A knit line is an imperfection of the seal material due to premature curing (scorching) of the rubber during molding (see Figure 162). It is for this reason that seal manufacturers must be extremely diligent in controlling material temperature fluctuations throughout the production process. Mold temperatures must be especially monitored so as not to inadvertently harm the material.

In shaft seals designed to retain grease or oil, a knit line that occurs along the primary lip, the secondary lip, the flex section, the seal O.D., or the endface can be a major concern. Less problematic – though still potentially dangerous – are knit lines seen in the interior or on the exterior of the seal.

See Table 49 for a diagram of these seal components.

POSSIBLE MANUFACTURING DEFECTS MAIN PAGE