In ISO 2393, the std for preparing test sheets, the press is required to exert a pressure of not less than 3.5 MPa (500 psi) on the cavity area of the mould. This is intended to minimise flash and to ensure low thickness tolerances. For some soft compounds 1.5 to 2.5 MPa has been found acceptable but for mouldings containing a metal insert a pressure up to 8 MPa has sometimes been advised. Obviously the pressure should never be excessive to avoid mould wear or damage, and lower values will be needed for mould materials less robust than steel.
The effect of the above pressure ranges on properties is acknowledged to be small or negligible, but studies of very high pressures of at least 350 MPa (50000 psi) have been found to change modulus and even to bring about crosslinking in the absence of curatives. For a review of such work do have a look at Martin Bellander's paper High Pressure Vulcanization issued by the Swedish Royal Institute of Technology in 1998. Relevant studies go back to Wilkinson and Gehman in JIEC, 1949, 41, 841-46 and their 1954 patent, US 2682082.