Disclaimer: I haven't tried either of these and they might be very dangerous.
#1-
How stiff are mold halves made out of MDF? What I was thinking was that it might be possible to to away with the press frame. One inch tubular climbing webbing is rated at ~4k lbs tensile strength, though I think knots and bends decrease this somewhat. Assuming that the MDF molds are stiff enough you could place loops of webbing around them to hold them (perpendicular to the length of the ski) together when you pressurize the bladders. 15 loops would have a breaking point of ~60k lbs.
Say with your ski in the mold and the bladder deflated the circumfrence of the mold is 50" and with the bladder partially inflated 55". You could slide loops of webbing with a circumference of 55" over the mold, and maybe put some grooves in the sides of the mold that face awar from the ski for the webbing to sit in.
Nylon webbing does strech somewhat so I think that this would help equalize the force, since they're not all going to be identical.
Thoughts?
#2-
Why not build an "autoclave"? I was looking at a piece of PVC pipe and it's stamped rated to 280 psi which is really high, though it's only 3" diameter. Looking at this page for some pipe company PVC goes up to 24" in diameter and 120 psi (or 210 psi for higher quality) which gives a nice margin of safety (shrapnel is bad for your health). (PVC is plastic, which means heating it makes it WEAKER, the psi rating seem to be for a pipe at a temp of 78 deg F or sort of room temperature.)
The problem with this is the pipe needs to be big enough for you, at the very least, to put a bottom mold for one ski with the ski on it (and a vacume bag you vent to the outside of the pipe) into the pipe. It seems that the largest pipe that's easy to get (say at Home Depot) is 6" which -might- be big enough thought it would be pretty tight. Though now that I think about it [help me out here engineering/physics guys] since the forces on the mold and ski are uniformly distributed the mold only needs to be stiff enough to not warp under the forces of the ski wanting to go back to flat. I just reread that last sentence and even though I don't remember my physics too well it doesn't seem right. Anyway, so having a much thinner mold might make 6" pipe work. And you could allways just chain two (or more really) of these together to do two skis at once.
Thoughts?
Two nonstandard press ideas
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Mdf isn't that stiff or strong to be used without proper support. Without the steel beams, it would probably bend and break with just a little pressure. The strap idea may be good to reinforce the beams to minimize any deflection. However, there is probably more strech in the straps than the beam deflects.
I have looked into the pvc autoclave idea a while back. The problem with pvc is that it doesn't do well at even slight elevated temperatures. At 100F it only has 51% of it's original strength and at 140f it has 22%. It will start to sag and lose its shape quickly. Also the price of just one 24" pvc pipe is so much that you could build a steel presses and still come out ahead. The nice thing about a vacuum press is the mold doesn't have to be as sturdy, but you probably still need a 16" pipe to make it fit. Maybe if you can find a large steel pipe at a surplus yard, that might work better than pvc.
intresting ideas though.
kelvin
I have looked into the pvc autoclave idea a while back. The problem with pvc is that it doesn't do well at even slight elevated temperatures. At 100F it only has 51% of it's original strength and at 140f it has 22%. It will start to sag and lose its shape quickly. Also the price of just one 24" pvc pipe is so much that you could build a steel presses and still come out ahead. The nice thing about a vacuum press is the mold doesn't have to be as sturdy, but you probably still need a 16" pipe to make it fit. Maybe if you can find a large steel pipe at a surplus yard, that might work better than pvc.
intresting ideas though.
kelvin