As can be seen from the seal cross-section shown in Fig. 14.2, shaft seals are complex shapes that require advanced mold design and molding techniques (see Section 7.3 for discussion of fluoroelastomer molding). For some time, most shaft seals were made in the United States by compression molding. Injection molding of shaft seals is prevalent in Europe, and is being used increasingly in the United States. An advantage of compression molding is that preforms (usually rings cut from extruded tubing) are used that closely approximate the amount of stock required for the final parts, so compound waste is minimized. For injection molding, the amount of cured stock in the central sprue and runner (actually a thin sheet leading to the seal lip) is often large compared to the stock required for the final part, so the waste of high-cost fluoroelastomer may be high. Such waste is reduced in modern injection molding designs.

This is almost similar to that of ‘A’ type seal. But this has the metal case placed outside. When the housing is rough, temperature is high and working conditions are severe, this type is preferred

Table 14.1. Fluoroelastomers Used in Oil Aging⁵

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However, rubber-cased seals do have limitations. For example, during installation, shear stresses can build up between the seal and housing wall, causing the seal to release itself.