Debulking dry fiber before infusion

[RPM7]

Is it necessary to debulk a dry stack of carbon fiber plies prior to resin infusion?
Most literature I've read states that debulking is necessary for prepregs, as it helps remove the air trapped by the resin between the plies.
I'm just not sure whether this would be necessary for a dry stack of plies.

If debulking is necessary for a dry stack, would you recommend using a bristle roller, or is that only appropriate for piercing the air pockets in wet resin?

Thanks

 

[berkshire]

You are going to pull a vacuum on the stack before starting resin infusion . That by itself should de bulk the plies.
B.E.

You are judged not by what you know, but by what you can do.

 

[Compositepro]

It appears you are asking about a wet lay-up rather than vacuum-bagging a dry preform. In wet lay-up debulking is done during lay-up while the resin is being applied. Debulking of dry preforms with a roller is generally not effective because the fibers will spring back up without a binder or resin to hold them in place. When laying-up dry fibers there is some debulking that naturally occurs as part of the lay-up process, with fingers and rollers pressing fabric into place on a mold. There is friction between fiber in a fabric. When rolling or pressing fabrics into place, some of the friction is overcome to compact them closer together, and then friction will prevent full rebound of the fibers. Over-working of fabrics will increase the bulk of fibers due to entanglement and loss of alignment.

Debulking is simply the process of reducing the bulk volume of a fiber preform. A bristle roller does not "pierce air pockets". The bristle ends press on fibers, compacting them closer together. This displaces resin and air out from between the fibers. This displaced resin and air can flow up between the bristles. Since air has a far lower viscosity than resin, it flows out more rapidly.

 

[RPM7]

Thanks for the replies, you've answered my question.

Is there a way to improve the consolidation of the dry fibres and prevent wrinkling or voids?
I will essentially have a sandwich construction of dry fibre laminate at the bottom, followed by a think foam filler (closed cell), and then another dry laminate stack at the top.
It's a very large structure and is vacuum infused.
I'm concerned that there will be a lot of air pockets and poor resin impregnation and a low Vf.
Are there are any measures that can be taken to improve the manufacturing process.

 

[berkshire]

One way might be to do the job in two operations, do the outer skin first with peel ply added, add the foam and the inner skin in the second operation.

I realize that you are trying to save time and money doing a one step infusion process. One thing to do, if it is possible with your mold system, is to make sure the resin is infused from the bottom of the mold where possible so that the natural buoyancy of the air bubbles allows them to rise to the top of the mold and your bleeder pipes/ manifolds. Also make sure there is enough slack in your bagging system so that you do not get voids and bridges as the vacuum pulls and compacts the dry fiber. If you cannot get enough compaction with atmospheric pressure alone you may have to autoclave. if you have an oven big enough.
B.E.

You are judged not by what you know, but by what you can do.