After the rubber parts are removed from the moulds, they contain thin flash, a result of the elastomer flowing in multisection mould. This flash is typically removed by exposing the parts to a cryogenic tumbling process. This process is carried out in a machine known as cryogenic tumbler. In this machine liquid Nitogen is used to bring the temperature down upto -1960 C. Rubber components are put in the machine and the machine is then switched on. Due to the cooling effect, these components become brittle. The flash being the thinner section brakes easily and the cleaned items come out without damaging/affecting the profile of the seals.
Flash should not be removed by grinding or knifing operation due to the reasons given here. When the rubber is heated, pressed and cured in a mould, the surface contact between the metal mould and the outermost layer of the rubber seal, where the temperature is maximum when compared to the core of the seal, ensures that a case hardened, smooth, compact outer layer is formed above a comparatively soft, rough, loosely bound inner core. This seal when offered to a grinding wheel in order to remove the flash, will slightly eat into the case hardened outer layer, thus exposing the softer loosely bound inner layer, however carefully grinded by an expert. This seal when used in a hydraulic system or a fuel system will let loose the exposed minute rubber particles and contaminate the system. Over and above this, as more and more loose rubber particles come out of the seal, it will make a minute passage for the fluid to seep through, thus reducing the efficiency of the hydraulic system. Not realising this internal minute leakage, some other rectification is attempted by the technician in order to partially restore the efficiency. Over a period of time, a combination of several such inefficiencies of different seals and also other parts, say in a hydraulic booster, will ultimately lead to premature withdrawal of the component without assigning any one specific reason for its failure.