O-ring

Chapter 1: What is an O-Ring?

An O-ring is a round elastic loop that is used as a seal for static and dynamic applications. Their main purpose is to serve as a seal between structures such as pipes, tubes, in pistons, and cylinders. O-rings are made of various materials depending on how they will be used and are highly pliable. When placed between two surfaces, they block the leakage of liquids or gases.

When used as a static seal, an O-ring remains stationary to contain pressure or seal a vacuum. The dynamic form of O-ring can be reciprocating or rotating. O-rings are a self-energizing seal that applies pressure inside a tube or pipe to form a seal.

Chapter 2: How O-Rings are Made

The production and manufacturing of O-rings involves the use of extrusion or injection, compression, and transfer molding. The extrusion part of the process is used to shape elastomers for the molding process.

Selecting the Mold

The mold for O-rings has two halves. The material is compressed between the two sections. The choice of the O-ring mold depends on the desired diameter. Since the material expands when it is compressed, the groove width should be 1.5 times its diameter. For custom O-rings, new mold tools are computer designed and produced to fit any size that is required. O-ring blanks are cut from steel using a lathe.

For immediate production of O-rings, spliced and vulcanized O-ring production can be used and does not involve the use of a tooled die but is made from extruded elastomer cord.

Material Selection

Choosing the correct material for the O-rings application is important to ensure its proper performance. The chemical compatibility, temperature resistance, and miscellaneous other factors determine the type of material to be chosen as well as its application.

O-rings are made from various types of elastomers with the more typical ones being PTFE, Nitrile (Buna), Neoprene, EPDM Rubber, Fluorocarbon (VitonTM), and Silicone with silicone being used for high temperature applications. The chart below is a short list of a few O-ring materials and their properties. Elastomers get their performance and characteristics from the materials that are mixed in them.

Extrusion

During the extrusion process, the elastomer is fed into an extruder that heats the material and forces it through a die. The process produces the desired configurations to be placed in the mold in lengths of cord. The die selected for the extrusion process is chosen according to the diameter of the finished O-ring.

Molding

There are three molding processes used in the production of O-rings, which are compression, transfer, and injection.

Compression 

Compression molding is used when there is a need for a high volume small non-standard O-rings. With compression molding, the extruded material is placed in the mold cavity and held at a high temperature under pressure, which forces it to take the shape of the mold.

Transfer

Transfer molding is a middle ground between compression and injection molding. In the transfer process, material is forced into the mold, while the mold is closed resulting in higher dimensional tolerances and less environmental impact. Uniform pressure is used to completely fill the mold. The material for molding may be solid and be placed in the transfer pot from which it is forced into the preheated mold.

Injection

The injection process involves pre-heating the material, which is forced under pressure through an injection nozzle. The material enters the enclosed mold through a series of sprues. The molded material is left to cool and harden to the configuration of the mold cavity.

Post Mold Curing

Post mold curing enhances the physical properties and performance of the molded O-ring. Post curing exposes the O-ring to elevated and increased temperatures as a means of improving its characteristics. It assists in the cross linking process and improves tensile strength, flexibility, and the heat distortion temperature above what would happen if it were cured at room temperature.

Spliced and Vulcanized Extrusions

Another process used for the manufacture of extruded cord is spliced vulcanization that does not use a die to create O-rings. Spliced vulcanized O-rings are made from extruded cord that is cut and bonded. They are used for static sealing applications, quick production runs, or when only a few O-rings are needed. They are made from a wide variety of elastomers and come in any size.

To form a spliced and vulcanized O-ring, the extruded cord is cut to the proper length after which the cut ends are joined using a bonding agent. The bonded and joined ends are placed in a high temperature mold to form a molecular bond at the joint.

Spliced and vulcanized O-rings are for static use only and must not come in contact with moving parts. They are not recommended for dynamic applications. Spliced and vulcanized O-rings are produced in small quantities and are ideal for short manufacturing runs.

Finishing

After the O-rings are molded, they will have excess material around the sides where the molds meet. This material, known as flash, has to be removed for the O-ring to have the proper shape and size. Flash can be removed using three processes to give the O-ring its perfectly round shape.

Drumming – The O-rings are placed in a rapidly rotating drum that contains stones. As the drum turns, the excess material is removed by rubbing against the stones.

Buffing – For larger O-rings that cannot be placed in a drum, the flash is removed by the abrasive action of a buffer.

Cryogenic – Cryogenic deflashing is a computer controlled process that uses nitrogen gas to freeze the O-rings causing the flash to freeze and later be removed with grit. It is a safe clean process that removes outer diameter (OD) and inner diameter (ID) flash.

Curing

Once the O-rings are deflashed, they need to be cured. How long the O-rings are in the curing oven depends on the type of elastomer and can vary from a few hours to a day. The purpose of this step is to stabilize the finished O-rings and drive off any by contaminants from the production process.