Injection Molded Seals

A lip seal is a thin flexible edge that sits slightly inside the shaft it surrounds, so it has to stretch onto it. That interference is what holds the lip against the surface, and how much of it there is decides everything else. More interference seals better against pressure and against surface imperfections. Less interference drags less, runs cooler, and lasts longer.

Friction is not just an efficiency question. A lip running dry heats up, and a warm lip loses stiffness, so it presses less and seals worse while wearing faster. Low friction materials such as acetal and lubricated grades, a smooth counter surface, and the film of fluid the seal is holding back all keep the lip cool enough to keep working.

The counter surface matters as much as the seal. A shaft that is too rough tears the lip. A shaft that is polished mirror smooth can be worse, because it gives the lip nothing to hold a lubricating film in, so it runs dry. A moderate finish with fine marks running around the shaft rather than along it is what seals best.

Many molded parts do not carry a seal at all, they carry a groove for one. That groove is a precision feature: its depth sets how much the O-ring is squeezed, its width gives the ring somewhere to move, and its corners keep the ring from being pinched or rolled during assembly. A lead-in taper into the bore stops the ring being cut as the parts come together.