplastic elastomers (TPE). While both offer unique advantages, liquid silicone injection molding stands out for its specific benefits and considerations.

Liquid silicone rubber (LSR) injection molding is a transformative manufacturing method used to produce a high volume of pliable, durable, intricate, and precise silicone rubber parts. We'll explore how this process works, its key advantages, and some of the limitations we need to be aware of when considering liquid silicone injection molding for our projects.

Key Takeaways

• What is Liquid Silicone Rubber (LSR)?: LSR is a high-purity, two-part platinum-cured thermoset elastomer known for its low viscosity. Its chemical backbone is made of siloxane bonds (silicon-oxygen), which are stronger than the carbon bonds in traditional rubbers, granting it superior thermal stability and chemical inertness.
• The Process: Liquid silicone injection molding involves pumping two liquid components (Part A with a platinum catalyst and Part B with a cross-linker) from a closed system into a mixing unit. The mixed material is then injected into a heated mold cavity where it cures into a solid, finished part. This automated process is clean, fast, and ideal for high-volume production.
• Core Advantages: The primary benefits include exceptional precision for complex parts with tight tolerances, rapid cycle times, and superior material properties. LSR parts are biocompatible, hypoallergenic, and resistant to extreme temperatures, UV light, and chemicals, making them suitable for medical, automotive, and consumer goods.
• Significant Limitations: The main drawbacks are higher initial costs, particularly for tooling and specialized machinery. The raw material itself is often more expensive than alternatives like TPEs. Processing can be complex, requiring precise control to avoid defects like "flash," and silicone is not easily recyclable through conventional methods.
• LSR vs. TPE: The choice between LSR and TPE depends on the application's demands. LSR, a thermoset, excels in high-temperature environments and applications requiring medical-grade biocompatibility and excellent compression set. TPE, a thermoplastic, is generally more cost-effective, easier to process, and recyclable.

Understanding Liquid Silicone Rubber (LSR) in Liquid Silicone Injection Molding

To fully appreciate the pros and cons of liquid silicone injection molding, we must first understand the star of the show: the material itself. Liquid Silicone Rubber, or LSR, is not just another flexible polymer; it's a high-performance thermoset elastomer with a unique chemical structure and properties that set it apart from other Injection-Molding-Materials. Its characteristics are central to why this manufacturing process is chosen for some of the most demanding applications across various industries.

What is Liquid Silicone Rubber (LSR)?

At its core, Liquid Silicone Rubber (LSR) is a synthetic polymer that starts as a two-part liquid system and, when mixed and cured, transforms into a solid, flexible, and highly durable material. It is a high-performance thermoset rubber, meaning that once it is cured (cross-linked), it cannot be re-melted and reshaped like a thermoplastic. This curing process is what locks in its superior properties. One of the defining features of LSR is that it is a platinum-cured elastomer.

The Unique Composition of Liquid Silicone Rubber

The remarkable properties of LSR stem directly from its unique molecular structure. The main chemical chain, or "backbone," is composed of alternating silicon and oxygen atoms, known as a siloxane bond (-Si-O-Si-). This siloxane bond is significantly stronger than the carbon-carbon bonds found in most organic rubbers and plastics, with a binding energy of 433 kJ/mol compared to 355 kJ/mol for carbon bonds. This fundamental difference is what gives LSR its exceptional heat resistance, chemical stability, and resistance to UV and ozone degradation.

LSR is supplied as a two-component system:

• Part A: This component contains a platinum catalyst.
• Part B: This part contains methylhydrogensiloxane, which acts as a cross-linker, and an alcohol inhibitor to prevent the reaction from starting prematurely.

When these two liquid components are mixed—typically in a 1:1 ratio—the platinum catalyst initiates an additive curing process. This reaction forms cross-links between the polymer chains, converting the liquid into a solid elastomeric network without generating any harmful by-products. Fillers, like specially treated silica, are often added to the formulation to improve mechanical properties such as tear strength. Pigments can also be added during the mixing process to achieve specific colors.

Why Choose Liquid Silicone Rubber for Injection Molding?

Manufacturers choose LSR for injection molding for a host of reasons, primarily driven by its unique combination of processing and performance characteristics.

• Low Viscosity: LSR's liquid state and low viscosity allow it to flow easily, filling intricate and complex mold cavities with thin walls and fine details that would be difficult to achieve with thicker materials. This enables the production of highly detailed parts with exceptional precision.
• Biocompatibility: LSR is a highly pure material that is non-toxic, hypoallergenic, and can comply with strict medical standards like USP Class VI and ISO 10993. This makes it one of the few materials suitable for medical devices that come into contact with the human body, as well as for food-grade products and baby care items.
• Temperature Resistance: LSR parts exhibit outstanding performance across an extreme temperature range. They remain flexible and functional at temperatures as low as -60°C and can withstand continuous heat up to 250°C (482°F) without degrading.
• Chemical Inertness: The stable siloxane backbone makes LSR chemically inert, meaning it does not react with most chemicals and is resistant to water, oxidation, and oils.
• Durability and Elasticity: LSR has an excellent compression set, meaning it can be compressed for extended periods and will return to its original shape once the force is removed. This property, combined with great tear and tensile strength, ensures long-term durability for seals, gaskets, and other functional components.

These qualities make liquid silicone rubber a go-to choice for producing high-quality, high-performance parts through the injection molding process.

The Liquid Silicone Injection Molding Process Defined

Liquid Silicone Injection Molding (LIM), is a thermoset process designed to produce durable, high-precision silicone parts in medium to high volumes. The process is highly automated and happens within a closed system, which is crucial for maintaining the purity of the material and ensuring part consistency. The key components of an LSR injection molding machine include supply drums, a metering unit, a static mixer, an injection unit, and a heated mold.

Here is a step-by-step breakdown of how the process works:

1. Material Preparation and Pumping: The two primary liquid components of LSR, Part A (platinum catalyst) and Part B (cross-linker), are housed in separate containers, often referred to as supply drums or plungers. A metering unit precisely pumps the two liquids in an exact 1:1 ratio. If color is required, a third stream for liquid pigment is added at this stage.

2. Mixing: The precisely metered streams of Part A and Part B are pumped through a static mixer. This device contains a series of baffles that repeatedly divide and recombine the material streams, ensuring a complete and homogenous mixture without introducing air bubbles. The mixture remains at a low temperature to prevent curing from beginning prematurely.

3. Injection: The mixed, still-liquid silicone is then transferred to the machine's injection unit. From there, it is injected under controlled pressure through a nozzle and a runner system into a heated, sealed mold. Unlike thermoplastic injection molding where the material is heated in the barrel and cooled in the mold, the LSR process uses a cooled barrel and a hot mold. This keeps the material liquid until it enters the final cavity.

4. Curing (Vulcanization): The mold is heated to a specific temperature, typically between 150°C and 200°C (302°F and 392°F). This high temperature initiates the platinum-catalyzed chemical reaction, causing the LSR to cross-link and solidify, or 'vulcanize', into its final, solid elastomeric shape. This curing process is very fast, often taking only 30 seconds to a couple of minutes, depending on the part's size and thickness.

5. Demolding and Ejection: Once the curing cycle is complete, the mold opens automatically, and the finished part is ejected. The part is now a fully formed, solid component that retains its shape and properties. Depending on the application, some parts may undergo a post-curing process, where they are baked in an oven to further enhance mechanical properties and remove any trace volatiles.

This highly controlled and automated process is a key reason for LSR's reputation for consistency and precision in high-volume manufacturing.

The Remarkable Advantages of Liquid Silicone Injection Molding

Liquid silicone injection molding is a leading manufacturing process for a reason. It offers a powerful combination of precision, speed, and superior material characteristics that make it an invaluable choice for demanding industries like medical, automotive, and consumer electronics. The advantages extend from the efficiency of the production floor to the performance of the final product.

Precision and Efficiency in Liquid Silicone Injection Molding

One of the most significant benefits of LSR injection molding is its unparalleled ability to produce highly precise and complex parts with remarkable efficiency. This is a direct result of the material's properties and the nature of the automated process.

• Exceptional Precision and Complex Geometries: The very low viscosity of liquid silicone rubber allows it to flow effortlessly into the most intricate and detailed areas of a mold. This enables the creation of parts with very thin walls, micro-features, and complex geometries that would be impossible to achieve with higher-viscosity materials. The process ensures high dimensional accuracy and consistency, making it ideal for high-volume production where uniformity is non-negotiable.

• Fast Cycle Times: The platinum-cured chemistry of LSR allows for very rapid vulcanization inside the heated mold. Cycle times are exceptionally fast, often ranging from just 30 to 60 seconds, a stark contrast to the several minutes required for traditional compression molding of silicone. For parts with a wall thickness of around 1-2 mm, the curing time can be as short as 5-20 seconds. This rapid turnover significantly increases production throughput.

• High Level of Automation: The entire liquid silicone injection molding process, from material pumping and mixing to part ejection, can be fully automated. This high degree of automation minimizes the need for manual labor, reducing operational costs and the potential for human error. Automated systems ensure precise control over all process parameters—such as temperature, pressure, and dosing ratios—which leads to highly repeatable and consistent part quality. A single operator can often oversee multiple machines, further boosting efficiency.

• Flashless and Low-Waste Production: Modern LSR molding can be engineered to be virtually "flashless," meaning little to no excess material escapes the mold cavity at the parting line. This is achieved through high-precision tooling and excellent process control. The closed-loop system and efficient cold-runner technology (which keeps the material liquid until the last moment) also minimize material waste, an important factor given the higher cost of LSR.

Exceptional Material Properties of Liquid Silicone Injection Molded Parts

The parts produced through LSR injection molding inherit the outstanding properties of the material itself. These characteristics ensure reliable performance even in the most challenging environments.

Property	Description	Key Benefits
Biocompatibility	LSR is inherently non-toxic, hypoallergenic, and does not support bacterial growth. It can meet stringent regulatory standards like USP Class VI and ISO 10993.	Ideal for medical devices, healthcare applications, baby care products, and food-contact items.
Thermal Stability	Parts maintain their mechanical properties and flexibility over an extremely wide temperature range, typically from -65°C to 200°C (-85°F to 392°F), with some grades handling even higher temperatures.	Suitable for automotive components near engines, kitchenware for baking, and devices used in extreme climates.
Chemical Resistance	LSR is highly resistant to water, oxidation, UV radiation, ozone, and a wide array of chemicals.	Ensures durability and longevity for outdoor applications, seals exposed to fluids, and parts that undergo repeated sterilization.
Excellent Sealing & Low Compression Set	LSR exhibits a very low compression set, meaning it returns to its original shape after being under prolonged compressive stress.	Perfect for creating durable, long-lasting seals, gaskets, and O-rings that maintain their sealing force over time.
Electrical Properties	LSR is an excellent electrical insulator with high dielectric strength, making it ideal for protecting sensitive electronic components.	Used for electrical connectors, insulators, keypads, and protective casings in the electronics industry.
Clarity and Aesthetics	Standard LSR grades are naturally translucent, allowing for high clarity. They can be easily pigmented to create parts in a wide spectrum of colors.	Enables the production of clear medical tubing or lenses, as well as aesthetically pleasing consumer products.

Versatility and Applications of Liquid Silicone Injection Molding

The combination of manufacturing efficiency and superior material properties makes LSR injection molding incredibly versatile. It has become a cornerstone technology in numerous industries for creating critical, high-performance components.

• Medical and Healthcare: Due to its biocompatibility and sterilizability, LSR is used for respiratory masks, seals for medical devices, catheters, syringe stoppers, implantable components, and soft-touch grips for surgical instruments.
• Automotive: In the automotive sector, LSR's resistance to high temperatures and fluids makes it perfect for wire harness seals, connector seals, gaskets for engine and transmission components, and vibration-dampening parts.
• Consumer Goods and Electronics: LSR is found in a wide variety of consumer products, including kitchenware like baking molds and spatulas, baby bottle nipples and pacifiers, waterproof seals for smartwatches and outdoor electronics, and soft, durable keypads.
• Industrial Applications: The durability and sealing capabilities of LSR are leveraged in industrial settings for pump seals, electrical enclosure gaskets, and components for ruggedized data acquisition devices.

This broad applicability underscores why liquid silicone injection molding is a preferred method for manufacturers looking to produce reliable, high-quality, and cost-effective flexible parts in high volumes.

Exploring the Limitations of Liquid Silicone Injection Molding

While liquid silicone injection molding offers a suite of impressive advantages, it's not the ideal solution for every project. Understanding its limitations, particularly concerning cost, process complexity, and environmental impact, is crucial for making an informed manufacturing decision. These factors represent the primary trade-offs for achieving the high performance and precision that LSR provides.

Cost Considerations in Liquid Silicone Injection Molding

The most significant barrier to adopting LSR injection molding is often the initial and ongoing financial investment. These costs can be broken down into three main areas: tooling, materials, and equipment.

1. High Tooling and Mold Costs: Tooling represents one of the largest upfront expenses. Molds for LSR must be manufactured to extremely high standards of precision to handle the material's low viscosity and prevent flash. Key factors driving up mold costs include:

• Complexity: Molds for parts with intricate geometries, undercuts, or micro-features require more complex engineering and machining, significantly increasing the price. A simple, single-cavity mold might cost between $5,000 and $10,000, but a high-precision, multi-cavity mold for a medical device can easily exceed $50,000 to $100,000.
• Materials and Construction: LSR molds are typically made from high-grade steel to withstand the heated curing process. They often require specialized components like cold runner systems and shut-off valves to control material flow, which adds to the expense.
• Number of Cavities: While multi-cavity molds increase production throughput, they also multiply the complexity and cost of the tool.

2. Raw Material Price: The cost of the liquid silicone rubber itself is another major factor.

• Higher Price per Kilogram: General-purpose LSR typically costs more per kilogram than many standard thermoplastic elastomers (TPEs). In 2025, prices for general-purpose LSR can range from $10 to $20 per kilogram.
• Specialty Grades: The price increases substantially for specialized grades. Medical-grade LSR, which requires higher purity and biocompatibility certifications, can cost $25 to $40 per kilogram, while optical or other high-performance grades can be even more expensive.
• Higher Density: LSR also has a specific gravity that is about 20% higher than most TPEs, meaning you get fewer parts per pound of material, further impacting the per-part cost.

3. Equipment and Processing Costs: LSR injection molding requires specialized machinery. These machines are equipped with precision dosing pumps, static mixers, and advanced control systems, representing a higher capital investment than standard thermoplastic injection molding presses. The operating cost for these machines can range from $40 to $100 per hour, which includes energy for heating the mold and running the dosing units.

While the high-speed, automated nature of the process can offset some of these costs in high-volume production runs, the initial financial hurdle remains a significant consideration for startups and low-volume projects.

Processing Complexities in Liquid Silicone Injection Molding

Achieving the high-quality results that LSR is known for requires navigating a narrow and demanding process window. Expertise is critical, as several processing challenges can arise if not properly managed.

• Risk of Flash: The extremely low viscosity of LSR is a double-edged sword. While it allows for filling intricate details, it also means the material can easily escape through the smallest imperfections or gaps in the mold, creating a thin film of excess material known as "flash". Preventing flash requires exceptionally precise and well-maintained molds, perfect parting line seals, and finely tuned injection pressure and clamping force. Even minimal flash can interfere with a part's function, especially for seals, and often requires a secondary removal process, which adds time and cost.

• Material Contamination and Curing Inhibition: The platinum-catalyzed curing process of LSR is highly sensitive to contamination. Contact with certain chemical compounds—such as sulfur, tin, amines, and peroxides—can inhibit or completely stop the curing reaction. This can leave the part sticky, weak, or uncured. To prevent this, the entire process must be meticulously clean, from the material supply lines to the mold itself. The use of a closed-loop system is essential to avoid exposure to environmental contaminants.

• Air Entrapment and Venting: Because the molding process happens so quickly, air can become trapped within the mold cavity, leading to voids or bubbles in the finished part. This compromises both the structural integrity and the cosmetic appearance. Proper mold design with adequate venting is critical to allow air to escape as the liquid silicone fills the cavity. In some cases, a vacuum may be applied to the mold just before injection to completely evacuate all air.

• Specialized Equipment and Expertise: As mentioned, LSR molding cannot be done on standard thermoplastic injection machines. It requires specialized equipment with unique components like dosing pumps and static mixers. Operating this machinery and troubleshooting issues like flash, short shots (incomplete parts), or burn marks requires a high level of technical expertise and experience specifically with silicone materials.

Navigating these complexities successfully is the key to leveraging the full potential of liquid silicone injection molding.

Environmental and Recycling Aspects of Liquid Silicone Injection Molding

The environmental profile of silicone is complex and often debated. While it offers advantages over traditional plastics in some areas, it also presents significant challenges, particularly at the end of its life cycle.

Durability as a Form of Sustainability: One of the key arguments for silicone's sustainability is its incredible durability. Products made from LSR are highly resistant to heat, chemicals, and physical degradation, giving them a very long lifespan. This longevity reduces the need for frequent replacements, which in turn cuts down on the consumption of raw materials and the generation of waste compared to less durable, single-use plastic items. A high-quality silicone part can function for years without losing its properties.

Challenges in Recycling: Despite its durability, the end-of-life disposal of silicone is a major drawback.

• Not Recyclable in Conventional Systems: Silicone is a thermoset, not a thermoplastic. It cannot be simply melted down and reformed into new products like PET plastics. As a result, silicone is not accepted in most municipal curbside recycling programs. If placed in a standard recycling bin, it will almost certainly end up in a landfill.
• Specialized Recycling Processes: Silicone is technically recyclable, but it requires specialized, energy-intensive industrial processes. These methods typically involve grinding the material down and then using high heat or chemical processes to break the polymer down into silicone oil, which can then be purified and used to create new silicone.
• Lack of Infrastructure: The facilities capable of performing this type of recycling are very rare. This lack of widespread infrastructure means that the vast majority of silicone products are not recycled and are instead sent to landfills.

Biodegradability and Environmental Impact: Silicone is not biodegradable. Like plastic, it will persist in the environment for hundreds of years. However, unlike many plastics, high-quality silicone does not break down into micro-particles that are known to contaminate waterways and harm wildlife. When incinerated, it reverts to harmless components like silica, carbon dioxide, and water vapor, without releasing the toxic chemicals associated with burning some plastics.

While the raw material for silicone, silica, is abundant, its production is an energy-intensive process. A study commissioned by the Global Silicones Council found that the energy savings and greenhouse gas reductions enabled by the use of silicone products in various applications outweigh the environmental impacts of their production and disposal by a factor of nine. Still, the challenge of creating a truly circular lifecycle for silicone remains a significant environmental limitation.

Liquid Silicone Injection Molding Versus Thermoplastic Elastomers (TPE)

When designing a product that requires a flexible, rubber-like material, engineers often face a choice between Liquid Silicone Rubber (LSR) and Thermoplastic Elastomers (TPE). While both can be processed via injection molding, they are fundamentally different materials with distinct properties, processing requirements, and costs. Understanding these differences is key to selecting the right material for your application.

Key Differences in Material and Process: Liquid Silicone Injection Molding vs. TPE

The most fundamental distinction between LSR and TPE lies in their material classification: LSR is a thermoset, while TPE is a thermoplastic. This single difference dictates how they behave when heated and determines their core properties and processing methods.

1. Material Chemistry:

• Liquid Silicone Rubber (LSR): LSR's structure is based on a backbone of silicon and oxygen atoms (siloxane). It is a thermoset, which means it undergoes an irreversible chemical cross-linking reaction (vulcanization or curing) when heated. Once it solidifies, it cannot be melted and reshaped. This permanent chemical bond gives LSR its superior stability and resilience.
• Thermoplastic Elastomer (TPE): TPE is a blend of polymers, typically a mix of a plastic and a rubber material. Its structure is based on a carbon backbone. As a thermoplastic, it softens and melts when heated and hardens when cooled, a process that is fully reversible. No chemical bonding occurs during processing, which allows it to be re-melted and recycled.

2. Injection Molding Process: The difference in chemistry leads to an inverse processing approach during injection molding.

• LSR Molding: Involves injecting a cool, two-part liquid material into a very hot mold (typically 175-205°C). The heat in the mold triggers the chemical curing reaction, solidifying the part. This process requires specialized machinery with mixing and metering units.
• TPE Molding: Involles heating solid plastic pellets in the machine's barrel until they melt. This molten material is then injected into a cool mold, where it solidifies upon cooling. This process uses standard thermoplastic injection molding equipment.

3. Core Property Comparison:

Here is a table summarizing the key differences in properties between LSR and TPE:

Feature	Liquid Silicone Rubber (LSR)	Thermoplastic Elastomer (TPE)
Material Type	Thermoset	Thermoplastic
Chemical Basis	Silicon-oxygen backbone	Carbon-based polymer blend
Heat Resistance	Excellent; stable at very high and low temperatures (-100°F to 400°F / -73°C to 204°C).	Moderate; softens at high temperatures, performance limited at temperature extremes.
Compression Set	Excellent; returns to its original shape very effectively after compression. Ideal for long-term seals.	Fair to good; more likely to deform permanently under sustained pressure compared to LSR.
Chemical Resistance	Superior resistance to a wide range of chemicals, solvents, and oils.	Moderate chemical resistance; vulnerable to certain oils and solvents.
Recyclability	Not easily recyclable; requires specialized facilities.	Fully recyclable; scrap material can be re-melted and reprocessed.
Biocompatibility	Excellent; inherently pure and hypoallergenic, often meeting medical-grade standards (USP Class VI).	Varies by grade; some medical grades are available, but not all TPEs are biocompatible.
Processing	Requires specialized LSR injection molding machines; fast cure times but the process can be more complex.	Uses standard thermoplastic injection molding equipment; generally easier to process.
Cost	Higher raw material cost and often higher tooling investment.	Lower raw material cost and uses standard, less expensive tooling.
Overmolding	Bonds well with high-temperature engineering plastics.	Bonds easily to a wider range of common plastics due to lower processing temperatures.

When to Choose Liquid Silicone Injection Molding Over TPE

The decision to use LSR over TPE is almost always driven by performance requirements. While TPE is a more cost-effective and easier-to-process material for many general-purpose applications, there are specific scenarios where the superior properties of LSR are non-negotiable.

Choose Liquid Silicone Rubber (LSR) when:

• High-Temperature Performance is Critical: If the part will be exposed to continuous high temperatures, such as in automotive engine compartments, industrial machinery, or for bake-ware, LSR is the only choice. TPE will soften and lose its shape under such conditions.
• Superior Sealing is Required: For applications demanding a reliable, long-lasting seal that will not deform under pressure, LSR's excellent compression set makes it the clear winner. This is crucial for gaskets, O-rings, and other sealing components.
• Medical-Grade Biocompatibility is Mandatory: When the part will have contact with human skin or tissue, especially for implants or other invasive medical devices, LSR's inherent purity and ability to meet strict standards like USP Class VI is essential.
• Chemical Resistance is a Must: If the component will be exposed to aggressive solvents, oils, or chemicals, or will undergo repeated sterilization cycles, LSR's chemical inertness provides the necessary durability.
• Extreme Low-Temperature Flexibility is Needed: LSR maintains its flexibility in deep-freeze conditions where most TPEs would become brittle and crack.
• The Product Demands the Highest Level of Quality and Durability: For premium products where long-term performance and reliability justify a higher initial cost, LSR is the preferred material.

In essence, while TPE is an excellent material for many cost-sensitive consumer products, LSR is the high-performance option for applications where failure is not an option and where superior thermal, chemical, and mechanical properties are paramount.

Conclusion

In summary, liquid silicone injection molding offers a powerful combination of precision, durability, and versatility, making it a preferred choice for numerous applications requiring high-performance flexible parts. Its ability to produce complex components with rapid cycle times and exceptional material properties—such as biocompatibility and extreme temperature resistance—sets it apart. While it comes with specific considerations regarding higher initial costs for tooling and raw materials, along with processing complexities that demand expertise, its unique properties often justify the investment for critical applications where performance cannot be compromised.

Understanding the nuances of liquid silicone injection molding, including its distinct advantages over materials like TPE, allows us to make informed decisions for our manufacturing needs. It's a choice that hinges on balancing budget with the non-negotiable performance requirements of the final product. For those seeking reliable and high-quality liquid silicone injection molding services, SOMI Custom Parts stands ready, connecting you with premium manufacturers worldwide to bring your innovative designs to life. We encourage you to share this article with your network and join the conversation about the future of advanced manufacturing.