Silicone Rubber Shore A Hardness Selection Guide
Choosing the wrong Shore A hardness for a silicone rubber part is one of the most common causes of field failure. A seal that is too soft extrudes under pressure, leaks at low compression, and tears during installation. A part that is too hard loses elasticity, develops high compression set, and cracks at low temperatures. For engineers, OEM designers, and procurement specialists who specify silicone rubber seals, gaskets, and molded components, Shore A is the single most referenced property on a data sheet and the most misunderstood. It will help you read the number correctly, match it to mechanical performance, and select the right range for your application.
Shore A Hardness Basics and How Silicone Rubber Is Measured
Shore A hardness measures the indentation depth of a truncated cone under a defined spring force, following ASTM D2240 or ISO 7619. The scale runs from 0 (gel-like) to 100 (rigid plastic). Most commercial silicone rubber parts fall between 10 Shore A (squeezable gel) and 80 Shore A (semi-rigid). Liquid Silicone Rubber (LSR) is typically molded between 20 and 70 Shore A, while High Consistency Silicone Rubber (HCR or HTV) covers a wider range from 10 to 85 Shore A. Higher numbers mean stiffer material, lower numbers mean softer and more flexible.
When you compare data sheets, always check three things: the standard (ASTM D2240 vs ISO 7619), the dwell time (typically 1 s, 3 s, or 5 s, and softer readings drop with longer dwell), and the sample thickness (a 6 mm plaque gives different values than a 2 mm part). Newsil reports Shore A on molded plaques at 3 s, 6 mm thickness, per ASTM D2240, to keep values comparable across product lines.
| Material Family | Typical Shore A Range | Common Molding Process | Typical Applications |
|---|---|---|---|
| Liquid Silicone Rubber (LSR) | 20 – 70 | LIM injection molding | Medical seals, baby care, electrical |
| High Consistency Rubber (HCR / HTV) | 10 – 85 | Compression, transfer molding | Gaskets, O-rings, keypads |
| High-Translucency / Optical LSR | 30 – 60 | LIM, polished tooling | Optics, LED lenses, light guides |
| Fluorosilicone (FVMQ) | 30 – 75 | LIM, HCR molding | Fuel system seals, aerospace |
How Shore A Hardness Affects Mechanical Properties
Hardness is a proxy for crosslink density, and almost every mechanical property shifts with it. As Shore A increases, tensile strength and tear resistance generally rise, while elongation and compression set recover improve. Understanding the trade-offs prevents over-specifying (paying for tear strength you do not use) and under-specifying (a soft 30 Shore A that extrudes at 10 bar). The table below summarizes typical values for a general-purpose platinum-cured silicone; specialty grades (high-tear, low-modulus, low-compression-set) can shift these numbers by 10–20%.
| Shore A | Tensile Strength | Elongation at Break | Tear Resistance (Die B) | Compression Set (22 hr / 175 °C) |
|---|---|---|---|---|
| 20 | 5 – 7 MPa | 600 – 800 % | 10 – 15 kN/m | 25 – 40 % |
| 40 | 7 – 9 MPa | 500 – 700 % | 20 – 30 kN/m | 15 – 25 % |
| 60 | 8 – 10 MPa | 350 – 500 % | 30 – 45 kN/m | 10 – 18 % |
| 80 | 8 – 11 MPa | 150 – 300 % | 35 – 50 kN/m | 12 – 22 % |
Use this rule of thumb when reading the table: 20–30 Shore A for soft, conformable, low-pressure seals and vibration pads; 40–50 Shore A for general static gaskets and food-contact closures where balance matters; 50–60 Shore A for dynamic seals with moderate pressure (up to ~10 bar) and for molded keypads that need tactile feel without bottoming out; 60–80 Shore A for static load-bearing and structural elements. Compression set typically improves up to ~60 Shore A, then plateaus or worsens at very high hardness because stiff networks relax less on a percentage basis but start to crack under repeated deflection.
Selection Guide: Matching Hardness to Application
The fastest way to narrow your choice is to start from the working environment and work backward to the Shore A value. Below is a four-step workflow used by Newsil applications engineers, followed by recommended ranges for the most common part types we supply.
- Define the function: static seal, dynamic seal, cushion/damper, or structural element.
- Quantify the operating pressure, compression ratio, temperature range, and any chemical or sterilization exposure.
- Set the compression ratio target (typically 15–25% for static gaskets, 10–15% for dynamic O-rings).
- Select a Shore A range, then validate with a prototype compression set and leak test.
| Application | Recommended Shore A | Key Drivers |
|---|---|---|
| Medical fluid-handling seals (ISO 10993) | 30 – 50 | Biocompatibility, low compression set, gamma/EtO sterilization stability |
| Food-contact gaskets and bottle seals (FDA, LFGB) | 40 – 60 | Tight reseal, repeated compression, odor neutrality |
| Automotive vibration dampers and engine mounts | 30 – 50 | High damping factor, broad temperature (-40 °C to 200 °C) |
| Electrical connector grommets and cable penetrators | 50 – 70 | IP67/IP68 sealing, mechanical support, flame rating UL94 |
| High-pressure static O-rings (above 10 bar) | 60 – 80 | Anti-extrusion, low friction, tight groove fill |
If you are unsure, start in the middle of the recommended range. For instance, a food-contact bottle closure usually feels right at 50 Shore A: soft enough to seal a slightly warped finish, firm enough to survive thousands of opening cycles. A medical fluid seal for a syringe pump plunger often sits at 40 Shore A, where compression set under 22 hr / 150 °C is still under 15% and the part can be sterilized by autoclave to 134 °C without losing elasticity.
Frequently Asked Questions
What Shore A hardness should I use for a silicone rubber O-ring under 10 bar pressure?
For static O-rings at pressures up to about 10 bar, select 50 to 70 Shore A to resist extrusion and keep compression set under 20% after 22 hr at 150 °C. Above 10 bar, step up to 60 to 80 Shore A and consider a back-up ring to prevent gap extrusion.
Does a higher Shore A mean a stronger silicone rubber part?
Higher Shore A usually increases tensile strength and tear resistance, with general-purpose silicone rising from roughly 5 MPa at 20 Shore A to about 9 to 11 MPa at 60 to 80 Shore A. However, elongation at break drops sharply, from 600 to 800% at 20 Shore A to 150 to 300% at 80 Shore A, which means stiffer parts are less forgiving of stretch and sharp bend radii.
How does temperature affect my Shore A choice for silicone rubber seals?
Silicone softens slightly as temperature rises and hardens as it drops, but the effect is small compared with most organic rubbers. For continuous service above 200 °C or below -40 °C, allow 5 to 10 Shore A of margin: pick 50 Shore A instead of 60 Shore A for high-temperature gaskets to keep sealing force, and pick 40 to 50 Shore A instead of 30 Shore A for low-temperature dynamic seals to avoid overstress at the stiffer end of the curve.
Conclusion and Next Step
Shore A is the starting point of every silicone rubber specification, not the end of it. Match the hardness to the function (static, dynamic, dampening, structural), respect the compression ratio limits of your groove design, and always validate with a prototype test that mimics the real pressure and temperature cycle. Newsil supplies LSR and HCR silicone rubber compounds from 10 to 85 Shore A, with full ASTM D2240, tensile, elongation, tear, and compression set data on every data sheet. Send your drawing, application details, and target hardness range to our technical team and we will return a material recommendation, prototype samples, and a quotation within two business days.
Related Reading
- Silicone Rubber Gaskets: Material Selection and Design Guide
- Optically Clear LSR: Equivalent Grades, Light Transmission and LIM Molding Guide
- HTV Silicone Rubber Compounding and Mixing Process: Two-Roll Mill Procedure and Batch QC
- Low-Volatility, Oil-Bleeding-Free LSR for Automotive and Medical Parts
- Silicone Rubber Compounds for Industrial Applications
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