How to Choose LSR Liquid Silicone Raw Material

Introduction

Liquid Silicone Rubber (LSR) has become an irreplaceable material in modern manufacturing, with applications spanning medical devices, baby products, automotive components, consumer electronics, and industrial seals. Its unique combination of biocompatibility, thermal stability, chemical resistance, and design flexibility makes it a preferred choice for high-performance end products. However, selecting the right LSR raw material for a specific application is far from a one-size-fits-all process. The wide range of commercially available LSR grades – from Wacker Chemie’s ELASTOSIL® series to Momentive’s LSR product lines – brings both options and confusion for product designers and manufacturers.

At 橡楚（湖北）橡胶有限公司, our engineering team has decades of combined experience in LSR product development and injection molding, working with a full spectrum of commercial LSR raw materials from leading global suppliers. We are based at 湖北省鄂州市鄂城区经济开发区凡口街道内河巷54号, and operate under an ISO 9001 certified quality management system to ensure consistent material evaluation and finished product performance. In this article, we will break down our systematic material review logic, walking through the core criteria that guide our selection process from leading brand LSR series to custom formulations. Whether you are developing a new medical device or optimizing an existing consumer product, this framework will help you avoid costly material selection mistakes and end-product failures.

Core Application Requirements: The Foundation of LSR Selection

All LSR material selection starts with a clear definition of end-use requirements. No single LSR grade can meet every performance need, so aligning material properties with application demands is the first step in our review process. We break this down into two core categories of requirements: regulatory compliance and mechanical performance.

Regulatory and Safety Compliance

Regulatory compliance is non-negotiable for most LSR applications, and failing to meet the required standards can lead to product recalls, legal penalties, and reputational damage. Different regions and industries have distinct compliance requirements, so we first map the target market and application to narrow down eligible LSR grades.

Common compliance frameworks we evaluate against include:

Both Wacker ELASTOSIL® and Momentive LSR series offer a wide range of pre-complied grades that meet most global regulatory requirements. For example, Wacker’s ELASTOSIL® LR 5040 series is fully certified for food contact and medical applications, while Momentive’s LSR 3xxx series is designed for high-compliance baby and consumer products. Our engineering team always cross-checks the latest compliance certificates from the raw material supplier before including a grade in our review, and we only source raw materials from authorized distributors to avoid counterfeit or non-compliant material.

Mechanical and Physical Performance Requirements

After confirming regulatory compliance, we evaluate mechanical and physical properties against the end-use application. Key parameters we assess include:

1. Hardness: LSR hardness is typically measured on the Shore A scale, ranging from 10 Shore A (very soft, gel-like) to 80 Shore A (firm, rubber-like). Some high-modulus LSR grades can reach up to 50 Shore D. The required hardness depends entirely on the product function:

• Soft tactile products (baby pacifiers, watch straps, massage heads): 10–30 Shore A
• Seals and gaskets: 30–50 Shore A
• Structural components, keypads, durable industrial parts: 50–80 Shore A

Hardness also affects compression set – softer LSR typically has higher compression set, which is a critical consideration for long-term sealing applications.

1. Tensile strength and elongation at break: These properties determine how much stress the LSR part can withstand before breaking. For dynamic applications (such as wearable device straps that are stretched repeatedly), we require a minimum tensile strength of 6 MPa and elongation at break of 400% to avoid premature fatigue failure. For static seals, a tensile strength of 4 MPa is often sufficient.
2. Compression set: Compression set measures how well an LSR part recovers its original shape after being compressed over time. For sealing applications, a low compression set (less than 15% after 22 hours at 150°C) is required to maintain a consistent seal over the product’s lifespan. High-performance LSR grades from Wacker and Momentive typically offer compression set values below 10% for heat-cured parts.

Table 1 below compares the typical mechanical properties of common LSR grades from Wacker ELASTOSIL® and Momentive LSR series:

Processing Compatibility: Matching LSR to Molding and Production Capabilities

Even if an LSR grade meets all performance requirements, it can still lead to production delays, high scrap rates, and poor part quality if it is not compatible with your existing molding equipment and process. Our engineering team always evaluates processing compatibility early in the selection process to avoid downstream issues. We focus on two core processing factors: injection molding behavior and curing system compatibility.

LSR Injection Molding Flow Behavior

Liquid silicone rubber is processed via liquid injection molding (LIM), which relies on consistent flow behavior to fill complex mold cavities completely without defects. The key parameter that determines flow behavior is the viscosity of the uncured A and B components. LSR viscosity typically ranges from 100,000 mPa·s to 1,000,000 mPa·s, measured at room temperature before mixing.

Our team matches viscosity to part design and mold design as follows:

1. Thin-walled parts (<1 mm wall thickness) and complex micro-features: We select low-viscosity LSR (100,000–300,000 mPa·s) to ensure complete cavity filling at moderate injection pressures. For example, micro medical components such as catheters and valve seats require low viscosity to reproduce fine detail without short shots. Wacker’s ELASTOSIL® LR 5040/10 has a viscosity of ~200,000 mPa·s, making it an ideal choice for thin-walled medical parts.
2. Medium to thick-walled parts (1–5 mm wall thickness): Medium-viscosity LSR (300,000–600,000 mPa·s) offers a good balance of flow and cured mechanical properties. This viscosity range is suitable for most consumer products such as baby bottle nipples, kitchen utensils, and keypads.
3. Large structural parts (>5 mm wall thickness): Higher viscosity LSR (600,000–1,000,000 mPa·s) is preferred, as it reduces flash formation and simplifies degating after molding. Higher viscosity LSR also typically has higher cured tensile strength, which is beneficial for large structural components.

Another flow-related property we evaluate is the mold release behavior. Some LSR grades are formulated with internal mold release agents, which reduce the need for external mold spraying and shorten cycle time. For high-volume production, this is a major advantage. However, for applications that require secondary bonding (such as LSR-over-molding onto plastic or metal), internally released LSR can reduce bond strength, so we select non-internally released grades for over-molding applications. Both Wacker and Momentive offer LSR grades with and without internal mold release, so we can match the formulation to the production process.

Curing System and Cycle Time Compatibility

All commercial LSR raw materials are two-part platinum-catalyzed systems, consisting of a base component (Part A) and a crosslinker/catalyst component (Part B), mixed at a standard 1:1 ratio by weight. However, different formulations have different curing kinetics, which affects production cycle time and requires different molding temperature settings.

We evaluate two key curing parameters: pot life after mixing and cure time at molding temperature.

1. Pot life: Pot life is the amount of time the mixed LSR remains flowable at room temperature before it begins to crosslink. For most LIM applications, mixed LSR is pumped directly from the static mixer to the mold, so a pot life of 1–4 hours is sufficient. For manual dispensing or low-volume production, longer pot life (4–8 hours) is preferred to avoid material waste.
2. Cure time at molding temperature: Most LSR cures fully in 30–90 seconds at 160–180°C. Higher performance LSR grades for automotive applications often require higher molding temperatures (180–200°C) and longer cure times to reach full physical properties. However, for high-volume consumer product production, faster cure grades (full cure in 20–30 seconds at 170°C) reduce cycle time and lower per-part production cost.

A critical factor we always test for is cure inhibition. Platinum-catalyzed LSR can be inhibited by certain contaminants, including nitrogen, sulfur, tin, and amine additives that are common in some rubber compounds and plastics. If you are over-molding LSR onto a plastic substrate that contains these inhibitors, the LSR will not cure properly at the interface, leading to poor bond strength and sticky surfaces. When selecting LSR for over-molding applications, we work with suppliers to select inhibition-resistant LSR grades, and we always run small-scale bonding tests before full production. Wacker and Momentive both offer specialized inhibition-resistant LSR grades for over-molding applications, which are our go-to options for multi-material parts.

Environmental Endurance: Matching LSR to Long-Term Operating Conditions

The performance of LSR can change significantly over time when exposed to different operating environments. Our material review process always includes an evaluation of long-term environmental endurance, based on the expected operating conditions of the end product. We focus on two core areas: thermal stability and chemical resistance.

Thermal Stability Across Operating Temperature Ranges

LSR is known for excellent thermal stability compared to organic rubbers, but different LSR grades have different maximum and minimum continuous operating temperatures. We match thermal stability to the application as follows:

• Low-temperature applications (cryogenic storage, aerospace components, cold-chain seals): We select high-purity LSR grades formulated to remain flexible down to -60°C, with no brittling or cracking after long-term exposure. Low-temperature resistant LSR typically has a lower crosslink density, which maintains flexibility at sub-zero temperatures.
• Standard temperature applications (-40°C to 150°C continuous operation): Most general-purpose LSR grades from Wacker and Momentive meet this requirement, with less than 10% change in hardness and tensile strength after 1000 hours of heat aging at 150°C. This covers the majority of consumer and medical applications.
• High-temperature applications (engine gaskets, cooking utensils, industrial baking molds): We select high-performance heat-resistant LSR grades that can withstand continuous operation at up to 200°C, and intermittent exposure up to 250°C. These grades are formulated with special heat stabilizers that prevent chain scission and degradation at high temperatures. For example, Momentive’s LSR 47xx series offers excellent long-term heat stability for high-temperature food contact applications.

Table 2 below summarizes the typical aging performance of different LSR grades after 1000 hours of heat exposure:

Chemical and Media Resistance

Chemical resistance is a critical requirement for LSR parts that come into contact with oils, fuels, cleaning agents, or other chemicals during use. LSR has excellent resistance to water, polar solvents, and common cleaning agents, but it has poor resistance to non-polar hydrocarbons such as gasoline and mineral oil, unless specially formulated.

Our evaluation of chemical resistance is tailored to the media the part will contact:

1. Contact with water, body fluids, food, and common cleaning agents: All medical and food-grade LSR grades from Wacker and Momentive offer excellent resistance, with less than 5% weight change after 7 days of immersion at room temperature. No special formulation is required for these applications.
2. Contact with automotive fluids (engine oil, coolant, fuel): Standard LSR will swell by 20–50% when exposed to hydrocarbon fluids, which leads to loss of mechanical properties and seal failure. For these applications, we select specialty fluorinated LSR (F-LSR) grades, which offer much higher resistance to hydrocarbon swelling. Fluorinated LSR can have as low as 5% weight change after immersion in mineral oil for 7 days, compared to 30% for standard LSR. Both Wacker and Momentive offer fluorinated LSR grades for automotive and industrial sealing applications.
3. Long-term outdoor exposure: LSR has excellent inherent UV and ozone resistance compared to organic rubbers, so most standard LSR grades maintain more than 80% of their tensile strength after 2 years of outdoor exposure. For outdoor applications that require long-term color stability, we select LSR grades formulated with UV stabilizers to prevent discoloration.

How 橡楚（湖北）橡胶有限公司 Engineers Structure Material Review: A Step-by-Step Framework

After covering the individual evaluation criteria, we want to share the full systematic review framework our engineering team uses for every new project, when selecting between Wacker ELASTOSIL® and Momentive LSR series (or other brand LSR grades). This process eliminates subjective selection and ensures we find the optimal balance of performance, compliance, processing, and cost.

Step 1: Requirements Gathering and Initial Screening

We start by collecting all requirements from the customer, including:

• End-use application and target market
• Regulatory and compliance requirements
• Expected operating temperature range and environmental exposure
• Required mechanical properties (hardness, tensile strength, compression set)
• Part design features (wall thickness, complexity, over-molding requirements)
• Annual production volume and target cost per part

Using this information, we screen out any LSR grades that do not meet non-negotiable requirements (such as regulatory compliance) to create a shortlist of 2–3 candidate grades. For example, if the project is for a medical implant that requires USP Class VI compliance, we automatically narrow the shortlist to Wacker ELASTOSIL® LR 5000 series medical-grade LSR and Momentive LSR 3000-50 series medical LSR, eliminating all general-purpose grades.

Step 2: Technical Evaluation and Small-Scale Testing

For the shortlisted candidates, we conduct small-scale testing in our in-house laboratory to verify material properties and processing behavior. Our testing includes:

1. Curing test: We test the cure kinetics at our standard molding temperature to confirm cycle time and check for any cure inhibition issues, especially for over-molding projects.
2. Mechanical property testing: We cast test plaques and measure hardness, tensile strength, elongation at break, and compression set to confirm the properties match the supplier’s technical data sheet.
3. Environmental aging testing: For applications with extreme operating conditions, we conduct accelerated heat aging or chemical immersion testing to verify long-term performance.
4. Molding trial: We run a small-scale molding trial with the candidate material to check for flow, flash, demolding behavior, and part quality.

Step 3: Cost-Benefit Analysis and Final Selection

After testing, we conduct a cost-benefit analysis of the candidate materials to select the final grade. We do not automatically select the highest performance (or lowest cost) material – we select the material that meets all requirements at the lowest total cost. For example:

• If two candidate grades meet all performance requirements, and Wacker ELASTOSIL® has a 10% higher material cost but delivers a 15% lower scrap rate and 10% faster cycle time, we will recommend the Wacker grade for high-volume production, as the total per-part cost is lower.
• If the project is a low-volume custom part where material cost is the primary concern, and a Momentive general-purpose grade meets all requirements, we will recommend the Momentive grade to reduce overall project cost.

At 橡楚（湖北）橡胶有限公司, we maintain long-term partnerships with authorized distributors of both Wacker and Momentive LSR raw materials, so we can access competitive pricing for all grades, and we pass these cost savings on to our customers