Characteristics, Process and Application of Low Hardness High Elasticity Silicone Rubber Parts

Introduction

Low hardness high elasticity liquid silicone rubber (LSR) parts, defined as components with Shore A hardness ≤ 20 and permanent compression set < 5% after 22 hours of aging at 100°C, occupy a unique niche in advanced elastomer applications. Unlike conventional silicone rubbers that typically trade off softness for mechanical resilience, this specialized LSR formulation combines a durometer as low as Shore 00 30 with elongation at break exceeding 800%, making it ideal for use cases that demand conformal contact, repeated dynamic deformation, and long-term structural stability. Driven by growing demand from medical wearables, consumer electronics haptics, and automotive sealing systems, global production of low hardness high elasticity silicone parts is projected to grow at a 7.2% CAGR through 2030, with high-purity medical-grade variants accounting for 41% of total market volume. This article systematically analyzes the material performance characteristics, precision manufacturing process controls, and real-world application performance of these components, providing a technical reference for product designers and process engineers seeking to leverage LSR’s unique property profile.

Core Material Performance Characteristics of Low Hardness High Elasticity Silicone Rubber Parts

The exceptional functional performance of these components stems from tailored molecular structures and formulation adjustments that resolve the traditional tradeoff between low hardness and mechanical integrity. Unlike general-purpose LSR, which relies on high crosslink density to achieve structural stability, low hardness high elasticity LSR uses a bimodal polymer chain distribution: 65–75% of the matrix consists of long-chain polydimethylsiloxane (PDMS) with molecular weight ≥ 600,000 g/mol to deliver elasticity, while 25–35% is short-chain branched PDMS to reduce crosslink density and lower hardness. This structure, combined with specialized reinforcing fillers and curing systems, delivers a set of properties unavailable in other soft elastomers such as thermoplastic polyurethane (TPU) or natural rubber.

Key Mechanical and Environmental Performance Metrics

To quantify the unique performance of low hardness high elasticity silicone parts, Table 1 compares their core parameters to conventional soft elastomers under identical testing conditions per ISO 37 and ISO 815 standards.

The most notable differentiator is the ultra-low compression set: even after 10,000 cycles of 50% compressive deformation, low hardness high elasticity LSR parts retain 96% of their original thickness, compared to 78% for TPU and 85% for natural rubber. This durability under repeated loading makes them far more suitable for dynamic applications. Additionally, their low glass transition temperature (Tg = -118°C) ensures they remain flexible even in extreme cold environments, eliminating the brittleness failure common to TPU parts at temperatures below 0°C.

Functional Property Advantages for Target Use Cases

Beyond baseline mechanical metrics, low hardness high elasticity silicone parts exhibit three functional advantages that address unmet needs in high-growth industries:

1. Conformal contact performance: With a modulus as low as 0.03 MPa at 10% strain, these parts conform to irregular surface topographies with < 10 kPa of contact pressure, making them ideal for skin-facing medical sensors that require consistent signal acquisition without causing user irritation. In clinical trials for continuous glucose monitors (CGMs), sensors using 10A LSR adhesive layers reported 27% fewer signal dropouts compared to those using acrylic adhesives, as the LSR conforms to skin micro-movements without losing contact.
2. Low hysteresis dynamic response: The energy loss coefficient of these components is < 0.08 during cyclic deformation between 10% and 50% strain, meaning they deliver consistent rebound performance with minimal heat buildup even at 10 Hz operating frequencies. This property is critical for haptic feedback actuators in consumer electronics, where consistent force output across thousands of actuation cycles directly impacts user experience.
3. Chemical and aging resistance: The fully cured PDMS matrix is inert to most common chemicals, including skin oils, hand sanitizers (70% isopropyl alcohol), and automotive fluids. After 1000 hours of UV aging per ASTM G154 Cycle 1, the parts retain 92% of their original tensile strength and show no discoloration, outperforming TPU which typically loses 30% of its mechanical properties under the same conditions.

Precision Manufacturing Process Controls for Low Hardness High Elasticity Silicone Parts

The unique formulation of low hardness high elasticity LSR introduces significant manufacturing challenges compared to standard LSR: its low viscosity before curing increases the risk of flash, its low crosslink density requires precise curing parameter control to avoid under-curing or post-curing deformation, and its softness makes demolding and post-processing difficult without inducing permanent damage. Achieving consistent part quality requires end-to-end process optimization from material preparation to final inspection, with tight tolerances on process parameters that are 2–3 times stricter than for conventional LSR production.

Injection Molding Process Optimization

Injection molding is the dominant production method for high-volume low hardness high elasticity silicone parts, as it enables precise control over material distribution and curing uniformity. The key process adjustments relative to standard LSR molding are outlined below:

1. Metering and mixing system calibration: Due to the low viscosity of the base polymer (typically 10,000–20,000 cP for the A/B components, compared to 50,000–80,000 cP for standard 40A LSR), the metering pump must have a dosing accuracy of ±0.5% to ensure consistent 1:1 mixing ratio of A and B components. A dynamic mixer with 8+ mixing elements is required to avoid inhomogeneities that cause localized variations in hardness across the part.
2. Mold design and temperature control: Molds for low hardness LSR require a parting line surface finish of Ra ≤ 0.2 μm and a maximum parting line gap of < 0.005 mm to prevent flash, as the low-viscosity material can flow into gaps as small as 0.01 mm. The mold must be uniformly heated to 120–140°C (10–20°C lower than standard LSR curing temperatures) with a temperature variance of ≤ ±2°C across the cavity to avoid uneven crosslinking. For parts with wall thickness < 0.5 mm, a cold runner system with needle valve gates is mandatory to reduce material waste and avoid gate vestige defects that would compromise surface smoothness.
3. Injection and curing parameter tuning: Injection speed must be limited to 50–100 mm/s to avoid shear heating that can cause premature curing of the low-viscosity material, while injection pressure is typically 80–120 bar, 30–40% lower than for standard LSR. Curing time is 10–30% longer than for conventional LSR of the same wall thickness, as the low crosslink density requires additional time to form a stable network. Table 2 summarizes the recommended molding parameters for 10A LSR parts with 1 mm wall thickness.

Demolding and Post-Processing Quality Control

The extreme softness of low hardness high elasticity silicone parts makes demolding the highest-risk step for quality defects, as excessive ejection force can cause permanent deformation or tearing. Three specialized demolding methods are used for these components, selected based on part geometry:

1. Compressed air demolding: For thin, flat parts such as sensor adhesive layers and keyboard pads, low-pressure (2–3 bar) filtered compressed air is applied to the cavity edge to lift the part without contact, eliminating the risk of surface damage. This method achieves a 99% demolding success rate for parts with surface area < 100 cm².
2. Soft ejector pin demolding: For parts with undercuts or complex geometries, ejector pins made of 50A polyurethane are used instead of steel pins, distributing the ejection force over a larger area to avoid puncturing or deforming the soft LSR. The ejector speed is limited to < 10 mm/s to reduce impact force.
3. In-mold coating demolding: For parts requiring medical-grade surface smoothness (Ra ≤ 0.1 μm), a thin layer of food-release silicone coating is applied to the mold cavity every 500 cycles, reducing demolding force by 60% compared to uncoated molds.

Post-processing for these parts has been largely streamlined to avoid damage: traditional deflashing methods such as tumbling or cryogenic trimming are not suitable, as they can cause surface abrasion or permanent compression marks. Instead, precision laser trimming with a 1064 nm fiber laser is used to remove any residual flash, with a cutting accuracy of ±0.02 mm and no heat-affected zone on the part material. Final quality inspection includes 100% hardness testing using a Shore A durometer with a 0.1 unit resolution, as well as automated vision inspection to detect flash, air bubbles, and surface defects as small as 0.03 mm.

Application Performance and Case Studies Across Industries

The unique combination of softness, elasticity, and biocompatibility has led to the widespread adoption of low hardness high elasticity silicone parts in three high-growth sectors, each with distinct performance requirements that cannot be met by conventional elastomers. The following case studies quantify the performance improvements delivered by these components in real-world use cases.

Medical Wearable and Implantable Components

The medical device industry is the largest consumer of low hardness high elasticity silicone parts, accounting for 38% of global demand, driven by the need for skin-friendly, long-wear components that do not compromise patient comfort or device performance. Two of the most common applications are:

1. Continuous Glucose Monitor (CGM) adhesive layers: Top-tier CGM manufacturers use 8–10A LSR as the skin contact layer for 7-day and 14-day wear sensors, replacing acrylic pressure-sensitive adhesives (PSAs) that cause skin irritation in 12–18% of users. The LSR layer’s low contact pressure (≤ 8 kPa) conforms to skin curvature and moves with the user during exercise, bathing, and sleeping, reducing edge lift by 32% compared to acrylic PSAs. In a 2022 clinical study of 240 type 1 diabetes patients, CGMs using LSR adhesive layers had a 94% wear success rate over 14 days, compared to 79% for PSA-based sensors, with only 3% of patients reporting mild skin redness, compared to 21% for the PSA group.
2. Implantable soft tissue spacers: 15A high-purity LSR is used for orthopedic soft tissue spacers that are implanted between bones and tendons to reduce friction and prevent adhesion after surgery. The material’s low modulus matches that of surrounding muscle tissue, while its high elasticity allows it to deform with joint movement without losing shape. Accelerated aging tests show these spacers retain 95% of their mechanical properties after 5 years of implantation, with no evidence of degradation or leachable substances, meeting USP Class VI and ISO 10993-5 biocompatibility requirements.

Consumer Electronics Haptic and Interface Components

The consumer electronics sector is the fastest-growing market for low hardness high elasticity silicone parts, with demand growing at 11% CAGR as device manufacturers prioritize immersive haptic feedback and ergonomic user interfaces. Key applications include:

1. Haptic actuator diaphragms for smartphones and AR/VR controllers: 10–15A LSR diaphragms are used in linear resonant actuators (LRAs) to deliver high-amplitude, low-hysteresis haptic feedback. The material’s low energy loss allows the actuator to achieve 30% higher force output at the same power input compared to conventional 30A LSR diaphragms, while its high tear strength prevents failure after 10 million actuation cycles. For AR/VR controllers, these diaphragms enable textured haptic feedback that simulates the feel of physical objects, with a response time of < 5 ms that is critical for immersive virtual experiences.
2. Waterproof sealing gaskets for wearables: 18A LSR gaskets are used to seal smart watch sensor arrays and charging ports, providing IP68 water resistance while maintaining conformal contact with the device housing even after repeated drops from 1.5 m onto concrete. Unlike rigid plastic or TPU gaskets, the low hardness LSR compensates for housing tolerance variations of up to 0.1 mm, eliminating the risk of water ingress caused by housing deformation. Accelerated aging tests show these gaskets retain their sealing performance after 1000 hours of exposure to sweat and 5000 charging cycles, with no permanent compression set.

Automotive Sealing and NVH Reduction Components

The automotive industry accounts for 22% of global demand for low hardness high elasticity silicone parts, driven by the growth of electric vehicles (EVs) that require improved noise, vibration, and harshness (NVH) reduction and sealing for high-voltage components. Key applications include:

1. Battery pack sealing gaskets for EVs: 15A LSR gaskets are used to seal EV battery pack housings, providing protection against dust and water ingress while accommodating thermal expansion and contraction of the battery module during charge and discharge cycles. The material’s service temperature range of -60°C to 220°C ensures it remains flexible even in extreme cold and during fast charging events that raise battery temperatures to 60°C. Unlike EPDM rubber gaskets, LSR gaskets do not degrade when exposed to battery electrolytes, reducing the risk of sealing failure over the 15-year service life of the vehicle.
2. Interior NVH damping pads: 5A LSR (Shore 00 50) is used as a damping layer between automotive interior trim panels and the vehicle frame, reducing road and wind noise transmitted into the cabin by 4–6 dB compared to conventional foam damping materials. The material’s high elasticity absorbs vibration across a wide frequency range (20–20,000 Hz), making it particularly effective for reducing high-frequency noise from EV powertrains that are not masked by internal combustion engine noise.

Conclusion

Low hardness high elasticity silicone rubber parts represent a major advancement in elastomer technology, resolving the longstanding tradeoff between softness and mechanical resilience to deliver performance unavailable in conventional LSR, TPU, or natural rubber. Their unique material properties, including elongation at break up to 1200%, compression set < 5%, and biocompatibility, make them indispensable for applications ranging from medical wearables to EV battery sealing. While manufacturing these components requires tighter process controls than standard LSR, including precision mold design, optimized injection parameters, and specialized demolding processes, the performance benefits they deliver far outweigh the additional production complexity. As demand for soft, durable, biocompatible components continues to grow across industries, ongoing material formulation advancements – including self-healing low hardness LSR and electrically conductive variants – are expected to expand their application scope further, enabling new innovations in wearable medical technology, immersive AR/VR interfaces, and next-generation electric vehicles. For product designers and engineers, specifying low hardness high elasticity LSR can unlock significant improvements in user comfort, device reliability, and long-term performance, making it a material of choice for advanced elastomer applications in the 2020s and beyond.