Pressure in compression molding

[examorph]

What are the effects of pressure in compression molding, is it just there to stop the molding die top from lifting due to thermal expansion of the rubber in the tool or does it have any other advantages?

Are there any calculations that can be made to determine which tool or component requires how much pressure?

 

[stancom]

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.

 

[acspain]

Elastomer compound pressure of 2000 psi/mold is one rule of thumb that I used in the past to determine the size of the press required for a mold-set. Ultimately, at the time, we didn't know the pressure any particular mold reached during the compression molding process. The press force does three things, one it forces the elastomer prep into the cavity and to fill the cavity. Second is to hold the mold closed to enable proper flash removal and since some compounds would foam, to prevent foaming. Third, certain part dimensions will be off if the mold is not fully closed possibly leading to inferior part performance.

So long as the cavity was filled, the part was considered good. Higher or lower pressures was not identified as a reason for a part failing a specification.

I hope that helps.

 

[examorph]

Is there no hand calculation that can be used to determine what pressure is required? if not, could a simulation be ran to analyse this?

 

[acspain]

There is no hand calculation that I know of. Remember that rubber/elastomer is considered incompressible with poisson's values very close to 0.5, think 0.495. Rereading your initial post, compression molds are not closed until the last instant, let say about 0.005 inch from closed so that the elastomer is more or less free to thermally expand. Once closed the compound should be up to temperature (with no additional thermal expansion) and beginning to cure.

There are mold filling simulation software packages available however they are mostly geared to plastics and injection molding. Some simulation packages do have the ability to model elastomers/rubber but characterizing the material for the simulation can be challenging as curing may need to be taken into account or somehow dealt with.

Have a good day!