Performance and Applications of Low-hardness High-elastic Silicone Rubber Parts

Introduction

Low-hardness high-elastic silicone rubber (LSR) parts, defined as components with Shore A hardness ranging from 10 to 30 and permanent compression set below 7% under 70°C for 22 hours, represent a high-performance niche in the silicone elastomer product family. Unlike conventional LSR materials that typically have Shore A hardness of 40 to 70, these specialized formulations balance ultra-low modulus, high elongation at break, and exceptional recovery performance, making them irreplaceable in applications requiring gentle contact, repeated deformation resistance, and long-term reliability.

The global demand for low-hardness high-elastic LSR parts is projected to grow at a CAGR of 8.2% from 2024 to 2030, driven by expanding requirements in wearable medical devices, consumer electronics haptics, and automotive seal systems. Unlike thermoplastic elastomers (TPE) with similar hardness ranges, these silicone parts maintain their elastic properties across a temperature window of -60°C to 180°C, and exhibit no leaching of plasticizers or residual monomers even after 1000 hours of accelerated aging. This article systematically analyzes the core performance metrics, material formulation and manufacturing optimization strategies, and high-value application scenarios of low-hardness high-elastic LSR parts, providing actionable guidance for material selection and component design in industrial practice.

Core Performance Metrics and Comparative Advantages of Low-hardness High-elastic LSR Parts

The unique value of low-hardness high-elastic LSR parts stems from their combination of three non-negotiable performance attributes: ultra-low hardness, consistent elastic recovery, and environmental stability. Unlike blended low-hardness elastomers that sacrifice durability for softness, these LSR components achieve their performance through controlled crosslinking density and polymer chain modification, resulting in a balanced property profile that outperforms competing materials in demanding use cases.

Key Mechanical Performance Parameters

The mechanical properties of low-hardness high-elastic LSR parts are tightly specified to meet application requirements, with critical metrics varying slightly across different hardness grades. Table 1 summarizes the standard performance parameters of three common commercial grades of these components, compared to a conventional Shore A 40 LSR and a Shore A 25 TPE for reference.

As shown in the table, the most distinctive advantage of low-hardness high-elastic LSR parts over TPE alternatives is their extremely low compression set, even at elevated temperatures. For applications requiring repeated compression or deformation, such as seal gaskets or wearable contact pads, the 6-8% compression set of LSR parts at 120°C ensures consistent performance over thousands of cycles, while TPE parts would suffer permanent deformation and functional failure within 100 cycles. Additionally, the 700-950% elongation at break of these LSR parts allows them to withstand excessive deformation during assembly or use without tearing, a critical requirement for components that are installed via snap-fit or subject to accidental overstress.

Environmental and Biocompatibility Advantages

Beyond mechanical performance, low-hardness high-elastic LSR parts exhibit exceptional stability across harsh environmental conditions and meet strict regulatory requirements for skin and food contact, further expanding their application scope.

For medical and wearable applications, these LSR parts are typically formulated without plasticizers, phthalates, or volatile organic compounds (VOCs), meeting ISO 10993-5 cytotoxicity requirements, USP Class VI certification, and EU REACH Annex XVII restrictions. Unlike TPE parts that may leach residual monomers or processing aids during prolonged skin contact, low-hardness LSR parts show no measurable leachables even after 30 days of immersion in artificial sweat at 37°C, making them suitable for long-term wearable medical devices such as continuous glucose monitor (CGM) adhesive pads and neurostimulation electrode interfaces.

In terms of environmental resistance, these LSR parts maintain 90% of their original tensile strength and elastic recovery after 1000 hours of UV exposure (QUV-A 340 nm, 0.89 W/m² irradiance) and 500 hours of immersion in automotive fluids including gasoline, engine oil, and coolant. This performance far exceeds that of TPE and natural rubber parts, which typically lose 30% or more of their mechanical properties under the same aging conditions. For outdoor and automotive applications, this stability translates to a 5-10 year service life without cracking, hardening, or loss of sealing performance.

Formulation Optimization and Manufacturing Process Control for Low-hardness High-elastic LSR Parts

Producing low-hardness high-elastic LSR parts with consistent performance requires careful optimization of both the base material formulation and the injection molding process. The primary challenge in formulating these materials is reducing hardness without compromising crosslinking uniformity, as excessive amounts of low-molecular-weight silicone oil additives can lead to migration, increased compression set, and surface tackiness. Similarly, the low viscosity of low-hardness LSR formulations introduces unique challenges in injection molding, including flash, incomplete filling, and inconsistent curing.

Material Formulation Design Principles

The performance of low-hardness high-elastic LSR parts is determined by four core formulation components: vinyl-terminated polydimethylsiloxane (PDMS) base polymer, hydride crosslinkers, reinforcing fillers, and functional additives. Each component is tailored to balance hardness, elasticity, and long-term stability:

1. Base Polymer Selection: Low-hardness LSR formulations use high molecular weight (60,000 to 120,000 g/mol) vinyl-terminated PDMS with a low vinyl content (0.05 to 0.15 mol%), compared to 0.2 to 0.5 mol% for conventional LSR. The longer polymer chains and lower crosslink site density reduce the modulus of the cured elastomer, while maintaining a continuous crosslinked network that prevents permanent deformation. For LSR grades with Shore A hardness below 15, a small proportion (10 to 15 wt%) of vinyl-functional silicone resin is added to improve tear strength without increasing hardness.
2. Crosslinking System Tuning: The hydride crosslinker content is adjusted to a Si-H:Vi molar ratio of 1.2:1 to 1.5:1, slightly lower than the 1.5:1 to 2.0:1 ratio used in conventional LSR. This reduces crosslinking density to achieve lower hardness, while still ensuring complete reaction of vinyl groups to avoid uncrosslinked oligomer migration. For applications requiring minimal compression set, a small amount (0.5 to 1 wt%) of a platinum complex inhibitor is added to extend curing time at low temperatures and reduce crosslinking heterogeneity.
3. Reinforcing Filler Modification: Fumed silica with a specific surface area of 150 to 200 m²/g is used as the reinforcing filler, at a loading of 15 to 25 wt% compared to 30 to 40 wt% for conventional LSR. The silica surface is treated with hexamethyldisilazane (HMDS) to improve compatibility with the PDMS matrix, reducing filler agglomeration that can lead to inconsistent hardness and increased hysteresis. This modified filler system improves tensile and tear strength without increasing the modulus of the cured material.
4. Anti-tack and Migration Control Additives: To avoid the surface tackiness common in low-hardness LSR, 1 to 3 wt% of a high-molecular-weight (20,000 to 30,000 g/mol) methylphenyl silicone oil is added, rather than the low-molecular-weight silicone oil used in conventional soft elastomer formulations. This high-molecular-weight additive is co-crosslinked into the polymer network during curing, preventing migration to the surface over time and reducing compression set by 2 to 3 percentage points compared to formulations using non-reactive silicone oil.

Precision Injection Molding Process Control

The low viscosity (10,000 to 30,000 cP at 25°C) of low-hardness LSR formulations requires specialized process control to avoid manufacturing defects and ensure consistent part performance. Key process parameters are optimized as follows:

1. Metering and Mixing: A two-component precision metering system with a dynamic mixing ratio accuracy of ±0.5% is required, as even small deviations in the crosslinker ratio can lead to 3 to 5 points of variation in Shore A hardness and a 20% increase in compression set. The mixing chamber temperature is maintained at 15 to 20°C to prevent premature curing of the low-viscosity material before injection.
2. Injection Parameter Optimization: Injection speed is set to 50 to 80 mm/s, lower than the 100 to 150 mm/s used for conventional LSR, to avoid shear heating that can cause premature curing and uneven crosslinking. Injection pressure is controlled at 80 to 120 bar, with a holding pressure of 30 to 50 bar applied for 5 to 10 seconds after filling to compensate for material shrinkage during curing. For parts with wall thickness below 0.5 mm, a vacuum venting system with a pressure of <10 mbar is installed in the mold to eliminate air entrapment that can cause surface defects and reduced elastic recovery.
3. Curing and Demolding Control: The mold temperature is set to 110 to 130°C, 10 to 20°C lower than conventional LSR processing temperatures, to slow the curing rate and ensure uniform crosslinking throughout the part thickness. Curing time is adjusted based on part thickness, with a general guideline of 30 seconds per mm of wall thickness, compared to 20 seconds per mm for conventional LSR. To avoid deformation during demolding, the mold is designed with a 3 to 5° draft angle, and a pneumatic ejection system is used instead of rigid ejector pins to prevent indentation or tearing of the soft material.
4. Post-curing Process: For applications requiring minimal compression set and no extractable residues, parts undergo a post-curing process at 150°C for 2 to 4 hours in a forced-air oven. This step removes unreacted monomers and completes the crosslinking reaction, reducing compression set by 1 to 2 percentage points and reducing extractable content to below 0.1 wt%, meeting the requirements of medical and food contact applications.

Key Application Scenarios and Design Guidelines for Low-hardness High-elastic LSR Parts

The unique combination of softness, elastic recovery, and environmental stability makes low-hardness high-elastic LSR parts the material of choice for three high-growth application segments, each with specific design requirements to maximize performance.

Medical Wearable and Implantable Components

In the medical device sector, low-hardness high-elastic LSR parts are used in applications requiring long-term skin contact, gentle tissue interface, and reliable performance under repeated deformation. Common applications include:

• Wearable Device Contact Pads: CGMs, Holter monitors, and transcutaneous electrical nerve stimulation (TENS) units use LSR 10A to 20A grade contact pads to conform to irregular skin surfaces, reducing pressure points and improving signal stability. The 950% elongation at break allows the pads to stretch with skin movement during exercise, while the <5% compression set ensures consistent adhesion and contact pressure over 7 to 14 days of wear. For these applications, parts are designed with a 0.3 to 0.5 mm thick contact layer and a microstructured surface (100 to 200 μm diameter pillars) to improve skin breathability and reduce irritation.
• Surgical Instrument Grips and Seals: Minimally invasive surgical instruments use LSR 20A to 30A grade grips that provide a non-slip surface even when wet with blood or saline, while reducing hand fatigue for surgeons during long procedures. The low compression set of these parts ensures that dynamic seals in surgical robotic arms maintain leak-free performance after 10,000+ cycles of articulation, with no particulate generation that could contaminate the surgical field.
• Implantable Soft Tissue Interfaces: Short-term implantable devices such as urinary catheters and nasal cannulas use LSR 10A to 15A grade components that match the modulus of mucosal tissue, reducing irritation and inflammation during use. These formulations are typically modified with barium sulfate to enable X-ray visibility, while maintaining elastic recovery and biocompatibility.

Design guidelines for medical LSR parts include:

• Specify post-curing to ensure extractable content <0.1 wt% for long-term skin contact applications
• Avoid sharp corners in part design to reduce stress concentration during repeated deformation, with a minimum fillet radius of 0.5 mm
• For parts requiring adhesive bonding to textiles or polycarbonate housings, use a plasma surface treatment to improve bond strength, as the low surface energy of untreated LSR can lead to adhesive failure.

Consumer Electronics Haptic and Sealing Components

In the consumer electronics industry, low-hardness high-elastic LSR parts enable improved haptic feedback and water resistance for smartphones, wearables, and audio devices:

• Haptic Button and Actuator Dampers: Smartphones and smartwatches use LSR 15A to 25A grade dampers between the haptic actuator and device housing to transmit tactile feedback while isolating vibration from other internal components. The ultra-low modulus of these parts allows the actuator to operate at maximum efficiency, while the consistent elastic recovery ensures uniform haptic feedback over 1 million+ actuation cycles. Compared to TPE dampers, LSR parts show no hardening or loss of damping performance after 1000 hours of aging at 85°C/85% relative humidity.
• **Ear Tip and Wearable Interface Components: True wireless stereo (TWS) earbuds use LSR 10A to 20A grade ear tips that conform to the unique shape of the user’s ear canal, providing passive noise cancellation of 20 to 30 dB and improved comfort during 8+ hours of continuous wear. The low compression set of these parts ensures a consistent seal even after repeated insertion and removal, with no permanent deformation that would reduce noise isolation performance. High-end models use a dual-hardness design, with a LSR 10A contact layer and a LSR 40A structural core to balance comfort and retention.
• Electronic Device Sealing Gaskets: Waterproof smartphones and smartwatches use LSR 20A to 30A grade gaskets for speaker grilles and charging port seals, which provide a reliable IP68 water resistance rating while minimizing sound attenuation for audio components. The soft LSR material requires 30 to 40% lower compression force than conventional LSR gaskets, reducing stress on thin plastic housing components and enabling thinner device designs.

Design guidelines for consumer electronics LSR parts include:

• Control part thickness tolerance to ±0.02 mm for sealing gaskets to ensure consistent compression force across the entire seal surface
• For ear tip applications, test for lipophilicity resistance to avoid swelling and hardening from contact with earwax and skin oils, with a maximum volume swell of 2% after 168 hours of exposure to artificial sebum
• Specify a surface roughness of Ra <0.2 μm for haptic components to improve tactile feel and reduce accumulation of dirt and skin oils.

Automotive Comfort and Sealing Components

In the automotive industry, low-hardness high-elastic LSR parts improve occupant comfort and long-term sealing performance, particularly in electric vehicles (EVs) where noise, vibration, and harshness (NVH) reduction is a critical design priority:

• NVH Dampening Components: EVs use LSR 20A to 30A grade dampers between the battery pack and vehicle chassis, as well as between electric motor components, to absorb vibration and reduce road and motor noise transmission into the cabin. The wide operating temperature range of LSR parts ensures consistent dampening performance from -40°C in cold climates to 120°C under peak battery operating conditions, with no loss of performance after 10 years of use. Compared to natural rubber dampers, LSR parts are 30 to 40% lighter, contributing to overall vehicle weight reduction and improved range.
• Door and Window Weather Seals: Premium vehicle models use LSR 15A to 20A grade secondary sealing layers on door and window weather seals, which provide a softer contact surface to reduce closing force and improve wind noise isolation. The UV resistance of LSR parts ensures no cracking or hardening after 10 years of outdoor exposure, while the low compression set maintains a consistent seal even after 100,000 door opening and closing cycles.
• Interior Touch Components: Automotive infotainment control knobs and armrest inserts use LSR 10A to 20A grade soft-touch