Material: Silicone
LSR and Its Characteristics
Silicone is a synthetic material derived from silicon, a chemical element found in quartz-containing sand (silicon dioxide, SiO₂). Through chemical processes, silicon dioxide is converted into silicone rubber, whose properties can be precisely tailored based on its molecular structure. This makes silicone one of the most versatile engineering materials in existence and is used in applications ranging from medical technology to the automotive industry.
Contents
What makes silicone special?
| Feature | Characteristic | Practical Relevance |
|---|---|---|
| Temperature Resistance | approx. -50°C to 180°C (continuous), up to 250°C for short periods | Use in engine compartments, sterilization, and ovens |
| Elasticity | Shore hardness range 10–95 Shore A, retains resilience even after long-term stress | Sealing performance is maintained throughout the product’s service life |
| Chemical Resistance | Resistant to diluted acids, alkalis, oils; limited resistance to concentrated solvents | Media contact in industry and life sciences |
| UV/Weather Resistance | Resistant to UV radiation and environmental influences throughout the entire product lifespan | Outdoor applications, automotive manufacturing |
| Biocompatibility | Depending on the material, compliant with FDA, KTW, ISO 10993, USP Class VI, and other common standards for medical, life sciences, food, and drinking water applications | Medical technology, pharmaceuticals |
| Electrical Insulation | Dielectric strength: 28–32 kV/mm | Electronics, cables, connectors |
What is LSR?
LSR stands for Liquid Silicone Rubber. The name itself describes the material’s key characteristics. Unlike traditional solid silicone rubber, LSR is supplied as a liquid and consists of two components that are mixed immediately before processing. The material is then injected into a heated mold, where it undergoes a chemical curing reaction and forms a solid, elastic silicone component.
This process differs fundamentally from plastic injection molding. Thermoplastic materials are heated and melted in the injection unit and then cooled in the mold to solidify. Liquid silicone rubber follows the opposite temperature profile: the material is kept cool during injection and cured inside a heated mold. This heat-triggered crosslinking process enables the efficient production of highly precise silicone parts with excellent consistency, quality, and performance.
The combination of liquid processing, heat-activated curing, and exceptional material consistency makes LSR ideally suited for highly automated, high-volume manufacturing of complex, precision-engineered silicone components.
LSR is one of three major silicone categories used in industrial manufacturing. The choice between Liquid Silicone Rubber (LSR), High Consistency Rubber (HCR), and Room Temperature Vulcanizing silicone (RTV) depends on factors such as production volume, component geometry, processing method, and performance requirements. The following overview highlights the key differences between these silicone materials and their typical applications.
What makes silicone temperature-resistant?
Silicone’s exceptional temperature resistance stems from its chemical structure. Its silicon-oxygen main chain (Si-O) has a significantly higher bond energy than the organic polymer chains found in conventional elastomers. Consequently, silicone maintains mechanical properties such as elasticity, sealing performance, and resistance to aging over long periods of time, even at high temperatures.
In short, LSR (liquid silicone rubber) is processed using injection molding and is ideal for producing high-precision, mass-produced parts.
LSR vs. HCR vs. RTV – A Comparison of the Three Types of Silicone
Not all silicones are the same. In industrial processing, there are three main types:
| Type | LSR (Liquid Silicone Rubber) | HCR/HTV (Solid/High Consistency Rubber) | RTV (Room Temperature Vulcanizing) |
|---|---|---|---|
| Processing Form | Liquid, two-component | Solid, malleable | Liquid/paste-like |
| Process | Injection molding, automated | Extrusion, compression molding | Manual casting, bonding, room-temperature curing |
| Typical Application | Mass-produced components, precision parts | Profiles, hoses, molded parts | Potting compounds, adhesives, small-batch production |
LSR is ideal for the automated mass production of complex, precise geometries, while HCR is commonly used for profiles. RTV is used for bonding and potting applications that do not require molds.
How does crosslinking work chemically?
LSR typically undergoes platinum-catalyzed addition crosslinking. Two components, usually in a 1:1 ratio, are mixed. A vinyl-functionalized polymer reacts with a hydrogen-functionalized crosslinking agent while heat is applied inside the mold. The advantage over condensation curing (as found in some RTV silicones) is that no byproducts are formed. This results in components with better dimensional accuracy and more reproducible processes, which is a decisive factor for mass production.
Put simply, you mix two liquids together, just as you would with a two-component adhesive. The heat in the mold causes the substances to bond firmly, forming a rubbery, elastic part without anything “left over” or escaping. This results in a particularly precise and highly repeatable outcome, which is important when producing a large number of identical parts.
The LSR Injection Molding Process in Detail
The injection molding process for liquid silicone involves several closely timed steps:
2C Injection Molding/Hard-Soft Composite
LSR is often not processed as a single material, but rather in combination with a thermoplastic carrier in a hard-soft composite or two-component injection molding process. This allows seals to be injection-molded directly onto a rigid housing component, eliminating the need for an additional assembly step. Typical applications include connectors with integrated seals, valves with plastic housings, and assemblies with dual functionality.
Silicone vs. Conventional Plastics
Whether a component requires silicone or a conventional thermoplastic depends primarily on the stresses it will undergo later on. Will it need to remain permanently elastic, withstand high temperatures, or function reliably for years? Thermoplastics soften when heated and can be molded, but silicone remains structurally stable under heat, UV radiation, and continuous mechanical stress. The following overview provides a direct comparison of the key differences.
| Characteristics | Silicone | Thermoplastics |
|---|---|---|
| Temperature Behavior | Very stable over a wide range | Limited |
| Elasticity | Permanently elastic | Often restricted |
| Resistance to Aging | Very high | Depending on the material |
| Chemical Resistance | High | Variable |
| Long-Term Behavior | Stable | May change |
Why is processing challenging?
The processing of silicone differs significantly from that of traditional plastics. While thermoplastics are melted and then solidified again, silicone is shaped through a chemical cross-linking reaction that occurs during processing.
Additionally, silicone is processed in liquid form, so even slight changes in process parameters can affect the component’s behavior. Mold design, temperature control, and process stability also play crucial roles.
This means that reliably manufacturing silicone components, especially in mass production, requires a comprehensive understanding of the material, its geometry, and the manufacturing process.
For silicone to be transformed into a functional, mass-produced component, several factors must work together perfectly:
- Component design and geometry: For example, designing wall thicknesses so that the material flows and cures evenly within the mold rather than cross-linking prematurely in thin areas or forming air pockets in thick areas.
- Material behavior and application: Selecting the appropriate Shore hardness, color, and certifications (e.g., biocompatibility) for the component’s intended use, not just for shaping purposes.
- Simulation and validation: Filling simulations predict weld lines and air entrapments before mold construction. This avoids costly correction cycles on the finished mold.
- Mold making: precise runner design and temperature control within the mold ensure the material cures simultaneously and uniformly in all cavities, which is critical for dimensional stability over thousands of cycles.
- Process design and automation: Optimization of injection pressure, mold temperature, and cycle time; integration of robotic handling for demolding and quality control in mass production. Only through the interplay of these factors can stable, cost-effective mass production be achieved.
This is exactly where starlim helps ensure that components are designed early on to meet mass-production requirements and are efficiently transitioned into production.
What is Shore hardness?
The hardness of silicone is measured using the Shore A scale (and the Shore 00 scale for very soft materials). The lower the value, the softer and more flexible the material. LSR components are typically manufactured within the 10-95 Shore A range. The hardness level is chosen based on the component’s sealing, damping, or functional requirements.
What is Compression Set?
It describes how well an elastomer returns to its original shape after undergoing prolonged deformation. Low compression set is important for seals, valves, and diaphragms because it ensures the sealing function is maintained under prolonged stress. Typical silicone materials achieve values ranging from about 5 to 25%.
Approvals & Standards
For sensitive applications, such as in medical technology or the food industry, silicone must meet specific standards:
- ISO 10993 – Biocompatibility for Medical Devices
- USP Class VI – Pharmaceutical Suitability
- EU 1935/2004 – Food contact in the EU
- FDA Compliance – Required for the U.S. market
- Cleanroom Manufacturing (ISO 14644-1 Classes 7 & 8 and GMP Classes C and D) – to prevent contamination
Typical Applications
Silikon wird in unterschiedlichsten technischen Anwendungen eingesetzt – insbesondere dort, wo Bauteile flexibel, dichtend und langfristig zuverlässig funktionieren müssen.
Silicone is used in a wide variety of technical applications—especially where components must be flexible, provide a seal, and function reliably over the long term.
Typical examples include:
- Seals
- Diaphragms
- Valves
- Functional systems with integrated properties
Requirements vary significantly depending on the industry:
Life Science:
High standards for cleanliness, biocompatibility, and functionality
e.g. Valves for infusion systems, seals for insulin pens
Mobility:
Resistance to temperature and environmental influences
e.g. Seals in the engine compartment, connector housings
Industry:
Durable and functional components for demanding operating conditions
eg. Diaphragm valves, sensor seals
Frequently Asked Questions
What is the difference between silicone and LSR?
Silicone is a material, while LSR (liquid silicone rubber) is a liquid form of silicone developed specifically for injection molding.
What does LSR stand for?
LSR stands for liquid silicone rubber, a two-component silicone processed in liquid form that cures inside the mold to form a solid, elastic component.
How do LSR, HCR, and RTV differ?
LSR is liquid and designed for automated injection molding. HCR (also known as HTV) is solid and processed by extrusion or compression molding. RTV cures at room temperature and is usually poured or bonded manually.
Is silicone food-safe?
Silicone can be formulated to be food-safe, but only if it meets the requirements of EU Regulation 1935/2004 or FDA regulations. This depends on the material, so not every type of silicone is automatically food-safe.
How temperature-resistant is silicone?
Depending on the formulation, it can withstand continuous exposure to temperatures ranging from approximately -50°C to 180°C.
What Shore hardness values are possible for LSR?
Values ranging from 10 to 95 Shore A are common, depending on the application.
How does the chemical cross-linking of LSR take place?
LSR is typically addition-cured, i.e., platinum-catalyzed. The two components react with each other in the mold under heat input without producing byproducts, ensuring dimensionally stable, highly reproducible parts.
Why are cold runner systems used in LSR injection molding?
In LSR injection molding, the material remains cool and uncured in the cold runner system until just before it enters the cavity. This prevents premature curing and minimizes material loss. As a result, cold runner systems are the state of the art in automated LSR mass production. In contrast, hot runner systems are commonly used in conventional thermoplastic injection molding.
What standards and approvals are relevant for silicone?
Depending on the application, they include ISO 10993 (biocompatibility), USP Class VI (pharmaceutical suitability), EU Regulation 1935/2004, FDA compliance for food contact, and cleanroom manufacturing according to defined ISO classes for contamination-sensitive components.
Is it possible to combine silicone with other materials?
In two-component injection molding, liquid silicone rubber (LSR) is often directly overmolded onto thermoplastic carrier parts (hard-soft composite), such as connectors with integrated seals.
Why is silicone used in medical, automotive, and industrial applications?
Silicone is characterized by its wide operating temperature range, high resistance to aging and weathering, long-term elasticity, and good electrical insulating properties. Additionally, the material can be formulated for applications requiring biocompatibility, food contact, or drinking water approvals. These properties make silicone a preferred material for technical components in the medical, industrial, and automotive sectors.
Glossary
- Addition curingThis is a platinum-catalyzed curing reaction in which two silicone components react with each other when heat is applied. This reaction does not produce any byproducts. This standard process ensures dimensionally stable, highly reproducible LSR components.
- BiocompatibilityThe property of a material to not cause harmful reactions when in contact with human tissue. For medical devices, biocompatibility is typically verified in accordance with ISO 10993.
- Cleanroom manufacturingProduction under controlled, low-particle environmental conditions (defined according to ISO classes) is necessary for contamination-sensitive components, e.g., in medical technology.
- Cold runnerRunner system in an injection mold in which the material remains cool and uncured until just before entering the cavity. This prevents premature curing of silicone, reduces material waste, and enables efficient automated mass production.
- Compression MoldingA manufacturing process in which solid silicone (HCR) is placed into a mold, pressed into shape, and cured using heat and pressure. It is an alternative to injection molding, especially for smaller production runs.
- Condensation curingAn alternative to addition curing in which decomposition products may form. It is used in some RTV silicones, for example.
- ElastomerA general term for rubber-like materials that deform significantly under load and then return to their original shape. Silicone is an elastomer.
- FDA-ComplianceConfirmation that a material meets the U.S. Food and Drug Administration’s requirements. This is relevant for food-contact materials and medical devices in the U.S. market.
- Hard-soft composite (2C injection molding)A component made of two materials, usually a rigid thermoplastic substrate to which LSR is injection-molded directly in a second step. This eliminates assembly steps and enables the creation of functional composite components.
- HCR / HTV (High Consistency Rubber / High-temperature vulcanizing silicone)A solid, kneadable silicone compound processed by extrusion or compression molding, for example. It is typically used for profiles, hoses, and molded parts.
- Hot runnerRunner system used in injection molding in which the material is kept at processing temperature all the way to the cavity. Hot runner systems are commonly used in thermoplastic injection molding to reduce material waste and support stable production processes.
- LSR (liquid silicone rubber):A two-component liquid silicone developed specifically for automated injection molding. It is injected into a heated mold where it hardens to form a solid, elastic part.
- Mold Temperature ControlThe process of controlling the temperature distribution within an injection mold. This is crucial for ensuring that the material cures evenly and simultaneously in all cavities.
- RTV (Room Temperature Vulcanizing)A silicone that cures at room temperature without the need for a heated mold. It is typically used as an adhesive or sealant in small-batch production and is usually applied manually.
- Shore hardnessA measure of the hardness or elasticity of elastomers, expressed in Shore A units. For very soft materials, the measurement is sometimes expressed in Shore 00 units. The lower the value, the softer the material.
- SiliconA chemical element extracted from quartz-containing sand (silicon dioxide, SiO₂), which is used as a raw material in silicone production.
- SiliconeA synthetic, silicon-based material whose molecular structure can be adjusted to create custom properties, such as temperature resistance, elasticity, and chemical resistance.
- USP Class VIA classification under the U.S. Pharmacopeia that confirms a material’s suitability for contact with drugs and medical applications.
- ViscosityA measure of the fluidity of liquid silicone before it cures. It affects how well the material fills fine mold contours during injection molding.
- VulcanizationThe process of chemically cross-linking a liquid or plastic raw material to transform it into a solid, elastic component.
