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The term “plastics” is generic way of describing a synthetic material made from a wide range of organic polymers. Organic polymers describes a man-made substance that is formulated using polymer chains to create what we commonly refer to as…(you guessed it), plastics.

Before plastic, leather had been used to create Backup ring devices behind O-rings. Leather allows fluids to be retained, providing lubrication for the O-ring when the system was running dry.

The problem with leather was that it could become dry and shrink away from the sealing service, exposing the elastomer to same pressure it was intended to protect against.

With the advent of polymers, a piece of plastic could be cut or formed into the exact shape to allow for zero extrusion gap, and for continued protection for the O-ring.

Some polymers were very brittle. Since they needed to be deformed to allow for installation into solid glands, the cut of the plastic could nibble at the O-ring, causing premature failure of the element it was supposed to be protecting.

The Revolution of PTFE

When PTFE moved out of the lab and into industrial use, it quickly found itself adjacent to the O-ring. PTFE offers extrusion resistance and, at the same time, doesn’t erode or nibble at the O-ring due to the “softness” of the polymer.(Hardness between 55 and 65 Shore D)

Given the composition of PTFE, or Teflon, it could be utilized as a sealing element to protect Backup rings and conform to the shaft. The bonus was it was generally easy on shafts (depending on the filler added to the PTFE).

There are some negative aspects to Teflon that needed to be overcome by early engineers. First, it has a fairly high rate of Thermal expansion which, by its own nature, could often times lose contact with the sealing surface. This meant some kind of loading was necessary to ensure contact.

PTFE is as tough as other polymers, so the fact that it could seal on a shaft made it vulnerable during installation for tears or nicks on sealing surface.

Second, if it were stretched during installation, the material had to be sized back to its original shape due to its poor elastic properties.

Eclipse Engineering has in-house capabilities to manufacture seals up to 55 inches in diameter, and over 100 inches through production partners.

While seals with huge diameters certainly grant their own significant levels of intricacy, here we’ll look at the other end of the spectrum: the micro-sized seals.

We won’t just look at a simple seal ring, but an inherently more complicated and geometrically detailed spring energized seal. As we’ll see, very small diameters make multiple manufacturing aspects more involved and challenging.

The Client’s Issue

A sealing solution in a customer's epoxy dispensing equipment. They needed an effective seal for the reciprocating rod responsible for the flow-control and metering of the epoxy while being dispensed.

Operating Conditions:

• Reciprocating Rod Seal
• Epoxy Dispensing Head
• Rod Diameter: 1.2mm [0.047”]
• Stroke Length: 6mm [0.236”]
• Cycle Rate: 15 per min
• Media: Epoxy
• Operating Pressure: 1,500 PSI
• Temperature: 70° to 150°F

In general terms, most viscous media sealing solutions have three things in common:

1. A variant of UHMW for the seal jacket,
2. heavy spring loading, and
3. multiple point contacts with increased interference.

In most cases, multiple nested V-Springs are incorporated to provide optimal load and energize the compound contact points on the seal. With this formula, we’ve had great success sealing media like epoxy, urethane, silicones and acrylics.

The heavy loading is necessary to effectively wipe the reciprocating rod. This is balanced with the correct material and design geometry to provide long wear life of the seal, which has the potential to be compromised under such loading.

The challenge in this case was to incorporate these same proven principles in a micro-sized seal.

The Eclipse Solution

Springs are an integral part of all sealing systems. A simple air cylinder has O-rings to seal in the air, and the O-ring exhibits spring-like qualities to ensure a good seal over a broad temperature range.

But what are the different types of springs and materials in sealing systems? And how do you choose the best for your application?

Metal Springs

Metal springs, such as the Cantilever and Canted Coil spring, are used to energize polymers such as Teflon and ultra high molecular weight polyethylene (UHMW) to allow sealing in a wide range of temperatures. Selecting the correct spring material is critical to the life of the seal.

Metal energized

Better known as Teflon in the industry, Polytetrafluoroethylene is widely used in practically every industry on and off the planet (and even beneath its surface!)

Medical Uses

This material’s primary claim to fame is its resistance to most chemicals. It inherently has an extremely low coefficient of friction, it’s easily machined from rods, tubes, or compression-molded shapes.

It’s one of the few polymers that are approved for medical implants due to its inertness to bodily fluids — the immune system principally ignores its presence in the body.

Moving away from the body, you’ll find PTFE or Teflon products in medical

When it comes to designing and developing seals, the aerospace and industrial industries need a basis to allow production anywhere in the world.

One of the first PTFE (Teflon) standards, AMS3678, describes Teflon and the addition of fillers. This was used in conjunction with Mil-R-8791, which is one of the Mil specs describing a backup ring device.

The origin of all these specs dates back to the creation of the O-ring.

The Origin of the O-Ring Patent

In 1939, Niels A. Christensen was granted a U.S. Patent for “new and useful improvements in packings and the like for power cylinders.” These referred to improved packing rings made of “solid rubber or rubber composition very dense and yet possessive of great liveliness and compressibility.” These products were suitable for use as packings for fluid medium pistons (liquid or air). The improved packing ring is the modern O-ring.

There was a progression of standards for the O-rings created by individual countries, such as AS568, BS 1806, DIN 3771, JIS B2401, NF T47-501, and SMS 1586. Eventually, AS568 became more accepted in the industry.

The backup ring was originally created to help improve the O-ring’s ability to resist extrusion. Teflon was widely used as one of the materials for backup ring devices. Standards were created to unify the production of this Teflon device.

The Progression of Mil Specs

The progression of standard changes has led to AMS3678/1 for Virgin PTFE through AMS3678/16. These standards describe a group of Virgin- and filled-PTFE materials accepted by the industry for manufacturing seals and back-up ring devices.

Mil-R-8791 was canceled in February 1982. This spec was superseded with AS8791, which eventually evolved into AMS3678.

AMS3678 is a tool used by customers and Teflon suppliers to create uniformity in the manufacturing and processing of seal and bearing materials. The standard is inclusive of most of the compounds upon which the industry was built.

When customers approach with an old “mil spec”, they are pushed to the new AMS spec which is currently active. Eclipse manufactures to the spec so their customers will have the confidence that they manufacture to a known standard.

When crossing custom materials from well-known sources, customers are driven to an accepted spec that is equivalent to the original source of the material. This helps customers sell their products with internationally-known materials rather than custom, home-grown compounds that are often intended to single source those materials.

There are several qualifications of the spec that suppliers must observe. This includes dimensional stability tests. This test ensures the material has been properly annealed, and that the seal or backup ring will fit and function as it was originally intended.

Eclipse is uniquely qualified to supply parts to the latest AMS3678 specification. They understand the scope of the specification which allows us to ship parts with fully traceable certification.

AMS3678 helps validate a material to a customer to ensure they get the same material processed the same way with each order. Beyond this, there are other ways to determine what makes a part process-capable.