When specifying silicone seals – whether O-rings, gaskets, boots, or hoses – one of the most critical properties to get right is the material’s Shore hardness. As a silicone parts manufacturer, we often guide engineers and purchasing agents through this choice. Shore hardness (measured by a durometer) directly affects how a seal will perform: it influences the seal’s flexibility, its ability to conform and seal under pressure, and its resistance to permanent compression. In this post, we’ll explain what Shore hardness means (with focus on the common Shore A scale), how durometer impacts sealing performance, flexibility and compression set, and how to choose the optimal hardness given your application’s pressure, groove design, fluid exposure, and temperature conditions. We’ll also look at examples – from soft silicone O-rings to rigid hoses – to illustrate choosing the right durometer for the job.
What is Shore Hardness (Shore A)?
Shore hardness is a standardized measure of how hard or soft a rubber or elastomer is, determined by how much it resists indentation by a given force. It is measured using a durometer instrument (shown above), which presses a spring-loaded indenter (a needle-like probe) into the material’s surface under controlled force. The durometer yields a number on a scale (typically 0 to 100) – higher numbers mean the material resists indentation more (i.e. it’s harder), while lower numbers indicate a softer, more easily indented material. For accuracy, hardness is tested on a clean, flat sample of sufficient thickness, since thin or curved parts can give lower readings than the true material hardness. In practice, engineers use Shore hardness as a quick gauge of a silicone’s firmness or flexibility, and it’s a key spec for seal materials.
shore hardness scales
The most common scale for silicone rubber is Shore A, which covers flexible elastomers. Shore A values run from 0 (gel-like softness) to 100 (extremely hard). Lower Shore A numbers indicate very soft, compliant silicones, whereas higher numbers indicate a stiffer, less flexible rubber. (By contrast, Shore D is used for much harder plastics or heavily filled rubbers, and Shore 00 for ultra-soft gels, but these are less typical for silicone seals.) For context, a very soft silicone (20–30 Shore A) is about as squishy as a rubber band or gum eraser, while a mid-range 50 Shore A feels like a pencil eraser. A standard 70 Shore A silicone – common for O-rings – has a firmness comparable to a car tire tread or shoe sole. At the upper end, a 90 Shore A silicone is quite rigid for a rubber, approaching the hardness of a hard plastic and offering minimal give under force. In short, Shore A hardness tells us how flexible or firm a silicone seal will be: a lower durometer means more softness and compliance, and a higher durometer means more stiffness and resistance to deformation.
How Durometer Affects Seal Performance
Choosing the right durometer is a balancing act. The hardness of a silicone seal will impact several key performance attributes:
Sealing and Conformability: Softer silicones compress and conform more easily to mating surfaces, which helps achieve a leak-tight seal even on rough or uneven surfaces. A low-durometer seal (say 40–50 Shore A) will deform under lower force, filling gaps and surface irregularities for a tight seal with minimal clamping pressure. In contrast, a harder seal (80+ Shore A) is less compliant – it won’t squeeze into imperfections as readily, so it may require higher force to seal. If a seal is too hard for the available compression force, it can lead to leaks because it doesn’t fully conform. Thus, softer durometers are often favored for ensuring good initial seal contact, especially in low-pressure or vacuum conditions, or when mating surfaces aren’t perfectly smooth.
Flexibility and Rigidity: Durometer corresponds to how flexible or rigid the silicone is. A lower-durometer (softer) silicone part can bend, stretch, or flex more easily without cracking. This is critical for dynamic seals and components that move or where flexibility is needed (for example, a bellows or a gasket on a lightly stiffened flange). Harder (higher Shore A) silicones are more rigid and maintain shape under load, which can be beneficial for structural support or high loads, but they will tolerate much less flexing. For instance, a silicone at 30–40 Shore A feels very elastic and can be stretched significantly, whereas at 80–90 Shore A it behaves almost like a hard plastic with minimal bend. Notably, hardness itself doesn’t change silicone’s inherent thermal or UV stability (silicone is resilient across a wide temperature range regardless of durometer), but it does affect low-temperature behavior – softer silicones stay flexible at colder temperatures, whereas very hard silicones can become even more unyielding and risk losing seal ability in extreme cold.
Compression Set and Wear Resistance: Hardness also influences how well a seal resists permanent deformation and wear. Softer silicone grades, while easier to compress, can be more prone to compression set – meaning if they are squeezed repeatedly or for long durations, they may not fully spring back to their original shape. In other words, a very soft O-ring might take a permanent “flat” set under sustained compression. Harder silicone compounds (e.g. 70–90 Shore A) tend to hold their shape under load and offer better resistance to extrusion, abrasion, and mechanical wear. They can withstand higher contact stresses without tearing or deforming. As a result, high-durometer seals generally endure high-pressure and high-friction conditions longer, whereas a soft seal might shear or extrude out of a gap under the same conditions. However, there is a trade-off: a very hard O-ring might resist extrusion but create more friction in dynamic applications and require more force to achieve a seal. The goal is to pick a hardness that minimizes permanent deformation (for longevity) while still providing enough elasticity to seal effectively cycle after cycle.
In summary, softer (lower-durometer) silicone seals provide better flexibility and conformability – crucial for sealing low-pressure or irregular interfaces – but they may wear faster or take a set under heavy stress. Harder (higher-durometer) seals offer greater strength, wear resistance, and support at high pressures, but can be too rigid to seal weakly clamped or uneven surfaces. The optimal durometer strikes the right balance for your application’s needs.
Pressure and Durometer Selection
One of the first considerations in choosing a silicone hardness is the operating pressure the seal must withstand. Pressure directly pushes on the seal and can force softer materials to deform or even squeeze out of the sealing gap (a failure mode known as extrusion). As a rule, higher system pressures call for higher durometer seals. A hard, high-durometer O-ring (for example 85–90 Shore A) is more resistant to extrusion and will require a greater force to be pushed into any clearance gap. In practical terms, if you have a hydraulic or high-pressure pneumatic application, a silicone O-ring at the upper end of Shore A hardness (or even a Shore D material, if extreme) will better maintain its shape under pressure and form a reliable seal without blowing out. By contrast, low-pressure systems benefit from softer seals – in a vacuum chamber or a low-pressure water seal, a 50 Shore A silicone gasket might seal perfectly with minimal bolt force, whereas a 90 Shore A piece might leak because there isn’t enough pressure to deform it into all the surface imperfections.
To illustrate, most standard O-rings are about 70 Shore A, which is a balanced hardness suitable for moderate pressures (this has become a de facto standard because it works for many common hydraulic and pneumatic systems). If the pressure increases significantly – for example, a high-pressure pump or an oilfield tool – engineers often specify a 90 Shore A O-ring, possibly along with anti-extrusion devices like backup rings, to prevent the seal from being forced out of its groove. On the other hand, for very low pressures or delicate equipment, a softer 50–60 Shore A seal can achieve a tight seal without requiring excessive compressive force. It’s important to note the trade-off: a harder seal (higher durometer) will withstand higher pressure before extruding, but its “squeeze” (deflection) at a given force is lower, meaning it may not seal well at low pressure or against rough surfaces. Always consider the maximum pressure and choose a hardness that can survive that pressure (consulting pressure-hardness curves or guidelines), and if in doubt, err toward harder for safety – but ensure your design can apply sufficient force to compress the harder seal. In extreme high-pressure scenarios, combining a high-durometer silicone with a rigid backup ring is a proven solution to prevent extrusion while still maintaining a seal.
Groove Design and Seal Hardness
The design of the seal’s groove or gland – including dimensions, tolerances, and surface quality – plays a significant role in determining the best durometer for your silicone seal. A key concept is available compression (“squeeze”) and clearance. In a well-designed groove with tight clearances and smooth, evenly mating surfaces, a relatively harder seal can be used because the seal only needs to fill a narrow, defined space. For example, between two precisely machined metal flanges with a thin gasket groove, you might choose a firm silicone that can resist high bolt loads – the surfaces are so flat that even a hard gasket will seal once compressed. Conversely, if the joint has a large gap or uneven surfaces, a harder seal might not deform enough to fill voids, causing leaks. In those cases, a softer silicone gasket is needed to compress and adapt to the irregularities. A general design rule for gaskets is to use the softest material that will still survive the mechanical forces, to ensure it can fill all gaps. For instance, an environmental enclosure with a flexible lid often uses a soft foam or low-durometer silicone seal, because the lid or door can warp slightly and the soft seal will take up that misalignment. On the other hand, a rigid pipe flange can accommodate a harder seal since the flanges won’t deform under tightening.
Another aspect of groove design is the clearance or extrusion gap on the high-pressure side of the seal. If the groove has a significant gap, a soft silicone will tend to extrude into that gap under pressure, leading to seal failure. A harder durometer material can resist this extrusion better. Designers often mitigate extrusion by tightening the clearance or adding features like retention rings, but selecting a higher durometer is a direct way to increase extrusion resistance in the face of a given groove gap. It’s also worth considering the percentage compression the seal will undergo. If your groove is designed to compress an O-ring by, say, 20% of its cross-section, a soft O-ring will require less force to achieve that compression than a hard O-ring. If the assembly has limited clamping force (for example, plastic components or delicate parts), you may need a softer seal to achieve the necessary squeeze with the available force. If your design can apply high force or if you have metal components, a harder seal could be feasible. Surface finish is another tie-in: rougher or less precise surfaces often demand softer seals that can flow into scratches or gaps, whereas polished surfaces can seal with harder seals. In summary, match the seal hardness to the groove design – tight, high-tolerance, and high-load designs favor harder seals, while looser or irregular designs require softer seals for a reliable seal.
Fluid Exposure Considerations
The fluids (or gases) that your silicone seal will be exposed to can influence which hardness and material formulation you should choose. While the Shore hardness itself doesn’t directly tell you a material’s chemical compatibility, it’s crucial to ensure that the chosen hardness is available in a silicone compound that can withstand the media in your application. Different silicone formulations (and related elastomers like fluorosilicone) offer varying resistance to oils, fuels, acids, or other chemicals. If your seal will contact aggressive fluids, prioritize a silicone grade formulated for chemical resistance; you may find that extremely soft grades aren’t offered in those specialty formulations, or conversely, that a fluid might swell or soften the rubber over time. Swelling is a key concern: some fluids (e.g. certain oils or solvents) can penetrate silicone and cause it to swell and soften. This effectively lowers the durometer of the seal in service, potentially making a once-firm seal turn soft or gelatinous. In such cases, you might compensate by choosing a slightly harder compound initially so that after swelling it still has acceptable firmness. On the other hand, fluids that extract oils or plasticizers could make a rubber harder or brittle over time, which might argue for using a softer compound that won’t become too stiff after exposure.
In general, silicone rubber is chosen for its broad chemical stability – it tolerates water, many chemicals, and temperature extremes better than most organic rubbers. But not all silicones are equal: for example, standard silicone can swell in petroleum oils or fuels, so a fluorosilicone might be specified for those environments (fluorosilicones have Shore A ranges similar to regular silicone, but maintain properties in fuels). When deciding durometer, ensure that the hardness you need is achievable in a chemically compatible grade. Often, medium hardness (50–70 Shore A) compounds are the most widely available in various specialty formulations, whereas very soft or very hard versions may be limited in chemical resistance options. Also consider the temperature of the fluid, since chemical attack can accelerate at high temperatures. For example, a silicone gasket in hot oil might need both a higher hardness (to resist extrusion at temperature) and a compatible compound (to resist oil swell). Always consult material compatibility charts and consider running immersion tests if the application is critical. In summary, choose a hardness that works in concert with the right silicone compound for the fluid – hardness alone won’t save a seal that degrades or swells in contact with the process media. It’s often a good practice to discuss your media and environment with the silicone manufacturer; they can recommend a compound (and hardness range) that offers the best performance in that chemical environment.
Temperature Considerations
Temperature extremes can dramatically affect how a silicone seal behaves, and thus should factor into hardness selection. Silicone elastomers are known for retaining flexibility in cold and resilience in heat compared to other rubbers, but even silicone will change stiffness with temperature. As temperature drops, rubber materials become harder (less flexible) and can eventually lose elasticity (the glass transition for silicone is very low, but practical flexibility can diminish in very cold conditions). In cold environments, a seal that is “just flexible enough” at room temperature might turn too rigid at operating temperature, failing to maintain a seal. Therefore, for low-temperature applications, a softer durometer is generally advisable – the silicone will stiffen up when it’s cold, effectively behaving like a higher durometer material. A softer compound can better maintain a seal when it’s, say, –50 °C, because it started with more give at ambient. For example, if you need a silicone O-ring to seal at –55 °C, a 50 Shore A silicone might remain pliable, whereas a 80 Shore A O-ring could become rock-hard and leak. Always check the low-temperature flexibility rating of the silicone; some silicone compounds are formulated to stay elastic in extreme cold, often those are softer to begin with or have special additives.
At the other end, high temperatures tend to soften elastomers. Silicone can handle high heat (200 °C or more for certain grades) without breaking down, but the material will be more easily deformed at those temperatures. If your application involves elevated temperatures, consider that the effective hardness of the seal will drop when hot – a seal that is 70 Shore A at room temp may behave like a much softer rubber at 150 °C. Thus, for high-temperature service, a higher durometer or heat-stabilized compound is often chosen so that the seal doesn’t become too floppy or prone to extrusion under heat. In fact, harder silicone materials are generally better at maintaining structural integrity at elevated temperatures. For instance, in an oven door gasket or engine compartment seal, using an 80 Shore A silicone can provide a firmer sealing force at operating temperature, whereas a 40 Shore A might become overly soft when the heat is on. Keep in mind that prolonged high heat can also lead to compression set in softer materials – the rubber relaxes and doesn’t rebound – so a slightly harder compound might give longer life in a high-temp compression seal. Manufacturers often specify a compression set resistance at temperature for different hardness grades; if a softer silicone shows high compression set at your max temperature, a harder one (or a different polymer) might be required.
It’s also important to consider thermal cycling (fluctuations between hot and cold). Repeated expansion and contraction can test a seal’s elasticity. Softer silicones usually handle thermal cycling well (due to their flexibility), but make sure the hardness you pick can handle the coldest condition without losing seal and the hottest without deforming. In summary, align the hardness with your temperature range: use softer durometers to preserve low-temp flexibility, and harder durometers to ensure high-temp stability and load-bearing. Remember that while hardness influences mechanical performance at temperature, it doesn’t change silicone’s innate heat tolerance – a 50 Shore A and 70 Shore A silicone made from the same base polymer will both survive the same temperature, but their mechanical behavior (stiffness or softness) at that temperature will differ. Often the “harder is better for heat” guideline holds true in practice for maintaining seal force. As always, consult data sheets: look at properties like torsional stiffness vs temperature or compression set to see how a given hardness grade performs across your temperature range.
Examples: Choosing Hardness for Different Silicone Parts
Let’s look at a few specific silicone components and how durometer choices typically play out for each:
O-Rings: Silicone O-rings are commonly produced in medium durometers around 70 Shore A, since this offers a versatile balance for most applications. A 70A silicone O-ring provides enough flexibility to seal well under moderate clamp force, while being firm enough to resist extrusion up to around 1,500 psi in a properly designed groove. For low-pressure systems or delicate hardware, softer O-rings (50–60 Shore A) can be used – these will seal with minimal force and conform to imperfect surfaces (for example, sealing plastic assemblies or glass components without cracking them). On the other hand, high-pressure O-rings (such as in hydraulic cylinders or oil & gas tools) often use 90 Shore A hardness for extra extrusion resistance. An 90A silicone O-ring can handle significantly higher pressures but requires more force to compress. It’s also common to see 90A O-rings in dynamic high-pressure applications, sometimes paired with backup rings if the gap is large. In summary, use the standard ~70A for general-purpose O-rings, go softer for low-pressure or easily damaged mating parts, and go harder (80–90A) for high-pressure or high-wear service where seal integrity is paramount.
Gaskets and Seals: Flat silicone gaskets and custom seals similarly tend to use mid-range hardness (50–70 Shore A) for most cases. Around 50–60A, a silicone gasket is soft enough to compress and fill gaps easily, which is great for sealing enclosures, HVAC ducts, or engine covers where the surfaces might not be perfectly aligned. Soft gaskets ensure that even with low bolt torque or slight warping, the seal can adapt and prevent leaks. A 70A gasket is a bit firmer – often used when a joint can be tightened firmly and you need a gasket that won’t squeeze out under higher pressure or loading. This hardness (70A) is roughly like a tire tread: tough but with slight give. For rough or uneven surfaces, you should lean toward the softer end so the gasket material flows into all the crevices. For example, a gasket on a lightly machined oil pan might be 50A silicone to accommodate surface roughness. If the gasket is exposed to high pressure or acting as a load-bearing seal, a harder silicone (80–90A) might be selected so it doesn’t extrude or tear. Keep in mind the earlier guideline: a gasket should be as soft as needed to seal, but as hard as needed to survive. Softer silicone gaskets will seal with less force but can extrude or suffer damage if the gap is too large or pressure too high. Harder gaskets can tolerate stress and abrasion (and they can double as vibration dampers or spacers in some designs), but require solid mating surfaces and sufficient compression force to seal. When specifying a silicone gasket, consider how flat the flanges are and how much bolt force is available – that will guide you to an appropriate hardness.
Silicone Boots and Bellows: Boots, bellows, and covers (such as dust boots for joints, accordion bellows for protecting rods, or connector boots) usually need to be highly flexible, since they must stretch, compress, or bend repeatedly in service. For these parts, a softer silicone (often around 30–50 Shore A) is ideal. At ~40 Shore A, a silicone boot can flex easily without cracking, enabling it to withstand the constant movement and vibration of, say, a steering rack dust cover or a protective cable boot. If a boot were too hard (say 80 Shore A), it would be too stiff to compress properly and could transfer excessive stress to the parts it’s meant to protect – or it might even tear because it can’t accommodate the motion. We find that many molded silicone bellows use compounds in the 40–60 Shore A range, which provides a good mix of elasticity and tear strength. These softer silicones are still very durable – it’s a misconception that soft silicone is fragile or weak. In fact, soft silicones are highly elastic and can endure stretching and compression repeatedly without losing their properties. For example, a 40A silicone bellows on a ventilator or pump can flex thousands of cycles. Only in cases where a boot also needs structural support or wear resistance (perhaps a large expansion joint in piping) might a harder (70–80A) silicone be used, often reinforced with fabric. But generally, for protective boots and bellows, choose a softer durometer so the part can do its job isolating movement and sealing out dust/water without adding too much resistance to the system.
Silicone Hoses and Tubing: Silicone hoses (such as automotive coolant or turbo hoses, medical tubing, or peristaltic pump tubing) cover a range of durometers depending on their purpose. A typical silicone hose is around 50–70 Shore A, providing a balance between flexibility and collapse resistance. For instance, a radiator hose might be ~60A – flexible enough to install and absorb engine vibration, but stiff enough to handle pressure and not balloon out. Thin-walled silicone tubing for laboratory or medical use might be even softer (30–50A) so it can bend easily and allow pumps to squeeze it (in the case of peristaltic pumps). At around Shore A 25–40, silicone tubing feels very soft, almost like a rubber band, which is perfect for gentle fluid transfer or applications where the tube must flex a lot without kinking. On the other end, reinforced high-pressure hoses can use harder silicone, e.g. 70–80 Shore A, often with fabric reinforcement. A 80A silicone tube is quite rigid (comparable to a garden hose or shoe sole in firmness) and can withstand higher internal pressures or vacuum without collapsing. The trade-off is that as hardness increases, the hose becomes less bendable. So for tight bends or dynamic movement, you want a softer (and possibly thinner) hose; for high pressure or vacuum, you want a harder, thicker-walled hose that won’t bulge or suck closed. When choosing silicone tubing hardness, consider the required bend radius and the pressure/vacuum: choose the lowest hardness that still meets the pressure requirement and won’t kink on the bends. For example, for a suction line that sees vacuum, a 70A hose might hold its shape where a 50A hose could collapse. But for a short connector tube with sharp bends, 50A might be necessary to allow installation without forcing the hose. In summary, match the tubing hardness to the handling needs – softer for flexibility and pump action, harder for pressure resilience.
Conclusion and Takeaway Advice
Specifying the right silicone hardness is crucial for a successful seal. As we’ve seen, Shore durometer affects how a silicone part will seal, move, and endure over time. There is no one-size-fits-all hardness – the choice should be guided by your application’s specific demands: pressure, mechanical design, environment, and functional requirements. A good approach is to select the softest material that will still reliably handle the stresses of the application. Going too soft can cause extrusion or wear problems; going too hard can cause sealing or assembly problems. It’s a balancing act, but understanding these principles will help you narrow down the optimal range.
Key takeaways:
Balance Flexibility and Strength: Aim for a hardness that is soft enough to ensure a good seal (fills gaps under the available pressure) but hard enough to resist extrusion, abrasion, or collapse. For many general-purpose seals, a mid-range ~70 Shore A silicone provides this balance. Softer compounds improve conformability; harder compounds improve durability – find the sweet spot for your design.
Consider Pressure and Clearance: Use higher-durometer silicones for high-pressure applications to prevent seal extrusion (e.g. 80–90A for hydraulic seals), and use lower durometer for low-pressure or vacuum seals that need to seal with minimal force. Always account for the gland clearance – large gaps or high pressures demand harder seals or auxiliary backup rings.
Match the Hardware and Surface: If your sealing surfaces are uneven or your assembly can’t apply large forces, choose a softer seal that will deform and seal readily. If you have rigid, well-supported grooves and can tighten bolts firmly, a harder seal can be used and will last longer under stress. Remember the guideline: the gasket/seal should be as soft as possible, but strong enough to survive the application.
Factor in Environment: Adjust hardness for extreme temperatures – softer for very cold (to stay flexible) and perhaps slightly harder for very hot (to avoid excessive softening). Ensure the silicone compound (at the chosen durometer) is compatible with any fluids it will contact. Chemical exposure can alter a seal’s effective hardness over time (through swelling or extraction), so choose materials accordingly and test if needed.
Test and Consult if Unsure: When in doubt, prototype and test seals of different durometers in your actual equipment. Small differences (50 vs 60 vs 70 Shore A) can have noticeable effects on sealing force and longevity. Also, consult your silicone manufacturer or seal supplier – they can often tweak a compound’s hardness within a certain range or recommend an optimal grade for your conditions. Leverage their expertise to get the best performance and avoid costly mistakes.
By carefully considering Shore hardness in light of pressure, design, fluid, and temperature factors, you can specify a silicone seal that delivers reliable performance and long service life. The right durometer ensures your O-rings, gaskets, boots, and hoses all function as intended – sealing when they should, flexing when they must, and holding up under the demands of your application. With this knowledge, you’ll be well-equipped to choose the optimal silicone hardness and achieve a successful seal every time.
