Let’s say I’m infusing a simple box and the mold area is 500 sqIn. If I stretch the material to fit the corners, the surface area I cut is what I’m trying to wet out, but if I notch the corners of the fabric to fit the box, I must deduct what I’ve cut out. How does stretching, (and compressing), the materials affect the properties of the casting? Aren’t the compressed corners in effect making a denser fabric in that area and less dense on the flats? Is that why stretched fabric seems to infuse slower in the corners? It becomes a more dense fabric there? Is stretched fabric a waste of fabric and matrix at the sake of aesthetics?
I’ve just found this interesting topic and maybe can add a new point to it. Beside the type, mass and orientation of the fibers the fiber volume fraction is one of the most important parameters when designing and calculating a composite layup. Usually the designer specifies a certain fiber volume fraction, which should be the optimum fraction. This optimum is not as much as possible but depends from many factors.
Given the fiber volume fraction it is quite easy to calculate the needed amount of resin (of course without the waste in peelply and tubes and such)
From my engineering point of view your calculation should be inverse. IMHO it would be more useful to calculate the necessary pressure to achieve a certain fiber volume fraction.
What do you think about that?
You can do it both ways. One does not rule out the other.
For optimum infusion quality the pressure needs to be as low as possible, although it is possible to throttle back the pressure afterwards. (will that result in the desired “springback” of the fiber? I do not know (I doubt)
On the other hand, in situations where it is more easy to play with pressure (autoclave) your comment is 100% valid.
I don’t want to hitchhike the Calculator thread, but some more thoughts here.
Sure you can, but what does make more sense, just switch on the pump and live with whatever fiber volume fraction it will bring out and thus not knowing the strength of your part in advance or trying to build a part to spec with the optimum fiber/matrix ratio?
Why? Can you explain this.
Yes, it will spring-back and this is the correct term. Spring-back happens with every infusion and is generally unwanted. You don’t even need to decrease the pressure, there will be a pressure gradient in the infused part of the stack rising towards the resin inlet. That’s why parts tend to be thicker at the inlet. It is getting more homogeneous gradually but at some point the resin viscosity is to high and matrix permeability to low.
In my opinion it is better to start infusion with a lower pressure to keep the permeability and flow area high and help a better wet out while the resin viscosity is still low. Later you can rise the pressure to compact your stack and distribute the resin evenly. Even better you’re doing with a double bag, since as long as the inlet is open and you have only one bag the pressure at the inlet tends to 0.
The autoclave gives you more pressure difference than the ambient pressure you can achieve with vacuum alone, so it has only a wider range. ∆P with autoclave is 0 to whatever it is built for (~10bar), ∆P with vacuum is 0 to ambient pressure (~1bar).
I guess this “as much as possible” attitude has it’s origins in the fact that most homebuilders with their hobby equipment (including me) are just able to generate a little less then enough pressure to get good and consistent fiber volume fraction.