1) Static (stationary tooling) seals are o-rings placed between tooling or fittings that do not move—such as cores trapped and sealed between plates, or cavities housed in a bore with external o-rings that seal against the I.D or walls of the bore.

2) Dynamic (rotating or reciprocating) seals are those where tooling is in motion against an o-ring or quad seal—such as unscrewing cores and cavities. Also in this class are hot runner molds with pneumatic or hydraulic valve gate components. The valve pin pistons contain o-rings that undergo higher temperatures (600 deg with some resins) and higher pressures (500 psi for hydraulic systems) than standard water pressure.
The probable cause list for static and dynamic o-ring failure in molds is as follows:

1) O-ring failure
Compression set, thermal and chemical degradation, incorrect hardness or elastomer

 2) Incorrect mold assembly
Wrong size, twisted, pinched, cut, missing or inadequate lubrication

 3) Tooling and plate condition, design
Cracked tooling, worn/pitted glands, incorrect stack dimensions, gland depth or surface finish, inadequate bolt patterns                      

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