Compression set is a phenomenon that causes most water leaks (not related to installation) in molds today. Simply put, this means that an o-ring has resided in a compressed state in the mold for so long that it has lost its ability to apply adequate force in the gland and overcome the applied water, air or hydraulic pressure.

This usually occurs with tooling that is difficult or time consuming to remove such as A and B side cavities or tooling requiring a complete mold disassembly to reach. In many shops, if there is no reason to remove this tooling (like a compulsory PM program), then it is overlooked in lieu of a speedy repair to get caught up with the red-tag mold count or to get the mold back into production.

Thermal degradation will accelerate this compressive set behavior such as when a hot runner system is fired up without turning on the water to the cavities. The compressed o-ring is then baked in this set condition—basically being “remolded” in the bore and reducing its ability to seal.

This is another in a long list of reasons why it is critical to follow mold start-up procedures and change o-rings after a predetermined amount of time, based on visual inspection of the o-rings at predetermined intervals to establish a cycle baseline.

Chemical degradation normally only occurs during mold cleaning operations when tooling is cleaned with the o-rings intact or in the bores. Some elastomers such as Nitrile and silicone are susceptible to swelling and softening if exposed to certain cleaning solutions such as acetone or MEK (methylethylketone). Caustic cleaners such as sodium hydroxide, a popular solution used in ultrasonic cleaning tanks will have the same effect on Fluorocarbons, so it is important to stay within the o-ring manufacturer’s recommendation of cleaning solution percentage.