Satori Seal Technical Blog

Rubber molecules are long polymer chains that are intertwined like a plate of spaghetti. In this state the rubber is sticky and not very strong or usable. By vulcanizing the rubber with a cure system, like sulfur, crosslinks are created between the individual polymer chains making the rubber stronger. The cure system links the carbon atoms in one polymer chain to the carbon atom in another polymer chain, kind of like rungs on a ladder. The more crosslinks that are created the stronger the rubber becomes and thus has higher tensile strength, compression set, elongation and tensile set properties. But too much crosslinking can make the rubber hard and brittle. Sulfur creates a carbon-sulfur-carbon (up to six atoms in length) between the polymer chains(2). Peroxide cure decomposes forming free oxy radicals on the polymer chain that promotes direct carbon-carbon bonding between the polymer chains. Peroxide cure creates stronger carbon-carbon crosslinks than the carbon-sulfur-carbon crosslinks created with sulfur cures. The shorter carbon -carbon crosslinks are more thermally stable and have better compression set properties. The longer sulfur crosslinks generally offer better tensile, elongation, fatigue and tear resistance (1).

Depending on the type of elastomer there can be several types of cure systems used to vulcanize rubber. The most commonly uses cure system is sulfur but other types of cures can be used depending on the type of elastomer and the physical properties desired. For instance, fluorocarbon uses either bysphenol or peroxide cure, chloroprenes (Neoprenes®) use metal oxides, silicone use peroxide, titanium, platinum or rhodium. But the most common known is EPDM with sulfur or peroxide cure.

(1) “The Rubber Formulary”, Peter A. Ciullo & Norman Hewitt

 (2) “Rubber Compounding, Chemistry and Application”, Brendan Rodgers

Tolerances have always been an issue with the majority of the drawings we receive for molded rubber seals. We commonly see tolerances on the drawing title bar that read something like .X = ±.03, .XX = ±.01, .XXX = ±.005. These tolerances typically will not work for molded rubber parts. Molded rubber tolerances are based on the length of the dimension. The longer the dimension the larger the tolerance that is needed.

The two most common industry standard tolerances used for molded rubber parts are ISO3302-1 and RMA Tolerances from the Rubber Manufacturers Association Handbook MO-1. When dimensioning a molded rubber part on a drawings we recommend using ISO3302-1 Table M2 or RMA-A2 as tables M2 and A2 have become standard for molded rubber parts.

ISO3302-1 and RMA tolerances are a set of 4 tables, A1-High Precision, A2-Precision, A3-Commercial, A4-Basic, with tolerances that vary in size depending on the length of the dimension. The table will have a “Fixed” or “Closure” tolerance for a dimensional range. Example: Above 0.40 and Below 0.63 Fixed tolerance is ±.008 and the closure tolerance is ±.010. Ok, so what is a fixed and closure tolerance? A closure tolerance is a dimension that is affected by the opening and closing of the mold. Take a look at an O-ring. Around the inside and outside diameter of the O-ring is a line. That line is a parting line and is caused by where the two halves of the mold come together. If the dimension is affected by the mold closure or flash thickness at the parting line then that dimension is a closure tolerance. When applying tolerances to closure dimensions, all closure dimensions for that part have the same tolerance as the largest closure tolerance. Lets say you have a part with 2 closure tolerances that are affected by the flash thickness, .500 and 1.125 inches. According to RMA-A2, the tolerance for each dimension is +/-.016. 0.500 would not be +/-0.010 per the table, instead it will be +/-0.016 based on the largest closure tolerance, in this case 1.125+/-0.016.

A fixed tolerance is a dimension that is not affected by the opening and closing of the mold. For instance the inside diameter of the O-ring. Fixed dimensions tolerance’s are by their own size per the table, unlike the closure tolerance.

The 1st thing to consider is how is this seal going to seal? Is it static, nothing moves once installed, or dynamic, the seal or sealing surface rotates or reciprocates in the application. Does the O-ring seal axially or radially. Axially is when the O-ring is squeezed from the sides perpendicular to the parting line or, if you think of the O-ring as a wheel, in line with the axle. Radially would be squeezed perpendicular to the axle or in line with the O-ring parting line. So, we have static axial, static radial, dynamic axial and dynamic radial O-ring seals. Each type of seal is going to have its own design criteria to consider. There are other O-ring seal types like thread seals, tapered seats, or boss fittings which we will not consider in this article. SAE.org is a great place to start your search for design criteria. They have many military and aerospace design documents available for sale. A great start is ARP1231, “Gland Design, Elastomeric O-Ring Seals, General Considerations.” This specification covers many aspects to consider in O-ring seal design. Aerospace recommended Practice ARP1232, ARP1233 and ARP1234 cover O-ring gland seal design for the AS568 series O-rings with an operating pressure up to 1500 psi. These ARP documents contain groove dimensions and stretch and squeeze specifications. These specifications are a great starting point for a custom O-ring seal. There are also design specifications for metric O-Rings, and for O-Rings with Backup Rings for pressures above 1500 psi.

We have had several cases over the years where the O-rings is breaking during installation. This could be an issue with Ultimate Elongation. You can find Ultimate Elongation on a Physical Property Data Sheet under Original Physical Properties. Elongation is the measure of how much a specimen stretches before it breaks. It is usually expressed as a percentage.

Case 1, High pressure spool on SCUBA regulators

SCUBA regulators have a spool that takes 2 small Nitrile O-rings. The manufacture wanted to switch compounds from Nitrile to Fluorocarbon (Viton®) for use with increased oxygen percentage, 32%-44%, in NITROX gas. When the Fluorocarbon O-rings were stretched over the spool they would break because they were being stretched well over 200% which exceeded Fluorocarbon compound’s Ultimate Elongation of 100% to 175%.  Nitrile compounds typically have an Ultimate Elongation between 250% to 400% which is why the Nitrile compound did not break. The manufactures solution was to stay with the Nitrile compound for this application.

Case 2, Stretching the O-Ring, Tensile Set

We visited a company that stretches the O-rings on mandrels for 1/2 to 1 hour to increase the ID  to make installing the O-ring easier. Some compounds were not returning to their original size after stretching making the assembly of the parts impossible. They need a compound with good Tensile Set properties. Tensile Set is the extension remaining after a specimen has been stretched and allowed to relax for a predefined period of time. Tensile Set is not normally tested or recorded on the Physical Property Data Sheet.

After testing several Nitrile compounds Satori was able to supply them with a compound that had good Tensile Set properties. They were able to stretch the O-rings on Mandrels and they would quickly return to their original size allowing for easy assembly of the parts.

When selecting a compound one thing to be aware of is how much you will need to stretch the rubber. Take into consideration the Ultimate Elongation properties  and when necessary, the Tensile Set properties. Silicone and Fluorocarbon compounds will usually have lower Ultimate Elongation properties.

What is Tensile Strength?

Ultimate tensile strength, tensile strength for short, is the maximum force a material can withstand without fracturing when stretched. It is the opposite of compressive strength. To understand this a little better, lets take a look at some other forces. Lets take a rectangular block of rubber. If you squeeze the small sides together this is compressive force. If you stretch the block, this is tension or tensile force. If you twist the block this is torsional force and if you apply a opposing force to the side on top and opposite side on bottom, this is shear force.

The last time I purchased a pair of shoes they came joined together with a piece of string. Instead of getting a pair of scissors, I opted to test my physical strength against that tensile strength of the string. If the string has a low tensile strength I should be able to pull and break the string easily. If it has a high tensile strength it will be much harder to break by pulling. So, let say you are pulling on your shoe laces and it breaks, you have just exerted a force greater than the ultimate tensile strength of the shoe laces. If you are unable to break the shoe laces then you can not exert enough force to overcome there ultimate tensile strength. Are you starting to understand what tensile strength is?

Why is Tensile Strength Important in Rubber?

Well, any time you have an application were you are pulling on the part, tensile strength is important to know. Whether you product is designed to break easily or not at all the tensile strength will let you know how the object will react to the tensional forces. A few rubber products that tensile strength are important would be bungee cords, rubber tie downs, drive belts. Some elastomeric compounds, like Silicone,  have a low tensile strength making them unsuitable for a dynamic a types of seal because they can fracture or tear easily.

Tensile strength is measured with a tensometer. A tensometer is special machine that is designed to apply a tensional force to a specimen, in our case a die cut dumbbell shape, and measure how much tensional force it takes to deform and fracture the specimen. The force is typical shown on a graph that shows how much force it takes to stretch the specimen to deformation and ultimately break.

Yes, 36 types of rubber, 1 natural and 35 synthetic, why so many? Good question. Different elastomers have different physical properties, temperature ranges and fluid compatibilities for the almost infinite number of applications they may be requires to function in, not to mention cost factors. I had an application were the temperature would dip down to a frigid -65°F and hit the other end of the temperature scale at over 300°F. Silicone was be a great start since its basic temperature range is -65°F to 400°F. Another example was when I rebuilt the anti-lock brake unit on my car. I used EPDM, with a peroxide cure, since EPDM is vary compatible with ester based brake fluids (DOT 3 and DOT 4). I couldn’t have used nitrile because brake fluid causes nitrile to swell and the seal would have quickly failed leaving me with no brakes–not a good idea on a corvette that can hit 170+ miles per hour, not that I’ve ever gone over the speed limit of course.

You might be getting to get the picture by now. Sometime material selection is not that easy and may require a little research. Here are some basic things to keep in mind when selecting a material. What is the temperature range while in the application? What fluids or chemicals will be coming in contact with the material? Are there any special physical properties that need to be met such as high tensile strength, soft durometer, ozone/weather resistance, low or high modulus, abrasion resistance, coefficient of friction requirements, etc. Visit http://www.satoriseal.com/compounds/satori_compounds.htm for a list of common compounds and uses.

For decades rubber was measured with a type A durometer gage. According to ASTM D2240, “Standard test Method for Rubber Property — Durometer Hardness,” you need a 6mm (.240 inch) min thick piece of rubber and large enough to be 12mm (.480 inch) away from the edge and previous test point. This and the geometry of the gage make it unsuitable for measuring small cross section O-rings or thin pieces of rubber.  Shore Instruments came out with the Type M gage, Micro O-Ring System,  specifically for measuring small cross section O-rings and thin pieces of rubber (not less than 1.25mm thick). This system was designed to give similar reading to the type A gage, but the gages will not yield exactly the same reading. It is not uncommon to get 4-5 points difference in the readings (given that all gages are properly calibrated). These scales are different, and according to Instron, there is no correlation between the Shore A and Shore M scales. In other words you can not measure a piece of rubber with a type M gage and convert the reading to type A scale and vice versa.

The major differences between the type A and Type M durometer gages are the indenter geometry (its shape) the spring force on the indenter and the size of the foot. The type A gage has a “Frustum Cone” indenter with 821 gram max spring force. The type M gage has a “Sharp 30° Angle” indenter with a 78 gram max spring force. The type M has a much smaller diameter indenter, .7874mm (.030 inches), with a sharp point as compared to the larger diameter type A indenter, 1.27mm (.050 inches) with a .79mm (.031 inch) diameter flat bottom instead of a point.

When you have a durometer Type A gage calibrated, after calibration it is accurate to only +/-2 points. After calibrating a type M gage it is accurate to +/-4 points. It’s  repeatability (variation caused by equipment) and reproducibility (variation caused by the operator) are not real accurate. Hardness instruments with a GR&R study of 10% to 30% is acceptable in most industries. Repeatability and reproducibility can be calculated by performing a Gage R & R Study. (Google Gage R & R and you will find several web sites that will describe how to perform this study). What this means is, it is not uncommon to get different readings from the same operator (repeatability) and different reading from two or more operators (reproducibility) on the same rubber samples.

Factors like temperature, humidity, the rate at which you apply the gage, how much pressure you apply to the gage will effect your readings. This is why we recommend using a durometer gage on a conveloader stand such as the Shore Instruments CV71200 Conveloader for the type A gage. The conveloader controls the rate of application, the amount of force applied to the gage and keeps the gage’s indenter perpendicular to the sample which help to increase repeatability and reproducibility of your readings.

Proper Storage
As rubber seals age the physical properties change and can cause the seals to be unusable. These changes are caused by many factors such as light, ozone, humidity, etc. These factors can cause the rubber to harden, soften, crack or cause other surface degradations. Proper storage is needed to reduce the effects these factors have on the rubber.
Proper storage is essential in extending the shelf life of O-rings. Rubber seals and molded rubber products, whether in bulk or in assemblies, should be places in sealed bags and kept in boxes out of sunlight and excessive temperatures and humidity. Temperatures should be higher than 59°F and below 100°F. 68°F-70°F is optimal with the humidity no greater than 75%, 65% for polyurethane seals.
When O-rings are exposed to extreme cold below their normal operating range they can harden and there shape can become distorted. The effects of extreme cold are not damaging to most elastomeric
seals and they usually return to their normal when they warm up. Exposure to excessive heat can accelerate the deterioration of the rubber. The effects caused by excessive heat usually are not reversible.
History of Age Control
Age control of elastomeric seals and assemblies started after World War II on hydraulic, fuel and lubrication seals on aircrafts. The first document on age control was release in 1958 and was a compilation of several studies on age control done since WWII. After several more studies and papers, MIL-STD 1523 was released in 1973 and gave 12 quarters as maximum shelf life. This was extended to 40 quarters in 1984 with the release of MIL-STD-1523A. This standard was cancelled in 1995 when the release of AS1933 was issued. AS1933, “Age Controls for Hose Containing Age- sensitive Elastomeric Materials” only addressed elastomeric hoses and seals were essentially released from control.
To meet the demand of contractors and address the confusion of age control of elastomeric seals since the cancellation of MIL-STD1523A, ARP5316 was issued and addresses shelf life, traceability,
proper storage and gives a reference source to work with.