Surface finish requirements for static and dynamic sealingWhat is hardware surface finish?​ Any surface can look (and even feel) perfectly smooth. However, look closely enough with high magnification and all surfaces will have a degree of fluctuation. The topography will look similar to a mountain range or the surface of the moon!The roughness of a surface is often linked to the way a surface is produced or machined, and any subsequent processes such as coating or platings.Just a couple of decades ago, the standard approach to assessing surface finish was to use comparison plates. These are small specimen sections of material produced by turning, grinding or milling. A fingernail comparison test with the finished part would then be used to determine which came closest.Historically, seal catalogues typically recommended a surface finish using just the Ra (metric) or CLA (Centre Line Average, inches) parameter. Little regard was given to what the seal was being expected to do.Ra simply, is the mean roughness. The average calculated from the peak heights and valley depths. A surface that is mostly spiked can have the same Ra value as one that is mostly troughed. However, each could have a very different impact on seal performance. Today, surface finish measuring equipment is more sophisticated and capable of tracing a surface finish using a diamond tipped stylus, or non-contact 3D laser scanning.These devices are now reasonably affordable and provide a sophisticated level of results from essentially a portable machine. These instruments can now analyse the surface roughness profile, trace and calculate a large range of parameters that define that profile. This includes Ra, Rt, Rz, Rmax, Rmr (bearing ratio) and even skewness parameters. What these are, and how they should be measured, have generally been well captured in standards such as ASME B46.1, ISO 4287, ISO 4288 and the upcoming ISO 21920. Why is surface finish important for sealing systems? To ensure seal effectiveness, a common assumption for specifying a hardware surface finish is the smoother the finish the better. However, this is not always the case. If the seal application is static and is sealing a low molecular size gas such as helium; then a very smooth surface would be preferable.This is often referred to as a “closed” surface (with no peaks and very few, shallow valleys). The seal mates tightly against the hardware, ensuring the leak path for the gas is reduced as much as possible. Even in this scenario, the required surface roughness can be influenced by other factors. These include the hardness (or rather, softness) of the seal material and the pressure being applied, both by the initial seal squeeze and the applied gas pressure.It’s important the interface between the seal and the hardware is well lubricated. This is critical for some dynamic applications for either seal friction (and hence noise, vibration and heat) or wear life.This is partly achieved by ensuring that the surface is more “open”, with small valleys or troughs to trap the lubricant -but no sharp peaks that would abrade the seal material. In these application cases, if the dynamic hardware surface (rod, shaft or piston bore) is too smooth, then the seal can wear prematurely. This can cause juddering, squeal and excessive heat build-up. Tribological performance consideration As a result, for dynamic applications it is important to consider all the factors affecting the tribological performance. These include the seal (design, material), the dynamic hardware (hardness, material type) and the fluid (viscosity, lubricity, contamination). The hardness of the dynamic hardware can also be a significant factor. Hardened or chromed steels at around the 50 Rockwell C level or less can often be polished by the seal itself. This means that even if the initial roughness is sub-optimal, good sealing performance is achieved after a brief period of bedding in.With the use of high hardness coatings such as HVOF and DLCs, the much softer seal material is very likely to be abraded by the hardware. In these cases, it’s essential to ensure that the optimal surface finish is met. This often requires a finer finish prior to the coating process (as the coatings can be rougher than the substrate material and post-process polishing is very difficult with such hard coatings). Any other factors to consider? Yes! Another thing to consider is the direction (or lay) of a particular surface finish. Machining marks, scratches, dents or other hardware damage that cut across a sealing contact face are likely to result in leakage. This is compared to (for example) concentric machining marks in the base of an axial face-seal O-ring groove. It is important to consider specifying a lay direction when applying surface finish requirements to the hardware design specifications and drawings. What hardware surface finish should you specify? Despite complexities involved in hardware surface topography, the sealing industry has developed a comprehensive set of recommendations. These can vary a little between different manufacturers and suppliers. Given the specificity of the seal design and material, they’re best found within the specific literature for the products considered for the application.This is a good starting point, but for each individual application it's worth considering whether these specifications should be addressed. For example, specifications could be tightened, or perhaps even loosened to ensure that performance targets can be met. This would reduce the need to over-specify the surface finish - because this can add unnecessary processing cost to a part.The practicalities involved in accessing the surface to measure also need consideration. It’s notoriously difficult to check the sidewalls of an O-ring groove, especially in a rod/ID sealing configuration. Cylinder bores are often less tightly defined due to stylus/probe access issues compared to rods and shafts.Importantly, whoever is required to produce and check a particular surface finish has both the equipment and understanding to do so. This is very important if using relatively modern and less well known parameters such as Rmr (or Tp, bearing ratio).Further detail can also be found in the ISO and ASME standards regarding measuring settings. For example, filtering/cut-off values, stylus tip radius, measuring, evaluation and tracing lengths. When validating a supplier’s measurements, it’s important these machine settings are identical. Failure to do so will result in differing surface finish values obtained on the same part. Those standards are not specific to sealing applications. Generally, we’d recommend a cut-off filter (λc) of 0.8, unless measuring very fine (< 0.1 µm Ra) surface finishes where 0.25 would be more suitable, with a 2 micron stylus tip in all cases.Specifying the right surface finish for the hardware components that contact a seal can be complex and daunting. Ensuring an optimum finish and right-first-time seal performance, will be achieved. This is with core understanding, and consideration of relevant parameters for the given application.

Are all elastomers the same?Are all elastomers the same? Material hardness This is often the least considered property when selecting a material. In sealing applications, elastomer material hardness can impact assembly loads, seal friction and extrusion resistance. Softer rubber can better accommodate surface imperfections (especially when sealing low pressure gas). This means softer seal compounds can be used effectively against rough hardware surface finishes. Harder compounds will have greater wear resistance in dynamic applications.Most of the common base elastomers are produced in 60 to 80 Shore hardness range. These tend to be produced in increments of 5 hardness points. It's important to remember the hardness tolerance of a material is typically +/-5 points, therefore 70 and 80 hard nominal materials could both legitimately have a hardness of 75 Shore A.With possible hardness down to 30 Shore A (or even lower), silicone rubber lends itself to applications requiring very soft seals. An example of which is where seal compression forces need to be low to enable easy manual assembly of components. NBR, HNBR and FKM materials can be formulated up to 95 Shore A hardness for highly demanding high pressure and temperature applications; for example applications found in the Oil & Gas industry. Chemical compatibility Unlike PTFE seals (given very few chemicals that will attack and breakdown the material) elastomer seal materials must be carefully selected. This is so the properties are not affected by any fluids or gasses that the seals come into contact with. This includes the main media that the seal is retaining or excluding, additionally any contaminants (such as cleaning fluids or bi-products of reactions between any of these fluids, the seal material or the surrounding hardware).Even a seemingly straightforward application often involves a long list of chemicals and compatibility must be checked with the elastomer. Most seal suppliers publish compatibility tables or like us, have interactive tools to help check this. Even then, it’s often not the brand name oil that’s listed. For example, often at the more basic chemical level, so some knowledge of what’s actually in the fluid is required. Recent developments in health, safety and environmental requirements have made this a little easier. However, some fluid manufacturers can still be a little reluctant to declare the full list of chemicals in their products.Manufacturers sometimes give guidance on general compatibility with elastomer groups, but this can be a little generic. Additionally, not all grades in a specific group will have the same resistance to a particular fluid. Again, this can be especially relevant to NBRs and HNBRs with a variable ACN content. Sometimes the reactions can be dramatic and swift, with the rubber shrinking or swelling in a matter of hours when exposed to a non-compatible fluid and especially at elevated temperatures where the chemical reaction is accelerated. Reactions in chemicals In other cases, the reaction could be slower, but still causes sealing issues resulting in earlier system failure compared to using more capable elastomers. This is often the case with FFKM seals in applications where the cost of replacement or downtime is very high. The outstanding chemical resistance of these materials gives considerably longer service life compared to lesser grades, and it justifies the higher initial cost of the seal. In other cases, some reactions can be beneficial, for example where a small amount of elastomer seal swell in an oil over time can help to offset wear in dynamic O-ring energized PTFE slipper seals. Temperature range Outwardly, this material property seems straightforward, but elastomer technology is rarely that cooperative. All elastomer compounds will have a low temperature property stated on their datasheet. Either the TR10 (temperature at which a stretched and frozen sample recovers by 10% of its length) or the Tg (the glass transition temperature, at which all flexibility is fully lost).However, applied pressure increases the Tg by approximately 1°C for every 5 MPa. This is significant for even moderately high-pressure applications. Additionally, the flexibility required for successful sealing will vary between applications, making the correct material selection critical. At the other end of the spectrum, elastomers respond poorly when exposed to temperatures above their capability. This results in the material suffering irreversible degradation that reduces seal performance.Whilst guidance is given on maximum temperature capability for any specific elastomer grade, this is often in a benign air environment. Therefore the chemical impact of being exposed to hot fluids in the sealing application should be considered.Material groups tend to have well published temperature ranges. For example, the silicone family is able to reach -100°C (or even lower with special grades), and perfluoroelastomer (FFKM) grades are able to withstand 320°C (or even higher for short durations).Confusingly though, considerable variations can exist even within the same material families; especially with FKM where the more modern grades can now reach down to -50°C, with older technology grades only capable of -15°C. There are many NBR and HNBR grades where the acrylonitrile (ACN) content can be varied so that the low temperature limits can be between -30°C and -60°C, depending on the specific recipe formulation.This is just a short summary of three properties of elastomer materials. It’s clear that selecting the correct grade of material for a specific application is a difficult task, yet one that is absolutely critical to achieving the seal performance and lifetime required.

The importance of engineering tolerancesThe importance of engineering tolerances What tolerances are we concerned with?  We can start with the seal itself, and specifically the seal material, as almost all polymer seal materials contain multiple ingredients.  For PTFE and polyurethanes this is typically 2 or 3 different elements. However, for an elastomer material as many as 30 different ingredients can be used in the recipe. Each ingredient is weighed or measured before being added to the mixing machine, with a tolerance on the mass or volume of the solid or liquid ingredient. Digital advances in weighing and dosing equipment have improved tolerance ranges since synthetic rubbers were first commercially produced in the 1930’s. However, it’s important to remember that a small variation in material mix from batch to batch is inevitable. The mixing process (also now digitised to help reduce variation) is controlled by parameters which also have a tolerance. Pressures, temperatures, speeds and times are kept within certain limits and determined by the sophistication of the mixer and its control systems. To produce the finished seal, mould tools for polyurethane and elastomer seals are manufactured using CNC or electrical discharge machining techniques. These hold incredibly tight tolerances on the metal form. However, are will be variations in moulding pressure and temperature, combined with slight variations in material shrinkage rates. This results in tolerances on the finished parts that are significantly wider than the metal tool it was produced from. For machined seals, polymer seal materials tend to have high rates of thermal expansion. Together with their relative softness makes it difficult to maintain the same level of tolerance that can be achieved when machining metal components. Hardware tolerances When designing hardware for seal installation, tolerances are perhaps more obvious, and certainly where engineers can focus some attention. Plastic housings (whether injection moulded or machined), together with metal castings or pressings can pose tolerance challenges. Even for relatively straightforward machined metal housings, selecting the correct tolerance to apply is key. For many applications, a stack-up of tolerances is to be considered. This is together with tolerances of the assembly such as concentricity or misalignment (especially for dynamic sealing applications). Other considerations include bearing wear and the resulting increase in misalignment or runout as the equipment approaches the end of its target life. Why are tolerances important for sealing systems? A combination of theory and experience over decades, seal designers have developed an ideal set of installation conditions for many applications. From static flange or face sealing, through linear hydraulic cylinder seals and to rotary shaft sealing and more. Some of these applications have adopted standardised recommendations (such as ISO 3601 for O-rings and DIN 3760 for elastomer shaft seals). However, because no two applications are truly identical, many are based on the experience and preferences of the seal designer. Regardless, every application starts from a nominal condition, and the maximum and minimum tolerance conditions should always be considered. This is even in seemingly straightforward applications to ensure the seal is continuing to operate within it’s ideal set of conditions. Sometimes it’s assumed that tolerances only really come into play when looking at very small applications, with so-called “micro O-rings”. This is not the case - and we can illustrate this with a hypothetical application: A device containing a 25 mm rod, static, with an FKM O-ring sealing gas, whilst installed for 10 years untouched in the Arctic circle. However, it also needs to seal oil whilst installed in direct sun at the equator for 10 years, but with the rod removed and re-fitted several times a year. The device designer would prefer not to make two different variants. O-ring calculators working to ISO 3601 would recommend a 25.12 mm x 1. 78 mm O-ring fitted in a 27.8mm groove with f7 tolerance on the rod and H9 on the groove. However, in the cold application the worst-case squeeze is looking a little light to ensure long term gas sealing. Whilst in the high temperature location, the compression and groove fill are reaching levels that could accelerate compression set of the material and cause issues when the rod is disturbed and re-fitted. Moving the nominal sizes would help one installation but adversely impact the other. However, by tightening the tolerance on the rod and the groove, the concerns at the extreme conditions can be lessened. Approaching the seal supplier for tighter tolerance O-rings can improve the robustness of the design even further, as could incorporating a custom bonded seal-rod component which would remove one dimension and its tolerance completely. Such a hypothetical application rarely exists of course, but the approach is applicable to a surprisingly large number of applications which appear “standard” at first glance. What hardware tolerances should be specified? There is not a definitive answer to this question, but more of a recommendation that the tolerances for each component within an application are considered. Both the technical and financial impacts being evaluated. Sometimes applying a tighter tolerance can be achieved without any cost implications. In other cases it might slow the manufacturing process, or potentially require a different and more costly process (for example, grinding a rod in place of turning). Tolerances should be evaluated by the consequences of the complete sealing system not performing as intended at the absolute extremes of potential tolerance conditions. In applications where the impact of non-optimal performance may be less severe, a paper based, or theoretical tolerance evaluation is carried out. This could provide enough confidence the risk and impact of the problem arising is acceptable compared to the costs of preventing the risk further. In some safety critical applications in Life Sciences & Medical, Aerospace or Vehicle Control systems markets, testing of parts at the most extreme sizes may be required. This is to be certain that any combination of components from anywhere within their minimum or maximum range of tolerance sizes would still produce an assembly product which performs successfully in the application.

Polyurethane as a seal materialPolyurethane as a seal material The foam in your armchair. The strap on your wristwatch. The wheels on a supermarket shopping trolley and beyond; Polyurethane certainly has a diverse range of uses since its invention almost 85 years ago. Aside from day-to-day products; it is also a highly capable and versatile sealing material - and an option that is often overlooked. Here we explore a little deeper into the properties of polyurethane. Covering it's uses as a seal material and how it is manufactured. What is polyurethane?Is it rubber? Is it plastic? Rigid or flexible? The answer to all of those questions is – yes, it is all of these things! Polyurethane covers a group of materials; plastic polymers produced by the combination (or synthesis) of di-isocyanates with polyols and a chain extender.Thermoplastic Polyurethane (TPU) belongs to the Thermoplastic Elastomer (TPE) family. It was originally invented by Otto Bayer in 1937 and further developed during WWII as an alternative to rubber (which was difficult to source at the time). It can be formulated to produce different finished materials with an array of properties suitable for a wide range of applications. Where do we use polyurethane? In addition to seals, polyurethane is the basis of a diversity of products. These include varnish, foam mattresses, roller skate wheels and surfboards. There are literally hundreds of different types of polyurethanes, each made in a slightly different way to suit the demands of the final product. How are polyurethane materials manufactured? In a one-step process, the polyol (a compound containing multiple hydroxyl groups) is mixed with isocyanate (highly reactive low molecular weight chemicals) and a chain extender (low molecular weight diols or diamines). The result is a random copolymer with a physically cross-linked irregular molecular structure.In a two-step process, the polyol and isocyanate are mixed first to produce a pre-polymer. This is then mixed with the chain extender to produce a block copolymer with more regular molecular structure. This can result in improved and more consistent material properties in exchange for a slightly higher production cost.We use rigid polyurethanes for sealing products. The mixed liquid material is either cast into tubes from which seals can be machined on CNC lathes. Alternatively cast into bricks which are then chipped, mixed with dye (or other additives), and fed into injection moulding machines.  Why is polyurethane a good seal material?When formulated appropriately, polyurethane yields an impressive set of material properties that make it an ideal material for sealing products. It can be flexible which allows seals to be assembled into closed grooves. This capability also resists large deformation and makes them robust to survive without damage. In this respect it outperforms PTFE and is at least on a par with rubber.Polyurethane has excellent elastic behaviour, recovering almost instantly from deformation. Again, outperforming PTFE (no springs or rubber energisers needed to give the initial sealing contact stress) and matching rubber materials. However, unlike rubber it has a lower coefficient of friction, with very high abrasion resistance, tensile strength and stiffness. These properties mean it is often used without anti-extrusion/back-up rings at considerably higher pressures than even a 90-durometer elastomer O-ring would be capable of. How does polyurethane perform in tests? In abrasion tests, polyurethane has a quarter of the wear rates shown by typical rubber sealing materials. These include NBR (and considerably lower than even filled grades of PTFE). Its tensile and tear strengths are typically 3-5 times higher than rubber seal materials. Although it lacks the chemical resistance and temperature capability of PTFE, it is compatible with mineral oils.Polyurethane seal materials typically have a general operating temperature range of around -35°C to +110°C. However, more specialist grades can remain flexible down to -50°C, and other grades can push the upper limit to around 130°C.Because it can be injection moulded as well as machined from tubes, it lends itself to production of seals in both high and low quantities. This allows commercial flexibility at both prototype and production stages of a project.With complex chemistry, care must be taken to ensure sealing fluid resistance and temperature capabilities are always considered (as is the case when selecting any seal material). However, in many applications, polyurethane can often bring physical property and whole-life cost advantages over rubber or PTFE options. This can mean it bridges the gap in capabilities between these material groups.

Why the focus on PFAS?Why the focus on PFAS? PFAS is a blanket term used to describe Poly- and Per- fluoroalkyl substances. There are currently around 10,000 substances in existence that fit this description, with potentially more variants still yet to be produced. Some are already known to be harmful to human and animal health and the environment (such as PFOA and PFOS), and these specific PFAS are already controlled under legal restrictions. But in February 2023, The European Chemical Agency (ECHA) published a regulatory proposal to further restrict the manufacture, placing on the market, and use of all PFAS within the EU. What are the consequences for seal manufacturers? A broad ban on all PFAS would have a significant and direct impact on European industries and businesses that manufacture products using these substances; and will have consequences globally.One particular concern is the fluoropolymers material group. These elastomers could be caught in any blanket ban of PFAS without any exemption.These materials are essential in a wide variety of applications in the food, medical, pharmaceutical, clean energy, semiconductor, electronics, oil & gas, chemical, automotive and electric vehicle industries.These fluoropolymers include materials used in high performance sealing solutions, such as PTFE and PVDF plastics, and FKM, FFKM and FVMQ elastomers. What's the problem? There is substantial scientific evidence to show that fluoropolymers demonstrate unique characteristics that mean they do not pose significant risk to the environment, or to human health as they’re not bio-available, toxic or mobile.They do not dissolve in or contaminate water, or generate microplastics, and as a result cannot enter or accumulate in a person’s bloodstream.They meet the criteria specified by the Organisation for Economic Cooperation and Development (OECD) as “polymers of low concern” as they present no significant toxicity concerns, and do not degrade into other PFAS chemicals.Fluoropolymers far outperform other sealing materials in countless applications. To put it simply; they seal tighter, last longer and survive harsher application environments considerably better than any other available materials. Fluoropolymers add irreplaceable value to society, and contribute significantly to environmental sustainability. What do we do? We fully support restricting the use of PFAS chemicals that are known to be harmful to our health or our environment; but we believe grouping all PFAS within a restriction would be a mistake.Fluoropolymers do not have the toxicological and environmental profiles of other PFAS, and prolonged existence alone is not justification to restrict a substance under REACH.We already work proactively to ensure the products we put to market are safe, compliant with legislation, and utilise the latest sustainability developments to minimise our impact on the environment – during manufacture, in use and at end-of-life. As a company with high environmental, social and governance standards we will continue to do so, whenever and whatever new legislation comes into force.But we believe the sealing industry, it’s many customers, end users and indeed society as a whole, need specific derogations and exemptions for fluoropolymers (free of any time limitations) to be included in any legislation covering PFAS substances.To learn more about our key industries and our sealing solutions, read HEREAs we have done, we strongly encouraged any individual or organisation that may be impacted by a ban to engage with ECHA during their consultation period on this proposed restriction (which ended on 25th September 2023). 

Seals for cryogenic applicationsSeals for cryogenic applications Cryogenic sealing means controlling or sealing a media at very low temperatures. This process can be complex and advanced, and spans a range of markets; from pharmaceutical, chemical and refrigeration, to automotive and electronics. What are low temperatures for seals? A typical industrial sealing system will operate in the realms of a stable temperature rating. This is often sufficient for elastomer seals to cope with most of the time. For example, a general pump application running at -20°C to +80°C in air or water is perfectly suitable for a sealing solution manufactured in nitrile (NBR) elastomer. However, if a temperature specification dropped to -196°C in liquid Nitrogen; the sealing solution would have to be adapted dramatically. This would be changed to a more advanced polymer sealing solution. This is because general elastomer groups such as NBR, FKM, & VMQ are simply not suitable to perform at very low temperatures. This is attributed to the chemistry of the elastomers at a molecular level.All elastomers need a particular chemical makeup to be able to withstand a temperature range of the application. However current global elastomers on the market are not rated much more below -70°C. This means elastomers are not a viable choice for cryogenic applications with temperatures below -70°C (often requirements reach  -196°C or lower). Types of cryogenic sealing solutions Specialised sealing solutions are required for cryogenics conditions. Here at Ceetak, our engineers utilise a range of more advanced seal polymers to solve the most demanding and critical application setups.PTFE is often a go-to material due to its outstanding temperature rating of -260°C to +260°C and low friction characteristics. Additionally, it’s often used in conjunction with a metal spring energiser.The spring energiser acts to maximise the sealing force at the seal contact points and to maintain a tight seal. This is even when the very low temperature conditions are attempting to contract the seal away from the mating surfaces.Other common polymers suitable are UHMPWE and PTCFE for more specific applications. Our seals for cryogenic applications We offer a range of cryogenic sealing solutions for various markets and applications including; pumps, engines, couplings, cylinders and many more.Read about our full range of engineered materials HERE