LSR vs HCR: Choosing the Right Material for Silicone Product Development

LSR and HCR are both silicone elastomers, but take fundamentally different paths through manufacturing. One flows with low viscosity and cures in seconds under injection pressure; the other has a putty-like consistency that needs compression molding with longer cure times. Choosing between the two is rarely simple. Some applications clearly demand liquid silicone’s precision or favor HCR’s simpler tooling, but many designs fall somewhere in between. Production volume, part geometry, regulatory expectations, and whether a design is new or well established all factor into the decision, and understanding how these variables interact is crucial.

 

Understanding silicone elastomer materials: LSR vs. HCR fundamentals

Both of these materials differ in viscosity and processing method. These differences determine which contexts suit each best.

 

What are medical-grade silicone elastomers?

Silicone elastomers feature a silicon-oxygen backbone (Si-O-Si) that delivers biocompatibility, chemical stability, and temperature resistance from -13°F to 257°F. The material maintains stability through:

• Body fluid exposure
• Cleaning
• Sterilization (whether autoclave, gamma radiation, ethylene oxide, or e-beam)

 

Liquid silicone rubber (LSR)

LSR arrives as a low-viscosity, two-part system suitable for injection molding. The platinum-cure chemistry creates minimal extractables, which is critical for pharmaceutical and implantable devices.

ProMed processes this material from 10 to 80 Shore A, with cure times of 30 seconds to 3 minutes at 300 to 390°F. Equipment handles 0.5ccm to 180ccm shot sizes on 11 to 240 ton presses, yielding ±0.05 to 0.10mm tolerances with minimal post-processing.

 

High consistency rubber (HCR)

HCR has a putty-like consistency requiring compression or transfer molding at 347 to 392°F with 3 to 10 minute cycles. ProMed works with HCR from 20 to 80 Shore A.

This material brings decades of clinical history in long-term implantables, with extensive biocompatibility data supporting predicate device submissions.

 

Comparing material properties for device performance

Understanding material differences helps you design silicone parts that meet functional requirements over their lifetime.

 

Mechanical performance: Strength, flexibility, and durability

Both materials deliver necessary flexibility and durability, though formulations vary in stress response.

 

Durometer options and hardness ranges for medical uses

Shore A durometer spans from very soft 10A to firm 80A. LSR supports a wider range of softer durometers, which is often preferred for tissue-contact components. Typical selections include 20-30A for soft tissue interfaces, 40-50A for seals and gaskets, and 60-80A for parts requiring added structural support.

When standard grades fall short, ProMed develops custom formulations to meet specific performance requirements.

 

Tensile strength, tear resistance, and elongation properties

Injection-molded LSR and compression-molded HCR offer comparable mechanical strength, with the former typically ranging from 7 to 12 MPa in tensile strength and HCR falling between 6 and 10 MPa. Both achieve tear strength above 12 kN/m, making them well-suited for demanding mechanical environments.

Where they differ is elongation. Liquid silicone generally stretches farther, with elongation between 400 and 700%, while HCR ranges from 300 to 600%. That difference matters in components exposed to repeated flexing, stretching, or compression sealing, where recovery and fatigue resistance directly affect long-term performance.

 

Chemical resistance and thermal stability in medical environments

Both materials resist exposure to alcohols, body fluids, lipids, and common cleaning agents. They maintain dimensional stability through sterilization cycles and support continuous operating temperatures from approximately −13°F to 257°F.

 

Biocompatibility testing and regulatory requirements

Both materials are tested to ISO 10993 and USP Class VI standards, ensuring safety for cytotoxicity, sensitization, irritation, and systemic toxicity.

For long-term implants, six-month studies validate performance over time. Platinum-cure formulations generally produce lower extractables, simplifying toxicology analysis.

ProMed coordinates testing to support FDA submissions and maintain alignment with regulatory requirements.

 

Manufacturing processes and advanced molding capabilities

Material selection determines manufacturing approach. ProMed handles both standard and specialized techniques like micro-molding, multi-shot processing, and overmolding.

 

High-precision LSR injection molding process

Liquid silicone injection molding mixes two components at a 1:1 ratio, filling the mold under pressure. ProMed’s equipment, ranging from 11 to 240 tons, handles shots of 0.5ccm to 180ccm, from micro-molding (under 0.5 grams) to larger assemblies.

Key advantages:

• Class 7 clean room manufacturing
• Real-time statistical process control
• Minimal operator intervention
• Tight geometric tolerances
• Scalable to hundreds of thousands of units

 

HCR compression and transfer molding methods

HCR is processed through compression or transfer molding. Compression molding places pre-weighed material into open mold cavities and applies heat and pressure, while transfer molding forces material through runners into closed molds.

Both methods require deflashing but provide lower initial mold costs, making them suitable for development runs and lower-volume production.

 

Multi-component molding and overmolding technologies

Two-shot processes allow different durometers or colors to be combined in a single part, while overmolding bonds silicone to thermoplastics, metals, or other substrates. ProMed validates bond strength through process testing, ensuring consistent performance across complex, multi-material designs.

 

Micro-molding and specialty silicone applications

Micro-molding enables the creation of components under 0.5 grams with ±0.025 mm tolerances, ideal for minimally invasive devices and wearable technology. We also develop conductive formulations for EMI shielding and work with micronized particles for controlled drug release in pharmaceutical applications.

 

Silicone materials for pharmaceutical and drug-delivery devices

Pharmaceutical devices demand materials that preserve drug stability, support controlled release, and pass stringent extractables testing.

 

Drug-eluting devices and controlled release systems

Material choice directly influences drug diffusion rates, with softer durometers generally allowing faster release. Drug loading can occur:

• Throughout the matrix
• In reservoirs with controlling membranes
• On the surface

Compatibility testing ensures the material maintains stability with the drug, while in vitro dissolution studies confirm release kinetics for clinical correlation.

 

Combination product development and regulatory pathways

Combination products are classified by the FDA as drug-device or biologic-device systems, with lead center determination guiding the regulatory path. Extractables and leachables studies are more stringent than for device-only applications.

ProMed helps integrate device design and drug timelines, ensuring regulatory and functional alignment throughout development.

 

Selecting the right material for your medical device application

Application requirements should drive material selection. Part geometry, volume, regulatory pathway, and budget all play a role.

 

When to choose LSR for your medical device

This material is ideal for precision-driven designs and high-volume programs:

• Tight tolerances (±0.10 mm or better)
• Complex geometries and thin walls
• Multi-shot assemblies and overmolding
• Drug-delivery devices where purity and low extractables are critical
• Class 7 cleanroom processing

Short cycle times (30 seconds to 3 minutes) reduce per-part cost, and minimal post-processing cuts labor compared with deflashing HCR components.

 

When to choose HCR for your medical device

HCR suits use cases where robustness and established performance are key:

• Larger, thicker-walled components (over 5 mm)
• Long-term implantables with decades of clinical data
• Extrusion applications such as tubing and profiles
• Aerospace or ITAR-regulated projects
• Lower-volume programs (under 10,000 units)
• Development phases with likely design changes

HCR’s lower tooling investment and proven regulatory history make it well-suited for designs that prioritize durability and long-term compliance.

 

Scaling silicone products from prototype through production

Moving to production requires process validation, biocompatibility testing, tooling transitions, and quality systems. ProMed maintains knowledge retention between stages.

 

Material validation and biocompatibility testing protocols

Material characterization verifies properties across lots. ISO 10993 timelines vary:

• Cytotoxicity: 2 – 4 weeks
• Sensitization/irritation: 6 – 8 weeks
• Systemic toxicity: 8 – 12 weeks
• Implantation studies: 6+ months

These timelines often sit on the critical path, making early planning essential. Once material suitability is confirmed, process validation follows standard IQ, OQ, and PQ protocols to demonstrate repeatable, controlled manufacturing.

 

Transitioning from low-volume to high-volume manufacturing

Our process begins with single-cavity aluminum molds for 10 to 500 units to validate design and process. Trial runs of 500 to 5,000 units follow, with commercial tooling supporting 5,000+ units.

Automation, monitoring, and quality checks ensure consistency, while ProMed’s four facilities maintain capacity and continuity.

 

Design best practices and quality management

Design decisions influence which quality controls are critical. ProMed integrates engineering, manufacturing, and quality teams from project start to ensure consistent outcomes.

Design guidelines for injection-molded LSR:

• Wall thickness: Optimally 1 – 3 mm (0.5 – 10 mm possible)
• Draft angles: 2 – 5°
• Tolerances: ±0.10 mm standard, ±0.05 mm achievable
• Gate placement away from visible or critical surfaces
• Multi-shot parts require sufficient bonding surfaces
• Overmolding needs substrate prep and retention features

 

HCR design considerations:

• Wall thickness: Typically 2 – 10 mm
• Draft angles: 3 – 7° for compression molding
• Tolerances: ±0.20 mm for compression, tighter with transfer molding
• Parting lines placed in non-critical areas
• Design for accessible deflashing

 

Quality management integration:
ProMed’s ISO 13485 certification and FDA registration support robust production. This includes:

• Class 7 clean rooms for medical devices
• Real-time statistical process control
• First article inspection protocols
• Material traceability from receipt through finished goods
• CMM and optical measurements for complex geometries
• Certificates of Conformance and Analysis documenting all test results

 

Making the right decision for your project

Your choice depends on aligning material capabilities with project requirements. Part size, geometry, tolerance targets, and production volume help determine which material and tooling investment makes sense. Both materials meet ISO 10993 and USP Class VI standards, while budget and timelines shape practical decisions.

ProMed’s extensive experience as a contract manufacturer provides objective guidance, offering focused expertise and supporting early engagement to prevent redesigns. We provide design reviews, prototype tooling, and regulatory support, and our equipment — ranging from 11- to 240-ton presses with 0.5 ccm to 180 ccm shot sizes — accommodates a wide range of silicone parts for medical and pharmaceutical devices.

 

Frequently asked questions:

 

1)  What is the main difference between LSR and HCR silicone for medical devices?

LSR is a low-viscosity liquid molded via injection, achieving tight ±0.05 to 0.10 mm tolerances. HCR has a putty-like consistency processed through compression or transfer molding, typically ±0.20 mm. Both are biocompatible and handle -13 °F to 257 °F.

 

2)  Which material is better for implantable medical devices?

Both work for implants. Injection-molded LSR delivers precision for modern designs, while HCR offers decades of clinical data for predicate devices. ProMed supports both short- and long-term implants with biocompatibility testing and regulatory alignment.

 

3)  Can ProMed Molding manufacture components made of both materials?

Yes. Liquid silicone is processed on 11 to 240 ton presses with 0.5 to 180 ccm shots across 10 to 80 Shore A. HCR is molded via compression, transfer, or alternate methods from 20 to 80 Shore A.

 

4)  What production volumes are suitable for each material type?

HCR is cost-effective for runs under 10,000 units due to lower tooling investment. LSR becomes advantageous above 10,000 units and particularly above 50,000, where automation improves throughput and consistency.

 

5)  Can these materials be used for drug-eluting medical devices?

Both materials work for drug-delivery applications. LSR suits modern devices needing precise release kinetics and low extractables, while HCR offers a long history in implantables.

Material choice depends on drug compatibility, release profile, and regulatory pathway.

 

6)  What are the lead times from design to production for each material?

Prototype tooling takes 2-3 weeks for injection molds and 3 to 5 weeks for compression molds. Production tooling typically requires 12 to 16 weeks for injection and 6 to 10 weeks for compression. Biocompatibility testing can extend 3 to 12+ months, depending on scope.

 

7)  How does material selection affect FDA regulatory approval timelines?

Both typically require ISO 10993 biocompatibility testing. Platinum-cure formulations typically produce lower extractables, simplifying toxicology review, while HCR with historical clinical data can provide regulatory precedent. Early planning is essential.

 

8)  Can ProMed create multi-material or overmolded assemblies?

Yes. Two-shot molding combines durometers or colors. Overmolding onto thermoplastics, metals, or inserts creates integrated components. ProMed ensures proper substrate prep, bonding, and validation, supporting ergonomic grips, seals, and protected assemblies.

 

9)  What information do I need to provide ProMed to get started?

Provide intended use, tissue contact, sterilization method, part geometry (CAD with tolerances), mechanical requirements, production volume, timeline, and regulatory pathway. ProMed evaluates materials, process options, and provides cost and schedule guidance.

 

Conclusion

Choosing between these materials determines manufacturing approach, development costs, and regulatory path. Neither option is universally better; the right choice depends on geometry, tolerances, volumes, and regulatory strategy. ProMed’s experience with both means guidance based on what works — not what we’re selling.

Start early. Material selection during concept development prevents redesigns and keeps timelines on track. Contact ProMed at (763) 331-3800 or promedmolding.com/contact-us to discuss your project.

 

 

ALSO READ:

Silicone Molding Rapid Prototyping: What Are the Options?

Silicone Molding Services for Medical Devices: What You Need to Know

What to look for in a Contract Medical Manufacturing Partner?

Why Silicone Injection Molding is a Great Choice for Medical Manufacturing

Precision, Purity, and Performance: A Deep Dive into Medical-Grade Silicone Molding