Silicone O-Rings

Silicone o-rings are used in thousands of applications, like water-proof camera housings, enclosures, automobiles, medical devices and many more. Silicone rubber is available in durometers from 10 thru 70 durometer Shore A, however, 10 durometer silicone o-rings and 20 durometer silicone o-rings are best specified for gaskets with larger cross-sections since they are easily stretched. Silicone o-rings are also called silicone seals, and custom o-rings can be referred to as molded o-rings or pre-formed o rings.

The video below illustrates the benefits of using a pre-formed silicone o-ring to ensure a proper fit. Installing a bonded silicone o-ring cord can be extremely cumbersome and time consuming which can lead to poor installation. Twisting and pinching are common failure modes for o-rings and seals.

Pre-Formed Molded Silicone O-Ring Installation Demo

What is an O-Ring?

An o-ring is typically a ring shaped, molded elastomer with a circular cross section. O-rings can also be unshaped cord connected on the ends (similar to a rubber band) and made from other materials like plastics. They may be constructed with unique cross-sections such as T-shapes, L-shapes, etc. depending on the application’s requirements. O-rings are used for sealing out liquids and gases in both static and dynamic applications.

How a Silicone O-Ring Works

O-ring seals are capable of sealing at relatively high pressures because they are compressed or “activated” when pressurized – as the pressure is applied to the o-ring it deforms in the gland or groove. As a silicone o-ring deforms between two flat and smooth surfaces, it presses into the gland walls and seal joint. The compression energizes the seal and creates a mechanical barrier blocking fluids from passing through; this restrained condition allows a silicone o-ring to seal at higher pressures than many flat face gaskets. (View the blog post Seal Design – 3 Considerations for more details about design and material considerations.)

Non-Standard O-Ring Gland

An o-ring gland is the groove into which the o-ring is installed. The most common shape is a circle – hence the term “o-ring”. Standard round o-rings are readily available off the shelf from various sources. Stockwell Elastomerics manufactures custom o-rings, or pre-formed o-rings, for non-standard gland configurations (see photo) in OEM enclosures that have tight corners or sealing grooves around screw bosses. Trying to install a standard round o-ring into these types of custom enclosures can be time consuming and lead to o-ring / seal failures due to twisting and inconsistent assembly practices during installation.

Soft Silicone O-Rings

Stockwell Elastomerics makes silicone o-rings from liquid silicone rubber using liquid silicone injection molding, with durometers ranging from 10 – 70 Shore A. Silicone is one of the few rubber compounds commercially available in low or soft durometers. 30 durometer silicone o-rings offer good control of elongation and softness, while softer 10 durometer o-rings and 20 durometer o-rings are readily manufactured but caution should be taken since thin cross-section o-rings in low durometers can stretch easily during install.

Properties of Silicone O-Rings

• High and low temperature
• Very good compression set resistance
• Ozone and UV stable
• Excellent oxidation resistance
• Excellent water sealing
• Available in soft grades (10, 20, 30 durometer Shore A)
• Not very good abrasion resistance
• Moderate / poor chemical resistance (see fluorosilicone o-ring)

Custom Fluorosilicone O-Rings

Stockwell Elastomerics custom molds fluorosilicone rubber for the defense and aerospace sectors. Fluorosilicone rubber is more chemical resistant than silicone and has good high and low temperature properties. Fluorosilicone can be compression molded to custom o-ring geometries to offer better installation and fit in non-standard gland configurations.

Cost of Custom O-Ring Molds

While a custom o-ring may sound expensive, often the total value-add benefits outweigh the cost, resulting in a net savings. Unlike very expensive molds for injection molding plastic, the cost for a silicone injection mold usually ranges from $2000 – $5000 depending on size, geometry, and number of cavities. The speed of installation can easily offset the price of the molded custom o-ring. Further, if the silicone o-ring is at risk of twisting or improper installation, the cost of an o-ring/seal failure can be very high.