After an in-depth Web search and listening to numerous manufacturing reps speak in molecular detail, it is apparent that o-ring design/sealing is more complex than just a piece of rubber being squeezed between some tooling.

The crash of the shuttle Challenger was frequently used to illustrate how even the simplest of mechanical functions can bring down a goliath of engineering wizardry, if not properly design and maintained. And so it goes in plastics manufacturing. Although not life threatening (maybe), thousands of critical production runs are halted every day by the failure of this simple seal to do its job.

Typical o-rings are molded from a half dozen different elastomers available in hundreds of different compounds each with some type of application advantage. The majority of o-rings used in plastic injection molds are made of Buna-N (Niltrile) and Fluorocarbon (Viton). In some cases silicone and LSR are utilized in applications requiring exceptionally pliable or soft o-rings of low durometers (50 or less Shore “A” Hardness). These type o-rings work well in molds with pitted glands and plates. Most typical o-rings in mold applications measure 70–90 durometer.

If you require more scientific or specific information concerning elastomer and compound capabilities for heat, chemical or pressure requirements, there are several online resources who publish their own troubleshooting and selection guides for tooling engineers or mold designers. For our purposes, we will stick to dealing with typical o-ring issues as seen in most mold repair shops.