Let’s say one was making some .040" composite sheet, and instead of vacuum bagging, one was using mechanical clamping force like that which might be provided by a book press (see image in link below), is there a risk of overpressurizing, and if so, what is the consequence? If there is a maximum amount of pressure, what is the value?
http://shard3.1stdibs.us.com/archivesB/upload/8559/20/img_5172.jpg
I’ve done this before and to cut a long story short, you trap air.
I had to do flat sheets of Twintex and my consumables couldn’t take the heat required to cure TT. I did some trial runs by clamping the fabric between two plates of 20mm steel with bolts around the perimeter every 100mm, applied over 1000 psi of equal force by using a torque wrench and after curing it there were small voids where the threads met. I don’t know if this is because TT has huge threads, maybe a 2k carbon would have worked better.
In the end I got high temp consumables, vac bagged it over some stainless and it came out flawless.
I also use vac bagging to make flat sheet carbon, it comes out perfect every time:
Saying that, hot press moulding is essentially a high tech press that’s heated and its used to cure the part. Although I think it still uses a vacuum to draw air, it might involve an elastic membrane. I’ve never been anywhere near a hot press, maybe one of the experts can give you more info on that.
Thanks for the reply. Vacuum bagging is likely out of my range of options for the time being. I want to be able to use mechanical force. It sounds like one could apply more force than might be applied without vacuum, but perhaps not 1000psi 
My outer layer will be unidirectional, and perhaps it is more forgiving with respect to overclamping since there are no voids of the kind presented by a weave.
Overclamping doesn’t cause voids. Presses just have no means of removing air that is trapped in the layup when the two plates are pressed together. You can have air that is trapped within the carbon tow that migrates to areas of lower pressure (weave intersections for example). These will only show up when pressure is applied. The layup could “appear” to be voidless if no pressure is applied.
Well I am a little confused on trapped air and why it might be more of a problem with high clamping force. I recently learned of a product called Bleeder Lease from Airtech, and when used in conjunction with absorbent material like Airweave or Econoweave, it is supposed to allow excess epoxy to leach out under pressure, and I would have assumed trapped air would migrate to the surface of the Bleeder Lease (and the absorbent material) regardless of whether the force is mechanical or from vacuum. What am I missing here?
Is the bleeder designed to work under your anticipated working pressures? Some will just become compressed and not function properly.
I am not sure. Heck, I am not even sure of pressures yet My question was rather general, just wanting to get an idea of the hazards of overpressurizing.
Bleeder Lease is a silicone coated peel ply. Airweave and econoweave are standard breathers. Neither will help with trapped air without vacuum to suck the air out. Is trapped air an issue? What are you making?
Mainly 1" x 1" carbon fiber tube from unidirectional CF. I was hoping to use the two products in conjunction with wrapping under high pressure, or using aluminium angle with clamps to provide force. I know vacuum is the best way to go, but I am trying to work within a budget.
Get an aluminium tube with the external diameter that is equal to the internal diameter you need. Laminate the carbon around it and get some heat shrink tape from airtech, then just throw it in the oven. Mind you, vacuum pumps are pretty cheap, you can pick one up for 2 figures.
If you clamp wet fabric without mold sides, everything will squeeze OUT, including the fibers!!! I deal with compression molded plates often, and if you don’t have a perfect matching mold sides, then the resin will flow to any area it can. Any gaps, means it will flow there, and it will take the fabric with it.
This goes for thermoset and thermoplastic systems, wet layup, prepreg, fabric, and uni.
As for your idea of using angles and pressure, you will have the same issue, except the flow will be to the ends of the angle pieces, and the 90deg bend will be squashed thin.