I need to make a square sheet metal box to be used as a mold for foam expansion. 
Inside dimensions are 83 cms X 83 cms X 4 cms.
The sheet metal I am using is 2 mm thick.
The sides consist of  a frame made of 4cm X 4 cm  square section pipe.
The faces need to stay flat and parallel as the foam expands and
pushes outward in all directions.
What can I use in the way of stiffening ribs on the outside of the
faces and what would their dimesions have to be to prevent them from
bending outward under pressures of up to 20 PSI ?
I can increase the thickness of the sheet metal to a point but cannot
get too heavy because of handling problems.

I don't think that you realize how complex this problem is. A finite
element analysis is probably required for any meaningful results.

Is it appropriate to ask if you will be significantly constraining the
expansion of the foam in the box?
I could envisage that you would just be putting enough of the material
into the box so that it expanded to fill the box but not exert 20 psi.

Thank you very much for both of your comments.
1 - To: redhoss-ga
You are right. We dont have a clue of how complex the mathematics
are!Maybe a simple formula to calculate the deflection of I-beams
under  various loads might be close enough?

2 - To: myoarin-ga
In fact I dont think we will ever get even close to 20 PSI...we just
wanted to have a simple calculation method to figure out our
reinforcement requirements for any pressure and just picked out 20 psi
as an upper limit.
We do want the foam to completly fill the mold and harden to the
cavity shape...no outwards bowing of the mold faces.

A simple way of doin this would be to modle the skin a beam (NOTE not
the ribs).  Then calculate the bending over the length between an
external frame.  This will give you the deflection.  then reduce the
length of the "beam" until the central displacement is acceptable. 
this length will then be the basis for a grid of strengthening ribs
whose bending can be ignored if they are sufficiently large.

Although this may not be necessary the foam may be pumped in at 20PSI
but I doubt it would remain at 20PSI during solidification.  Therfore
assuming the 20PSI doen't take the metal skin past its elastic limit
the mould would "pop" back into the desired shape.

I can provide an anotated hand markup of the above answer.  

Mike

If I read your question correctly you made the box for a mold. 
In this mold you inject foam that will expand from the inside. In
every mold made the trick is not to close it so that the pressure of
20 psi never will be reached. So the trick is to drill holes in the
mold so that the surplus of the foam will go out of the mold. The
smart places of drilling those holes will be on the corners in such
way that the foam equaly get distributed en can harden. After
hardening the excess of foam is removed.

If this is not what you mea I need at least the type of material you
used. Alu, RVS 220 etc
Give responds
Prof Kees