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Royal Seals - Seal Material

Appropriate choice of seal material is very crucial in sealing systems to operate leak free and to maintain a long service life. Material performance and seal design are the two most crucial factors in terms of having the best sealing solution. Elastomers are one of the most widely used seal material. They possess properties like elasticity, resilience and chemical resistance which are ideal for sealing applications. Below is a brief description of the most widely used seal materials.

Important Seal Properties

Shore hardness

Hardness may be defined as the resistance to indentation under specific conditions. When judging the potential effectiveness of molded seal (processed), hardness is one of the most common criteria in the rubber industry. Though the elastomeric lip of most standard seals fall in the 70 to 95 Shore A range, the application in question will always govern the necessary hardness. Softer compounds offering less resistance may be perfectly fine for low-pressure seals, but highpressure seals will likely require a harder, tougher lip material.

Resilience

Resilience (also known as rebound) refers to a compound's ability to regain its original size and shape following temporary deformation. Though compounding may improve an elastomer in this area, it is largely an inherent property. As a general rule, resilience is most critical in dynamic seals. This can be seen, for example, when a shaft seal's elastomeric lip flexes (is distorted) to follow a shaft imperfection, and then returns to its original (“resting”) position. In many cases resilience is closely linked to flexibility.

Tear Strength

Tear resistance (or tear strength) is the ability of a material to resist the growth of a cut or nick when tension is applied. Tear resistance is an important consideration, both as the finished article is being removed from the mold and as it performs in actual service. Compounds with low tear resistance are at risk for installation damage, especially in designs featuring non-smooth areas (as with burrs, slots, threads) and/or sharp, non-radiused (non-rounded) corners.

Abrasion

Measured as a loss percentage based on original weight, abrasion resistance is the resistance of a material to wearing away by contact with a moving abrasive surface. On one hand where the cutting or nicking of a sealing lip is an instantaneous event, abrasive rubbing or scraping is much more progressive phenomenon that develops over time. Seals in motion are most susceptible to abrasion. Hard compounds tend to exhibit less abrasive wear than soft compounds, but use of a harder compound can also increase friction in dynamic seals, and increased friction generates seal-degrading heat.

Compression Set

Compression set is the end result of progressive stress relaxation, which is the steady decline in sealing force that results when an elastomer is compressed over a period of time. In terms of the life of a seal, stress relaxation is like dying, whereas permanent compression set is like death. Compression set tests find their widest use in selecting materials for use as O-rings, U cup seals and other seals that rely on an optimal degree of compression.