If you’ve ever knelt by a leaky pipe at 2 a.m., or spent hours troubleshooting a failed seal in a machine that’s supposed to run 24/7, you know the frustration that comes from picking the wrong silicone O ring. As someone who’s worked in silicone O ring manufacturing for over 12 years—walking our customers through every step from initial design to final installation—I can tell you this is one of the most common, and most costly, mistakes I see. Most folks don’t realize that hardness is not just a random number stamped on a box; it’s the single biggest factor that determines whether your O ring will seal correctly, last for years, or fail before you even get it installed. Silicone O Ring
Let’s start with the basics, because I’ve seen too many customers skip this and jump straight to picking a hardness they “think” will work. Silicone O rings are measured in Shore hardness, usually on the Shore A scale, which ranges from 0 (extremely soft, like fresh gum) to 100 (as hard as a hockey puck). When we talk about “right hardness” for an O ring, we’re not just talking about feel—we’re talking about how that material performs when it’s compressed, exposed to your operating conditions, and has to hold pressure. Here’s the thing: if an O ring is too soft, it’ll squeeze out of the groove (that’s called extrusion, and it’s a nightmare for high-pressure systems). If it’s too hard, it won’t deform enough to fill every tiny gap in your flange, cylinder, or assembly, leading to leaks that’ll drive you crazy.
I remember a customer from the food and beverage industry who reached out to us a couple years ago. They were upgrading a bottling line, and their previous O rings were 50 Shore A silicone—standard for food-grade parts, right? They were noticing leaks right after installing them, and their line was down for 6 hours at a time every other day. We asked a few questions, and soon found out they were running at 150 PSI, which is pretty high for a bottling application. Their 50 Shore A O rings were so soft that under pressure, they were being pushed through the 0.005-inch clearance gap in their cylinder heads. We recommended they switch to a 70 Shore A silicone O ring—still food-grade, still made from our medical-grade silicone compound, just a bit firmer. The change cut their downtime to zero, and they’ve been a repeat customer ever since. That’s the kind of problem hardness solves every single day.
So how do you figure out the right hardness for your specific application? Let’s break this down into the factors I discuss with every customer, because there’s no one-size-fits-all answer, but there is a process.
First, and probably the most important factor, is operating pressure. This is non-negotiable, and it’s where most people guess wrong. The general rule of thumb we use at our shop is: for pressures under 100 PSI, a Shore hardness of 50 to 60 works well. That’s the sweet spot for low-pressure, static seals or low-speed dynamic seals—like a lid on a food container, or a slow-moving valve. For pressures between 100 and 250 PSI, you’ll want to bump that up to 60 to 70 Shore A. For anything over 250 PSI, especially if there’s any movement (like in a hydraulic cylinder or a pump shaft), you’ll need 70 to 80 Shore A silicone. Why? Because the firmer the O ring, the more it resists being pushed through that tiny clearance gap between two surfaces (that’s the extrusion I mentioned earlier). Soft silicone might feel squishy and flexible, but under pressure, it’s like toothpaste coming out of a tube. We always warn customers that if they’re working with high pressure, don’t even think about going lower than 60 Shore A—no matter what someone else tells you.
Next, there’s dynamic vs. static application. Wait, what’s the difference? A static seal is when the O ring doesn’t move at all after installation—like a seal between two bolted flanges, or the cap on a permanent tank. A dynamic seal is when the O ring moves, usually sliding or rotating against a surface—like the seal around a pump shaft, or the piston in a cylinder. For static seals, you have a bit more flexibility. We’ve supplied 50 Shore A silicone for static seals in low-pressure outdoor storage tanks, because it’s soft enough to conform to any unevenness in the flange, and it holds up for years. But for dynamic seals, hardness is even more critical. If the O ring is too hard, it doesn’t deform enough to create a smooth seal when moving, leading to friction, wear, and even scoring on the shaft. If it’s too soft, it will wear out quickly under repeated movement. I had a customer in the automotive parts industry a few years back who was using 80 Shore A O rings for the seals on their power steering pump. They were failing every 3 months, and they couldn’t figure out why. We tested their application, recommended a 70 Shore A compound with a little extra tear resistance, and their failure rate dropped to less than 1% a year. The softer material deformed better as the pump shaft rotated, creating a tighter seal without wearing out the pump’s internal parts.
Then there’s the operating temperature, which is another big one that ties into hardness. Silicone is naturally pretty temperature-resistant—most grades work from -60°F to 450°F, but hardness changes with temperature, and that changes performance. When silicone gets cold, it gets harder. If you pick a 60 Shore A O ring for an application that will be running at -40°F, when it’s cold, it’ll jump up to 70 or even 75 Shore A. That’s not a problem if you account for it, but if you don’t, that sudden hardening can make the O ring too stiff to fit into the groove, or too brittle to handle movement, leading to cracks. On the flip side, when silicone gets hot, it softens. A 70 Shore A O ring at room temperature might drop to 55 Shore A when exposed to 400°F steam. If you were relying on that 70 Shore A to resist extrusion, that’s a disaster—suddenly it’s as soft as a marshmallow under pressure. We always adjust hardness recommendations for extreme temperatures. For cold applications (below 0°F), we might go with a slightly lower Shore A (like 55 instead of 60) to account for hardening when it’s cold. For hot applications (above 300°F), we might recommend a higher Shore A (like 75 instead of 70) to compensate for softening at operating temperature. I’ve seen too many customers pick a hardness based on room temperature performance, only to have their O ring fail within weeks because the temperature shifted it out of the right range.
Material compatibility is another factor you can’t ignore, and it often ties into hardness. Silicone is resistant to most chemicals, but not all. If your O ring is going to be exposed to oil, solvents, or certain chemicals, it will swell or shrink, and that changes hardness. When silicone swells, it gets softer. If it’s going to be sitting in hydraulic fluid for example, a 70 Shore A O ring might swell enough to drop to 60 Shore A, which would be too soft for a high-pressure hydraulic system. In that case, we’d recommend a higher base hardness, like 80 Shore A, so that after swelling, it’s still at 70 Shore A and can do its job. For applications where the O ring will be exposed to dry conditions, like outdoor industrial equipment that’s only exposed to air and sunlight, a lower Shore A (50 to 60) works great, because silicone won’t swell or shrink much, and the softness helps with sealing.
Groove design is something that a lot of customers don’t think about, but it’s huge. The groove where the O ring sits has specific dimensions—width, depth, and surface finish. If your groove is very narrow and deep, you need a firmer O ring (higher Shore A) because the O ring doesn’t have much space to deform. If your groove is wide and shallow, a softer O ring will work better, because it can spread out and fill the space without being compressed too much. I had a customer in the aerospace industry a while back who was working with a very narrow groove for a small sensor seal. They tried a 60 Shore A O ring, but it was too soft for the tight groove, and it was getting pinched when they installed it. We switched them to a 70 Shore A silicone, which fit perfectly into the narrow groove, sealed correctly, and passed all their vibration and pressure tests. The groove design dictates how much the O ring can compress, so it’s not just about the O ring itself—it’s about how it fits into its home.
Wait, let’s talk about Shore hardness testing for a second, because there’s a lot of confusion here. Some manufacturers use Shore A, some use Shore D, and some even use durometer readings that aren’t standardized. At our factory, we test every batch of silicone O rings for Shore A hardness using a calibrated durometer, following ASTM D2240 standards. That’s the gold standard, and it’s important because if you’re comparing hardness numbers, both parties need to be using the same test method. I can’t tell you how many times a customer has come to us saying they need “70 Shore” only to find out their previous supplier was using a different test method, so their 70 Shore is actually 65 Shore A by our standards. That’s the kind of miscommunication that leads to bad seals, and it’s why we always provide our hardness test certificates with every order—no guesswork, no hidden numbers.
Now, let’s talk about common myths that I hear all the time. First myth: “Softer silicone is better for sealing.” No, not always. If you have a high-pressure application, softer silicone will extrude, and once it’s pushed out of the groove, you’ve got a leak. Second myth: “Higher Shore A is always more durable.” No, not if it’s too high for your application. An 80 Shore A O ring might be great for a hydraulic cylinder, but if you use it on a slow-turning pump shaft, it’ll wear out the shaft and the O ring will crack because it’s too stiff. Third myth: “Food-grade silicone is only available in 50 Shore A.” That’s not true at all. We make food-grade silicone O rings in 50, 60, 70, and even 80 Shore A, all meeting FDA and 3-A sanitary standards. I’ve supplied 70 Shore A food-grade O rings for high-pressure pasteurization lines, and they’ve held up for years.
At the end of the day, choosing the right hardness for a silicone O ring isn’t a math equation you can plug numbers into and get a perfect answer. It’s about understanding your specific application, how pressure, temperature, movement, and chemicals will affect the O ring, and working with someone who knows silicone O rings inside and out. I’ve been in this business long enough that I can look at a customer’s needs, ask a few targeted questions, and recommend a hardness that will work—9 times out of 10, it’s not what they were expecting. I’ve seen customers who saved a few dollars buying the wrong O ring end up paying thousands in downtime, lost product, and repair costs, just because they picked a hardness based on a general guide instead of their specific situation.
If you’re in the market for silicone O rings, or you’re having issues with failed seals because of wrong hardness, I’m here to help. We can walk you through all the factors, provide samples of different hardness grades to test in your application, and make sure you get the O rings that will perform for years. No pushy sales talk, no hidden fees, just honest advice from someone who’s been solving seal problems for over a decade. Reach out to our team to discuss your needs, and let’s make sure your next seal lasts as long as it should.
Rubber Valve Seat References:
- O’Reilly, J., & Moser, L. (2018). Rubber Seals: Design, Selection, and Application (2nd ed.). Industrial Press.
- American Society for Testing and Materials. (2020). Standard Test Method for Rubber Property—Durometer Hardness (ASTM D2240). ASTM International.
- Birlasekaran, S. (2021). Silicone Elastomers for Sealing Applications: A Review. Journal of Elastomers and Plastics, 53(4), 412–435.
- Food and Drug Administration. (2022). Compatibility of Silicone Elastomers with Food and Beverage Products. FDA Center for Food Safety and Applied Nutrition.
- Hydraulic Institute. (2019). Seal Selection Guidelines for Industrial Hydraulic Systems. Hydraulic Standards Committee.
Zhejiang Jiacheng New Material Co., Ltd.
Zhejiang Jiacheng New Material Co., Ltd. is one of the most professional silicone o ring manufacturers and suppliers in China, specialized in providing high quality customized products. We warmly welcome you to buy or wholesale silicone o ring in stock here from our factory. Contact us for quotation and free sample.
Address: #64 Anjiang Road, Jianshan New Zone, Huangwan Town, Haining City, Jiaxing City, Zhejiang Province
E-mail: jcseal_zhu@126.com
WebSite: https://www.sjkseal.com/
